Abstract
Although bacteria play key roles in aquatic food webs and biogeochemical cycles, information on the seasonal succession of bacterial communities in lakes is still far from complete. Here, we report results of an integrative study on the successional trajectories of bacterial communities in a seasonally stratified lake with an anoxic hypolimnion. The bacterial community composition of epilimnion, metalimnion, and hypolimnion diverged during summer stratification and converged when the lake was mixed. In contrast, bacterial communities in the sediment remained relatively stable over the year. Phototrophic Cyanobacteria and heterotrophic Actinobacteria, Alphaproteobacteria and Planktomycetes were abundant in the aerobic epilimnion, Gammaproteobacteria (mainly Chromatiaceae) dominated in the metalimnion, and Chlorobi, Betaproteobacteria, Deltaproteobacteria, and Firmicutes were abundant in the anoxic sulfidic hypolimnion. Anoxic but nonsulfidic conditions expanded to the surface layer during fall turnover, when the epilimnion, metalimnion and upper hypolimnion mixed. During this period, phototrophic sulfur bacteria (Chromatiaceae and Chlorobi) disappeared, Polynucleobacter (Betaproteobacteria) and Methylobacter (Gammaproteobacteria) spread out from the former meta- and hypolimnion to the surface layer, and Epsilonproteobacteria dominated in the bottom water layer. Cyanobacteria and Planktomycetes regained dominance in early spring, after the oxygen concentration was restored by winter mixing. In total, these results show large spatio-temporal changes in bacterial community composition, especially during transitions from oxic to anoxic and from sulfidic to nonsulfidic conditions.
Highlights
Freshwater lakes provide vital ecosystem services to human society
The oxygen concentration recovered to near saturation throughout the entire water column in March, and the other physico-chemical parameters were essentially uniform over depth, indicating that mixing did reach the deeper parts of the lake in early spring
The ratio of dissolved inorganic nitrogen to phosphorus (DIN:DIP >75) greatly exceeded the canonical Redfield ratio of 16:1, which indicates that P was the major limiting factor for the phytoplankton spring bloom
Summary
Freshwater lakes provide vital ecosystem services to human society. As key players in biogeochemical cycles and water quality, bacteria in freshwater lakes have been studied extensively (Eiler and Bertilsson, 2004; Kent et al, 2007; Nelson, 2009; Shade et al, 2012). Gleason attributed a much greater role to chance events, and argued that succession is much less predictable than advocated by the Clementsian view Both viewpoints have been extensively debated over the years, in studies of succession of plants (Inouye and Tilman, 1995; Kreyling et al, 2011) and animals (Chase, 2010). Generation sequencing of DNA provided enhanced taxonomic resolution and further insight into community succession of bacteria in the epilimnion of seasonally stratified lakes (e.g., Eiler et al, 2012; Okazaki and Nakano, 2016). Comparative studies of bacterial succession in different water layers are still relatively rare (e.g., Shade et al, 2008; Garcia et al, 2013; Yu et al, 2014; Okazaki and Nakano, 2016), but could shed more light on the impact of seasonal stratification on successional trajectories
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