Bacterioneuston and Bacterioplankton Structure and Abundance in Two Trophically Distinct Marine Environments - a Marine Lake and the Adjacent Coastal Site on the Adriatic Sea.

Marine surface microlayer (SML) is a large and extreme marine environment with an important role in biogeochemical cycling and climate regulation. We explored the seasonal structure and abundance of bacterial assemblages in SML (bacterioneuston) and underlying water layer (ULW) (bacterioplankton) in eutrophic marine Rogoznica Lake and more oligotrophic coastal area of the adjacent Adriatic Sea. SML and ULW in each site were similar in pH, salinity, dissolved oxygen, oxygen saturation, and temperature. Rogoznica Lake was colder in winter and warmer in summer compared to the Adriatic Sea. Regarding nutrients, SML and ULW were notably different environments. SML was consistently enriched in nitrate, nitrite, orthophosphate, and total organic carbon than ULW in both investigated environments. Except in spring in Rogoznica Lake, bacterial abundance in SML was also significantly higher (p < 0.05) than in ULW. Both layers and sites show prominent seasonal variability. High-throughput 16S rRNA gene sequencing of DNA and cDNA revealed a considerable difference in bacterial assemblage structure, although study sites were < 200 m apart. Heterotrophs were predominant in both layers with pronounced spatial and temporal structural differences, except in autumn in Rogoznica Lake when, autotrophs became the dominant fraction under oxygen-deprived conditions. All these variations were driven by in situ conditions, the most important ones being total organic carbon and temperature (and additionally dissolved oxygen in Rogoznica Lake). This is especially important in terms of ongoing eutrophication, warming and deoxygenation, noticed not only in the Adriatic Sea and Rogoznica Lake but globally as well. Therefore, further structural and physiological changes in bacterioneuston and bacterioplankton assemblages can be expected.

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 for studying the microbial ecology of conditions resembling ancient Earth. The expansion and increasing global distribution of such environments also means that information about them serves to understand future phenomena. In this study, a long-term chemical dataset (1996-2015) was explored together with seasonal (in 2015) information on the diversity and abundance of bacterial and archaeal communities residing in the chemocline, monimolimnion and surface 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 nutrient concentrations. Long-term data indicated that Rogoznica Lake exhibited climate changes that could alter its physico-chemical features and, consequently, induce structural and physiological changes within its microbial community.