Diazotrophs: An Overlooked Sink of N2O

The ocean is the second-largest source of greenhouse gas nitrous oxide ((Formula presented.) O). However, its role as an (Formula presented.) O sink is severely overlooked. (Formula presented.) O fixation by diazotrophs has lately been proposed as a new pathway of (Formula presented.) O consumption. We investigated diazotrophic (Formula presented.) O consumption and examined the anthropogenic influence on (Formula presented.) O dynamics in the coastal northeastern Arabian Sea, a hotspot of (Formula presented.) O emissions. Our findings reveal that relatively unperturbed waters, unlike anthropogenically perturbed waters, are a modest net (Formula presented.) O sink (98 (Formula presented.) 29 (Formula presented.) saturation), contrary to previous reports. (Formula presented.) O fixation remains active in anthropogenically perturbed waters in contrast to (Formula presented.) 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 (Formula presented.) O fixation not only directly sequesters (Formula presented.) O but may correspond to 0.3 Tg C (Formula presented.) of global ocean net primary production.