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Microbial ecology of deep-sea hydrothermal vents : viruses, diversity and potential mortality Ortmann, Alice Catherine

Abstract

Deep-sea hydrothermal vents have been intensely studied since their discovery almost 30 years ago. These environments have been shown to harbour previously unknown organisms, which depend on chemosynthetic prokaryotes for much of their nutrition. Although high abundances of free-living prokaryotes have been documented from many hydrothermal vent sites, the fate of prokaryotic biomass has not been well studied. The initial goal of this thesis was to determine the abundance and distribution of viruses in hydrothermal vent environments and to determine if the abundances were high enough to support the occurrence of virus-mediated mortality of prokaryotes. This study clearly demonstrated that around active hydrothermal vent sites the virus abundance could be as high as values reported from coastal surface waters. Increased abundances within active hydrothermal fields and associated plumes above background seawater levels suggest that viruses are actively produced in these environments. The effects of viruses on a prokaryote community are dependent on the species composition of the microorganisms. Few studies have investigated the spatial distribution of prokaryote species around hydrothermal vents. In this study, several samples were collected and individually analysed using PCR-DGGE to determine the composition of the prokaryote communities, targeting both Bacteria and Archaea. From these fingerprints, it was demonstrated that the microbial communities around hydrothermal vents are extremely heterogeneous, with little similarity between samples collected from similar locations or environments. Sequencing of DGGE bands suggests common phylotypes among widespread hydrothermal vent sites. Through grazing, protists could cause mortality of prokaryotes. Because little is known about the presence of protists around hydrothermal vents, samples collected around a single sulphide structure were analysed using PCR-DGGE to determine the diversity of the eukaryote community in the water column. Sequencing of major bands was undertaken to determine the identity of the bands with the hopes of identifying previously unknown protists that could be grazers of prokaryotes. This study shows that DNA from the benthic invertebrate community dominates the eukaryote DNA around hydrothermal vents, suggesting viruses may be the main cause of mortality for free-living prokaryotes.

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