Ecophysiology Of The Cyanobacteria Synechococcus Spp. Under Seawater Salinity Conditions

As an ancient photosynthetic autotroph,cyanobacteria played a crucial role in the transformation of the Earth from an anaerobic to an aerobic environment.Evolutionary analysis has shown that the earliest cyanobacterial lineages inhabited terrestrial and freshwater environments,while later lineages acquired the ability to settle in semi-saline,marine and hypersaline environments.Salinity is a major environmental factor in determining the composition of microbial communities,and identifying differences between different microbial communities will increase understanding of microbial evolution and ecology.Currently,domestic and international research has focused on the mechanisms of tolerance to salt stress in freshwater cyanobacteria,while the mechanisms of how cyanobacteria diversify from low to high salinity and ultimately in the marine environment are still unclear.As a functional microorganism with important environmental and ecological benefits,i.e.contributing significantly to biogeochemical cycles,cyanobacteria may also cause algal blooms or red tides in the water column due to overpopulation.Therefore,investigating the effect of seawater salinity environment on the physiological ecology and aggregation of cyanobacteria can not only reveal the evolutionary adaptation process of cyanobacteria from freshwater to marine environment,but also provide new insights to solve the problem of algal bloom and apply the aggregation property of microorganisms to other biological treatment processes.The main contents and results of this study are as follows:(1)In this study,two model strains,marine cyanobacteria Synechococcus sp.PCC7002(PCC7002)and freshwater cyanobacteria Synechococcus elongates PCC7942(PCC7942),were selected and their physiological changes under different salinities were investigated in comparison.The results showed that the high salinity environment(2%seasalt or 3.5%seasalt)promoted the growth of PCC7002 and inhibited the growth of PCC7942.The reason for the opposite physiological traits of the two cyanobacteria originated from the effect of high salinity environment on photosynthesis.The high salinity environment inhibited the synthesis of photosynthetic pigments,causing disruption of the photosynthetic electron transport chain,which in turn inhibited the production of NADPH.This leaded to more electron transfer to oxygen,resulting in more reactive oxygen species(ROS)and oxidative stress in the cells,which inhibited the growth of PCC7942;whereas for PCC7002,the high salinity environment promoted the synthesis of photosynthetic pigments and its growth.In PCC7002,the high salinity environment promoted the synthesis of photosynthetic pigments and the activity of its photosynthetic electron transport chain,resulting in more NADPH and fewer superoxide radicals,which promoted the growth of PCC7002.(2)Excessive aggregate growth and reproduction of cyanobacteria directly contribute to the eruption of harmful algal blooms.Analysis of the aggregation of PCC7002 and PCC7942 in high salinity environments showed that high salinity environments promoted the aggregation of both cyanobacteria.The surface electric charge of both species was reduced in the high salinity environment,suggesting that the reduction in intercellular repulsion,together with enhanced hydrophobicity,promoted intercellular aggregation.Extracellular polymeric substances(EPS)of the cyanobacteria were also measured and although the total EPS and polysaccharide concentrations decreased in PCC7002 and increased in PCC7942,they both showed an increasing trend in total protein concentration.This result suggests that the increase in total protein concentration under high salinity environment may be the main reason for the enhanced aggregation of both cyanobacteria.(3)Transcriptome analysis allowed the detection of gene expression changes in PCC7002 and PCC7942 under high salinity conditions.The results showed that most of the genes in PCC7002 were up-regulated under high salinity conditions,while most of the genes in PCC7942 were down-regulated.Gene Ontology(GO)enrichment analysis showed that the ribosome function of both PCC7002 and PCC7942 was enriched with a large number of differentially expressed genes(DEGs),and the difference in protein expression due to the effect of hypersaline environment on the ribosome resulted in differences in physiological characteristics of the two cyanobacteria.The Kyoto Encyclopedia of Genes and Genomes(KEGG)metabolic pathway analysis showed the same results as the phenotypic assays.Genes related to the Na+/H+reverse transporter protein,which plays an important role in maintaining inorganic ion homeostasis during salt domestication,the synthesis of phase-soluble substances and fatty acid desaturases were up-regulated in PCC7002 in high salinity environments and down-regulated or not significantly changed in PCC7942.SYNPCC7002_A1954(A1954)and SYNPCC7002_A1548(A1548)in the cell signaling system were significantly upregulated in PCC7002,with A1954 encoding a histidine kinase(Hik)acting to sense salt signals and A1548 encoding a response regulator(Rre)plays a role in transmitting to salt signals.The RNA polymerase σfactors rpoD and rpoA were also significantly up-regulated in PCC7002.In contrast,the expression of all these related genes was down-regulated in PCC7942.