| Summary: | Understanding the processes that underlie patterns of microbial distribution is fundamental to the field of microbial ecology, but extremely challenging given the complexity of natural systems. Antarctica’s ice-free regions possess unique ecosystems of simple trophic structure, shaped by the harsh environmental conditions that typify the continent. The Dry Valleys comprise the largest of these regions and have one of the simplest food webs on the planet, making them a tractable system to comprehensively define relationships that influence microbial distributions at the landscape scale. The New Zealand Terrestrial Antarctic Biocomplexity Survey (nzTABS) was aimed at identifying factors that control, and can predict, biological distributions in the Dry Valleys. As part of nzTABS, the goal of the research presented in this thesis was to elucidate the factors that influence bacterial community structure in Dry Valley soils. This study explored how topographic, physicochemical, and spatial variation influence bacterial diversity and community structure across a Dry Valley landscape. Bacterial communities were characterized in 471 soil samples using automated ribosomal intergenic spacer analysis (ARISA). Diversity and community composition were most strongly related to variation in physicochemical soil properties, though significant relationships with topographic and spatial variables were also observed. This identified, for the first time, the influence of environmental variables on bacterial diversity and community composition across the landscape, and presents a structural equation model identifying those relationships. The phylogenetic diversity of bacterial communities in Dry Valley soils was also examined. High-throughput sequencing of 16S rRNA gene amplicons was used to analyze bacterial communities in 177 soil samples. This work identified significant relationships between the relative abundances of bacterial taxa and both abiotic and biotic variables, though these relationships explained only a small amount of community variation collectively. The relative abundances of several bacterial taxa were, however, significantly coupled to one another, suggesting that interactions between bacterial taxa may influence community compositions. Lastly, the bacterial composition of aerosols above the Dry Valleys was examined. High-throughput sequencing of 16S rRNA gene amplicons was used to analyze two aerosol samples collected in the Miers Valley, and their compositions were compared to those of previously characterized aerosols and soils from across the continent. Bacteria present in the aerosols were found to be distinct from those of local soils; instead, aerosol compositions were more similar to those of air samples reported from elsewhere on the planet. Importantly, these findings suggest that local redistribution of Dry Valley soil bacteria through atmospheric processes may be largely restricted to periods when high winds mobilize soil particles and associated biota. This study provides novel insights into the microbial ecology of the Dry Valleys. Despite the relative simplicity of the ecosystem, the factors that influence bacterial distributions within the Dry Valleys appear to be highly complex, and include a combination of abiotic and biotic drivers. Continued research will help to disentangle relationships that influence microbial community compositions in Antarctica’s ice-free ecosystems, and will improve understanding of processes that influence microbial community assembly globally.
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