Collaborative Research: Importance of Heterotrophic and Phototrophic N2 Fixation in the McMurdo Dry Valleys on Local, Regional and Landscape Scales - award #1246292

The McMurdo Dry Valleys of Antarctica represents the largest perpetually ice-free region on the continent. With the combination of prevailing katabatic winds and extremely low precipitation rates, they are arguably the oldest, coldest and driest deserts on Earth. Although the bulk mineral soils of the McMurdo Dry Valleys are extremely arid, during the brief austral summer months, wetted soils (hyporheic zone) adjacent to both glacial melt-water streams and lake systems become hotspots for microbial life and important zones for biogeochemical cycling. Cyanobacteria typically provide the primary structure to microbial mats that form within these wetted soils, and it has been assumed that cyanobacteria are the dominant N2 and CO2 fixing organisms within these communities and a major source of nitrogen and carbon to the entire Valley system due to wind-transport. Through our past research in the Miers Valley, we uncovered several novel findings related to wette d soils. For example, bacterial assemblages in wetted soils are highly divergent among the Miers Valley and smaller ponds nearby. Most importantly, molecular and activity based results indicate that diverse non-phototrophic bacteria make up a sizable fraction of phylotypes containing nitrogenase genes in the wetted microbial mats. This sub-population is active and can account for 50% of the N2 fixation activity in some mats. These surprising results are shifting the long-held paradigm that cyanobacteria are the primary N2 fixers in Dry Valley, and this distinction is important to understand, as phototrophic diazotrophs are a source of fixed carbon while heterotrophic diazotrophs are a sink. Intensive research was needed to truly understand the importance and drivers of heterotrophic N2 fixation across the entire Dry Valleys system, as each valley has its own unique characteristics. We sought to determine how universal our observations are by extending them to a broader range of sites across several contrasting valleys. We addressed the major questions as to what factors are responsible for the selection and dominance of non-phototrophic N2 fixers in these systems and how these groups respond to environmental variation (i.e. fall and spring light conditions). This research has now resolve the environmental factors driving diazotrophic community identity, diversity and biogeochemical significance in the wetted soils of the Valleys. Specifically, we have extend our current single valley study site to include a variety of Antarctic ice-free soil habitats, allowing the comparison among sites of differing latitude, temperature, elevation, and exposure to water. Modern genetic approaches (amplicon pyrosequencing, and quantitative PCR) and conventional activity-based assays (N2 fixation, sulfate reduction, bacterial productivity) were integrated with geochemical and soil property data (moisture, pH, conductivity, inorganic and organic nutrients, trace elements, etc.) to study the diversity, ecology, and function of N2 fixers that thrive in these extreme environments. Our results are now being integrated into a landscape wetness model that will help to elucidate the impact of phototrophic and heterotrophic diazotrophs on Dry Valley wide scale.