Environmental shaping of bacterial communities and functional gene diversity in the extreme Transantarctic Mountains

Ice free regions in Antarctica account for approximately 0.3% of the continental area and are patchily distributed. The extreme environmental conditions of these soil ecosystems have shaped a low diversity and simple trophic structure in which microorganisms face severe conditions including low water and nutrient availability, cold temperatures, freeze-thaw cycles, long periods of darkness in winter, and exposure to high levels of ultraviolet radiation in summer. In these areas, the diversity of microorganisms involved in key biogeochemical processes such as the nitrogen cycle is still largely unknown. This study is integrated in a multidisciplinary research team based in New Zealand (NZTABS) that focuses primarily on the microbial diversity of the Antarctic Dry Valleys. Given this, the K020 expedition to Victoria Valley in January 2013 sampled an area of approximately 300 km. Soil samples were collected, along with the corresponding metadata and subjected to automated ribosomal intergenic spacer analysis (ARISA), to characterize the abundance and richness of the existing microbial communities in the whole valley. Relationships between environmental and microbial data from a total of 86 stations revealed that diversity, abundance and activity of the Victoria Valley’s bacterial communities are subjected to strong spatial structuring due to the extreme heterogeneity in soil geochemical properties, being water availability identified as a main environmental constrain. In order to understand how water availability in these environments may affect bacterial diversity and phylogeny, soils from a transect with increasing distance from a water source were also sampled and the 16S rRNA gene was amplified and sequenced using Roche’s 454 pyrosequencing technology. Alpha and beta diversity metrics were computed and compared with measures of environmental variables which were also collected on site. Results revealed a clear shift in the microbial communities across the transect with a water availability gradient, namely a notorious replacement of members of the Proteobacteria phyla by members of the Actinobacteria phyla, as the soils became drier. Additionally, the sampling point with most unknown OTUs corresponded to the place with the highest values of conductivity and organic carbon production, along with the highest frequencies of members of phylum Bacteroidetes. Also, members of the Cyanobacteria phyla decreased their frequencies as the soils became drier, which likely caused a decrease in the frequencies of occurrence of other taxonomic groups, given the fact that Cyanobacteria are known to be the major nitrogen fixators in the Dry Valleys FCUP Environmental shaping of bacterial communities and functional gene diversity in the extreme Transantarctic Mountains vi and the bio-availability of this chemical element has proved to influence the abundance of other microorganisms. In parallel, this study also aimed to assess the distribution and phylogenetic affinities of a particular group of microorganisms with a prominent role in the nitrogen cycle. Taking advantage of the recently sampled Victoria Valley, the existence of ammonia-oxidizing bacteria (AOB) and archaea (AOA) was assessed in a high range of Antarctic soil environments, based on standard DNA molecular approaches (extraction, amplification, cloning and sequencing). We established relationships between the broad scale environmental gradients of the sampled locations and the relative diversity of ammonia oxidizing microbial communities. In addition to Victoria Valley, the Miers Valley, Beacon Valley, Upper Wright Valley, Battleship Promontory, and the DarwinHatherton Glacier region of the Darwin Mountains were the chosen sampling sites, from which DNA was extracted, amplified, cloned and sequenced. The gene coding for ammonia-monooxygenase (amoA) was the functional marker chosen and results revealed generally low AOB and AOA amoA gene diversity, with only 6 and 5 identified OTUs with the highest clustering cut-off, and, and 2 and 3 OTUs respectively with the lowest clustering cut-off. AOA OTUs were affiliated with the soil group 1.1b, along with culturable representatives of the genus Nitrososphaera. AOB OTUs were separated into two clusters, one affiliated with the genus Nitrosomonas, and another one affiliated with the genus Nitrosospira. In both groups of microorganisms there seems to exist a cosmopolitan OTU, which is the most abundant and has close affiliations with environmental clones from a wide range of environments. The observations reported in this study demonstrating a highly dispersed distribution of AOA and AOB within the Transantarctic Mountains reinforce the potential of nitrification processes in driving microbial functionality of these extreme ecosystems.