Microbial diversity and long-term geochemical trends in the euxinic zone of a marine, meromictic lake.

Hypoxic and anoxic niches of meromictic lakes are important sites to study the microbial ecology of conditions resembling ancient Earth. The expansion, and increasing global distribution, of such environments also means that information about them serve to understand future phenomena. We explored a long-term chemical dataset (1996 - 2015) along with seasonal (in 2015) information on the diversity and abundance of bacterial and archaeal communities residing the chemocline, monimolimnion and sediment of the marine meromictic Rogoznica Lake. The results of quantitative PCR assays, and high-throughput sequencing, targeting 16S rRNA genes and transcripts, revealed a clear vertical structure of the microbial community with Gammaproteobacteria (Halochromatium) and cyanobacteria (Synechococcus spp.) dominating the chemocline, Deltaproteobacteria and Bacteroidetes dominating the monimolimnion, and significantly more abundant archaeal populations in the surface sediment, most of which affiliated to Nanoarchaeota. Seasonal changes in the community structure and abundance were not pronounced. Diversity in Rogoznica Lake was found to be high, presumably as a consequence of stable environmental conditions accompanied by high dissolved carbon and nutrients concentrations. Long-term data indicated that Rogoznica Lake exhibits climate changes that can lead to development of highly specialized taxa, which could have deleterious consequences on functioning of the lake.