Establishment of phototrophic purple sulphur bacteria in microbial mat systems

Laminated microbial mat ecosystems, dominated by phototrophic microorganisms are widely distributed in geothermal, hypersaline and intertidal marine environments (Castenholz 1984; Cohen 1984; Jørgensen and Cohen 1977; Stal et al. 1985; Van Gemerden et al. 1989a, b). The laminated structure of these vertically stratified benthic ecosystems is the result of the development of the constituent microflora along physical and chemical gradients. Thus, in a typical intertidal marine microbial mat dense populations of diatoms, cyanobacteria, purple and green sulfur bacteria, sulfate reducing bacteria and colourless sulphur oxidising bacteria coexist and compete for light, CO₂ and sulfide in the top few millimetres of the beach surface. In these systems a green layer of oxygenic cyanobacteria and unicellular green algae overlies a pink/red layer of anoxygenic purple sulfur bacteria which in turn is underlain by a black layer composed of precipitated ferrous sulfide (FeS). Cyanobacteria are the key component of most microbial mats since they thrive in nutrient poor, unstable and hostile environments. Filamentous species belonging to the genus Oscillatoria, able to fix dinitrogen, are commonly found as the initial colonisers in such environments (Stal and Krumbein 1981). They not only enrich intertidal marine sediments with low molecular weight organic compounds as a consequence of photosynthate excretion and cell lysis but also play an important role in stabilising sediments and reducing evaporative water loss (Stal et al. 1985). The released organic carbon is rapidly mineralised by heterotrophic bacteria resulting in the development of anoxic conditions. In the absence of oxygen populations of sulfate reducing bacteria become established.