Slow Growth and High Substrate Affinity of Anammox Bacteria in an Oxygen Minimum Zone

A major percentage of total fixed nitrogen (N) loss in the oceans occurs within oxygen minimum zones (OMZ) via denitrification or anammox. Anammox has been suggested as the dominant N loss pathway in these regions, but the regulation of this process remains understudied in aquatic systems, with work to date mainly coming from laboratory-scale bioreactors. Sampling was undertaken in the anoxic, coastal basin of the Golfo Dulce, Costa Rica to assess the growth and substrate kinetics of anammox bacteria in a marine OMZ. Anammox rates showed a strong dependence on nanomolar concentrations of both NH₄⁺ and NO₂^-. A single Michaelis-Menten curve fitted for each substrate produced apparent half saturation constants (K_m) of 335 ± 292 nM NH₄⁺ and 167 ± 192 nM NO₂^- respectively. NO₂^- has a dual role to play in anammox metabolism, as an electron acceptor in the energy generating reaction and as an electron donor in the carbon fixation step; hence growth is associated with NO₃^- production. Using acetylene as an inhibitor for anammox, we were able to assess NO₂^- oxidation by anammox bacteria, producing a mean ratio of N₂ to NO₃^- production of 0.26 ± 0.04, which is in direct agreement with that observed in bioreactors. This allowed calculation of a carbon fixation rate, which when combined with enumeration of anammox bacteria through qPCR allowed us to determine the first doubling time for anammox bacteria (predominately Ca. Scalindua) in the marine environment. Doubling times at the peak of anammox activity fell in the range 65 to 123 days, which is at the high end of those seen in laboratory setups. Anammox bacteria have a high affinity for both NH₄⁺ and NO₂^- and are thus able to compete for these highly sought after substrates in OMZs. However, the slow growth of these bacteria will delay their response to injections of substrate, for example from inputs of fresh organic matter, and it will therefore ultimately influence their contribution to fixed N loss in the oceans.