Ecological and evolutionary strategies of archaeal, bacterial, and viral communities in deep-sea hydrothermal vents
Anderson, Rika Elizabeth
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The deep-sea hydrothermal vent habitat, formed by subsurface water-rock reactions that create high-temperature hydrothermal fluid, is dominated by physical, chemical, and mineralogical gradients. The mixing of cold, oxidized seawater with hot, reduced hydrothermal fluid produces environments that span a range of temperatures, pH, redox potential, chemical composition, and mineralogy, with constant fluid flux between these regions. Communities of archaea, bacteria, and viruses live across the gradients within these systems and are both exposed to and transported by these fluids. Since these conditions can push the boundaries of the limits for life, may represent conditions found on other planetary bodies, and are thought to have been important for the early evolution of life on this planet, the study of microbial adaptation to hydrothermal vents is of great astrobiological importance. This dissertation explores how these extreme gradients structure hydrothermal vent microbial and viral communities, and what evolutionary strategies are used by both cells and viruses in hydrothermal systems to adapt to these extremes. The first part of this dissertation address adaptation on the community level by examining microbial community structuring in various niches within the vent environment. First, I explore microbial niche partitioning across diffuse flow and plumes in hydrothermal vent systems, using a combination of microbial community profiling techniques and qPCR to demonstrate that certain microbial lineages are found in high abundance in particular conditions, but are far less abundant in other regions of the gradient. Second, I use 16S pyrotag sequencing to compare the structures of the rare and abundant biospheres across several hydrothermal vent systems worldwide. Through this I demonstrate that archaeal communities exhibit fundamentally different biogeographic patterning compared to bacterial communities. Whereas bacterial rare and abundant groups show similar biogeographic patterning, abundant archaeal groups are generally cosmopolitan and abundant everywhere but rare archaeal groups are biogeographically restricted. The second part of my dissertation focuses on adaptive strategies among viruses and their microbial hosts. I first demonstrate a novel method by which to identify potential hosts of a viral assemblage using metagenomics, showing that viruses in the vent system have the potential to infect a wide range of hosts. Finally, I use comparative metagenomics to demonstrate that the viral fraction in a high-temperature hydrothermal system is relatively enriched in energy-metabolizing genes, and present evidence suggesting that these genes are transferred by viruses as an adaptive strategy to enhance host metabolic plasticity in a dynamic environment. Taken together, this work indicates that the gradient-dominated nature of vent systems fosters a diverse microbial community through adaptation to particular niches, and that virally-mediated transfer of genes between these diverse hosts creates genomic plasticity to facilitate adaptation to the vent environment. In this sense niche partitioning drives these microbial lineages apart, while horizontal gene transfer allows them to borrow adaptive strategies from each other.
- Oceanography