The role of microbial mats in the production of reduced gases on the earlyEarth

The advent of oxygenic photosynthesis on Earth may have increased global bi ological productivity by a factor of 100-1,000 (ref. 1), profoundly affecti ng both geochemical and biological evolution. Much of this new productivity probably occurred in microbial mats, which incorporate a range of photosyn thetic and anaerobic microorganisms in extremely close physical proximity(2 ,3). The potential contribution of these systems to global biogeochemical c hange would have depended on the nature of the interactions among these mat microorganisms. Here we report that in modern, cyanobacteria-dominated mat s from hypersaline environments in Guerrero Negro, Mexico, photosynthetic m icroorganisms generate H-2 and CO-gases that provide a basis for direct che mical interactions with neighbouring chemotrophic and heterotrophic microbe s(4). We also observe an unexpected flux of CH4, which is probably related to H-2-based alteration of the redox potential within the mats. These fluxe s would have been most important during the nearly 2-billion-year period du ring which photosynthetic mats contributed substantially to biological prod uctivity(5) -and hence, to biogeochemistry-on Earth. In particular, the lar ge fluxes of H-2 that we observe could, with subsequent escape to space, re present a potentially important mechanism for oxidation of the primitive oc eans and atmosphere.