Microbial population dynamics along a terrestrial Antarctic moisture gradient

The McMurdo Dry Valleys in Antarctica are the largest ice-free soil region on the continent. The region is a hyper-arid polar desert with extremely limited moisture availability. Microbial colonization dominates this ecosystem but surprisingly little is known about how communities respond to changing moisture regimes. We utilized the natural model system provided by transiently wetted soil at lake margins in the Dry Valleys to interrogate microbial responses along a well-defined contiguous moisture gradient and employed fine-scale taxonomic resolution to disentangle responses between and within phyla. We identified a striking non-linear response among bacteria and fungi where at low moisture levels small changes resulted in a large impact on diversity. Once communities reached asymptote at higher moisture levels changes were less pronounced. We demonstrate that whilst the main drivers were deterministic, a switch in the major influence occurred from abiotic factors at low moisture levels to biotic interactions at higher moisture. Response between and within phyla was markedly different, highlighting the importance of fine-scale taxonomic resolution in community analysis. Furthermore we identify effects that appeared stochastic at broad taxonomic levels were in fact the result of deterministic factors when viewed at fine-scale resolution. Overall the findings provide critical new insight on the response to moisture and will allow better predictions of ecosystem resilience in the threatened McMurdo Dry Valleys system.