Relationships between functional groups of organisms in microbial mats

Abstract

Microbial mats develop under a wide range of environmental conditions, and can be found in hypersaline coastal lagoons, hot springs, alkalinelakes, and marine intertidal flats (Cohen 1984, 1989; Jørgensen and Cohen 1977; Javor and Castenholz 1981, 1984; Jørgensen et al. 1983; Bauld 1984; Stal et al. 1985; Nicholson et al. 1987; Pierson et al. 1987). These laminated ecosystems characteristically are dominated by only a few functional groups of microbes. The driving force of most microbial mats is photosynthesis by cyanobacteria (CyaB) and algae. Subsequently, dissimilatory sulfate-reducing bacteria (SRB), using excretion-, lysis-, and decomposition products of CyaB, produce sulfide. The sulfide can be reoxidized to sulfate by colorless sulfur bacteria (CSB) and purple sulfur bacteria (PSB). Aerobic heterotrophic organisms are functionally important as their activity leads to oxygen depletion, and fermentative organisms provide growth substrates for SRB. In microbial mats these metabolically different groups of microbes live together in a layer of 5–10 mm thickness. Their combined metabolic activities result in steep environmental microgradients, particularly of oxygen and sulfide. Sulfide is inhibitory for most oxygenic phototrophs. Sulfide production immediately underneath the layer of CyaB might inhibit their growth, and, consequently, that of the entire ecosystem. On the other hand, anaerobic PSB and SRB are hampered by oxygen.