ANALYSIS OF AQUATIC MICROBIAL COMMUNITIES IN EUTROPHIC AND HUMIC LAKES
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In this study, the microbial community structure and function of aquatic ecosystems were investigated. Illumina 16S gene tag sequencing was performed to profile the bacterial community of 7 humic lakes across multiple years of sampling, and shotgun metagenomic sequencing was performed to investigate the functional characteristics of humic and eutrophic lakes. Lakes are significant in the global carbon cycle. In particular, humic bog lakes are reservoirs for large quantities of terrestrially derived organic matter, and thus link the aquatic and terrestrial carbon cycles. The bacterial communities that inhabit these ecosystems are of great interest to researchers, as they are the primary respirators of detritus. While DOC quality can be a strong driver of bacterial communities within lakes, resource availability varies greatly with space and time. Here we sequenced the16S rRNA gene from over 700 samples from seven characteristic humic bog lakes to investigate the dynamics of bacterial community composition with time. Samples were taken weekly from each of the seven humic lakes during the ice-off season. For some lakes, communities from as many as four years were represented. This unique dataset allowed us to investigate the variability in diversity across temporal scales of years, seasons, weeks, and days, while maintaining the lens of spatial variation. The diversity of each lake could be characterized adequately after approximately 25 samples that were spaced evenly throughout the ice-off season. Unpredictable year-to-year variation of diversity was observed, although, the normal range of Shannon Diversity was near 4. Diverse spring and early summer communities have been observed for aquatic bacteria in prior studies, but here we identified that on average, the community is also changing at the highest rate during early summer weeks in these lakes. Mixing events, which alter the physical and chemical factors of the habitat, also correlated with times of rapid change of community structure. Conversely, the community changed at the slowest rate during the strongly stratified periods of mid-late summer. Metagenomic sequencing on two samples from one humic lake and three samples from a eutrophic lake revealed that the lakes were functionally most similar with space than season. The humic lake, which has a high concentration of humic substances from OM that is leached through a sphagnum mat, contained an overrepresentation of enzymes capable of degrading small organic acids and aromatic compounds. Presumably, photodegradation of humic substances are contributing the labile DOC pool for bacteria, and this represents a model for respiration of terrestrially derived C within aquatic ecosystems. Alternatively, the bacterial community in the eutrophic lake seemed suited for breaking down polysaccharides derived from algal biomass. Though more glycoside hydrolases were observed in the humic lake, the eutrophic community maintained a higher proportion of sulfatase enzymes that would putatively be active in degrading algal cell walls. With these studies, we have identified that the diversity and change of bacterial communities in aquatic ecosystems vary multiple temporal scales, but the trends are generally shared for similar lakes. We also have seen that the functional potential of the bacterial community is more stable with time than across trophic levels, and the differences between the humic and eutrophic lake are largely a factor of substrate preference.