Oxygen Tolerance Of Sulfur Reducing Bacteria In The Shuikou Reservoir And Xiangshan Bay Under Enrichment Culture Conditions

Sulfate-reducing bacteria(SRB),as facultative anaerobic bacteria,play an important role in the global sulfur and carbon cycle and the process of organic matter mineralization and formation of biogenic pyrite in lakes and coastal seas.It is essential to understand how SRB respond to different concentrations of dissolved oxygen(D.O.)in water columns.Nearly all SRB pure culture were performed under strictly anoxic conditions after they were isolated from ambient environment.Although pure cultures of SRB facilitate our understanding of the behavior of enzymes and mechanisms in response to oxygen exposure,there are deficiencies in representing the community structure,as SRB might lose any tolerance to oxygen after many generations of growth under strictly anoxic conditions.It remains largely unknown the relationship between the abundance of SRB and dissolved oxygen concentration,and if the complex and diverse microbial symbiosis of sulfur metabolism related bacteria work synergistically to increase SRB's tolerance to oxygen exposure in water columns.We selected a stratified lake,and a semi-closed estuary for sampling in order to compare the different behaviors of SRB in freshwater and saline environments.We adopted the strategy of enrichment cultures with selected medium favorable for the growth of SRB,with only one parameter,dissolved oxygen,strictly controlled the same as in situ.16S rDNA high-throughput sequencing and bioinformatics analysis were used to analyze the community composition,abundance and tolerance behavior of sulfate-reducing bacteria and their sulfur metabolism related bacteriaIn such enrichment cultures,genera Desulfovibrio and Desulfomicrobium were the main sulfate-reducing bacteria,and genera Shewanella and Sulfurospirillum were the associated sulfur metabolism-related bacteria with SRB in the water columns of lakes and coastal seas.In the enrichment cultures with water samples from a stratified freshwater reservoir,the absolute abundance of predominant SRB Desulfovibrio spp.and D.O.in a range from 1.84 to 5.00 mg/L were strongly negatively related(R'=0.96)To our best knowledge,this is the first quantitative measurement,indicating the predominant role of D.O.in determining the SRB abundance in enrichment cultures,laying the foundation for further investigation the relationship in situ in water columns.Such understanding would be essential to predict their evolution in lakes and coastal seas under increasing eutrophication and associated hypoxia.The upper D.O.limit in our enrichment cultures was 6.68 mg/L,significantly higher than those previously reported in pure cultures or co-cultures,most likely resulted from the contribution of coexisting sulfur metabolism-related bacteria which may provide ecological niches for Desulfovibrio spp.by consuming residual oxygen in the environment and thus increased Desulfovibrio spp.'s tolerance to oxygen exposure,in addition to its own adaptation strategy to high-oxygen environment.