Competing microbial groups in marine anoxic zones drive natural variability in ocean nitrogen loss.
Key Points & Overview
- Competition between aerobic and anaerobic microbes drives fluctuating ocean nitrogen (N) loss even in the absence of climate variability.
- Ecological fluctuations leave a detectable geochemical fingerprint that is found in ocean nutrient measurements.
- This mechanism of natural variability generates changes in the balance of autotrophic versus heterotrophic N loss, helping to reconcile conflicting views on the ocean’s N cycle.
Over vast regions of the low-latitude surface ocean, primary production is limited by the availability of nitrogen (N), a key macronutrient. The scarcity of N in the marine environment arises because of its conversion to inert N2 gas in small subsurface anoxic zones, where microbes respire bioavailable N in place of dissolved oxygen (O2). Climate variability alters the supply of O2 and organic matter to the anoxic zones, which are thought to be the primary factors modulating changes in N loss over time. However, microbial community structure is also hypothesized to play a key role in regulating N-cycling in marine anoxic zones.
In our study, we analyzed a data-constrained model of the microbial ecosystem in the world’s largest anoxic zone and found that species interactions drive local to basin-scale fluctuations in the rates of N loss even in the absence of climate variability. These fluctuations arise at the oxic-anoxic water boundary from an ecological mechanism: aerobic microbes consume O2 until anaerobic species are favored, but the latter cannot sustain their own anoxic niche, allowing O2 levels to rise until aerobic species are again dominant.
Biological oscillations have been predicted for nearly a century in idealized models, but are rarely demonstrated in a three-dimensional ocean circulation model. These fluctuations are predicted to leave a detectable geochemical fingerprint: infrequent ammonium spikes that are observed in nutrient measurements from the real anoxic zone. In addition to fluctuations in total N loss, ecosystem dynamics drive changes in the balance of autotrophic versus heterotrophic N removal processes, helping to reconcile conflicting views on which pathway dominates overall ocean N loss.