Date of Award
2024
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Oceanography
Specialization
Biological Oceanography
Department
Oceanography
First Advisor
Bethany D. Jenkins
Abstract
Diatoms, microscopic algae with silicified cell walls, play critical roles in supporting marine food webs and removing carbon dioxide from the atmosphere. This highly diverse group of phytoplankton impacts biogeochemical cycles by forming large blooms in coastal and high latitude open ocean ecosystems, transforming nutrients in the surface ocean and contributing to the biological carbon pump. Diatom taxa utilize varying strategies for growth, defense, nutrient acquisition, and metabolism to respond to their dynamic environments, and these unique traits influence large-scale ocean processes. Some diatoms form harmful algal blooms (HABs) in coastal regions by producing toxins that negatively impact marine life and public health. Understanding the ecological and environmental drivers of these toxic events is critical for ecosystem management and HAB mitigation. In the open ocean, diatom community composition and nutrient metabolism are thought to substantially influence the magnitude of carbon export, but a mechanistic understanding of these connections is lacking. Next generation sequencing technologies allow us to examine the genetic material of diatoms in various environments to explore species diversity and gene expression. This dissertation uses genomic and metatranscriptomic approaches to investigate diatom taxonomy and function in the context of coastal HABs and large open ocean carbon export events, which broadens our understanding of how cellular-scale changes impact critical ecosystem processes and biogeochemical cycles.
More than half of species in the Pseudo-nitzschia diatom genus can stimulate HABs by producing the neurotoxin domoic acid, which bioaccumulates in shellfish and negatively impacts marine organisms, shellfish harvest industries, and human health. In 2016 and 2017, Narragansett Bay, Rhode Island experienced its first shellfish harvest closures due to elevated domoic acid despite the consistent presence of Pseudo-nitzschia populations for more than 60 years. Using a DNA metabarcoding approach, we examined more than a decade of time series samples from Narragansett Bay to investigate changes in Pseudo-nitzschia species composition and environmental influences that may have contributed to these emergent toxic events. Several toxigenic Pseudo-nitzschia species were identified as year-round residents that may cause toxic blooms under certain conditions, with temperature playing a large role in shaping species composition. P. australis, a high toxin-producing species, was detected for the first time during the 2017 closure period and persisted for several years, suggesting it may have been introduced to Narragansett Bay from an offshore population and has the potential to stimulate future toxic blooms.
Little is known about Pseudo-nitzschia species assemblages and toxicity outside of Narragansett Bay on the Northeast U.S. continental shelf, an important region for fisheries that is warming more rapidly than other parts of the ocean. To investigate whether Pseudo-nitzschia species including P. australis enter Narragansett Bay from the Northeast shelf region and examine hypothesized ecosystem connectivity, we employed the same DNA metabarcoding approach on seasonal transects of the continental shelf and concurrent time series samples in Narragansett Bay. We observed greater similarity of Pseudo-nitzschia species assemblages between the two regions in winter than summer, suggesting seasonal differences in ecosystem connectivity. Spatiotemporal patterns of Pseudo-nitzschia occurrence revealed that toxigenic species may enter Narragansett Bay from the continental shelf more often in winter, which coincides with enhanced domoic acid production in offshore populations. Domoic acid detection in Narragansett Bay was associated with relatively low macronutrients, as well as warm sea surface temperatures in winter and cold temperatures in summer, which may indicate offshore water mass intrusions into the estuary. This study suggests that offshore toxigenic Pseudo-nitzschia populations could seed HABs in Northeast U.S. coastal ecosystems, particularly in winter.
The North Atlantic spring bloom, dominated by diatoms, is one of the largest primary productivity events globally. Following the bloom, nutrient depletion in the euphotic zone leads to large carbon fluxes to depth, the magnitude of which is influenced by diatom growth, metabolism, and community composition in the surface layer. A deckboard incubation experiment was carried out during the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) North Atlantic field campaign following the main diatom bloom to characterize taxon-specific metabolism in response to the limitation of silicon, nitrogen, and iron, common limiting and co-limiting nutrients in this region. In this experiment, diatoms exhibited the largest growth response to added silicon, supporting in situ silicon growth limitation. Diatom genera displayed diverging metatranscriptomic responses to nutrient stress, likely due to their unique competitive strategies for nutrient metabolism. Based on previous work, we identified several genes that may be closely associated with silicon limitation and production, which can be used to metabolically characterize in situ silicon stress in mixed diatom communities. These experimental diatom physiological profiles indicated ways that nutrient limitation may impact pathways and magnitude of carbon export, which will be verified by in situ metatranscriptomes and carbon flux measurements throughout the field campaign.
Together, these studies expand our understanding of diatom diversity and demonstrate the utility of genomic approaches in connecting metabolism of microorganisms to biogeochemical cycles that influence large ecosystems. This work leveraged multiple time series efforts, a long-term ecological research program, and an interdisciplinary multi-institution collaborative program to characterize changes in diatom community composition and metabolism in coastal and open ocean settings. These findings contribute knowledge for coastal management of toxic HABs and enable a broader understanding of diatom taxon-specific influences on carbon export, both of which are critically important within a changing climate.
Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.
Recommended Citation
Roche, Katherine M., "DIATOM TAXONOMIC AND FUNCTIONAL DIVERSITY ACROSS ENVIRONMENTAL GRADIENTS IN THE UPPER OCEAN" (2024). Open Access Dissertations. Paper 1700.
https://digitalcommons.uri.edu/oa_diss/1700