Microbial mats: A joint venture

Abstract

Microbial mats characteristically are dominated by a few functional groups of microbes: cyanobacteria, colorless sulfur bacteria, purple sulfur bacteria, and sulfate-reducing bacteria. Their combined metabolic activities result in steep environmental microgradients, particularly of oxygen and sulfide. The driving force of most microbial mats is photosynthesis by cyanobacteria and algae. Subsequently, sulfate-reducing bacteria, using excretion-, lysis-, and decomposition products of cyanobacteria, produce sulfide by the dissimilatory reduction of sulfate. The sulfide can be reoxidized to sulfate by colorless and purple sulfur bacteria. Colorless sulfur bacteria are chemotrophic organisms primarily oxidizing sulfide and other reduced forms of sulfur with oxygen to obtain energy. The oxidation of reduced sulfur species also provides reducing equivalents for the reduction of carbon dioxide to cellular carbon. The final product of sulfide oxidation is sulfate, with elemental sulfur, deposited extracellularly, as the principal intermediate. Purple sulfur bacteria primarily are anaerobic phototrophic organisms using sulfide and other reduced forms of sulfur exclusively as the electron donor for the reduction of CO 2 to cellular carbon. Usually, sulfur is temporarily stored intracellularly. The final product of the oxidation of reduced forms of sulfur is sulfate. The niches for these metabolically different groups of microbes in ecosystems with steep, often non-overlapping, gradients of oxygen and sulfide appear to be spatially separated. However, maximum viable counts of colorless sulfur bacteria and purple sulfur bacteria were both found in the top 5–10 mm of mats. Unexpectedly, viable counts of sulfate-reducing bacteria also peaked at the same depth horizon. Sulfide is inhibitory for most oxygenic phototrophs. Sulfide production immediately underneath the layer of cyanobacteria might inhibit their growth, and, consequently, that of the entire ecosystem. In microbial mats this effect is minimized by the combined action of colorless and purple sulfur bacteria. Colorless sulfur bacteria generally have a much higher affinity for sulfide than purple sulfur bacteria, however, in microbial mats, their activity is hampered by low oxygen supply rates. As shown by pure culture studies with colorless sulfur bacteria, sulfide is incompletely oxidized when oxygen is short in supply, resulting in the production of potential electron donors for purple sulfur bacteria, such as sulfur, thiosulfate and polysulfides. In the absence of purple sulfur bacteria, colorless sulfur bacteria would not be able to maintain a low sulfide concentration due to shortage of oxygen, which in turn would result in increased inhibition of oxygenic photosynthesis. It thus appears that the combined action of all four groups of functional microbes mentioned effectively results in optimal growth of these recent “stromatolites”.