Biphasic Behavior of Anammox Regulated by Nitrite and Nitrate in an Estuarine Sediment

Abstract

ABSTRACT The production of N 2 gas via anammox was investigated in sediment slurries at in situ NO 2 − concentrations in the presence and absence of NO 3 − . With single enrichment above 10 μM 14 NO 2 − or 14 NO 3 − and 15 NH 4 + , anammox activity was always linear ( P < 0.05), in agreement with previous findings. In contrast, anammox exhibited a range of activity below 10 μM NO 2 − or NO 3 − , including an elevated response at lower concentrations. With 100 μM NO 3 − , no significant transient accumulation of NO 2 − could be measured, and the starting concentration of NO 2 − could therefore be regulated. With dual enrichment (1 to 20 μM NO 2 − plus 100 μM NO 3 − ), there was a pronounced nonlinear response in anammox activity. Maximal activity occurred between 2 and 5 μM NO 2 − , but the amplitude of this peak varied across the study (November 2003 to June 2004). Anammox accounted for as much as 82% of the NO 2 − added at 1 μM in November 2003 but only for 15% in May 2004 and for 26 and 5% of the NO 2 − added at 5 μM for these two months, respectively. Decreasing the concentration of NO 3 − but holding NO 2 − at 5 μM decreased the significance of anammox as a sink for NO 2 − . The behavior of anammox was explored by use of a simple anammox-denitrification model, and the concept of a biphasic system for anammox in estuarine sediments is proposed. Overall, anammox is likely to be regulated by the availability of NO 3 − and NO 2 − and the relative size or activity of the anammox population.