Adaptive Response Of Sulfate Reducing Bacteria To Anaerobic Decomposition Of Cyanobacterial Bloom Biomass

Lake eutrophication can lead to black blooms,and the anaerobic decomposition process of black blooms will further lead to anaerobic water bodies,and even cause lake flooding problems and damage lake water ecosystems.Sulfate reducing bacteria（SRB）are important participants in the sulfur biogeochemical cycle,and can decompose algal soluble organic matter under anaerobic conditions,while reducing sulfate radical to soluble sulfide（∑H₂S）.To explore the adaptive response of SRB to the anaerobic decomposition of black blooms is helpful to further understand the important role of SRB in lake flooding and the adaptive mechanism of SRB to lake environmental changes.We selected Zhushan Bay,a typical algal bay in Taihu Lake,the third largest freshwater lake in China,as the research object.In June 2019,flooding occurred in Zhushan Bay（31°24.335’N,120°01.762’E）.Samples of surface overlying water（SW）,bottom overlying water（BW）and surface 1-10cm sediments were collected at this site.In July 2019,the water surface of this site was normal without lake flooding.SW,BW and surface sediment samples of this site were collected again.By analyzing the physical and chemical indexes of overlying water and sediment samples,combined with laboratory simulation experiments,the difference of SRB sulfate reduction activity in lacustrine flooding period and non-lacustrine flooding period was explored.Meanwhile,high-throughput sequencing technology and THREE-DIMENSIONAL fluorescence method were used.The changes of SRB community structure and the dynamic changes of the components and relative contents of colored dissolved organic matter（CDOM）during sulfate reduction were analyzed.The main conclusions of this study are as follows:（1）The concentrations of nitrogen and phosphorus in overlying water during lake flooding period were significantly higher than those in non-lake flooding period.A large amount of∑H₂S was produced in the surface overlying water（SW）during lacustrine flooding,and the maximum concentration was up to 1.24±0.12 mg L^-1,and∑H₂S diffused into the surface sediments by combining with iron to form solid sulfides.In addition,the addition of organic substrate acetic acid had no significant effect on the sulfate reduction rate（SRR）（P>0.05）,indicating that the anaerobic decomposition of black blooms provided sufficient organic matter for SRB in the overlying water,which may be the main reason why the SRR in Taihu Lake was significantly higher than that in many other semi-convective freshwater lakes.In addition,in this study,there was no significant difference in the specific sulfate reduction rate（cs SRR）between the overlying water and the overlying water（P>0.05）,indicating that the reductive metabolic capacity of SRB in the overlying water at different depths of freshwater lakes was similar and the physiological limit was narrow.（2）Microbial high-throughput sequencing results showed that there were significant differences in community structure of each SRB genus between lake flooding and non-lake flooding periods（P<0.05）.Desulfomicrobium and Desulfovibrio are the main SRBS in the overlying water of lake flooding,while Desulfobulbus,Desulfoobacca and Desulfatiglans are widely present in the sediments of lake flooding and non-lake flooding.The analysis of microbial coexistence network shows that SRB is one of the characteristic nodes in the microbial coexistence network,and the interaction between SRB and related functional microorganisms is diverse,and the strength of such interaction is not directly related to the relative abundance of SRB.In addition,the coexistence network also indicated that each SRB genus would combine with different bacteria to achieve specific functions,that is,the ecological role of each SRB genus in freshwater lake microbial community was different.SRB may maintain cellular physiological activity by regulating its own enzyme activity and metabolism,and adapt to lake flooding by altering its adjacent nodes.（3）The changes of the components and relative abundance of colored dissolved organic matter（CDOM）during the decomposition of algae-derived organic matter with the increase of sulfate concentration were studied in laboratory simulation experiment.The results showed that four fluorescence components of CDOM were produced during the anaerobic decomposition of black blooms.Component 1 was tyrosinoid,component 2 and component 3was tryptophan,and component 4 was humoid.Among the four components,the fluorescence intensity of components 1 to 4 decreased successively,and the control group（CK）without sulfate group had the highest content of component 1 tyrosine on day 8.Except tyrosine group1,the fluorescence intensity of the other three components increased first and then decreased.In addition,the sulfate concentration had no significant effect on the fluorescence components.High-throughput sequencing results showed that the species and relative abundance of SRB genera increased gradually with culture time in the CK group and the experimental group supplemented with 120mg L^-1 sulfate（120S）,indicating that the sulfate reduction reaction requires the interaction of multiple SRB genera,and the sulfate reduction reaction can also stimulate the growth of most SRB genera.In addition,SRB（such as Desulfovibrio,Desulfomicrobium,Desulfobulbus,Desulfohalovibrio,etc.）dominates in the number and relative abundance of microbial species.At the same time,there are differences in the distribution of community structure among different SRB genera.In each experimental group,the average relative abundance of Desulfovibrio and Desulfomicrobium was always the highest,and the relative abundance of Desulfovibrio was inversely proportional to sulfate concentration.In this study,the community structure of most SRB genera is positively correlated with the degree of sulfate reduction reaction,while Desulfovibrio growth is inhibited by sulfate reduction reaction.In conclusion,the anaerobic decomposition of black blooms after lake flooding provides sufficient organic substrates for SRB,promotes the sulfate reduction process,and has different effects on SRB activity and community structure in overlying water and sediments at different depths.In addition,the anaerobic decomposition of cyanobacterial blooms alters the interaction between SRB and other microorganisms.In this paper,the differences of SRB activity and community structure before and after lake flooding were studied.In the next step,the SRB activity and community change characteristics during the whole process of germination,development,formation and extinction of cyanobacterial blooms should be further explored.