Aridity-induced limit to photosynthesis and primary production in the Atacama Desert

Abstract

Identifying the dry limit to microbial photosynthesis contributes to our understanding of life in extreme environments and to the question of life, past or present, on Mars. Here we report on conditions in the Atacama Desert so dry that photoautotrophic microbes are virtually absent. Across a rainfall gradient, the fraction of translucent stones hosting cyanobacteria drops from 28% to 0.08% in the hyperarid core. The threshold for cyanobacterial survival correlates with mean rainfall of ≤5 mm/year, or 75 hours/yr of liquid water during light conditions suitable for photosynthesis. Across the gradient, cyanobacterial molecular diversity declines three-fold and organic carbon residence times increase by three orders of magnitude. The rare cyanobacteria in the core live slowly (3200 y turnover times) and survive in fertile refuges amidst an essentially abiotic landscape. Applied to Mars our results suggest that photosynthetic life is unlikely on the surface of that dry world.