ABSTRACT
DiversityStructurally, prokaryotic organisms are far simpler than eukaryotic cells. Yet, prokaryotes are functionally or metabolically diverse with regard to reactions they mediate or stresses they can endure. Prokaryotes do not possess membrane-bound organelles such as a nucleus, endoplasmic reticulum, mitochondria, or chloroplasts. Therefore, prokaryotes cannot separate metabolically incompatible biochemical processes into discrete compartments (Fig. 7.1). The inability to compartmentalize cellular processes poses a bottleneck for the biochemical and structural evolution of prokaryotes. As a result, prokaryotes have diversified with regard to biochemical abilities while remaining structurally simple. The prokaryotic solution to “compartmentalization” is the formation of associations with other organisms, which gives protections against potentially inhibitory environmental factors. These include exposure to adverse oxygen concentration, ultraviolet radiation, desiccation, and adverse pH value. The steep biogeochemical gradients that exist in mats allow and select for functional diversification such that diverse organisms having requirement for aerobic, microaerophilic, and anaerobic conditions may co-exist and contemporaneously function along a gradient. Such dramatic environmental changes occurring in a small spatial scale set up association that facilitate mutualistic nutrient, gas, and metabolite exchange. Associations reflect synergistic or syntrophic lifestyles where growth and biogeochemical cycling are conducted more effectively and efficiently than on an individual population basis. Such associations are also called microbial consortia. Members of the consortium maintain metabolic and ecological compatibility, as long as biogeochemical
and environmental gradients allow for individual niches to exist in a close proximity. Microbial mats typify these conditions and, accordingly, are the focus of research on consortial growth strategies in extreme environments.
