Lake Alatsee is a meromictic alpine lake with a maximum depth of 35 m and an 18 ha surface area located in Fuessen, Allgaeu, Germany. The lake´s geography and its steep-sided basin compared to the lake´s surface causes a permanent chemocline between 15 – 18 meters. It is of postglacial origin (ca. 8,000-10,000 years old) and below the chemocline the lake has been permanently anoxic and sulfidic throughout these years. The mixolimnion above the chemocline is oxygenated and the two layers remain unmixed. A characteristic feature of the water column is the existence of a well-defined layer in the chemocline formed by purple sulphur bacteria. This thick bacterial layer gives a reddish color to the water. On some rare occasions in the past, the lake turned anoxic to its surface and the layer of purple sulphur bacteria is said to have been extended to its oxygenated mixolimnion. The term “bleeding lake” was attributed to the lake by the locals who observed this phenomenon. Due to its remote location and the presence of the permanent anoxic monimolimnion, this lake is an ideal natural model ecosystem for studying anaerobic protistan freshwater communities that have probably been isolated in this ecosystem for thousands of years.

We were invited to study the protistan communities in this lake by F. Bruemmer and his scientific scuba-diving team from the University of Stuttgart and the GeoBio Center of the LMU Munich who focus on bacterial communities and the biogeochemistry of this unique ecosystem. Further studies are focused on the protistan predation on prokaryotic population and the molecular diversity of the microbial populations in Lake Alatsee.