Diazotrophs: An Overlooked Sink of N ₂ O

Abstract The ocean is the second‐largest source of greenhouse gas nitrous oxide (O). However, its role as an O sink is severely overlooked. O fixation by diazotrophs has lately been proposed as a new pathway of O consumption. We investigated diazotrophic O consumption and examined the anthropogenic influence on O dynamics in the coastal northeastern Arabian Sea, a hotspot of O emissions. Our findings reveal that relatively unperturbed waters, unlike anthropogenically perturbed waters, are a modest net O sink (98 29 saturation), contrary to previous reports. O fixation remains active in anthropogenically perturbed waters in contrast to fixation. We additionally provide evidence that the absence of control incubations leads to incorrect fixation rate estimates, further implying that oceanic dark carbon fixation rates might be overestimated. We suggest that O fixation not only directly sequesters O but may correspond to 0.3 Tg C of global ocean net primary production.