Approximately 30% to 50% of oceanic nitrogen-loss occurs within oxygen minimum zones (OMZs), via microbially-driven anammox and denitrification. While anammox rates are somewhat predictable in OMZs, denitrification occurs more sporadically, often in coastal regions. Recently, organic-rich sinking particles have been suggested as a niche for denitrification in OMZs, but so far denitrification rates and communities on individual particles remain uncharacterized. Here, we characterized denitrification rates and microbial communities associated with individual particles collected from the coastal Peruvian OMZ, a hotspot of marine nitrogen-loss. We show that denitrification is directly associated with particles and furthermore, that particle-associated denitrification can occur just below the oxycline in the OMZ, albeit at highly heterogeneous rates. Particles in the anoxic OMZ waters were extensively colonized by motile, chemotactic, and fast-growing denitrifying microorganisms that primarily originated from the surrounding OMZ waters, rather than surface waters. This colonization by denitrifiers appears to be facilitated by continuous particle fragmentation and reaggregation, which prolongs particle residence times and promotes microbial exchange between the particles. We suggest that particle colonization within anoxic waters and inter-particle community connectivity maintains denitrification activity on sinking particles, making them hotpots of nitrogen-loss in the dynamic waters of coastal OMZs.
 

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