engleski

Microbial communities in the Zmajevo oko (Rogoznica lake) during stratified and mixed conditions

Zmajevo oko is a seawater lake located on the Gradina Peninsula, approx. 100 m from the open Adriatic Sea. The lake has a circular shape with a maximum depth of 15 m. It is sheltered from the wind by high cliffs that prevent wind shear mixing. Compared with the open Adriatic Sea, lake Rogoznica is eutrophic. The relatively small freshwater input and the peculiar geochemical setting lead to the occurrence of seasonal stratification, accompanied by sulfidic and anoxic events. During such events, highly elevated sulfide concentrations and the formation of colloidal elemental sulfur, indicated by a milky turbidity, have been observed. The formation of S0 was related to the presence of a dense population of Chromatium like purple sulfur bacteria (PSB) on the sulfide/oxygen chemocline. The influence of sulfide and sulfur on microbial communities above and below this chemocline is still unknown. In this study, we used culture independent methods (comparative 16S rRNA gene analysis and Catalyzed Reporter Deposition - Fluorescence in situ hybridization) and 454-pyrosequencing of the 16S rRNA gene to determine the microbial community composition and its vertical distribution during and after stratification events. The total number of bacteria in the investigated period was between 3x 106 and 1 x 107. The surface layers of Zmajevo oko have shown to be dominantly populated by marine pelagic organisms, belonging to the Alpha- and Gammaproteobacteria and Bacteroidetes, as well as by oxygenic phototrophes, during both stratified and mixed conditions. According to pyrotag analysis, Alphaproteobacteria and Cyanobacteria were most abundant, followed by Bacteriodetes, Rhodobacteriaceae and Actinobacteria in the surface layer. The chemocline was dominated by GSB and Cyanobacteria. Surprisingly, only few PSB related 16S rRNA gene sequences were retrieved from the chemocilne layers. Instead, the chemocline occurring at a depth of ~9 m during stratified periods is strongly dominated by green sulfur bacteria (GSB) sequences, affiliating with Chlorobium sp. Deltaproteobacteria, most likely thriving on the reduction of S0 produced by the GSB layer, seem to dominate anoxic layers below the chemocline. When the water column is mixed, the GSB and Deltaproteobacteria populations seem to lose size and importance, and a community resembling those in lower depths replaces it.

microbial diversity; stratification; Rogoznica lake

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