Browsing by Author "Van Horn, David J."
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- Environmental controls over bacterial communities in polar desert soilsGeyer, Kevin M.; Altrichter, Adam E.; Van Horn, David J.; Takacs-Vesbach, Cristina D.; Gooseff, Michael N.; Barrett, John E. (Ecological Society of America, 2013-10)Productivity-diversity theory has proven informative to many investigations seeking to understand drivers of spatial patterns in biotic communities and relationships between resource availability and community structure documented for a wide variety of taxa. For soil bacteria, availability of organic matter is one such resource known to influence diversity and community structure. Here we describe the influence of environmental gradients on soil bacterial communities of the McMurdo Dry Valleys, Antarctica, a model ecosystem that hosts simple, microbially-dominated foodwebs believed to be primarily structured by abiotic drivers such as water, organic matter, pH, and electrical conductivity. We sampled 48 locations exhibiting orders of magnitude ranges in primary production and soil geochemistry (pH and electrical conductivity) over local and regional scales. Our findings show that environmental gradients imposed by cryptogam productivity and regional variation in geochemistry influence the diversity and structure of soil bacterial communities. Responses of soil bacterial richness to carbon content illustrate a productivity-diversity relationship, while bacterial community structure primarily responds to soil pH and electrical conductivity. This diversity response to resource availability and a community structure response to environmental severity suggests a need for careful consideration of how microbial communities and associated functions may respond to shifting environmental conditions resulting from human activity and climate variability.
- Factors Controlling Soil Microbial Biomass and Bacterial Diversity and Community Composition in a Cold Desert Ecosystem: Role of Geographic ScaleVan Horn, David J.; Van Horn, M. Lee; Barrett, John E.; Gooseff, Michael N.; Altrichter, Adam E.; Geyer, Kevin M.; Zeglin, Lydia H.; Takacs-Vesbach, Cristina D. (PLOS, 2013-06-18)Understanding controls over the distribution of soil bacteria is a fundamental step toward describing soil ecosystems, understanding their functional capabilities, and predicting their responses to environmental change. This study investigated the controls on the biomass, species richness, and community structure and composition of soil bacterial communities in the McMurdo Dry Valleys, Antarctica, at local and regional scales. The goals of the study were to describe the relationships between abiotic characteristics and soil bacteria in this unique, microbially dominated environment, and to test the scale dependence of these relationships in a low complexity ecosystem. Samples were collected from dry mineral soils associated with snow patches, which are a significant source of water in this desert environment, at six sites located in the major basins of the Taylor and Wright Valleys. Samples were analyzed for a suite of characteristics including soil moisture, pH, electrical conductivity, soil organic matter, major nutrients and ions, microbial biomass, 16 S rRNA gene richness, and bacterial community structure and composition. Snow patches created local biogeochemical gradients while inter-basin comparisons encompassed landscape scale gradients enabling comparisons of microbial controls at two distinct spatial scales. At the organic carbon rich, mesic, low elevation sites Acidobacteria and Actinobacteria were prevalent, while Firmicutes and Proteobacteria were dominant at the high elevation, low moisture and biomass sites. Microbial parameters were significantly related with soil water content and edaphic characteristics including soil pH, organic matter, and sulfate. However, the magnitude and even the direction of these relationships varied across basins and the application of mixed effects models revealed evidence of significant contextual effects at local and regional scales. The results highlight the importance of the geographic scale of sampling when determining the controls on soil microbial community characteristics.
- Microbial Community Responses to Increased Water and Organic Matter in the Arid Soils of the McMurdo Dry Valleys, AntarcticaBuelow, Heather N.; Winter, Ara S.; Van Horn, David J.; Barrett, John E.; Gooseff, Michael N.; Schwartz, Egbert; Takacs-Vesbach, Cristina D. (Frontiers, 2016-07-18)The soils of the McMurdo Dry Valleys, Antarctica are an extreme polar desert, inhabited exclusively by microscopic taxa. This region is on the threshold of anticipated climate change, with glacial melt, permafrost thaw, and the melting of massive buried ice increasing liquid water availability and mobilizing soil nutrients. Experimental water and organic matter (OM) amendments were applied to investigate how these climate change effects may impact the soil communities. To identify active taxa and their functions, total community RNA transcripts were sequenced and annotated, and amended soils were compared with unamended control soils using differential abundance and expression analyses. Overall, taxonomic diversity declined with amendments of water and OM. The domain Bacteria increased with both amendments while Eukaryota declined from 38% of all taxa in control soils to 8 and 11% in water and OM amended soils, respectively. Among bacterial phyla, Actinobacteria (59%) dominated water-amended soils and Firmicutes (45%) dominated OM amended soils. Three bacterial phyla (Actinobacteria, Proteobacteria, and Firmicutes) were primarily responsible for the observed positive functional responses, while eukaryotic taxa experienced the majority (27 of 34) of significant transcript losses. These results indicated that as climate changes in this region, a replacement of endemic taxa adapted to dry, oligotrophic conditions by generalist, copiotrophic taxa is likely.