f 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 Rogoznica's chemistry and hydrography have been 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. Our data suggest a tight coupling between the lake's chemistry and occurring microorganisms. During stratification, anoxygenic phototrophic sulfur bacteria were dominant at the chemocline and in the hypolimnion. After an anoxic mixing event, the anoxygenic phototrophic sulfur bacteria entirely disappeared, and the homogeneous, anoxic water column was dominated by a bloom of gammaproteobacterial sulfur oxidizers related to the GSO/SUP05 clade. 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.
Stratified sulfidic lakes offer a physically and chemically welldefined environment for the development of complex yet stable microbial communities along light, oxygen, and sulfide gradients (1-4). Their water column is divided into an oxic surface layer, the epilimnion, and an anoxic bottom layer, the hypolimnion (5). The interface between these layers is the chemocline. It is the zone of highest chemical reactivity in the lake, typically accompanied by elevated microbial activity. In the anoxic hypolimnion and the underlying sediments, sulfate reduction plays a key role in organic matter remineralization (e.g., see references 3 and 6), resulting in the production of hydrogen sulfide. Where sulfide and light are present but oxygen is absent, anoxygenic phototrophic green sulfur bacteria (GSB) and purple sulfur bacteria (PSB) oxidize sulfide and form extensive blooms with cell densities of up to 10 7 cells ml Ϫ1 (1, 3, 7-10). Chemotrophic sulfur oxidizers (e.g., Alphaproteobacteria, Gammaproteobacteria, and Epsilonproteobacteria) often occur in layers above or together with GSB and PSB at the chemocline (3, 9, 11-13). In the oxic, sulfide-free epilimnion, the microbial community composition is distinct from those in underlying anoxic waters (10,13,14). Lake Rogoznica is a seasonally stratified seawater lake on the Croatian Adriatic coast. Its hydrography and chemistry have been studied extensively throughout the last 2 decades. The epilimnion is similar to waters of the adjacent Adriatic Sea in temperature and salinity, while the hypolimnion is highly enriched in nutrients (NH 4 ϩ , up to 150 M; PO 4 3Ϫ , up to 22 M; SiO 4 4Ϫ , up to 400 M) and dissolved organic carbon (DOC) (up to 6 mg...