| Summary: | Microbial biogeography is being increasingly more studied, both in terms of genetic divisions and 'ecotype' variation. This thesis investigates the regional (100-1000s km) and local (10s m) distribution and diversity of microorganisms around hydrothermal vents and seamounts at the East Scotia Ridge and the Southwest Indian Ridge. Microbial communities were characterized using Illumina dye sequencing to de- termine taxon richness and diversity and flow cytometry to obtain cell counts. In addition I investigated the physicochemical environment (nutrients, organic carbon, salinity and temperature) in which the microorganisms persist. Typical deep-sea microorganisms were abundant at vents and below the euphotic zone on the seamounts. The surface layer of the seamounts contained typical open-ocean photoautotrophic organisms. Microbial communities were correlated to or- ganic carbon on both hydrothermal vents and seamounts. With microorganisms possi- bly having a large influence on carbon sequestration into the deep-sea from hydrothermal vents. On a local scale the hydrothermal vents had a relatively higher abundance of chemosynthetic Epsilonproteobacteria and the Gammaproteobacteria family SUP05, which were closely correlated to the redox potential in the vent effluent. This was prominent in both plumes from the East Scotia Ridge and the Southwest Indian Ridge, which also had distinct microbial community structures. Across the Southwest Indian Ocean the microbial communities were firstly segregated by depth. However, on a regional scale their physical environment primarily divided the microbial communities into three biological regimes the sub-tropical, con- vergence zone and the sub-Antarctic. The microbial community structure and biogeography is influenced by steep environmental gradients, displaying a distance-decay relationship between sampling locations. With environmental conditions persisting at different scales, from local (10s m) around the hydrothermal vents to regional (100-1000 km) between the seamounts, ...
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