Competition between purple and brown phototrophic bacteria in stratified lakes: sulfide, acetate, and light as limiting factors

Abstract

The affinities for sulfide and acetate under mixotrophic conditions have been determined for the brown Chlorobium phaeobacteroides and the purple Thiocapsa roseopersicina isolated from a bloom in Lake Kinneret (Israel) at a depth of about 18 m. C. phaeobacteroides exhibited a far higher affinity for sulfide than T. roseopersicina. For acetate, the opposite was observed. In light-limited continuous cultures, C. phaeobacteroides preferentially used sulfide, whereas in mixotrophic cultures of T. roseopersicina sulfide could be detected without detectable acetate. Competition experiments under increasingly severe light limitation resulted in co-existence of the two strains. Relatively high light intensities resulted in a dominance of T. roseopersicina over C. phaeobacteroides, whereas at lower intensities C. phaeobacteroides became dominant. However, at light intensities below 2 μEin · m−2· s−1, T. roseopersicina was completely excluded. At low light intensities, C. phaeobacteroides is able to grow at a much higher rate than T. roseopersicina. The maintenance rate constant μe of C. phaeobacteroides is −0.001 h−1, whereas that of T. roseopersicina is −0.011 h−1. However, high light intensities inhibit the growth rate of C. phaeobacteroides, but not that of T. roseopersicina. The explanation of the high numbers of C. phaeobacteroides in Lake Kinneret appears to be the combination of low light intensities and low sulfide concentrations. As a result, the incorporation of acetate is enhanced. The low numbers of T. roseopersicina can be explained by the high maintenance energy requirements of this organism, which exceed the available light at the depth of the bloom.