Vertical distribution of sulfate-reducing bacteria in the chemocline of Lake Cadagno, Switzerland, over an annual cycle

Sulfate-reducing bacteria were analyzed in the chemocline of meromictic Lake Cadagno, Switzerland, in March, June, August and October using in situ hybridization. Numbers of sulfate-reducing bacteria determined as the sum of cells hybridizing to Probes SRB385Db targeting Desulfobacteriaceae and SRB385 targeting Desulfovibrionaceae were similar at all samplings and accounted for 13 to 18% of the total microbial community, respectively. Abundance of cells detected with either probe, however, changed during the year, with cell numbers detected with SRB385Db being larger in early summer (June) and those detected with SRB385 being larger in late summer (October). Increasing cell numbers detected with SRB385Db were mainly caused by a yet uncultured and phylogenetically unidentified bacterium with a peculiar morphology ('Morphotype R') that followed the sulfide profile in June as well as in August with increasing numbers at increasing concentrations with depth. From the fraction of cells detected with SRB385, only a minor part represented a yet uncultured population without close cultured relatives. At all samplings, the majority of cells detected with SRB385 (93 to 99%) represented populations phylogenetically related to Desulfocapsa thiozymogenes DSM7269. Their cells were generally found in association with aggregates of small-celled phototrophic sulfur bacteria. This association was not specific for 1 of the 4 populations representing all small-celled phototrophic sulfur bacteria in Lake Cadagno. The association was also not obligate since non-associated cells were frequently found, especially in winter and early summer when limited light conditions caused by snow and ice cover reduced the abundance of small-celled phototrophic sulfur bacteria to about 50% of the values found in late summer. Nonetheless, the association between populations related to D. thiozymogenes and small-celled phototrophic sulfur bacteria suggests an ecological advantage to both organisms under appropriate environmental conditions.