The oceans are currently losing O₂ with many unconstrained impacts and feedbacks on marine biology, biogeochemical cycles, and climate. As O₂ concentrations decline, microorganisms transition to several possible NO_x^--based metabolisms: denitrification, anammox, and dissimilatory nitrite reduction to ammonium (DNRA), leading to either N-loss or N-retention, depending on pathway partitioning. This has important implications for ocean nutrient status and feedbacks on climate, but it remains unclear how these pathways are regulated in the transition to anoxia. Here, we use incubations of seawater collected from a model anoxic marine environment (Saanich Inlet, BC) with stable nitrogen isotope labeling (¹⁵N) to partition rates and pathways of microbial NO_x^--reduction in response to O₂ dynamics. By measuring rates as a monthly time series, we show that denitrification and anammox remain relatively consistent, whereas DNRA occurs in intermittent periods of extremely high activity following mixing events and is, overall, the dominant pathway of NO_x^--reduction. Additionally, by manipulating O₂ concentrations between 0.1-10 µM, we show that both DNRA and denitrification are unexpectedly stimulated by the addition of trace O₂. The three pathways have different thresholds and response patterns across this O₂ gradient, revealing interactions between competing pathways that could be otherwise overlooked. These findings challenge assumptions about the relationships between aerobic and anaerobic metabolisms and improve our capacity to predict microbially-driven responses to deoxygenation.

Primary Presenter: Julia Huggins, University of British Columbia (julia.a.huggins@gmail.com)

Authors:

Julia Huggins, University of British Columbia  (julia.a.huggins@gmail.com)

Celine Michiels, University of British Columbia  (ccmichiels@gmail.com)

Shea Thorne, University of British Columbia  (sthorne@eoas.ubc.ca)

Rachel Simister, University of British Columbia  ()

Steven Hallam, University of British Columbia  ()

Sean Crowe, University of British Columbia  ()