Through our earlier research of the biogeochemical importance of the primitive microbial mats in the Miers Valley of the Dry Valleys of Antarctica in the cycles of carbon and nitrogen, we uncovered several novel findings relating to nitrogen fixation in these seasonally wetted soils. Most importantly, both molecular and activity-based results indicate that a large diversity of non-phototrophic bacteria make up a sizable fraction of phylotypes containing nitrogenase genes and that these potential diazotrophs are also fixing nitrogen in these soils. This sub-population can account for over 50% of the nitrogen fixation activity in some mats. These surprising results have shifting the long-held paradigm that cyanobacteria are the primary nitrogen fixers in Dry Valley soils, and this distinction is an important one to understand, as phototrophic diazotrophs are a source of fixed carbon while heterotrophic diazotrophs are a sink. Intensive research is needed to fully understand the importance and drivers of heterotrophic nitrogen fixation across the entire Dry Valleys system, as each valley has its own unique characteristics.

During the current project period, we sought to determine how generalizable our observations are by extending them to a broader range of sites across several contrasting valleys including the Taylor, Garwood and Victoria Valleys. We also addressed the major questions as to what factors are responsible for the selection and dominance of non-phototrophic nitrogen fixers in these systems and how these groups respond to environmental variation (e.g. light, temperature and degree of wetting). Our research is helping resolve the relative importance of these environmental factors driving microbial mat communities with respect to microbial (and specifically diazotrophic) community identity, diversity and biogeochemical significance in the wetted soils of the valleys. While community structure appears to vary somewhat among the sites, general principals gleaned from our original studies appear to apply as non-phototrophic, and specifically sulfate reducing bacteria contribute substantially to nitrogen inputs in these systems. We also noted the distribution of a key photosynthetic gene across the range of mats sampled. In general, while mat morphology varies among sites depending on the degree of wettedness, community composition appears to be robust and similar among all sites. 

Specifically, we have extended our current single Valley study site to include a variety of Antarctic ice-free soil habitats, thereby allowing comparison among sites of differing latitude, temperature, elevation, and exposure to water.  We have seen that nitrogen fixing cyanobacteria are not universal in microbial mats, for example around the lake in Garwood Valley, but that microbial activity (e.g. thymidine uptake) is occurring in all wetted areas, sometimes at substantial rates.  Sulfate reduction rates, which to our knowledge have never been measured in the mats of the Dry Valleys, are measurable in the microbial mats alone, but at relatively low rates.  However, this activity confirms the possibility of nitrogen fixation from non-diazotrophic actors in the mats.

Last Modified: 10/26/2017
Modified by: Douglas G Capone