Sinking diatom aggregates provide carbon to drive microscale denitrification in a bulk oxygenated ocean

Sinking marine particles drive the biological gravitational pump that naturally sequesters carbon dioxide from the atmosphere. Ubiquitous throughout the ocean, these particles are largely composed of phytoplankton that aggregate together or are repackaged by zooplankton into pellets that sink to the deep. Despite their small size, the compartmentalized nature of these particles promotes intense localized metabolic activity by the bacteria lucky enough to colonize them. Due to their sheer numbers, these microscale interactions can change the chemistry of the bulk ocean and impact global biogeochemical budgets. As soon as phytoplankton-derived particles are exported from the surface ocean, the fate of the carbon depends on the lability and availability of the carbon, the diffusive supply of oxidants from the bulk, and the development of microbial communities throughout the aggregate. Here we show with a model experimental system that aggregates composed of marine diatoms -- important primary producers substantially contributing to global carbon export -- can support active denitrification even among bulk oxygenated water ill-conducive to anaerobic metabolisms. We further show the primary nitrite maximum could be formed, in part, due to dissimilatory reduction of nitrate and nitrite occurring at anoxic microsites within such particles. Particle-based denitrification and other anaerobic metabolisms can change the global budget of elemental cycles important for life and climate across the oceans.