Interaction Between Microalga And Bacteria Under High Salt Stress Condition

With the human activities and climate change,freshwater environments are increasingly threatened by salinization.Salt is an important factor affecting biological growth,and has important influence on the physiological and ecological activities such as individual growth,photosynthetic efficiency and element cycle.However,there are many unsolved questions on the interaction between algae and other organisms,and how these interactions impact the adaptation of algae to salt stress.In order to explore the effect of high salt on the interactions between algae and bacteria,we studied the microbial community structure and interspecies relationships using algae--bacteria co-culture system under salt stress condition.In bacterial isolation tests,we also obtained several strains that can significantly promote microalgae salt tolerance,followed by preliminarily exploring the molecular mechanism with gene expression studies.Our data deepened the understanding of the dynamic changes of microbial communities under different salt stress conditions,and how bacteria promoter the salt tolerance of microalgae.And we provided scientific basis for further transformation and utilization of microorganisms to assist freshwater phytoplankton to resist salt stress.The main results are as follows:(1)Using high-throughput sequencing of 16S rRNA,we found that the high salt environment had an important effect on the microbial community composition and interspecific relationship under the condition of laboratory culture.Salt stress can reduce the a diversity of microbial organisms;high salt can cause large changes in microbial community structure.Microorganisms such as Xanthomonadaceae,Pseudomnadaceae,Acidisoma,etc,are positively correlated with green alga,Chlamydomonas reinhardtii in their genome or plastid 16s rRNA gene read numbers.Some of these positive correlations remained under high salt conditions,suggesting that these microorganisms might play a significant role on salt-tolerant of C.reinhardtii.(2)By enrichment and strains isolation,we obtained 12 bacterial strains which can enhance the salt tolerance of C.reinhardtii.Under the high salt stress,these bacteria could effectively protect chlorophyll content and the maximum quantum yield of photochemistry in PSII(Fv/Fm)against damage of high salt concentration.(3)Among these bacterial strains,ZX29 presents a high protection ability for algal salt tolerance.The chlorophyll content and the maximum quantum yield of photochemistry in PSII(Fv/Fm)of C.reinhardtii were significantly improved when the bacteria were coinoculated.Its highest homologous 16S rRNA gene is from Rhodococcus.(4)In the single inoculation of C.reinhardtii,high salt stress can decrease the expression of Photosynthetic system II light composite gene 2.2(Plhc).By contrast,the co-inoculation of bacteria ZX29 can rescue the expression change,suggesting that interspecific interactions play an important role in the regulation of gene expression during algal adaptation to stress.