NO formation by nitrite-induced (chemo)denitrification in coastal marine sediment.

Nitrous oxide (NO) is a potent greenhouse gas that also contributes to stratospheric ozone depletion. Besides microbial denitrification, abiotic nitrite reduction by Fe(II) (chemodenitrification) has the potential to be an important source of NO. Here, using microcosms, we quantified NO formation in coastal marine sediments under typical summer temperatures. Comparison between gamma-radiated and microbially-active microcosm experiments revealed that at least 15-25% of total NO formation was caused by chemodenitrification, whereas 75-85% of total NO was potentially produced by microbial N-transformation processes. An increase in (chemo)denitrification-based NO formation and associated Fe(II) oxidation caused an upregulation of NO reductase (typical nosZ) genes and a distinct community shift to potential Fe(III)-reducers (Arcobacter), Fe(II)-oxidizers (Sulfurimonas), and nitrate/nitrite-reducing microorganisms (Marinobacter). Our study suggests that chemodenitrification contributes substantially to NO formation from marine sediments and significantly influences the N- and Fe-cycling microbial community.