Title: Anoxic aggregates in the ocean - hotspots for nitrogen, sulfur and trace metal cycling
Abstract: Anoxic conditions are uncommon in the open ocean, and mostly confined to the cores of oxygen minimum zones. When oxygen runs out, a suite of alternative electron acceptors are used, leading to denitrification and, rarely in open waters, sulfate reduction. Anoxic conditions have been shown to develop inside millimeter-scale aggregates, establishing microscale gradients that could sustain diverse microbial communities along a sequence of redox niches. We develop a simple model of organic particles that predicts the occurrence of denitrification and sulfate reduction as a function of particle properties and seawater chemistry. We apply the model to a realistic population of particles sinking through the water column, and use it to estimate the potential for particle-bound denitrification and sulfate reduction in the global ocean. The results suggest that anoxia inside aggregates may be common throughout low-oxygen waters, expanding the niche of denitrification beyond anoxic regions, and sustaining pockets of sulfate reduction in otherwise non-sulfidic waters. Globally, particle microenvironments can double the rate of water column denitrification, and explain trace metal deficits in upwelling regions. Our results also imply a decoupling of denitrification from the dynamic variability of tropical anoxic zones, changing the sensitivity of the marine N cycle to deoxygenation in a warming climate.