Microbial diversity in a seasonally stratified marine lake (Lake Rogoznica). LakeRogoznica

Most stratified sulfidic holomictic lakes become oxygenated after annual turnover. In contrast, Lake Rogoznica on the eastern Adriatic coast has been observed to undergo a period of water column anoxia after water layer mixing and establishment of holomictic conditions. Although Lake Rogoznicas’ chemistry and hydrography are studied extensively, it is unclear how the microbial communities typically inhabiting the oxic epilimnion and a sulfidic hypolimnion respond to such a drastic shift in redox conditions. We investigated the impact of anoxic holomixis on microbial diversity and microbially mediated sulfur cycling in Lake Rogoznica with an array of culture-independent microbiological methods. We identified a tight coupling between the lake’s chemistry and occurring microorganisms. During stratification, anoxygenic phototrophic green sulfur bacteria (GSB) were dominant at the chemocline and in the hypolimnion. After an anoxic mixing event the GSB population entirely dissapeared, and the homogeneous, anoxic water column was dominated by a bloom of GSO/SUP05 clade-related Gammaproteobacteria. This study is the first report of a community shift from phototrophic to chemotrophic sulfide oxidizers as a response to anoxic holomictic conditions in a seasonally stratified seawater lake.