WHOI Theses

Permanent URI for this collection

https://hdl.handle.net/1912/1172

WHOI's educational role, at the graduate level, was formalized in 1968 with a change in its charter and the signing of an agreement with the Massachusetts Institute of Technology for a Joint Program leading to doctoral (Ph.D. or Sc.D.) or engineer's degrees. Joint master's degrees are also offered in selected areas of the program. Woods Hole Oceanographic Institution is also authorized to grant doctoral degrees independently.

New theses are added as they are published.

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Browsing WHOI Theses by Author "Apprill, Amy"
  • Thesis
    Examining coral reef ecosystem dynamics using microorganisms and metabolites
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2023-06) Becker, Cynthia C. ; Apprill, Amy
    Microorganisms and metabolites are foundational to the success and productivity of biodiverse and economically important coral reef ecosystems and are also tightly connected. Metabolites are small organic compounds produced by reef organisms and are the chemical currencies exchanged by unicellular microorganisms (bacteria and archaea) within the seawater. Although central to reef biogeochemical cycling, we still lack fundamental information on the dynamics of these components of reefs. In this dissertation, I analyzed microorganisms in Caribbean coral reef habitats over temporal, spatial and reef health gradients as well as metabolites in a spatial reef study. In Chapter 2, I applied a rapid sequencing methodology to corals afflicted with the lethal stony coral tissue loss disease and identified specific microorganisms which were biological indicators of the disease. In Chapter 3, I investigated the dynamics of microorganisms over short temporal tidal and diurnal cycles, as well as spatially across US Virgin Island (USVI) coastal habitats. In these habitats, I found tidal cycles were driving changes in microbial communities within mangroves, but diurnal patterns were more important in reef habitats. In Chapter 4, I examined reefs over a longer temporal scale by contributing to the building of a 7-year time-series of USVI reef ecology and found that reef water microorganisms were predictive of hurricane and stony coral tissue loss disease impacts. Finally, in Chapter 5, I combined analyses of untargeted and targeted metabolomics, microbial taxa, and functional genes from metagenomics across 300 km of reefs in Florida, in addition to microorganisms in healthy and diseased corals. With this unprecedented combination of ‘omics datasets, I found that biogeographic zones, environmental features, and underlying habitat characteristics were related to microbial and metabolite features in the reef ecosystem. Further, I identified microorganisms and metabolites which were characteristic of specific reef biogeographic zones. Collectively, my work advances our understanding into the dynamics of microorganisms and metabolites in biodiverse coral reef habitats across natural temporal and spatial gradients and in the face of unprecedented stress and disturbance.
  • Thesis
    Variations in marine microbiomes: from habitat to host
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2022-09) Ma, Lei ; Apprill, Amy
    Microorganisms are the dominant life form on Earth and inextricably tied to the ecology and evolution of all multicellular life, including marine animals. As the importance of microorganisms to our conception of life gains prominence, animals (and other macroorganisms) are increasingly viewed as “holobionts”, an assemblage of the host plus all its symbiotic microbes. This dissertation examines holobiont biology from the perspective of the microbial communities that live in and around marine hosts. Using both amplicon and metagenomic sequencing, I study the microbiomes of reef-associated seawater and Atlantic killifish to better understand habitat and host effects on microbiome structure. In two Caribbean reef systems, I used examined the biogeography of reef water microbes. I found that the microbiome of reef seawater varies with reef system and individual reefs but that microbiomes within individual reefs were similar to each other and did not vary with benthic composition. The regionalism of reef seawater microbiomes was further assessed upon incorporation of global scale data from five additional studies, which revealed that microbial communities were more distinct with increasing geographic distance. These results contribute to our understanding of the coral holobiont’s microbial environment and can inform monitoring efforts for reef health. Atlantic killifish populations can be categorized as sensitive or tolerant to industrial pollutants based on history of pollutant exposure. Thus, they are an excellent “natural laboratory” for understanding the combined effect of environment and host on microbiome composition. I examined the gut microbiomes of two populations of wild fish as well as captive fish originating from each of these wild populations. I found that living in and adapting to polluted waters can impact microbiome composition and structure, resulting in a microbiome that appears more disordered. Additionally, captivity resulted in a complete turnover of dominant microbial taxa, indicating the environment plays a large role in shaping killifish gut microbiomes. This dissertation demonstrates that diverse systems, from coral reefs to killifish, can benefit from a better understanding of its associated microorganisms. For holobiont studies, these results highlight the importance of considering the context of microbial communities, from environment to host population.