Doctor of Philosophy (PhD)
Oceanography and Coastal Sciences
Deoxygenation of marine and coastal waters, both caused and exacerbated by a suite of abiotic and biotic factors, has been documented for more than sixty years. The incidence and severity of low-oxygen areas is predicted to expand globally in the coming decades. Despite their colloquial moniker “dead zones,” low-oxygen areas contain thriving and diverse microorganisms that have significant impacts on global biogeochemical cycles including contributing to positive feedback loops that further expand low-oxygen areas. While the diversity and activity of these microorganisms is beginning to be elucidated, the ecology of their viruses is particularly understudied. In this dissertation, I present three studies that focus on viral ecology within distinct low-oxygen areas ranging from seasonally hypoxic to permanently anoxic and highlight their potential biogeochemical impacts. The first study investigated viruses in a permanently anoxic oxygen minimum zone using quantitative microscopy and metagenomes of extracellular viruses to characterize viral communities. I found that viruses here remained present and lytically active throughout the water column and dominant community functions shifted towards potentially reducing host stress caused by low oxygen. Secondly, I estimated microbial abundances and viral lytic and lysogenic activity in the northern Gulf of Mexico during its annual hypoxic event to investigate how these parameters varied based on changes in salinity, nutrients, and oxygen concentration. I found that the Mississippi River input negatively impacted many of these parameters, suggesting that the sudden changes in salinity and nutrients had a greater impact on microbial communities than oxygen concentration in this area. Lastly, I leveraged viruses present in microbial metagenomes and metatranscriptomes to characterize the viral communities of Saanich Inlet, a seasonally anoxic fjord, over several years. Variations in community composition largely mirrored patterns previously observed in prokaryotic communities, and the expression of several key genes suggested that viruses in this area may sporadically yet directly affect nutrient cycling. These studies contribute to several key questions of ecology, including “who is there?” and “what are they doing?”. Answering some of these baseline ecological questions is essential to understanding the impacts of marine viruses, particularly in understudied low-oxygen areas.
Jurgensen, Sophie, "Ecology of Viruses in Marine and Coastal Oxygen-depleted Waters" (2023). LSU Doctoral Dissertations. 6173.
Rabalais, Nancy N
Available for download on Saturday, May 23, 2026
Environmental Microbiology and Microbial Ecology Commons, Oceanography and Atmospheric Sciences and Meteorology Commons