Doctoral Dissertations

Date of Award

8-2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Microbiology

Major Professor

David Talmy

Committee Members

Andrew Steen, Mike A. Gilchrist, and Erik R. Zinser

Abstract

Microbial community structure in the oligotrophic ocean has evolved so that organisms unable to survive in monoculture can flourish in this harsh, nutrient deplete environment. The oligotrophic ocean contains high concentrations of hydrogen peroxide (H2O2), a type of harmful reactive oxygen species, which is produced when light hits pigmented detrital material. Prochlorococcus is the most numerically abundant photosynthetic organism in the oligotrophic ocean and is estimated to perform half of the carbon fixation there, yet it cannot live in monoculture at the level of H2O2produced in one day of its natural environment. The lesser streamlined sister of Prochlorococcus, Synechococcus, as well as heterotrophic bacteria are able to survive the H2O2of this environmentbut cannot outcompete Prochlorococcus for low nutrients concentrations here. Prochlorococcus has evolved dependence on other microorganisms, requiring them to constantly detoxify ambient H2O2. Thus, allowing Prochlorococcus to survive in mixed-culture communities but not to competitively exclude the community members without facing H2O2 damage. Mechanisms that control the oceanic carbon cycle are an essential area of current and future climate change research, yet they are complex and poorly understood. Mathematical models accounting for various biological and physical processes allow for the assessment of factors that exert control on carbon and nutrient cycling. Current phytoplankton global models attribute 25% of global production to Prochlorococcus but exclude H2O2 as a source of microbial death. Beginning with a simple Monod-based representation of H2O2 dynamics, this thesis provides a basal quantitative groundwork to assess H2O2 detoxification and damage in globally important plankton genera. The tradeoffs between growth advantage (Prochlorococcus) and H2O2 detoxification (Synechococcus) were found to control the toggle between competitive and coexistent behavior. The leakiness of H2O2 detoxification was found to be a key driver of coexistence both dimensionless and spatially/ temporally resolved frameworks. The stepwise build of this model keeps resource competition and coexistence theories rooted in the (micro)biology while allowing for quantitative ecological evaluation of H2O2 impacts on globally relevant microbial dynamics.

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