Bacterioneuston and bacterioplankton dynamics in two adjacent but trophically distinct marine environments on the Adriatic coast. Bacteria in marine surface microlayer and underlying water

Marine surface microlayer (SML) is large and extreme marine environment with an important role in biogeochemical cycling and climate regulation. We explored seasonal dynamics of bacterial assemblages in SML (bacterioneuston) and underlying water layer (ULW) (bacterioplankton) in eutrophic marine Rogoznica Lake environment and more oligotrophic coastal area of the adjacent Adriatic Sea. While both layers 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. Except in spring in Rogoznica Lake, bacterial abundance in SML was also significantly higher (p <0.05) than in ULW. Both layers show prominent seasonal variability. High-throughput 16S rRNA gene sequencing of DNA and cDNA revealed a considerable difference in bacterial assemblage structure and dynamics, although study sites were <200 m apart. Heterotrophs were predominant in both layers with pronounced spatial and temporal dynamics, except in autumn in Rogoznica Lake when, autotrophs became dominant active 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 in future.