Filed: August 11, 2023 RECOMBINANT BACTERIUM PRODUCING CATIONIC BIOPOLYSACCHARIDE FLOCCULANT AND ITS APPLICATION RELATED APPLICATIONS [0001] This application claims priority to and the benefit of the earlier filing date of China Application No.202210961117.0, filed on August 11, 2022 (and published as CN115960798A on April 14, 2023); and of U.S. Provisional Application No.63/490,454, filed March 15, 2023. Each of these earlier filed applications is incorporated by reference herein in its entirety. FORMAL SEQUENCE LISTING [0002] A computer readable text file, entitled "A248-0035PCT_seqs.xml" created on or about July 25, 2023, with a file size of 16 KB, contains the formal sequence listing for this application and is hereby incorporated by reference in its entirety. FIELD OF THE DISCLOSURE [0003] The disclosure relates to the field of biosynthesis and to the technical field of cationic biological polysaccharide flocculant modification, and to a preparation method and application of a high-molecular-weight cationic biological polysaccharide flocculant prepared by bacterial fermentation. BACKGROUND OF THE DISCLOSURE [0004] At present, the flocculants commonly used in industry can be divided into two types: inorganic flocculants and organic flocculants. The inorganic flocculants include alum (aluminum sulfate), ferric chloride, ferrous sulfate, and lime. Among them, aluminum salt flocculants have problems such as light flocs formed, poor sedimentation performance, and residues that are harmful to human body; iron salt flocculants also have problems such as effluent residue and secondary pollution. A small amount of acrylamide monomer remaining in polyacrylamide in organic flocculant has neurotoxicity and strong carcinogenicity. It is worth noting that with the continuous development of society, people's requirements for the environment are constantly improving, making sustainability an important consideration including for sewage treatment technology. Therefore, compared with inorganic flocculants, the advantages of natural flocculants with environmentally friendly characteristics are becoming more and more obvious. At present, natural flocculants with good biocompatibility mainly include starch, gelatin, cellulose, sodium alginate, chitosan, tannin, and gum. Filed: August 11, 2023 [0005] The flocculation mechanism of flocculants for suspended solids in sewage mainly includes charge neutralization, electrostatic charge repair, polymer bridging, and polymer adsorption. Without chemical modification, most natural flocculants are non-ionic, which limits the use scenarios of these flocculants. Chitosan, with its high molecular weight and high cation density, possesses a series of attractive physical, chemical, and biological properties. Due to the amino and hydroxyl functional groups contained in chitosan, it exhibits unique polycationic, chelating and film-forming properties. When chitosan is dissolved in an acidic environment, the amino groups on the chain can be protonated, and the polymer becomes a cationic polymer that can interact with various types of molecules. At present, chitosan has a wide range of applications, such as biomedicine, cosmetics, papermaking, sewage treatment, agricultural or pharmaceutical applications, etc. Its non-toxic, biocompatibility and degradability characteristics make it widely concerned in water treatment. In the field of water treatment, especially in the field of adsorption and flocculation, chitosan is a very valuable biocompatible material. [0006] According to existing literature reports, under weakly acidic conditions, chitosan has excellent removal efficiency for various dyes, such as acid black 1, acid violet 5, reactive black 5, acid blue 92, and Congo red. Under acidic conditions, in addition to flocculating anion suspended particles, removing colored wastewater, and reducing the turbidity of drinking water, chitosan can also be used as a chelating agent to remove a variety of heavy metal ions in industrial wastewater, especially for Mn (II), Zn (II), As (V), As (III), Cu and Cr plasmas have high removal rates. Chitosan can also inhibit the growth of various microorganisms including algae. Chitosan inhibits bacteria in several ways. The cell wall is usually composed of peptidoglycan, and the abundant carboxyl groups on the peptidoglycan often make it negatively charged, and the positively charged chitin flocculant can be tightly bound to the negatively charged bacterial cell wall under the action of electrostatic force. In combination, chitosan can limit the movement of bacteria through cross- linking after combining with bacteria. While chitosan inhibits microorganisms, it exhibits lower toxicity in mammalian cells. [0007] While chitosan has many advantages in the field of water treatment, it also has some problems, which affect its application range. Two reasons for currently limiting the use of chitosan are cost and low solubility. SUMMARY OF THE DISCLOSURE [0008] Cationic polysaccharide chemicals similar to chitosan can be synthesized through synthetic biology methods, and the cost can also be reduced by adjusting the molecular structure to increase its water solubility. More importantly, under the double carbon target for carbon Filed: August 11, 2023 neutrality, CO ₂ can be used as a substrate to participate in the synthesis of target products through synthetic biological methods. [0009] Disclosed herein are recombinant bacteria useful for producing cationic biological polysaccharide flocculant, the produced polysaccharide flocculant, methods of production and applications thereof. Embodiments of the recombinant bacterium use Citrobacter as the starting strain, to which is introduced a gene encoding ribulose carboxylase 1,5-bisphosphate, a gene encoding ribulose phosphorylase, the aceB gene of malate synthase is repressed or knocked out (e.g., inactivated), and the gene encoding Bacillus rhodopsin (bacteriorhodopsin) gene is introduced. The polysaccharide flocculant produced by the strain has good antibacterial, flocculation and other functions, and the production process is simple, easy to control, biodegradable, excellent performance, wide application range, high application value and broad market prospect. [0010] Provided herein in a first embodiment is a recombinant Citrobacter bacterial strain, including: a gene encoding a ribulose 1,5-bisphosphate carboxylase, a gene encoding a ribulose phosphorylase, an inactivated malate synthase aceB gene, and a gene encoding a bacteriorhodopsin. In examples of this embodiment, the recombinant Citrobacter bacterial strain includes one or more of: the gene encoding the ribulose 1,5-bisphosphate carboxylase includes a sequence at least 80% identical to the sequence of SEQ ID NO: 1; and/or the gene encoding the ribulose phosphorylase includes a sequence at least 80% identical to the sequence of SEQ ID NO: 5; and/or the gene encoding the bacteriorhodopsin includes a sequence at least 80% identical to the sequence of SEQ ID NO: 7. In further examples, the recombinant Citrobacter bacterial strain includes one or more of: the ribulose 1,5-bisphosphate carboxylase includes a sequence at least 80% identical to the sequence of SEQ ID NO: 2; and/or the ribulose phosphorylase includes a sequence at least 80% identical to the sequence of SEQ ID NO: 6; and/or the bacteriorhodopsin includes a sequence at least 80% identical to the sequence of SEQ ID NO: 8. [0011] Also provided are embodiments in which, in the recombinant Citrobacter bacterial strain, the ribulose 1,5-bisphosphate carboxylase includes the sequence of SEQ ID NO: 2; and/or the ribulose phosphorylase includes the sequence of SEQ ID NO: 6; and/or the bacteriorhodopsin includes the sequence of SEQ ID NO: 8. By way of example, the recombinant Citrobacter bacterial strains in embodiments have a ribulose 1,5-bisphosphate carboxylase that includes the sequence of SEQ ID NO: 2; a ribulose phosphorylase that includes the sequence of SEQ ID NO: 6; and a bacteriorhodopsin that includes the sequence of SEQ ID NO: 8. Filed: August 11, 2023 [0012] Another embodiment is a method of making the recombinant Citrobacter bacterial strain of embodiment 1 or another embodiment herein, which method includes: knocking out or otherwise inactivating function a Citrobacter aceB gene in the genome of a Citrobacter spp. bacteria; and inserting into the genome of the Citrobacter spp. bacteria: a nucleic acid sequence encoding a ribulose 1,5-bisphosphate carboxylase; a nucleic acid sequence encoding a ribulose phosphorylase; and a nucleic acid sequence encoding a bacteriorhodopsin. The introduced nucleic acids may be joined to each other, or not; they may be introduced concurrently or in sequence in any order. In examples of this embodiment, the method further including inserting into the genome of the Citrobacter spp. bacteria a nucleic acid sequence encoding a marker, such as a selectable marker or a counter selectable marker. [0013] Specific examples of the provided methods of making a recombinant Citrobacter bacterial strain including: construction of a CBB cycle and the knockout of a aceB gene, including: cloning of cbbM and prkA genes; amplify ~ 500bp homology regions upstream and downstream of aceB (GEN2534) by PCR; using the pkd4 plasmid as a template, amplify the KanR resistance gene fragment with FRT sites at both ends by PCR, and use it as a recombinant clone screening marker; connecting the fragments to the aceB upstream homology arm, cbbM, ldh promoter, prkA, KanR resistance fragment with FRT site, and aceB downstream homology arm in sequence to obtain gene targeting fragments; using a γ-Red or other recombination system, targeting fragment into the competent Citrobacter with pkd46 plasmid via electroporation, and screening the recombinant strain after incubation for a period of time; introducing bacillus rhodopsin into the Citrobacter genome, using the bacillus rhodopsin gene of Halobacterium salinarum but codon- optimized and transferred to Citrobacter by electroporation, and resistance screening and PCR screening the resultant transformants, to obtain a single-crossover strain(s); and inoculating single-crossover strain(s) on to LB plates to screen for double-crossover strains. [0014] Also provided are methods of producing a cationic biopolysaccharide (such as a cationic biopolysaccharide that is useful as a flocculant), the method including: growing a recombinant Citrobacter bacterial strain as described herein, such as a recombinant strain according to embodiment 1, in a fermentation medium including or to which is introduced a carbon source thereby producing a bacterial culture, and extracting from the bacterial culture the cationic biopolysaccharide. By way of example, extracting the cationic biopolysaccharide from the bacterial culture in embodiments includes alcohol washing of the bacterial culture or of a supernatant produced by centrifugation of the bacterial culture. [0015] The provided methods of producing a cationic biopolysaccharide may further include: inoculating the recombinant Citrobacter bacterial strain into a fermentation medium, providing the Filed: August 11, 2023 culture with carbon dioxide as a carbon source, and allowing the culture to ferment under culture fermentation conditions, to produce the cationic biopolysaccharide. [0016] In various examples of the methods of producing a cationic the carbon source during fermentation includes one or more of: carbon dioxide, fatty acids, fatty alcohols, monosaccharides, and oligosaccharides. In specific instances, the carbon source includes carbon dioxide. Optionally, the carbon dioxide is filtered through a 0.22 micron filter membrane and may be passed into the fermenter to maintain the carbon dioxide in a headspace above the fermenter at a pressure of, for instance, 0.2Mpa. [0017] In examples of the methods of producing a cationic biopolysaccharide, the culture fermentation conditions include one or more of: fermented at 35-37°C, mixing rotation speed 50- 300 rpm, pH controlled at 7.0 with 1M sodium hydroxide solution, inoculum size 5%, fermentation time for at least 15 hours, at least 20 hours, at least 25 hours, at least 30 hours, or more than 30 hours; and/or anaerobic or substantially anaerobic fermentation. By way of example, the fermentation medium may further include: caprylic acid 2 g/L, sucrose 5 g/L, yeast extract 10 g/L, peptone 5 g/L, ammonium chloride 1 g/L, and trace elements (e.g., ZnSO47H2O 2.8 g/L, H3BO4 9.4 g/L, CuSO45H2O 5g/L, and MnCl24H2O 4.6 g/L). [0018] Also provided herein are cationic biological polysaccharide flocculants, prepared by fermentation of a recombinant Citrobacter bacterial strain as provided herein, such as in of any one of embodiments 1-5, or prepared by the method of any one of embodiments 10-17. [0019] Yet another embodiment is a method of precipitating (flocculating) contaminants from an aqueous sample, the method including contacting the aqueous sample with an amount of a cationic biological polysaccharide as provided herein, such as the cationic biological polysaccharide flocculant of embodiment 18. By way of example, the aqueous sample from which contaminants are removed can be or contain a wastewater. Wastewaters include sewage, industrial waste streams, household waste streams, and the like. BRIEF DESCRIPTION OF THE DRAWINGS [0020] The accompanying drawings are used to provide a further understanding of the present disclosure, and constitute a part of the description, and are used together with the embodiments of the present disclosure to explain the present disclosure, and do not constitute a limitation to the present disclosure. [0021] FIG.1 illustrates the γ-Red genetic expression system, where the aceB gene is replaced by cbbM and prkA. Filed: August 11, 2023 [0022] FIGs.2A-2D is a series of liquid chromatography-mass spectrometry tracings, illustrating detection of the reference molecule chitosan (FIG.2A) and the acid hydrolysis monomer of the synthetic product of recombinant Citrobacter NK02-2 (FIGs.2B-2D), as exemplified herein. This series illustrates HPLC-MS analysis of the cationic polymer mononer; it demonstrates the monomer belongs to the glycosylamine series. [0023] FIG. 3 is the infrared spectrum detection of the polysaccharide synthesized by recombinant Citrobacter NK02-2. [0024] FIG.4 is an image of the cationic polysaccharide flocculation effect of an embodiment of the present disclosure. [0025] FIG.5 illustrates treatment of sludge from Jilin Petrochemical Wastewater Plant (from left to right polyaluminum, cationic polyacrylamide, chitosan, polysaccharide of the present disclosure). [0026] FIG.6 illustrates treatment of sludge from Jilin domestic sewage plant (from left to right polyaluminum, cationic polyacrylamide, chitosan, polysaccharide of the present disclosure). [0027] FIG.7 illustrates treatment of sludge from Xinjiang Oilfield Sewage Plant (from left to right polyaluminum, cationic polyacrylamide, chitosan, polysaccharide of the present disclosure). SEQUENCES [0028] The nucleic acid and/or amino acid sequences described herein, and provide herewith in the formal Sequence Listing, are shown using standard letter abbreviations, as defined in 37 C.F.R. §1.822. Only one strand of each nucleic acid sequence is shown, but the complementary strand is understood as included in embodiments where it would be appropriate. [0029] SEQ ID NO: 1 is the nucleic acid sequence of a cbbM gene: aagttaagaggcaagaatgcatcatcaccatcaccacatggaccagtcatctcgttacgt caatctggcgctcaaggaagaggatct gatcgccggcggcgagcatgtgctttgtgcctatatcatgaagcccaaggccggatatgg ctatgtggcgaccgcggcgcatttcgcc gccgagagttcgacgggcaccaacgtcgaggtctgcaccaccgacgatttcacccggggc gtcgacgccctggtctatgaggtgga cgaggcccgcgagctgaccaagatcgcctatccggtggctttgttcgaccgcaacatcac cgacggcaaggcgatgatcgcctcgtt cctgacgctcaccatgggaaacaaccagggtatgggcgacgtggaatacgccaagatgca cgatttctatgtgcccgaggcttatcg cgccctgtttgatggcccgagcgtcaatatctcggccctgtggaaagtgctggggcggcc cgaggtcgacggcggtctggtcgtcggc acgatcatcaagccgaagctcggcctgcgtcccaagcccttcgccgaggcctgccacgcc ttctggctgggcggcgacttcatcaag aacgacgagccccagggcaatcagcccttcgcccccttgcgcgacaccatcgccctggtc gccgacgccatgaggcgggcccag gacgagaccggcgaggccaagctgttctcggccaatatcaccgccgacgatcccttcgag atcatcgcccgtggcgagtatgtgctg gagaccttcggcgagaacgcctcgcatgtcgccttgctggtcgacggctatgtcgccggc gccgcggcgatcaccacggcgcgccg ccgcttccccgataacttcttgcattatcaccgggctggccacggcgccgtcacctcgcc ccagtccaagcgcggctataccgccttcg Filed: August 11, 2023 tccattgcaagatggcccgccttcaaggcgccagcggcatccacaccggcaccatgggct ttggcaagatggaaggcgagtccag cgaccgcgccatcgcctatatgctgacccaggacgaggcccaggggccgttctaccgtca atcctggggcggcatgaaggcttgta cgccgatcatcagcggcggcatgaacgccctgcgcatgcccggcttcttcgagaacctgg gtaatgccaatgtcatcttgaccgccgg cggcggcgccttcggccatatcgacggcccggtggccggggcgcggtcgttgcgtcaagc ctggcaagcctggcgggacggggtt ccggttctggactatgcccgcgagcacaaggaactggcccgcgccttcgagtccttcccc ggcgacgccgaccagatctatccggg ctggcgcaaggccctgggcgtcgaggacacccgcagcgcccttccggcgta [0030] SEQ ID NO: 2 is the amino acid sequence of the 1,5-bisphosphate ribulose carboxylase encoded by the cbbM gene shown in SEQ ID NO: 1: MADRLRATYRVRATAAAIEARAKGLAVEQSVEMPLSAIDDPAVLDGIVGVVEEITERGED CFDV RLALATATTGGDAGQLFNMLFGNSSLQDDTVLLDIDLPDDLLASFGGPNIGAAGLRARVG ASAD RALTCSALKPQGLPPDRLADLARRMALGGLDFIKDDHGMADQAYAPFASRVGAVAAAVDE VN RQTGGQTRYLPSLSGHLDQLRSQVRTGLDHGIDTFLIAPMIVGPSTFHAVVREFPEAAFF AHPT LAGPSRIAPPAHFGKLFRLLGADAVIFPNSGGRFGYSSETCQAVAEAALGPWGGLHASLP VPA GGMSLARVPEMIATYGPDVIVLIGGNLLEARDRLTEETAAFVASVAGAASRGCGLAP [0031] SEQ ID NO: 3 is the nucleic acid sequence of a prkA gene: atgcatcatcaccatcaccacagcaagccagatcgtgttgttttgatcggcgttgccggt gactccggttgcggcaaatcaaccttccta aatcgccttgccgacttgtttggtacggaattgatgacggtcatctgcttggatgactat cacagtctcgatcgcaagggccggaaggaa gcaggcgtaacggctttggatccccgcgccaacaactttgacttgatgtatgaacaggtc aaggcgttgaagaacggcgaaacgatc atgaagccgatctacaaccatgaaaccggcttgatcgatccgcccgaaaaaatcgaaccc aatcgcatcattgtgatcgagggtctg catccgctttacgacgagcgcgtgcgtgaactgctcgatttcagcgtttacctcgacatc gatgacgaagtcaaaatcgcttggaagat ccaacgcgatatggcagaacgcggccactcctacgaagatgtcctcgcctcgatcgaagc gcgccgccctgacttcaaggcctaca ttgagccccagcgtggccatgcggacatcgtcatccgcgtcatgccgacccagctaatcc ccaatgacaccgagcgcaaggtgctg cgggtgcagttgatccaacgggaaggccgcgatggttttgagccggcttacctgttcgac gaaggttcgaccatccagtggacgccct gcggtcgtaagctgacctgctcctatccgggcattcgcttagcctacggccctgacacct actacggtcacgaagtctcagtgcttgag gtcgacggtcagttcgagaacctcgaagagatgatctacgtcgagggccacctcagcaag accgacacgcagtactacggtgagtt gacccacctgctgctacagcacaaagattacccgggttcgaacaacggcacgggtctgtt ccaagtgctgaccggcctgaaaatgc gggcggcctatgagcgtttgacctcccaagcagcacccgtcgccgctagtgtctag [0032] SEQ ID NO: 4 is the amino acid sequence of the phosphoribulokinase encoded by the prkA gene shown in SEQ ID NO: 3: MSVKHPIIAITGSSGAGTTSVTRTFEQIFRREGVNAAVVEGDSFHRNDRKAMKIAMAEAQ KAGN ANFSHFGPEANLFEELETLFRTYGETGGGRRRLYLHNDEEAAPFAQEPGTFTPWEDLPES DLL FYEGLHGAVVTDTVDVAQHADLKIGVVPVINVEWIQKLHRDRAARGYSTEAVTDTILRRM PDYV HYICPQFTRTDVNFQRVPLVDTSNPFVARHVPSADESFVVIRFRDPKGIDFPYLLNMLND SFMS RPNTIVVPGGKMELSMQLIFTPFIWRFMDKRARALGR (SEQ ID NO: 4) Filed: August 11, 2023 [0033] SEQ ID NO: 5 is the nucleic acid sequence of a bacteriorhodopsin (bop) gene, codon opt: atgttggagttattgccaacagcagtggagggggtatcgcaggcccagatcaccggacgt ccggagtggatctggctagcgctcggt acggcgctaatgggactcgggacgctctatttcctcgtgaaagggatgggcgtctcggac ccagatgcaaagaaattctacgccatc acgacgctcgtcccagccatcgcgttcacgatgtacctctcgatgctgctggggtatggc ctcacaatggtaccgttcggtggggagca gaaccccatctactgggcgcggtacgctgactggctgttcaccacgccgctgttgttgtt agacctcgcgttgctcgttgacgcggatca gggaacgatccttgcgctcgtcggtgccgacggcatcatgatcgggaccggcctggtcgg cgcactgacgaaggtctactcgtaccg cttcgtgtggtgggcgatcagcaccgcagcgatgctgtacatcctgtacgtgctgttctt cgggttcacctcgaaggccgaaagcatgcg ccccgaggtcgcatccacgttcaaagtactgcgtaacgttaccgttgtgttgtggtccgc gtatcccgtcgtgtggctgatcggcagcga aggtgcgggaatcgtgccgctgaacatcgagacgctgctgttcatggtgcttgacgtgag cgcgaaggtcggcttcgggctcatcctc ctgcgcagtcgtgcgatcttcggcgaagccgaagcgccggagccgtccgccggcgacggc gcggccgcgaccagcgactga [0034] SEQ ID NO: 6 is the amino acid sequence of the bacillus rhodopsin encoded by the bop gene shown in SEQ ID NO: 5: LWLGTAGMFLGMLYFIARGWGETDGRRQKFYIATILITAIAFVNYLAMALGFGLTFIEFG GEQHPI YWARYTDWLFTTPLLLYDLGLLAGADRNTIYSLVSLDVLMIGTGVVATLSAGSGVLSAGA ERLV WWGISTAFVLLYFLFSSLSGRVANLPSDTRSTFKTLRNLVTVVWLVYPVWWLVGSEGLGL VGI GIETAGFMVIDLVA [0035] SEQ ID NO: 7 is the nucleic acid sequence of a Malate synthase aceB gene: atgactgaacaggcaacaacaaccgatgaactggctttcataaggccgtatggcgagcag gagaagcaaattcttactgccgaagc ggtagaatttctgactgagctggtgacgcattttacgccacaacgcaataaacttctggc agcgcgcattcagcagcagcaagatattg ataacggaactttgcctgattttatttcggaaacagcttccattcgcgatacggactgga aaattcgtggtattcccgcggacttacaagat cgtcgagtcgagataactggcccggttgagcgcaagatggtgatcaacgcactgaacgcc aatgtgaaagtctttatggccgatttcg aagattcactggcaccagactggaacaaagtgatcgacgggcaaattaacctgcgcgatg cggttaacggcaccatcagctatacc aatgaagcaggcaaaatttatcagctcaagcccaatccagcggttttgatttgtcgggta cgcggtctgcacttgccggaaaaacatgt cacctggcgtggtgaggcaatccccggtagcctgtttgattttgcgctctatttcttcca caactatcaggcactgttggcaaagggcagc ggtccctatttctatctgccgaaaacccagtcctggcaggaagcggcctggtggagcgaa gtcttcagctatgcagaagatcgctttaa tctgccgcgcggcaccatcaaggcgacgttgctgattgaaacgctgcccgccgtgttcca gatggatgaaatccttcacgcgctgcgt gaccatattgttggtctgaactgcggtcgttgggattacatcttcagctatatcaaaacg ttgaaaaactatcccgatcgcgtcctgccag acagacaggcagtgacgatggataaaccattcctgaatgcttactcacgcctgttgatta aaacctgccataaacgcggtgcttttgcg atgggcggcatggcagcgtttattccgagcaaagatgaagagcgcaataaccaggtgctc aacaaagtaaaagcggataaagcg ctggaagccaataacggtcacgatggcacatggattgctcacccaggccttgcggatacg gcaatggcggtattcaacgacattctc ggctcccgtaaaaatcagcttgaagtgatgcgcgaacaagacgtgccgattactgccgat cagctgctggcaccttgtgatggtgaat gcaccgaagaaggtatgcgcgccaacattcgcgtggcagtgcagtacatcgaagcatgga tctccggcaacggctgcgtgccgatt tatggcctgatggaagatgcggcgacggctgaaatttcccgtacctcaatctggcagtgg atccatcatcaaaaaacgttgagcaatg gcaaaccggtgaccaaagccttgttccgc Filed: August 11, 2023 [0036] SEQ ID NO: 8 is the amino acid sequence of the malate synthase encoded by the aceB gene shown in SEQ ID NO: 7: MTEQATTTDELAFIRPYGEQEKQILTAEAVEFLTELVTHFTPQRNKLLAARIQQQQDIDN GTLPD FISETASIRDTDWKIRGIPADLQDRRVEITGPVERKMVINALNANVKVFMADFEDSLAPD WNKVI DGQINLRDAVNGTISYTNEAGKIYQLKPNPAVLICRVRGLHLPEKHVTWRGEAIPGSLFD FALYF FHNYQALLAKGSGPYFYLPKTQSWQEAAWWSEVFSYAEDRFNLPRGTIKATLLIETLPAV FQM DEILHALRDHIVGLNCGRWDYIFSYIKTLKNYPDRVLPDRQAVTMDKPFLNAYSRLLIKT CHKRG AFAMGGMAAFIPSKDEERNNQVLNKVKADKALEANNGHDGTWIAHPGLADTAMAVFNDIL GSR KNQLEVMREQDVPITADQLLAPCDGECTEEGMRANIRVAVQYIEAWISGNGCVPIYGLME DAA TAEISRTSIWQWIHHQKTLSNGKPVTKALFRQMLGEEMKVIASELGEERFSQGRFDDAAR LME QITTSDELIDFLTLPGYRLLA [0037] SEQ ID NO: 9 corresponds to positions 99-298 of SEQ ID NO: 4. GRRRLYLHNDEEAAPFAQEPGTFTPWEDLPESDLLFYEGLHGAVVTDTVDVAQHADLKIG VVP VINVEWIQKLHRDRAARGYSTEAVTDTILRRMPDYVHYICPQFTRTDVNFQRVPLVDTSN PFVA RHVPSADESFVVIRFRDPKGIDFPYLLNMLNDSFMSRPNTIVVPGGKMELSMQLIFTPFI WRFM DKRARALGR DETAILED DESCRIPTION [0038] Grease in kitchen waste undergoes β-oxidation of fatty acids to produce a large amount of reduced hydrogen and acetyl-CoA. Under the pressure of reducing hydrogen, the present disclosure utilizes Citrobacter and uses carbon dioxide as an electron acceptor to convert a molecule of acetyl-CoA, carbon dioxide and two molecules of reduced hydrogen are converted into pyruvate, and then carbon dioxide is introduced into the synthesis of polysaccharides through gluconeogenesis. In this process, a large amount of ATP is consumed in the process of assimilating carbon dioxide and synthesizing cationic polysaccharides by Citrobacter. However, the shortage of ATP under anaerobic conditions limits the polysaccharide synthesis of Citrobacter. Therefore, described herein is construction of a recombinant Citrobacter, which can synthesize a large amount of target cationic polysaccharides under anaerobic conditions with carbon dioxide as a substrate. The synthesized polysaccharide flocculant has good antibacterial and flocculation functions. The disclosure provides embodiments with simple production process, easy control, biodegradability, excellent performance, wide application range, high application value, and broad market prospect. [0039] To solve the technical problem that the shortage of ATP under anaerobic conditions limits the polysaccharide synthesis of Citrobacter, introduction of bacillus rhodopsin can enable Filed: August 11, 2023 Citrobacter to synthesize ATP by using the proton concentration difference formed by light energy, thereby promoting its growth and accelerating its metabolite(s) accumulation. In addition, the Citrobacter also can utilize hydrogen, which is coupled to the synthesis of ATP during the process of converting hydrogen into intracellular reduced hydrogen. It has been reported that in Citrobacter, every molecule of hydrogen assimilated will be accompanied by the formation of 0.4 molecule of ATP. Excess reduced hydrogen further drives the rate of carbon dioxide utilization. [0040] To maximize the use of hydrogen and light energy, cbbM and prkA genes were introduced into Citrobacter to build a CBB cycle for fixing carbon dioxide. Hydrogen and ATP were used to further increase the amount of carbon dioxide fixed and reduce the amount of organic carbon. In addition, aceB, a key gene of the glyoxylate cycle in this strain, can convert two molecules of acetyl-CoA into one molecule of carbon dioxide and one molecule of pyruvate. Not only the endogenous carbon dioxide is produced, but also the utilization efficiency of organic carbon is further reduced, so the gene needs to be knocked out to reduce the formation of endogenous carbon dioxide. [0041] Therefore, the present disclosure provides a recombinant Citrobacter, which is obtained by combining a gene encoding 1,5-bisphosphate ribulose carboxylase (for example, cbbM gene), a gene encoding phosphoribulose kinase (for example, PRKA /PKA gene) was introduced into the bacterium Citrobacter, and then the original malate synthase aceB gene was knocked out; finally, the gene (for example, bop gene) encoding bacillus rhodopsin was introduced to obtain. [0042] In the recombinant Citrobacter of the present disclosure, the starting bacterium Citrobacter can be Citrobacter NK02. [0043] Though illustrated herein with one exemplar Citrobacter strain, it is believed that any species (e.g., C. koseri, C. freundii, C. amalonaticus, C. farmeri, C. youngae, C. braakii, C. werkmanii, C. sedlakii, C. rodentium, C. gillenii, and C. murliniaeand) of genus Citrobacter bacteria can be used. Thus, the methods, procedures, and compositions illustrated herein enable additional recombinant Citrobacter spp. strains. [0044] In the recombinant Citrobacter of the present disclosure, the gene encoding ribulose 1,5- diphosphate carboxylase may be able to catalyze the carboxylation reaction of ribulose 1,5- diphosphate with carbon dioxide or the reaction with oxygen. As the gene for the oxidation reaction, examples thereof may include the cbbM gene derived from the photoheterotrophic bacterium Rhodospirillum rubrum. Preferably, the gene encoding ribulose carboxylase 1,5- bisphosphate can be as described in SEQ ID NO: 1, or SEQ ID NO: 1 is substituted and/or deleted by one or several nucleotide residues and/or a DNA molecule derived from SEQ ID NO: 1 added and having the same function. Filed: August 11, 2023 [0045] A representative cbbM gene sequence is shown in SEQ ID NO: 1. [0046] Further, the ribulose 1,5-bisphosphate carboxylase may be as described in SEQ ID NO: 2, or the amino acid sequence shown in SEQ ID NO: 2 is substituted and/or deleted by one or several amino acid residues and/or a protein derived from SEQ ID NO: 2 added and having the same function. [0047] The amino acid sequence of the 1,5-bisphosphate ribulose carboxylase encoded by the cbbM gene is shown in SEQ ID NO: 2. [0048] In the recombinant Citrobacter of the present disclosure, the gene encoding ribulose phosphorylase may be able to catalyze the phosphorylation of ribulose 5-phosphate by ATP to produce ribulose 1,5-diphosphate. An example thereof may include the gene of PRKA/PKA from the photoheterotrophic bacterium Rhodospirillum rubrum. Preferably, the gene encoding phosphoribulokinase can be as described in SEQ ID NO: 3, or SEQ ID NO: 3 undergoes substitution and/or deletion and/or addition of one or several nucleotide residues and has the same Functional DNA molecule derived from SEQ ID NO: 3. [0049] A representative prkA gene sequence is shown in SEQ ID NO: 3. [0050] In the recombinant Citrobacter of the present disclosure, preferably, the phosphoribulokinase can be as described in SEQ ID NO: 4, or the amino acid sequence shown in SEQ ID NO: 4 is substituted by one or several amino acid residues and/or or a protein derived from SEQ ID NO: 4 that is missing and/or added and has the same function. A representative shorter version is provided in SEQ ID NO: 9. [0051] The amino acid sequence of the phosphoribulokinase encoded by the prkA gene is shown in SEQ ID NO: 4. [0052] In the recombinant Citrobacter of the present disclosure, the gene encoding bacteriorhodopsin may be a gene capable of synthesizing pigment proteins such as bacteriorhodopsin. An example thereof may include the gene of bop1,2 from Halobacterium salinarum. Preferably, the gene encoding bacteriorhodopsin can be as described in SEQ ID NO: 5, or SEQ ID NO: 5 undergoes substitution and/or deletion and/or addition of one or several nucleotide residues and has the same function A DNA molecule derived from SEQ ID NO: 5. [0053] A representative bacteriorhodopsin gene sequence is shown in SEQ ID NO: 5. [0054] In the recombinant Citrobacter of the present disclosure, preferably, the bacteriorhodopsin can be as described in SEQ ID NO: 6, or the amino acid sequence shown in SEQ ID NO: 6 is substituted by one or several amino acid residues and/or A protein derived from SEQ ID NO: 6 that has been deleted and/or added and has the same function. Filed: August 11, 2023 [0055] The amino acid sequence of bacillus rhodopsin encoded by the bop gene is shown in SEQ ID NO: 6. [0056] In the recombinant Citrobacter of the present disclosure, the gene encoding malate synthase can catalyze the synthesis of malate from acetyl-CoA and glyoxylate (S). This enzyme belongs to the class of acyltransferases in transferases and converts acyl groups to alkyl groups during the transfer process. An example thereof may include the gene of aceB from Citrobacter. Preferably, the gene of the malate synthase can be as described in SEQ ID NO: 7, or SEQ ID NO: 7 has undergone substitution and/or deletion and/or addition of one or several nucleotide residues and has the same function DNA molecule derived from SEQ ID NO: 7. [0057] A representative malate synthase aceB gene sequence is shown in SEQ ID NO: 7. [0058] In the recombinant Citrobacter of the present disclosure, preferably, the malate synthase can be as described in SEQ ID NO: 8, or the amino acid sequence shown in SEQ ID NO: 8 is substituted by one or several amino acid residues and/or A protein derived from SEQ ID NO: 8 that has been deleted and/or added and has the same function. [0059] The amino acid sequence of the malate synthase encoded by the aceB gene is shown in SEQ ID NO: 8. [0060] Steps (which are described in an illustrative order, though that order is not essential) of a representative construction process of the recombinant Citrobacter of the present disclosure: [0061] The first step: construction of CBB cycle and knockout of aceB gene [0062] 1.1 Cloning of cbbM and prkA genes: Firstly, the genome of the photoheterotrophic bacterium Rhodospira rubrum was extracted, and its cbbM gene and prkA gene were cloned by PCR. Then, the Citrobacter lactate dehydrogenase (LDH) promoter was cloned by PCR, and frozen. [0063] 1.2 Amplify the 500 bp homology regions upstream and downstream of aceB (GEN2534) by PCR. [0064] 1.3 Using the pkd4 plasmid as a template, the KanR resistance gene fragment with FRT sites at both ends was amplified by PCR as a recombinant clone screening marker. [0065] 1.4 Then connect the fragments by Gibson Assembly Connect the aceB upstream homology arm, cbbM, ldh promoter, prkA, KanR resistance fragment with FRT site, and aceB downstream homology arm in sequence and use high-fidelity enzymes Amplify to obtain gene targeting fragments. Filed: August 11, 2023 [0066] 1.5 Using the γ-Red system, the targeting fragment was sent into the competent Citrobacter NK02-1 carrying the pkd46 plasmid by means of an electroporator, and the recombinant strain Citrobacter NK02-1 was screened after incubation for a period of time. [0067] The second step: the introduction of bacteriorhodopsin. Using the bacillus rhodopsin gene of Halobacterium salinarum as a template, a codon-optimized bacillus rhodopsin gene fragment was synthesized from a long fragment. After PCR amplification, it was connected with the 1000 bp fragments of the upper and lower reaches of the Citrobacter ldh gene by over-lap PCR, and ligated by enzyme digestion, and the fragment was connected to the pMMB67EH vector. This recombinant plasmid was then transferred into Citrobacter by electroporation. After resistance screening and PCR screening, the strains with recombinant plasmids were transferred to a 42°C incubator to screen for single exchange strains. The single-crossover strain was screened and then inoculated on LB plates containing 6% sucrose to screen the double-crossover strain Citrobacter NK02-2. [0068] Step 3: Synthesize the cationic polysaccharide flocculant by anaerobic fermentation using the above-mentioned recombinant Citrobacter bacterium. Representative culture conditions are as follows: 35-37°C, 50-300 rpm, pH=7.0, and the fermentation time is 30 hours. The medium components used are as follows: carbon source 1-100 g/L, yeast extract 10 g/L, peptone 5 g/L, ammonium chloride 1 g/L, etc. Carbon sources include one or more carbon sources such as carbon dioxide, fatty acids, fatty alcohols, monosaccharides, and oligosaccharides, among which carbon dioxide is considered a highly beneficial carbon source. Anaerobic condition can be beneficial, because production will then consume less energy. [0069] Step 4: After the fermentation is finished, the fermentation liquid is centrifuged at 4000 g for 10 minutes to remove bacteria, then an equal volume of ethanol is added for coagulation, and the precipitate is collected after centrifugation at 12,000 g for 20 minutes. The primary product of the polysaccharide flocculant was obtained, and after repeated washing with ethanol for 3 times, a colorless white precipitate was obtained, which was the cationic polysaccharide flocculant. [0070] Step 5: The structure identification of the obtained cationic polysaccharide flocculant is carried out by infrared broad spectrum, liquid phase mass spectrometry, nuclear magnetic resonance and other means. [0071] Antibacterial and water purification flocculation experiments were carried out on the obtained cationic polysaccharide flocculant to verify its performance. The obtained cationic polysaccharide flocculant structure is a cationic polysaccharide flocculant with sugar amine as the main unit, and its molecular weight can reach 20-10 million. The aqueous solution of the Filed: August 11, 2023 polysaccharide flocculant shows a positively charged cationic polysaccharide flocculant, which has good antibacterial and flocculation and other functions. [0072] The disclosure describes construction of a recombinant citric acid bacterium (exemplified by Citrobacter) and utilizes carbon dioxide to synthesize a cationic polysaccharide particularly useful as a flocculant. The constructed strain and its cationic polysaccharide flocculant have important application value, including in cleaning wastewaters (that is, water that has been used in the home, in a business, or as part of an industrial process) of unwanted contaminants. The cationic polysaccharide is a product with high added value, uses carbon dioxide as a carbon source, and is environmentally friendly; anaerobic cultivation requires less energy consumption. [0073] Though exemplified herein with a KanR resistance marker, it will be recognized that which genetic marker is used is irrelevant to the production of the desired cationic polysaccharide preparation. Thus, other markers can be used. For instance, a genetic construct of the disclosure can include a gene encoding a selectable marker and/or counter-selectable marker. In particular embodiments, cells expressing a selectable marker can grow in the presence of a selective agent or under a selective growth condition. Examples of selectable markers include antibiotic resistance markers (e.g., chloramphenicol resistance, erythromycin resistance, ampicillin resistance, carbenicillin resistance, kanamycin resistance, spectinomycin resistance, streptomycin resistance, tetracycline resistance, bleomycin resistance, and polymyxin B resistance), markers that complement an essential gene (e.g., diaminopimelic acid auxotrophy (dapD), thymidine auxotrophy (thyA), proline auxotrophy (proBA), glycine auxotrophy (glyA), carbon source auxotrophy (TpiA)), chemical resistance (e.g., tellurite resistance, Fabl for triclosan resistance, bialaphos herbicide resistance, mercury resistance, arsenic resistance), and visual markers (e.g., green fluorescent protein (GFP), luciferase, β-galactosidase (lacZ)). In particular embodiments, a genetic construct of the disclosure includes a chloramphenicol acetyl transferase resistance gene (CAT) operably linked to a chloramphenicol responsive promoter (PCAT) and terminator (TCAT) from the Staphylococcus plasmid pC194 (Horinouchi & Weisblum, J Bacteriol. 150(2): 815-825, 1982). In particular embodiments, cells may be positively selected that have lost expression of a counter-selectable marker (i.e. cells expressing a counter-selectable marker are selected against). Examples of genes encoding counter-selectable markers include: sacB (gene encoding levansucrase that converts sucrose to levans, which is harmful to bacteria); rpsL (strA) (encodes the ribosomal subunit protein (S12) target of streptomycin); tetAR (confers sensitivity to lipophilic compounds such as fusaric and quinalic acids); pheS (encodes the α subunits of Phe- tRNA synthetase, which renders bacteria sensitive to p-chlorophenylalanine, a phenylalanine analog); thyA (encodes thymidilate synthetase, which confers sensitivity to trimethoprim and Filed: August 11, 2023 related compounds); lacY (encodes lactose permease, which renders bacteria sensitive to t-o- nitrophenyl-β-D-galactopyranoside); gata-1 (encodes a zinc finger DNA-binding protein that inhibits initiation of bacterial replication); and ccdB (encodes a cell-killing protein that inhibits bacterial gyrase). [0074] The Exemplary Embodiments and Examples below are included to demonstrate particular embodiments of the disclosure. Those of ordinary skill in the art should recognize in light of the present disclosure that many changes can be made to the specific embodiments disclosed herein and still obtain a like or similar result without departing from the spirit and scope of the disclosure. Exemplary Embodiments. [0075] 1. A recombinant Citrobacter bacterial strain, including: a gene encoding a ribulose 1,5-bisphosphate carboxylase, a gene encoding a ribulose phosphorylase, an inactivated malate synthase aceB gene, and a gene encoding a bacteriorhodopsin. [0076] 2. The recombinant Citrobacter bacterial strain of embodiment 1, wherein one or more of: the gene encoding the ribulose 1,5-bisphosphate carboxylase includes a sequence at least 80% identical to the sequence of SEQ ID NO: 1; and/or the gene encoding the ribulose phosphorylase includes a sequence at least 80% identical to the sequence of SEQ ID NO: 5; and/or the gene encoding the bacteriorhodopsin includes a sequence at least 80% identical to the sequence of SEQ ID NO: 7. [0077] 3. The recombinant Citrobacter bacterial strain of embodiment 1, wherein one or more of: the ribulose 1,5-bisphosphate carboxylase includes a sequence at least 80% identical to the sequence of SEQ ID NO: 2; and/or the ribulose phosphorylase includes a sequence at least 80% identical to the sequence of SEQ ID NO: 6; and/or the bacteriorhodopsin includes a sequence at least 80% identical to the sequence of SEQ ID NO: 8. [0078] 4. The recombinant Citrobacter bacterial strain of embodiment 1, wherein: the ribulose 1,5-bisphosphate carboxylase includes the sequence of SEQ ID NO: 2; and/or the ribulose phosphorylase includes the sequence of SEQ ID NO: 6; and/or the bacteriorhodopsin includes the sequence of SEQ ID NO: 8. [0079] 5. The recombinant Citrobacter bacterial strain of embodiment 4, wherein the ribulose 1,5-bisphosphate carboxylase includes the sequence of SEQ ID NO: 2; the ribulose phosphorylase includes the sequence of SEQ ID NO: 6; and the bacteriorhodopsin includes the sequence of SEQ ID NO: 8. Filed: August 11, 2023 [0080] 6. A method of making the recombinant Citrobacter bacterial strain of embodiment 1, including: knocking out or otherwise inactivating function a Citrobacter aceB gene in the genome of a Citrobacter spp. bacteria; and inserting into the genome of the Citrobacter spp. bacteria: a nucleic acid sequence encoding a ribulose 1,5-bisphosphate carboxylase; a nucleic acid sequence encoding a ribulose phosphorylase; and a nucleic acid sequence encoding a bacteriorhodopsin. [0081] 7. The method of embodiment 6, further including inserting into the genome of the Citrobacter spp. bacteria a nucleic acid sequence encoding a marker. [0082] 8. The method of embodiment 7, wherein the marker is a selectable marker or a counter-selectable marker. [0083] 9. The method of embodiment 6, including: construction of a CBB cycle and the knockout of a aceB gene, including: cloning of cbbM and prkA genes; amplify ~ 500bp homology regions upstream and downstream of aceB (GEN2534) by PCR; using the pkd4 plasmid as a template, amplify the KanR resistance gene fragment with FRT sites at both ends by PCR, and use it as a recombinant clone screening marker; connecting the fragments to the aceB upstream homology arm, cbbM, ldh promoter, prkA, KanR resistance fragment with FRT site, and aceB downstream homology arm in sequence to obtain gene targeting fragments; using a γ-Red or other recombination system, targeting fragment into the competent Citrobacter with pkd46 plasmid via electroporation, and screening the recombinant strain after incubation for a period of time; introducing bacillus rhodopsin into the Citrobacter genome, using the bacillus rhodopsin gene of Halobacterium salinarum but codon-optimized and transferred to Citrobacter by electroporation, and resistance screening and PCR screening the resultant transformants, to obtain a single-crossover strain(s); and inoculating single-crossover strain(s) on to LB plates to screen for double-crossover strains. [0084] 10. A method of producing a cationic biopolysaccharide, including: growing the recombinant Citrobacter bacterial strain of embodiment 1 in a fermentation medium including a carbon source to produce a bacterial culture, and extracting from the bacterial culture the cationic biopolysaccharide. [0085] 11. The method of embodiment 10, wherein extracting the cationic biopolysaccharide from the bacterial culture includes alcohol washing of the bacterial culture or of a supernatant produced by centrifugation of the bacterial culture. [0086] 12. The method of embodiment 10, including: inoculating the recombinant Citrobacter bacterial strain into a fermentation medium, providing the culture with carbon dioxide as a carbon Filed: August 11, 2023 source, and allowing the culture to ferment under culture fermentation conditions, to produce the cationic biopolysaccharide. [0087] 13. The method of embodiment 10, wherein the carbon source during fermentation includes one or more of: carbon dioxide, fatty acids, fatty alcohols, monosaccharides, and oligosaccharides. [0088] 14. The method of embodiment 10, wherein the carbon source includes carbon dioxide. [0089] 15. The method of embodiment 12, wherein the culture fermentation conditions include one or more of: fermented at 35-37°C, mixing rotation speed 50-300 rpm, pH controlled at 7.0 with 1M sodium hydroxide solution, inoculum size 5%, fermentation time for at least 15 hours, at least 20 hours, at least 25 hours, at least 30 hours, or more than 30 hours; and/or anaerobic or substantially anaerobic fermentation. [0090] 16. The method of embodiment 10, wherein the fermentation medium includes: caprylic acid 2 g/L, sucrose 5 g/L, yeast extract 10 g/L, peptone 5 g/L, ammonium chloride 1 g/L, trace element ZnSO47H2O 2.8 g/L, H3BO49.4 g/L, CuSO45H2O 5g/L, and MnCl24H2O 4.6 g/L. [0091] 17. The method of embodiment 14, wherein the carbon dioxide is filtered through a 0.22 micron filter membrane and passed into the fermenter to maintain the carbon dioxide in a headspace above the fermenter at a pressure of 0.2Mpa. [0092] 18. A cationic biological polysaccharide flocculant, prepared by fermentation of the recombinant Citrobacter bacterial strain of any one of embodiments 1-5, or prepared by the method of any one of embodiments 10-17. [0093] 19. A method of precipitating contaminants from an aqueous sample, including contacting the aqueous sample with the cationic biological polysaccharide flocculant of embodiment 18. [0094] 20. The method of embodiment 19, wherein the aqueous sample include wastewater. Example 1: Construction of CBB cycle and knockout of aceB gene [0095] This Example describes representative methods for modifying Citrobacter spp. to form a recombinant Citrobacter bacterial strain. The γ-Red system (illustrated in FIG. 1) was used to replace the aceB gene in the Citrobacter with cbbM and prkA encoding sequences from the photoheterotrophic bacterium Rhodospira rubrum. [0096] Cloning of cbbM (e.g., SEQ ID NO: 1) and prkA genes (e.g., SEQ ID NO: 3): The genome of the photoheterotrophic bacterium Rhodospira rubrum was extracted, and its cbbM gene and Filed: August 11, 2023 prkA gene were cloned by PCR. Then, the Citrobacter lactate dehydrogenase (LDH) promoter was cloned by PCR, and frozen. [0097] The 500 bp homology regions upstream and downstream of the Citrobacter aceB (GEN2534) were amplified by PCR. [0098] Using the pkd4 plasmid as a template, the KanR resistance gene fragment with FRT sites at both ends was amplified by PCR as a recombinant clone screening marker. Other art-known markers could readily be used, as will be recognized. [0099] Using Gibson Assembly® Connect, the aceB upstream homology arm, cbbM, LDH promoter, prkA, KanR resistance fragment with FRT site, and aceB downstream homology arm were connected in sequence and high-fidelity enzymes used to amplify the assembled gene targeting fragments. [0100] Using the γ-Red system (see FIG.1), the targeting fragment was inserted into competent Citrobacter NK02-1 carrying the pkd46 plasmid by means of electroporation, and the recombinant strain Citrobacter NK02-1 was screened after incubation for a period of time. Example 2: Introduction of Bacteriorhodopsin. [0101] This Example describes a representative method for inserting bacteriorhodopsin into a recombinant Citrobacter bacterial strain. [0102] Using the bacillus rhodopsin (bacteriorhodopsin) gene of Halobacterium salinarum as a template, a codon-optimized bacillus rhodopsin gene fragment (SEQ ID NO: 5) was synthesized from a long fragment. After PCR amplification, it was connected with the 1000 bp fragments of the upper and lower reaches of the Citrobacter ldh gene by over-lap PCR, and ligated by enzyme digestion, and the fragment was inserted into the pMMB67EH vector. The recombinant plasmid was then transferred into Citrobacter NK02-1 by electroporation. After resistance screening and PCR screening, the strains with recombinant plasmids were transferred to a 42°C incubator to screen for single exchange strains. The single-crossover strain was screened and then inoculated on LB plates containing 6% sucrose to screen the double-crossover strain Citrobacter NK02-2. Example 3: Batch fed-batch fermentation of recombinant Citrobacter NK02-2 in a 5L fermenter to produce cationic polysaccharide flocculant [0103] This Example describes representative culture conditions for the recombinant Citrobacter bacterial strain, and initial characterization of the polysaccharide flocculant harvested from it. [0104] The recombinant Citrobacter strain NK02-2 was grown under the following culture conditions: 35-37°C, rotation speed 50-300 rpm, pH controlled at 7.0 with 1M sodium hydroxide Filed: August 11, 2023 solution, inoculum size 5%, fermentation time 30 hours, under anaerobic culture. The components of the fermentation medium used were as follows: octanoic acid 2 g/L, sucrose 5 g/L, yeast extract 10 g/L, peptone 5 g/L, ammonium chloride 1 g/L, and trace elements (e.g., ZnSO ₄ 7H ₂ O 2.8 g/L, H ₃ BO ₄ 9.4 g/L, CuSO ₄ 5H ₂ O 5 g/L, MnCl ₂ 4H ₂ O 4.6 g/L). Carbon dioxide was provided as a necessary carbon source; it was filtered through a 0.22 micron filter membrane and passed into the fermenter from a steel cylinder to maintain the pressure of carbon dioxide in the space above the fermenter at 0.2 Mpa. [0105] After the fermentation, the polysaccharide was deposited from the water phase of the fermentation medium by alcohol washing. By way of example, the fermentation broth was centrifuged at 4000 g for 10 minutes to remove bacterial cells, then an equal volume of ethanol was added for coagulation, and the precipitate was collected after centrifugation at 12,000 g for 20 minutes. [0106] The primary product of the polysaccharide flocculant was obtained, and after repeated (e.g., 3 times) washing with ethanol, a colorless white precipitate was obtained. This was the cationic polysaccharide useful as a flocculant; its fermentation yield was 26.5 g/L, measured using a dry weight method. Through gas chromatography detection, the carbon dioxide fixation rate is 54.5%. After conversion, every ton of the cationic polysaccharide synthesized can fix 545 kg of carbon dioxide. [0107] Analysis of the obtained polysaccharide flocculant was carried out by means of infrared broad spectrum, liquid phase mass spectrometry, Zeta potential, and other means for structural identification and performance testing. To detect its monomers, the polysaccharide was acid- hydrolyzed at 105 °C at pH=2 for 3 hours, using hydrochloric acid. The obtained acid hydrolysis liquid was analyzed by liquid chromatography-mass spectrometry, as shown in FIGs. 2A-2D. From the illustrated results, the monomer of the polysaccharide is mainly acetylglucosamine. From the results of infrared spectrum analysis in FIG.3, the absorption peak at 2936.27 cm-1 illustrates the strong stretching vibration of the C-H bond [0108] Bond strength stretching vibration, the absorption peaks around 1652.23 and 1542.57 cm-1 indicate the presence of acetamido (-NH ₂ COCH ₃ ), and the absorption peak at 1408.33 cm- 1 are the characteristic absorption peak of -COO-. To sum up, -C-H chain, -NH ₂ COCH ₃ , -COO- are characteristic groups of sugars, so the detected molecules are glycosaminoglycans. From the test of the zeta potentiometer, in the 0.1% purified solution, the polysaccharide has a zeta potential of 9.43, which shows that it is positively charged. [0109] Molecular weight determination. HPLC-Gel permeation chromatography was used to carry out molecular weight determination of the cationic polysaccharide, by using a Waters™ Filed: August 11, 2023 Ultrahydrogel chromatographic column. Commercially available chitosan preparations (with molecular weights of 6 kDa, 25 kDa, 40 kDa, 270 kDa, and 670 kDa respectively) were used to determine the standard curve. As shown, the molecular weight of the cationic polysaccharide produced herein is between 30 million and 1 million. [0110] Table 1 Residence time and molecular weight of the main peak of liquid chromatography of samples Dwell time Mol. Wt. Relative content (min) (Da) (according to peak area ratio) Example [0111] This Example describes analyses of the minimum inhibitory concentration (MIC) of the cationic polysaccharide flocculant against six tested pathogenic organisms. [0112] Using the well known filter paper agar diffusion method, a filter paper sheet containing a certain concentration of drug was pasted on the surface of the solid agar medium and inoculated with the test strain. The drug in the filter paper sheet is allowed to diffuse into the agar, and the concentration of the drug being highest nearest the filter paper and gradually decreasing away from it through diffusion. When the drug concentration increases to a certain value, it can inhibit it growth of the inoculum, and a transparent circle without bacterial growth appears around the filter paper, that is, the bacteriostatic zone. Different types of bacteria lead to formation of different inhibition zones. The diameter of the inhibition zone directly reflects the sensitivity of the tested bacteria to the drug. The larger the diameter, the greater the antibacterial effect of the compound/preparation on the tested bacteria, and the stronger the inhibitory effect. [0113] Through this experimental process, it was determined that the polysaccharide significantly inhibits the growth of rice fever, bacterial wilt pathogen, Paenibacillus polymyxa, rice smut, Staphylococcus aureus, Pseudomonas, and its inhibitory effect increases (becomes stronger) with increasing concentration. [0114] The MIC of the polysaccharide on the six microorganism strains was tested. Experiments show that the MIC values against Oryzae fever, Ralstonia solanacearum, Paenibacillus polymyxa, Aspergillus oryzae, Staphylococcus aureus, and Pseudomonas are 0.021%, 0.015%, 0.012%, 0.023%, 0.018%, and 0.022%, respectively. Thus, the cationic polysaccharide flocculant Filed: August 11, 2023 produced from engineered Citrobacter strains as described herein has significant antibacterial effects at a low concentration of about 200 ppm. Example 5: Flocculation effect [0115] This Example describes a representative method for testing flocculation effect provided by the disclosed preparations. [0116] The obtained cationic polysaccharide was formulated into a 0.2% solution as a mother liquor. Aqueous sludge without flocculant was taken from different sewage treatment plants to test the flocculation effect. Since this polysaccharide produced herein is a cationic polysaccharide, polyaluminium, cationic polyacrylamide, and chitosan, from different manufacturers in the market were used for comparison. The moisture content of the filtered mud cake was selected, and the moisture content measured by the gravimetric method, and the flocculation rate was measured by the solid content method in water to characterize the flocculation effect. [0117] In the illustrated comparative flocculation experiment, the effective substance concentration of each flocculant was set at 30 ppm. The results of the test are shown in FIG.4. The cationic polysaccharide produced from the engineered Citrobacter described herein has a very noticeable flocculation effect, and the sludge particles after flocculation are large and clearly visible. [0118] In the performance comparison test of different flocculants (FIGs. 5-7), the cationic polysaccharide flocculant of the present disclosure has significant flocculation effect. This demonstrates this cationic polysaccharide flocculant has advantages for use in treating sewage sludge from different sources, from industrial sludge to biological sludge. [0119] After testing the flocculation rate after flocculation and the water content of mud cake after filtration, the cationic polysaccharide flocculant of the present disclosure has higher sludge flocculation rate and lower water content of mud cake, compared to the commercial comparison preparations; see Table 2, below. From the perspective of flocculation rate, the higher the value, the cleaner the suspended solids in the water are treated. Judging from the water content of the filter mud cake, the lower the water content of the mud cake, the more conducive it is to the reduction treatment of dewatered sludge. The water content of the sludge (wet weight) after filtration by general flocculants is greater than 60%, and it is difficult to process it to below 60%. [0120] From the experimental results presented here, the performance of the present disclosure was very prominent, which illustrates benefits realized from the cationic polysaccharide of the present disclosure. Molecular weight composition, large molecular weight cationic Filed: August 11, 2023 polysaccharides capture and bridge, gather more solid content, and form large flocs; small molecular weight cationic polysaccharides reduce the water content between large flocs. [0121] Table 2 Comparison of flocculation effects of different flocculants. Flocculant Flocculant Mud cake moisture rate % content（%） [0122] , t intended to limit the present disclosure. Although the present disclosure has been described in detail with reference to the foregoing embodiments, for those skilled in the art, the technical solutions recorded in the foregoing embodiments may be modified, or some technical features thereof may be equivalently replaced. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present disclosure shall be included within the protection scope of the present disclosure. X. Closing Paragraphs [0123] As will be understood by one of ordinary skill in the art, each embodiment disclosed herein can comprise, consist essentially of or consist of its particular stated element, step, ingredient or component. Thus, the terms “include” or “including” should be interpreted to recite: “comprise, consist of, or consist essentially of.” The transition term “comprise” or “comprises” means has, but is not limited to, and allows for the inclusion of unspecified elements, steps, ingredients, or components, even in major amounts. The transitional phrase “consisting of” excludes any element, step, ingredient, or component not specified. The transition phrase “consisting essentially of” limits the scope of the embodiment to the specified elements, steps, ingredients, or components and to those that do not materially affect the embodiment. [0124] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present disclosure. At the very least, and not as an attempt to limit the application Filed: August 11, 2023 of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. When further clarity is required, the term “about” has the meaning reasonably ascribed to it by a person skilled in the art when used in conjunction with a stated numerical value or range, i.e. denoting somewhat more or somewhat less than the stated value or range, to within a range of ±20% of the stated value; ±19% of the stated value; ±18% of the stated value; ±17% of the stated value; ±16% of the stated value; ±15% of the stated value; ±14% of the stated value; ±13% of the stated value; ±12% of the stated value; ±11% of the stated value; ±10% of the stated value; ±9% of the stated value; ±8% of the stated value; ±7% of the stated value; ±6% of the stated value; ±5% of the stated value; ±4% of the stated value; ±3% of the stated value; ±2% of the stated value; or ±1% of the stated value. [0125] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. [0126] The terms “a,” “an,” “the” and similar referents used in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention. [0127] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member may be referred to and claimed individually or in any combination with other members of the group or other elements found herein. It is anticipated that one or more members of a group may be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the Filed: August 11, 2023 specification is deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims. [0128] Certain embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Of course, variations on these described embodiments will become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventor expects skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. [0129] Furthermore, numerous references have been made to patents, printed publications, journal articles, other written text, and web site content throughout this specification (referenced materials herein). Each of the referenced materials are individually incorporated herein by reference in their entirety for their referenced teaching(s), as of the filing date of the first application in the priority chain in which the specific reference was included. For instance, with regard to chemical compounds, nucleic acid, and amino acids sequences referenced herein that are available in a public database, the information in the database entry is incorporated herein by reference as of the date of an application in the priority chain in which the database identifier for that compound or sequence was first included in the text. [0130] It is to be understood that the embodiments of the invention disclosed herein are illustrative of the principles of the present invention. Other modifications that may be employed are within the scope of the invention. Thus, by way of example, but not of limitation, alternative configurations of the present invention may be utilized in accordance with the teachings herein. Accordingly, the present invention is not limited to that precisely as shown and described. [0131] The particulars shown herein are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of various embodiments of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for the fundamental understanding of the invention, the description taken with the drawings and/or examples making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. Filed: August 11, 2023 [0132] Definitions and explanations used in the present disclosure are meant and intended to be controlling in any future construction unless clearly and unambiguously modified in the example(s) or when application of the meaning renders any construction meaningless or essentially meaningless. In cases where the construction of the term would render it meaningless or essentially meaningless, the definition should be taken from Webster's Dictionary, 11th Edition or a dictionary known to those of ordinary skill in the art, such as the Oxford Dictionary of Biochemistry and Molecular Biology, 2 ^nd Edition (Ed. Anthony Smith, Oxford University Press, Oxford, 2006), and/or A Dictionary of Chemistry, 8 ^th Edition (Ed. J. Law & R. Rennie, Oxford University Press, 2020).