The prokaryotic solution to the packaging problem is the formation of associations with other organisms the promote protection from potentially inhibitory environmental factors. These include exposure to adverse oxygen concentrations, ltraviolet radiation, desiccation, adverse pH, and Eh. The steep biogeochemical gradients that exist in mats allow and select for functional diversification such that fully organisms possessing aerobic, microaerophilic, and anaerobic requirements 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 may be termed microbial consortia. Functionally, a consortium exceeds the sum of its parts. Members of the consortium maintain metabolic and ecological compatibility, as long as biogeochemcal and environmental gradients allow for individual niches to exist in close proximity. Microbial mats typify these conditions and, accordingly, are the focus of research on consortial growth strategies in extreme environments.