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UBC Theses and Dissertations

Microbial nitrogen metabolisms and nitrogen recycling through DNRA in low-oxygen marine waters Huggins, Julia Anne


Microorganisms regulate global nitrogen (N) availability by conducting complex series of metabolisms that transform N compounds and can produce bio-unavailable forms of N (N-loss). N-loss primarily occurs in the absence or near-absence of oxygen (O₂), and thus O₂ emerges as an important regulator of N availability. Low-O₂ pelagic marine environments are among the largest venues for N-loss and they are currently expanding as a result of ocean deoxygenation, driven by climate change. These changes have potential to alter N availability in the ocean and create feedbacks on climate, but the outcomes are difficult to predict, partly because microbial N metabolisms are complex. Three different N metabolisms can occur under low-O₂ conditions: two metabolisms (denitrification, anammox) result in N loss, but another (DNRA) recycles N in the environment. We know relatively less about DNRA and the ways O₂ can influence N loss and recycling. In this thesis I conduct experiments in a model low-O₂ marine environment to create new knowledge about N-recycling and the ways microbial N metabolisms respond to changing O₂. In chapter 2, I test for metabolisms that can consume N₂O across a range of O₂ conditions to determine whether N₂O is lost through denitrification or recycled. I find that N₂O cannot be recycled, but N₂O-loss can still be active in the presence of low O₂. In chapter 3, I test the importance of DNRA in marine waters and generate new information about microorganisms that conduct DNRA. I find that DNRA is highly variable and can be the dominant metabolism, overall. I also find that DNRA is conducted by facultatively aerobic organisms and is most active in intermittently-oxygenated environments. In the fourth chapter I test whether low O₂ can directly regulate N loss and recycling. I find that O₂ can regulate all three metabolisms and potentially stimulate denitrification and DNRA. The observations that DNRA can be the dominant metabolism in pelagic marine environments and that O₂ can stimulate N metabolisms are two novel findings that, if widely applicable, could alter models and forecasts for deoxygenation. I suggest future work to test extensibility and build on these results.

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