Biphasic behavior of anammox regulated by nitrite and nitrate in an estuarine sediment

The production of N-2 gas via anammox was investigated in sediment slurries at in situ NO2- concentrations in the presence and absence of NO3-. With single enrichment above 10 mu M (NO2-)-N-14 or (NO3-)-N-14 and (NH4+)-N-15, anammox activity was always linear (P < 0.05), in agreement with previous findings. In contrast, anammox exhibited a range of activity below 10 mu M NO2- or NO2-, including an elevated response at lower concentrations. With 100 mu M NO3-, no significant transient accumulation of NO2- could be measured, and the starting concentration of NO2- could therefore be regulated. With dual enrichment (1 to 20 mu M NO2- Plus 100 mu M NO3-), there was a pronounced nonlinear response in anammox activity. Maximal activity occurred between 2 and 5 mu M NO2-, but the amplitude of this peak varied across the study (November 2003 to June 2004). Anammox accounted for as much as 82% of the NO2- added at 1 mu M in November 2003 but only for 15% in May 2004 and for 26 and 5% of the NO2- added at 5 mu M for these two months, respectively. Decreasing the concentration of NO3- but holding NO2- at 5 mu M decreased the significance of anammox as a sink for NO2-. 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 NO3- and NO2- and the relative size or activity of the anammox population.