Green sulphur bacteria as a component of the photosynthetic plankton community in small dimictic humic lakes with an anoxic hypolimnion

High bacteriochlorophyll (BChl) concentrations in the anoxic water layers of some humic lakes have indicated that green sulphur bacteria (GSB) may be ecologically significant. The abundance and spatial distribution of GSB were therefore addressed in 13 small humic lakes using fragment analysis and sequencing of PCR-amplified 16S rRNA genes. GSB were detected from lakes where the photosynthetically active radiation was at least 1.1 µE m-2 s-1 at the oxic-anoxic boundary layer. In these lakes, 13 to 42% of the 16S rRNA gene sequences of the anoxic water column were assigned to GSB. The spatial distribution of GSB was tightly correlated with the spectrophotometrically measured BChl concentration during the summer season. Maximum BChl concentrations were observed in the uppermost part of the anoxic water layer, covering most of the chlorophyll pigment in these lakes. The GSB of the humic lakes typically belonged to a phylogenetically homogenous group closely related to Chlorobium clathratiforme.