Evidence for anaerobic ammonium oxidation process in freshwater sediments of aquaculture ponds

The anaerobic ammonium oxidation (anammox) process, which can simultaneously remove ammonium and nitrite, both toxic to aquatic animals, can be very important to the aquaculture industry. Here, the presence and activity of anammox bacteria in the sediments of four different freshwater aquaculture ponds were investigated by using Illumina-based 16S rRNA gene sequencing, quantitative PCR assays and 15N stable isotope measurements. Different genera of anammox bacteria were detected in the examined pond sediments, including Candidatus Brocadia, Candidatus Kuenenia and Candidatus Anammoxoglobus, with Candidatus Brocadia being the dominant anammox genus. Quantitative PCR of hydrazine synthase genes showed that the abundance of anammox bacteria ranged from 5.6 × 104 to 2.1 × 105 copies g−1 sediment in the examined ponds. The potential anammox rates ranged between 3.7 and 19.4 nmol N2 g−1 sediment day−1, and the potential denitrification rates varied from 107.1 to 300.3 nmol N2 g−1 sediment day−1. The anammox process contributed 1.2–15.3 % to sediment dinitrogen gas production, while the remainder would be due to denitrification. It is estimated that a total loss of 2.1–10.9 g N m−2 per year could be attributed to the anammox process in the examined ponds, suggesting that this process could contribute to nitrogen removal in freshwater aquaculture ponds.