Characterization of a dense, purple sulfur bacterial layer in a meromictic salt lake

Abstract

In meromictic Mahoney Lake (British Columbia), purple sulfur bacteria formed a dense, 10‐cm‐thick layer at the primary chemocline. The dominant species (Amoebobacter purpureus) reached a maximal cell concentration of 4 × 108 cells ml‒1, 27% of which were viable. A maximal concentration of 20,900 µg bacteriochlorophyll a liter‒1 was determined—the highest concentration ever found in a natural body of water. Steep vertical gradients of conductivity, light, and sulfide concentration were detected across the bacterial layer, while the concentration of oxygen increased above it. Integrated primary production of the layer was 15 mg C m‒2 h‒1. Bacteriochlorophyll‐specific assimilation rates reached maxima at the top of the layer. Anoxygenic photosynthesis was limited by reduced sulfur compounds at the top of the layer, whereas strong attenuation of irradiance resulted in light limitation for most of the cell population below. The sulfide flux from below the layer accounted for <37% of the sulfide oxidized in the layer. Thus >63% of the sulfide oxidized during anoxygenic photosynthesis must have been produced within the layer. This calculation agrees well with sulfide production rates measured in situ (12.4 µmol liter‒1 h‒1). In contrast to other lakes, considerable uptake of [14C]acetate by purple sulfur bacteria and comparatively high numbers of purple nonsulfur bacteria indicate an important role of dissolved organic matter and a short cycle of carbon and redox compounds in the dense purple microbial layer.