COEXISTENCE OF AEROBIC CHEMOTROPIC AND ANAEROBIC PHOTOTROPHIC SULFUR BACTERIA UNDER OXYGEN LIMITATION

The aerobic chemotrophic sulfur bacterium Thiobacillus thioparus T5 an d the anaerobic phototrophic sulfur bacterium Thiocapsa roseopersicina M1 were co-cultured in continuously illuminated chemostats at a dilut ion rate of 0.05 h(-1). Sulfide was the only externally supplied elect ron donor, and oxygen and carbon dioxide served as electron acceptor a nd carbon source, respectively. Steady states were obtained with oxyge n supplies ranging from non-limiting amounts (1.6 mol O-2 per mol sulf ide, resulting in sulfide limitation) to severe limitation (0.65 mol O -2 per mol sulfide). Under sulfide limitation Thiocapsa was competitiv ely excluded by Thiobacillus and washed out. Oxygen/sulfide ratios bet ween 0.65 and 1.6 resulted in stable coexistence. It could be deduced that virtually all sulfide was oxidized by Thiobacillus. The present e xperiments showed that Thiocapsa is able to grow phototrophically on t he partially oxidized products of Thiobacillus. In pure Thiobacillus c ultures in steady state extracellular zerovalent sulfur accumulated. i n contrast to mixed cultures. This suggests that a soluble form of sul fur at the oxidation state of elemental sulfur is formed by Thiobacill us as intermediate. As a result, under oxygen limitation colorless sul fur bacteria and purple sulfur bacteria do not competitively exclude e ach other but can coexist. It was shown that its ability to use partia lly oxidized sulfur compounds, formed under oxygen limiting conditions by Thiobacillus, helps explain the bloom formation of Thiocapsa in ma rine microbial mats.