Geochemical Characterization of Coastal Pond and Adjacent Soil Organic Matter from Two Distinct Field Areas of the McMurdo Dry Valleys, Antarctica

The McMurdo Dry Valleys (MDV), Antarctica contains some of the few ice-free oases on the continent, and is one of the coldest, driest places on earth. Despite the harsh conditions, the MDV supports an abundance of biological communities within soil and aquatic systems. Bioavailable carbon is present in these systems in the form of ancient and modern sources. An additional source of bioavailable carbon that has largely been ignored may be represented by desiccated and disseminated microbial mats (coastal pond organic matter, or CP-OM) originating in ephemeral coastal meltwater ponds of the MDV. The goal of this study is to more fully characterize CP-OM so that it may be recognized in a variety of environmental samples. Modern (active) mat, relict (desiccated) mat, and soil samples were collected from four ponds and along transects adjacent to these ponds at two field localities, Hjorth Hill Camp and Garwood Valley Camp. Bulk isotopic, elemental and phospholipid fatty acid (PLFA) analyses of the three sample types were performed. Results indicate that CP-OM is present in the soil reservoir at both localities, even at sample locations distant from coastal ponds. The soil reservoir at Garwood Valley has a greater contribution of CP-OM than at Hjorth Hill. Total organic carbon (TOC) measurements show high concentrations of organic matter close to pond edges, with TOC concentrations decreasing with distance. PLFA analyses demonstrate all three sample types are composed of a three-tiered community system dominated by cyanobacteria. Additionally, a shift from minor bacterial community composition in modern and relict mats towards greater community composition in soil samples is evident. Bulk carbon and nitrogen isotopic signatures vary between sample types and field locations, however large-scale implications indicate nitrogen-fixing cyanobacteria are present within ponds under CO2-limited conditions. Variations in carbon and nitrogen isotopic values for relict mat and soils are a result of varying degrees of in situ microbial processes and bulk source matter utilization. A baseline geochemical characterization for CP-OM and adjacent soils has been created, and can be used in conjunction with previous work for future studies of ecosystem functioning and linkages within the MDV