Direct biological fixation provides a freshwater sink for NO.

Nitrous oxide (NO) is a potent climate gas, with its strong warming potential and ozone-depleting properties both focusing research on NO sources. Although a sink for NO through biological fixation has been observed in the Pacific, the regulation of NO-fixation compared to canonical N-fixation is unknown. Here we show that both NO and N can be fixed by freshwater communities but with distinct seasonalities and temperature dependencies.