Diversity And Denitrifying Potential Of Microbial Communities Associated With Cyanobacterial Aggregates In Lake Taihu

Cyanobacteria blooms caused by water body eutrophication are a common threat to lake ecosystems worldwide.In addition to human intervention by reducing nutrient input,nitrogen removal processes occurring in situ has attracted increasing attention.Denitrification is the most important nitrogen removing pathway in eutrophic lakes,and may occur both at the sediment-water interface and within cyanobacteria aggregates(CAs)in the water column during bloom outbreak.However,there has been little understanding of the flux and mechanisms of denitrification process of the latter case.This study took CAs from lake Taihu as the research object,employed amplicon sequencing of 16S rRNA genes,¹⁵N isotope labeling and indoor microcosm incubation experiments,as well as bacterial isolation and purification,to analyze the microbial diversity within CAs,to reveal the potential and seasonal dynamics of denitrification driven by CAs,to quantify the abundances of denitrification-related functional genes,and to isolate denitrifying bacterial strains from CAs.This study comprehensively revealed mechanisms of the microbially mediated denitrification in CAs of lake Taihu and quantified its potential contribution to nitrogen removal in the lake.The major results include the follows:(1)Continuous sampling and microbial community analysis of the CAs in lake Taihu was carried out.Chroococcales dominated the cyanobacterial communities in lake Taihu,followed by Nostocales.The bacterial communities of CAs were dominated by Proteobacteria.The relative abundance of Alpha-proteobacteria gradually decreased over time,while that of the Beta-proteobacteria increased over time.The community composition within CAs in June and July was significantly different from the rest of the months.The?-diversities were lower in the former months,and the differing taxa were mostly related to Alpha-proteobacteria.Environmental factors such as water physical-chemical properties also had significant impacts on the microbial community structure within CAs.(2)Through the indoor microcosm incubation experiments,¹⁵N-NO₃^-supplementation combined with membrane injection mass spectrometry(MIMS)revealed that CA-driven denitrification potential may reach 0.14?mol N/L·?g chl.The potential of CA-driven denitrification within the entire lake Taihu was estimated to be up to 0.9?mol N/L·day,suggesting that CA-driven denitrification could have a significant contribution to nitrogen removal of lake Taihu.Denitrification rates by CAs varied across months,reaching the highest in August.The abundances of denitrification-related functional genes also fluctuated with time.In general,nosZ ? gene showed the highest abundances,followed by nirS,and nirK had the lowest.nosZ ? gene was not detected.Ammonia oxidizers were dominated by ammonia oxidizing archaea(AOA).The water physical-chemical factors significantly impacted the abundance distribution of the functional genes.(3)A total of 62 denitrifying bacterial strains were obtained from CAs.Based on16S rRNA gene sequencing and phylogenetic analysis,these strains were attributed to nine genera,showing a high diversity of CA-associated denitrifying bacteria.The medium used for strain isolation had a significant impact on the resulting taxonomy of obtained isolates.Based on the results above,this study revealed the diversity of microbial communities within CAs of lake Taihu,its seasonal dynamics and influencing factors;quantified the potential of CA-driven denitrification and its potential contribution to nitrogen removal in lake Taihu;elucidated the abundances,dynamics and influencing factors of denitrification-related functional genes within CAs;and finally,obtained a range of denitrifying bacterial strains,which provided the fundament for further exploration of the denitrification mechanisms within CAs.