Microbial competition for N intermediates drives oscillating N loss from marine oxygen deficient zones

Abstract:

Small oxygen deficient zones (ODZs) below the ocean surface play a major role in regulating marine biological productivity, through the removal of bioavailable nitrogen (N). Recent observations have shown diverse microbial communities affecting N metabolic pathways, but the large-scale consequences of this complexity are unknown. Here we investigate the key microbial interactions and their importance for N loss in a circulation and ecosystem model of the largest ODZ, in the North Pacific. The microbial model replicates observed chemical distributions, but exhibits fundamentally distinct behavior from traditional models based on chemistry alone. The ecosystem undergoes local oscillations that create large variability in the regional rate of N loss, even in a steady circulation. These N loss fluctuations occur at the boundary between oxic and anoxic waters, where miniscule fluctuations in scarce resources are driven by competitive dynamics and dramatically shift the balance between aerobic and anaerobic bacteria. The coexistence of these populations and their competing geochemical effects reduces overall rates of N loss while expanding the scale of anoxic waters. Intrinsic ecosystem oscillations also cause dramatic swings in the ratio of heterotrophic to autotrophic N loss pathways, possibly underlying the wide variations observed in nature.