Effect of conversion of brackish marshes to aquaculture ponds on comammox Nitrospira communities in the Min River Estuary

Sediment ammonia oxidation from aquatic ecosystems plays a critical role in global nitrogen cycle, including nitrous oxide (N2O) emissions. Conversion of coastal marshes to aquaculture ponds has been frequently shown to significantly influence N2O emissions, but the corresponding effect on ammonia oxidation and ammonia-oxidizing microorganisms remains elusive. Here, we compared the newly discovered complete ammonia oxidation (comammox, CMX) abundance and community structure, and potential ammonia oxidation rate between brackish marshes and the nearby converted shrimp aquaculture ponds in Southeast China over a 10-month period. We hypothesized that aquaculture reclamation would decrease the abundances of AOA, AOB and CMX Nitrospira and potential ammonia oxidation rate due to a decrease in oxygen and substrate supply. We also hypothesized that temporal variations in ammonia oxidizer abundance and ammonia oxidation potential would be stronger in the marsh, thanks to its more exposed environment, than in the aquaculture ponds. The results would advance our understanding of how seasons and landscape change impact ammonia oxidizer communities and ammonia oxidation in coastal areas.