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 represe nting pioneer and often the only biota inhabiting extreme aquatic and terre strial environments. Laminated mats and aggregates exemplify such communiti es. The fossil record indicates that these associations represent the earli est extant inhabitants and modifiers (i.e. anoxic to oxic conditions) of th e Earth's biosphere. Present-day consortia flourish in physically and chemi cally stressed environments, including nutrient-deplete, hypersaline, calci fied, desiccated and high-irradiance ecosystems ranging from the tropics to polar regions. Consortial members exhibit extensive metabolic diversificat ion, but have remained structurally simple. Structural simplicity, while ad vantageous in countering environmental extremes, presents a 'packaging prob lem' with regard to compartmentalizing potentially cross-inhibitory aerobic versus anaerobic growth processes. To circumvent these metabolic constrain ts, phototrophic cyanobacteria and microheterotrophs orient along microscal e chemical (i.e. O-2, PH, Eh) gradients to meet and optimize the biogeochem ical processes (C, N, S cycling) essential for survival, growth and the mai ntenance of genetic diversity, needed to sustain life. Microscale ecophysio logical, analytical, molecular (immunological and nucleic acid) techniques have helped to develop a mechanistic basis for understanding consortial gro wth and survival under extreme environmental conditions on Earth. Consortia are ideal model systems for developing a process-based understanding of th e structural and functional requirements for life in extreme environments r epresentative of the Earth's earliest biosphere and possibly other planets.