Bahaman Stromatolites

From studies on the Bahaman stromatolites and associated microbial mats a great deal of information on microbial mat lithification and stromatolite formation has been elucidated.

Carbon, nitrogen, oxygen and sulfur cycles have been studied (Pinckney et al., 1995). The cyanobacteria, aerobic heterotrophs, anoxygenic phototrophs, sulfate-reducers, sulfide-oxidizers and other anaerobic fermenters have been identified as the key metabolic groups within the stromatolite community (Dupraz and Visscher, 2005). Certain species of sul-fate-reducing bacteria have been identified by 16 S rDNA sequencing, and their location within the community pinpointed using fluorescent in situ hybridisation (Baumgartner et al., 2006).

The surface population of the stromatolites was found to cycle between several community types (Reid et al., 2000). Initially a pioneer community of the filamentous cyanobacterium Schizothrix sp. formed, which bound and trapped carbonate sand grains. This was followed by a heterotrophic bacterial biofilm containing abundant extracellular polymeric substances (EPS) developed, during which time cyanobacterial EPS production and bacterial sulfate reduction regulated calcium carbonate precipitation (Kawaguchi and Decho, 2002; Decho et al., 2005; Visscher et al., 1998). Finally a climax community developed, dominated by the endolithic cyanobacterium Solentia sp. which strengthened the lithified layer by fusing adjacent carbonate grains via microboring (Macintyre et al., 2000). From this current information regarding mat lithification and stromatolite formation processes in the Bahamas, it is possible to infer analogous processes that also may have occurred within Precambrian stromatolites. In addition to the study of stromatolite communities, the important question of why some mats lithify while others do not, has also been addressed. Possible answers include the need for an uncoupling of the metabolism of the key functional groups by spatial and/or temporal separation, and the influence of physicochemical properties such as calcium carbonate saturation indices and iron availability (Visscher et al., 1998; Dupraz and Visscher, 2005).

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