Anaerobic dark energy generation in the mat-building cyanobacterium Microcoleus chthonoplastes
Microbial mats are characterized by strong diel fluctuations in oxygen concentration which to a large extent can be attributed to the physiology of the cyanobacteria (Revsbech et al. 1983). In the light these organisms carry out oxygenic photosynthesis resulting in oxygen supersaturation of the mat. In the dark the primary mode of energy generation is respiration of endogenous glycogen (Smith 1982). However, in well-established microbial mats diffusion of oxygen in the mat will normally be insufficient to cover the demands and, as a result, the mat will turn anoxic. Evidently, the cyanobacteria have to switch to another way of energy generation in order to survive. Organisms isolated from such habitats were investigated for their mechanisms of anaerobic dark energy generation and turned out to be capable of fermentation. Forinstance, Oscillatoria limnetica, which thrives in the sulfide-rich hypolimnion of Solar Lake (Sinai) displays a homolactic fermentation using endogenous storage glucan as the substrate (Oren and Shilo 1979). Alternatively, this organism is capable of anaerobic respiration using elemental sulfur as the electron acceptor. Another mat-building cyanobacterium, Oscillatoria limosa, carries out a heterolactic and homoacetic fermentation simultaneously (Heyer et al. 1989). In the presence of elemental sulfur this organism stops producing hydrogen and sulfide is produced instead. Oscillatoria terebriformis was isolated from a hot-spring microbial mat and produces lactate form exogenous glucose or fructose (Richardson and Castenholz 1987).
KeywordsOxidative Pentose Phosphate Pathway Fermentative Metabolism Carbon Recovery Soluble Hydrogenase Sulfur Respiration
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