Cyanobacteria in deserts — life at the limit?

Cyanobacteria are primitive phototrophic prokaryotes whose long evolutionary history dates back to the Proterozoic era. Their ubiquity on the planet and dominance in hot and cold deserts is a measure of their ecophysiological resilience and adaptability. They have been studied extensively as part of exobiological research into the limits of life in the Solar System. Desert cyanobacterial communities tolerate desiccation that results from acute water deficiency. and they accumulate compatible solutes to counteract osmotic stresses which result from freezing and high salinities. They also accumulate trehalose as a water replacement mechanism to maintain the functional integrity of membranes during anhydrobiosis. Cyanobacteria tolerate high and low extremes of temperature. Their capacity for screening excessive solar radiation (PAR and UVb) by synthesis of “sunscreen” biochemicals whilst retaining a capacity for shade-adaptation. makes them eminently suited for colonisation of diverse lithic habitats. They pioneer the development of microphytic soil crusts which stabilise mobile desert soils. They colonise fissures in rocks as chasmolithic colonists and penetrate the fabric of porous, translucent rocks to provide the primary-producing basis of endolithic communities ranging from the hottest deserts to the cold Dry Valleys of Antarctica. They biodegrade these rocks to create soils which they enrich and inoculate. Their ability to survive at the limits of life on the surface of the Earth is now being studied as an analogue for past life on Mars ? the ultimate desert.