Contribution of ammonium oxidizing archaea and bacteria to intensive nitrification during summer in Mutsu Bay, Japan

Abstract

Nitrification, in which ammonium is oxidized to nitrate via nitrite, is a regeneration process of bioavailable nitrogen. Ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) carry out the ammonium oxidation. Here we measured the ammonium oxidation rates (AORs), distinguishing those of AOA (VA) and AOB (VB), for two years in Mutsu Bay, a subarctic semi-enclosed bay. Nitrate and ammonium were depleted in the upper layer in summer, but a high concentration of nitrate was observed at the bottom (below 40 m), of up to 7.8 μM with a strong vertical gradient within several meters of the seafloor. A High AOR of up to 0.17 μM d−1 were observed in the bottom waters. The AOA were responsible for most of the total AOR, but the VB exceeded the VA just above the seafloor (0.3 m) in September. Removal of the suspended solids (SS) did not change the AOA’s oxidation activity but eliminated the AOB’s. This finding suggested that AOA were free-living whereas AOB were attached to the SS’s surface. It is also suggested that the sediment was this SS’s source. When ammonia mono-oxygenase, amoA, of AOA and AOB, was quantified, the copy number of AOA-amoA was an order of magnitude higher than that of AOB-amoA throughout the study period. A significant correlation (p < 0.05) was found between the AOB-amoA copy number and VB whereas no correlation was found between the AOA-amoA copy number and VA suggesting the AOA-amoA copy number did not always represent active cells. Ammonia oxidation was occurring near the bottom waters of Mutsu Bay during the summer, and it was accompanied by a dynamic balance reflecting the difference in the AOA’s and AOB’s lifestyles, free-living and particle-associated, respectively.