Controlled Carbon Transport And Stress Resistance Mechanisms Of Arthrospira Platensis In CO₂ Fixation From Flue Gas

The reduction of high concentrations of CO₂ in coal chemical flue gas by Arthrospira platensis is of great significance for the development of a low-carbon economy and achievement of carbon neutrality.However,the cellular response to 99.99% CO₂ in coal chemical flue gas is still unclear,the pathway of carbon transmembrane transportation under coal chemical flue gas has not been revealed,and the strain with high enzyme activity and strong tolerance to trace impurities in flue gas has been in urgent need.In this paper,the competitive pathway between the active transport of bicarbonate and the diffusion and utilization of CO₂ was analyzed.By constructing CO₂ carriers,screening mutant strains with nuclear mutagenesis,and regulating cellular stress resistance mechanisms,the carbon fixation rate in A.platensis was significantly promoted.The cellular carbon-controlled transport mode of A.platensis under 99.99% CO₂ has been revealed by transcriptomics.The expression of key proteins for bicarbonate active transport(BCT1)was up-regulated by about 50%.Correspondingly,to maintenance the sodium ion gradient inside and outside the cell membrane,the expression of Na+/H+ antiporter(Nha S3)was upregulated by 34%.In contrast,the expression of the protein related to CO₂ diffusion and utilization(NDH-1)was down-regulated by 34%.Hence,the active transport of bicarbonate was enhanced and the affinity to bicarbonate was improved,while the diffusion and utilization of CO₂ were limited under coal chemical industry flue gas conditions.The carbon-controlled transport mode caused a partial lack of CO₂ in the carboxylase body,and ribulose-1,5-disphosphate(Ru BP)was catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase(Rubisco)to produce glycolate-2-P,a by-product of oxygenation and known as a photosynthetic inhibitor.Therefore,the expression of key enzymes(Ggt,etc.)in the photorespiration pathway of A.platensis cells was up-regulated by 82% for the metabolism of glycolate-2-P.The maximum quantum yield and the effective quantum yield of photochemical energy conversion in photosystem II were increased by 19.1%and 12.0%,respectively,providing more energy molecule ATP and NADPH for cells.The synthesis of lipopolysaccharide in the cell membrane structure was up-regulated,reducing the permeability of the cell membrane to survive with the external 99.99% CO₂ condition,while increasing the negative charge of the cell membrane to stabilize the cell membrane structure.The zeolite imidazole framework-8(ZIF-8)with zinc cores was synthesized and used as the CO₂ carrier in the algae solution.The CO₂ molecules could be adsorbed at the unsaturated metal active sites in ZIF-8,so that the CO₂ diffusion and conversion into dissolved bicarbonate could be effectively strengthened.Carbon in flue gas from coal-chemical industry could be made available throughout cultivation to promote biomass productivity of A.platensis.The CO₂ carrier ZIF-8 with particle size of 719 nm performed the largest pore area of 351.8 m2/g and exposed the most unsaturated metal active sites,which promoted CO₂ conversion into HCO3-by 72.9%(compared to control condition without ZIF-8)to 133.6 mmol/L.The increased HCO3-concentration enhanced thylakoid membrane proportion in cell cross-sectional area by 1.3 times to 78.3%,which resulted in enhancement of photosynthesis in A.platensis cells.Relative electron transport rate increased by 9.4% accordingly,which was attributed to the improvement of chlorophyll a concentration by 110%.The carbon fixation rate using ZIF-8 carrier with particle size of 719 nm markedly increased by 64.0% to 1.07 g/L/d.In order to make the CO₂ carrier easy to separate and reuse when harvesting biomass,the magnetic iron-based organic framework MIL-100(Fe)was used as an improved CO₂ carrier,so the trace leached iron ions could promote the synthesis of light-harvesting pigment chlorophyll a,and further increase the average carbon fixation rate of A.platensis to 1.12 g/L/d.In order to promote phenol tolerance and the carbon fixation rate of cells during CO₂ sequestration in coal chemical flue gas,?-ray irradiation from cobalt-60(60Co)was used to mutate A.platensis to generate efficient strains.A phenol-tolerant strain Mutant 11 k was obtained with11000 Gy of gamma irradiation.The phenol degradation efficiency of the mutant was 34.7%higher than that of wild strain,attributing to the increased activity(by 25.4%)of phenol hydroxylase that catalyzed phenol to generate catechol.The catechol 1,2-dioxygenase activity in mutant cells was enhanced by 39.5% while catechol 2,3-dioxygenase was weakened by 23.8%,revealing that the ?-ketoadipate pathway plays a more important role than 2-hydroxymuconic semialdehyde pathway in phenol degradation.The activity of superoxide dismutase and catalase increased by 6.1% and 39.8%,respectively,reducing peroxide damage and maintaining rapid cell growth.The contents of chlorophyll-a and maximum quantum yield of photosystem II in mutant cells were higher than those in wild strain.When the phenol concentration was 4.2 mg/L,the average carbon fixation rate of Mutant 11 k increased by 22.4% to 0.72 g/L/d.When the phenol concentration was 84 mg/L,the growth of wild strain almost stopped,and the mutant could still achieve a carbon fixation rate of 0.51 g/L/d.The effects of different CO₂ concentrations on phenol degradation and cell metabolism were explored.A high CO₂ concentration locally around the cells promoted the secretion of extracellular polymeric substances(EPS).The stimulated humic acid-like organics(main ingredients in EPS observed by excitation-emission matrix fluorescence spectroscopy)with strong adhesive properties,worked as protective barriers to limit phenol transportation into the cells and created a microenvironment suitable for cells growth.The activity of phenol hydroxylase released into extracellular matrix was increased by 112.7% to enhance extracellular phenol degradation under 99.99% CO₂.The decreased abundances of intracellular superoxide dismutase and malondialdehyde suggested that elevated CO₂ level protected cells by alleviating phenol toxicity and regulating intracellular redox state.Furthermore,the effective quantum yield of photosystem II was promoted by reducing the damage on photosynthetic membranes under 42mg/L phenol,leading to an increased carbon fixation rate by 35.1% to 0.66 g/L/d during the first four days under 99.99% CO₂.