Cyanobacterial-bacterial mat consortia: examining the functional unit of microbial survival and growth in extreme environments.

Cyanobacterial-bacterial consortial associations are taxonomically complex, metabolically interactive, self-sustaining prokaryotic communities representing pioneer and often the only biota inhabiting extreme aquatic and terrestrial environments. Laminated mats and aggregates exemplify such communities. The fossil record indicates that these associations represent the earliest extant inhabitants and modifiers (i.e. anoxic to oxic conditions) of the Earth's biosphere. Present-day consortia flourish in physically and chemically stressed environments, including nutrient-deplete, hypersaline, calcified, desiccated and high-irradiance ecosystems ranging from the tropics to polar regions. Consortial members exhibit extensive metabolic diversification, but have remained structurally simple. Structural simplicity, while advantageous in countering environmental extremes, presents a 'packaging problem' with regard to compartmentalizing potentially cross-inhibitory aerobic versus anaerobic growth processes. To circumvent these metabolic constraints, phototrophic cyanobacteria and microheterotrophs orient along microscale chemical (i.e. O2, pH, Eh) gradients to meet and optimize the biogeochemical processes (C, N, S cycling) essential for survival, growth and the maintenance of genetic diversity, needed to sustain life. Microscale ecophysiological, analytical, molecular (immunological and nucleic acid) techniques have helped to develop a mechanistic basis for understanding consortial growth and survival under extreme environmental conditions on Earth. Consortia are ideal model systems for developing a process-based understanding of the structural and functional requirements for life in extreme environments representative of the Earth's earliest biosphere and possibly other planets.