Eolian transport of biomass from ephemerally wetted soils, associated with summer glacial meltwater runoffs and lake edges, to low‐productivity areas of the Antarctic Dry Valleys (DV) has been postulated to be an important source of organic matter (fixed nitrogen and fixed carbon) to the entire DV ecosystem. However, descriptions and identification of the microbial members responsible for N2 fixation within these wetted sites are limited. In this study, N2 fixers from wetted soils were identified by direct nifH gene sequencing and their in situ N2 fixation activities documented via acetylene reduction and RNA‐based quantitative PCR assays. Shannon‐index nifH diversity levels ranged between 1.8 and 2.6 and included the expected cyanobacterial signatures and a large number of phylotypes related to the gamma‐, beta‐, alpha‐, and delta‐proteobacteria. N2 fixation rates ranged between approximately 0.5 and 6 nmol N cm−3 h−1 with measurements indicating that approximately 50% of this activity was linked with sulfate reduction at some sites. Comparisons with proximal dry soils also suggested that these communities are not ubiquitously distributed, and conditions unrelated to moisture content may define the composition, diversity, or habitat suitability of the microbial communities within wetted soils of the DVs.

Carbon-fixation is a critical process in severely oligotrophic Antarctic Dry Valley (DV) soils and may represent the major source of carbon in these arid environments. However, rates of C-fixation in DVs are currently unknown and the microorganisms responsible for these activities unidentified. In this study, C-fixation rates measured in the bulk arid soils (<5% moisture) ranged from below detection limits to ∼12 nmol C/cc/h. Rates in ephemerally wet soils ranged from ∼20 to 750 nmol C/cc/h, equating to turnover rates of ∼7–140 days, with lower rates in stream-associated soils as compared to lake-associated soils. Sequencing of the large subunit of RuBisCO (cbbL) in these soils identified green-type sequences dominated by the 1B cyanobacterial phylotype in both arid and wet soils including the RNA fraction of the wet soil. Red-type cbbL genes were dominated by 1C actinobacterial phylotypes in arid soils, with wetted soils containing nearly equal proportions of 1C (actinobacterial and proteobacterial signatures) and 1D (algal) phylotypes. Complementary 16S rRNA and 18S rRNA gene sequencing also revealed distinct differences in community structure between biotopes. This study is the first of its kind to examine C-fixation rates in DV soils and the microorganisms potentially responsible for these activities.

Recently, there has been a dramatic expansion of studies of major histocompatibility complex (MHC) variation aimed at discovering functional differences in immunity across wild populations of diverse vertebrate species. Some species with relatively low genetic diversity or under strong directional selection by pathogens have revealed fascinating cases of MHC allelic disease linkage. More generally in genetically diverse species, however, these linkages may be hard to find. In this paper, we review approaches for assessing functional variation in MHC and discuss their potential use for discovering smaller-scale intraspecific spatial and temporal patterns of MHC variation. Then, we describe and illustrate an approach using the structural model to produce a population composite of variation in antigen-binding regions by mapping population-specific substitutions onto functional regions of the molecule. We are producing models of variation in major histocompatibility (MH) loci for populations of non-migratory fish (killifish, Fundulus heteroclitus) resident at sites that vary dramatically in environmental quality. We discuss the goal of relating MH population variation to functional differences in disease susceptibility such as those inferred by observations of parasitic infection and direct measurement of bacterial challenges in the laboratory. Our study has focused on relatively well-studied killifish populations, including those resident in a highly disturbed, chemically contaminated estuary and nearby less contaminated sites. Population-specific genetic changes at MHC antigen-binding loci are described, and evidence relevant to functional implications of these changes is reviewed. Population-specific patterns of variation in antigen-binding regions in combination with a range of assessments of immune function will provide a powerful new approach to reveal functional changes in MHC.

Cyanobacterial mats in the Antarctic Dry Valleys are photosynthetic microbial ecosystems living at the extreme of conditions on Earth with respect to temperature, light, water and nutrient availability. They are metabolically active for about 8 weeks during the austral summer when temperatures briefly rise above freezing and glacial and lake melt waters are available. There is much to learn about the biogeochemical impact of mats in these environments and the microbial communities associated with them. Our data demonstrate that these mats attain surprisingly high rates of carbon (CO 2) and dinitrogen (N 2) fixation when liquid water is available, in some cases comparable to rates in warmer temperate or tropical environments. C and N 2 fixation in Dry Valley mats in turn substantially elevate dissolved organic C and inorganic N pools and thereby promote enhanced microbial secondary production. Moreover, the microbial community fingerprint of these mats is unique compared with the more ubiquitous dry soils that do not contain mats. Components of the heterotrophic microbiota may also contribute substantially to N inputs through N 2 fixation.