The Phototrophic Sulfur Bacteria of the Lake Cadagno (Switzerland)

Lake Cadagno is a crenogenic meromictic lake located at 1921 m.a.s.l. in the southern Swiss Alps (46°33'N, 8°43'E) in the catchment area of a dolomite vein rich in gypsum. The chemistry of the lake is directly influenced by the particular geology of this valley. The water that percolates through these erodible sedimentary rocks becomes enriched in minerals and enters the lake through underwater springs. The inflow of this denser water produces a stable stratification of the lake and establishes meromixis. The transition zone between the oxygenic mixolimnion and the anoxygenic monimolimnion that is rich in salts is known as the chemocline and is characterized by steep gradients of oxygen, sulfide, and light. Generally, the chemocline that is positioned at a depth of approximately 12 m coincides with the presence of a dense community of anaerobic phototrophic sulfur bacteria. This community includes purple sulfur bacteria (PSB; family Chromatiaceae) of the genera Chromatium, Lamprocystis, Thiocystis and Thiodictyon and green sulfur bacteria (GSB; family Chlorobiaceae) of the genus Chlorobium. These phototrophic microorganisms fix CO2 via anaerobic photosynthesis.