Hyporheic Zone

The idea for the project came from discussions between the three Professors and came to fruition through a successfully funded proposal submitted to the US National Science Foundation (NSF) Office of Polar Programs (OPP). The project started near the end of 2008 and will come to completion mid-2011.

The team devoted to the project has been assembled from 3 different universities who are all concurrently working on the project. Two deployments (Jan. and Dec. 2009) into the Dry Valleys have completed through the US Antarctic Program; each trip consisted of a 4 person team deployed into the Miers Valley for about 3 weeks. Through continued collaboration with the nzTABS group  we have had the exceptional chance to be hosted by the New Zealand team during our time in Miers Valley, integrating into a fully functional field camp during both field deployments, encompassing cooking equipment, human waste facilities and a large, temporary field-hut that was utilized as a makeshift laboratory by both the NZ and US teams.

Although the bulk mineral soils of the Dry Valleys (DV) are extremely arid, during the brief Antarctic summer months, temperatures within the DV become warm enough to melt exposed surfaces of glaciers resulting in the formation of melt-water streams that flow periodically (approx. 4 to 10 weeks) during the summer months. Wetted soils adjacent to the melt-water streams and wet soils surrounding lakes, defined as “hyporheic zones”, extend up to several meters on either side of the water source and are hotspots for microbial life and biogeochemical cycling. The microbial communities that form in these hyporheic zones survive the winter months in a desiccated state and are re-activated through hydration by summer melt-waters. As a result these communities can form large concentrations of responsive biomass even under extreme in situ environmental conditions. It has been hypothesized that these communities may represent a major source of nitrogen and carbon to the entire DV ecosystem through wind transportation of mat detritus to the low productivity bulk arid soils. Therefore, the microorganisms fixing both N2 and CO2 within these hyporheic mat communities may represent keystone communities supplying both nitrogen and carbon to the arid DV desert soils. However, investigations into the microbial members, biogeochemical activities and the genetic underpinnings responsible for these transformations in wetted DV soils are lacking. Therefore, through the current project we intended to fill this knowledge gap, specifically studying nitrogen and carbon dynamics and associated microbial communities that proliferate along glacial melt-water streams and lake edges in the Miers Valley locale of the DV.

To date, our analyses have uncovered several novel findings relating to both N2 and CO2 fixation in wetted soils of the DV. We have revealed the identities and activities of N2 and CO2 fixers in the DV, and most importantly, have proven that a large diversity of non-phototrophic N2 fixing bacteria exist in these habitats that can account for over 50% of the N2 fixation activity. These surprising results are in contrast to the commonly held belief that cyanobacteria are the primary N2 fixers in DV soils. Comparison between the microbial communities in wet and arid soils also indicate that although some microbes are shared, a large proportion of the microbial members are specific to their environment and are not present in the contrasting wet or dry soil biotope. Likewise, there appears to be CO2 fixing microbes that are specific to their respective wet and dry environments which may be indicative of specific adaptation to their contrasting habitats. Our results also indicate that these wetted soil systems harbor microbial cell concentrations that often exceed values found in temperate zone streams with N2 fixation rates that are similar to rates reported in temperate and tropical marine systems.
During our final sampling expedition and with the help of the NZTABS team a portion of the Adams Glacier melt-water stream in the Miers Valley was diverted to an area of arid soil. The goal of the experiment was to observe how an associated, yet non-hyporheic, microbial community responds both spatially and temporally to the long-term presence of liquid water. Microbial response (change in community structure and activities) in the wetted soils and nutrient levels were followed over a 7 week time period. Analyses are currently in progress, with preliminary results indicating a rapid change (< 24 hours) in community structure due to the wetting events.

Future of the project:

Our novel findings have encouraged us to continue our research momentum. We are also in the position to apply our newly developed methodologies to an extended sampling area so that we can truly define the range and influence these important microorganisms have throughout the entire Antarctic Dry Valley ecosystem. Specifically, through continuation of our current New Zealand collaboration, we plan to extend our current single Valley study site (Miers Valley) to include a variety of Antarctic ice-free soil habitats; thereby allowing the comparison among sites of differing latitude and Dry Valleys that do not include a lake system. As such, we plan to submit an application to renew this research and if successful, we hope to provide a more accurate portrayal of the microbial communities in wetted soils of the entire Dry Valley ecosystem, their contribution to large-scale nitrogen and carbon dynamics, with an ultimate goal of resolving the environmental factors driving microbial community identity, diversity and biogeochemical significance in the wetted soils of the Dry Valleys.

In addition to the three Professors mentioned above, major contributors to the project include, Thomas Niederberger from the University of Delaware, Alex Parker and Joëlle Tirindelli from San Francisco State University and Jill Sohm and Troy Gunderson from the University of Southern California.