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Phototrophic sulfur bacteria in the chemocline of Lake Cadagno, Switzerland, were dominated by purple sulfur bacteria before 1998, but their composition shifted after a presumed disturbance to consist of mainly green sulfur bacteria. This study focused on comparative analyses of the distribution of the green sulfur compared to the purple sulfur bacteria by analyzing specific populations along fine scale depth profiles of the chemocline of Lake Cadagno. Water samples were collected from the chemocline on October 14th, 1998, and on September 28th, 2004. A detailed analysis of chemocline depth profiles revealed that total biomass of phototrophic sulfur bacteria was three times higher in 2004 than in 1998. The three-fold increase of biomass was entirely due to increments in abundance of one population of green sulfur bacteria, identified as Chlorobium clathratiforme. Abundance of purple sulfur bacteria remained unchanged with respect to overall numbers in the chemocline, but also with respect to distribution of different populations in depth in the chemocline. Aggregates of small-celled purple sulfur bacteria decreased in size about four-fold, but remained associated with sulfate-reducing bacteria of the genus Desulfocapsa. Compared to 1998, these had increased in numbers about three times in 2004, resulting in about ten times higher numbers per aggregate. These results demonstrate long-term effects of a presumed disturbance in autumn 1999 and 2000, on environmental conditions and on green sulfur and sulfate-reducing bacteria in the chemocline, however, without changes in the abundance and in the distribution of purple sulfur bacteria.
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