Biogeography and impact of nitrous oxide reducers in rivers across a broad environmental gradient on emission rates.

Microbial communities that reduce nitrous oxide (NO) are divided into two clades, nosZI and nosZII. These clades significantly differ in their ecological niches and their implications for NO emissions in terrestrial environments. However, our understanding of NO reducers in aquatic systems is currently limited. This study investigated the relative abundance and diversity of nosZI- and nosZII-type NO reducers in rivers and their impact on NO emissions. Our findings revealed that stream sediments possess a high capacity for NO reduction, surpassing NO production under high NO/NO- ratio conditions. This study, along with others in freshwater systems, demonstrated that nosZI marginally dominates more often in rivers. While microbes containing either nosZI and nosZII were crucial in reducing NO emissions, the net contribution of nosZII-containing microbes was more significant. This can be attributed to the nir gene co-occurring more frequently with the nosZI gene than with the nosZII gene. The diversity within each clade also played a role, with nosZII species being more likely to function as NO sinks in streams with higher NO concentrations. Overall, our findings provide a foundation for a better understanding of the biogeography of stream NO reducers and their effects on NO emissions.