Diversity of and interactions among sulphur bacteria in microbial mats

Abstract

Microbial mats are found along the outflow of continental thermal springs, on marine littoral sediments, in thalassic and athalassic (inland) hypersaline ponds and lakes, and in the deep-sea along hydrothermal vents. Hydrothermal vent microbial mats consist of chemotrophic sulphur bacteria thriving in the dark on sulphide which is mainly supplied by geochemical processes. However, it has been shown that sulphate reduction in these systems does occur up to 110 °C (Jørgensen et al. 1992), thus indicating that complete sulphur cycling also takes place. Microbial mats that are exposed to sunlight often comprise dense populations of oxygenic and anoxygenic phototrophic microorganisms together with chemoorganotrophs and chemolithotrophs. In habitats where the sulphur cycling is not a dominant process, microbial mats often originate from an association of cyanobacteria with anoxygenic filamentous phototrophic bacteria. For example, in thermal mats, it was shown that the Chloroflexus-like filamentous bacteria incorporated glycolate which was a major excretion product of the cyanobacteria found in the same mats (Bateson and Ward 1988). This commensalistic relationship is a nice example of a positive interaction of two organisms coexisting in the same environment. In other microbial mat environments, sulphide oxidation plays a predominant role. In the absence of geochemically formed sulphide, sulphide formation results mainly from the activities of sulphur and sulphate-reducing bacteria. In this paper, we discuss the biodiversity of sulphur bacteria and their ecological interactions in microbial mats.