Bristow Laura A, Dalsgaard Tage, Tiano Laura, Mills Daniel B, Bertagnolli Anthony D, Wright Jody J, Hallam Steven J, Ulloa Osvaldo, Canfield Donald E, Revsbech Niels Peter, Thamdrup Bo
Nordic Center for Earth Evolution, Department of Biology, University of Southern Denmark, 5230 Odense M, Denmark;
Department of Bioscience, Arctic Research Centre, Aarhus University, 8000 Aarhus C, Denmark;
Proc Natl Acad Sci U S A. 2016 Sep 20;113(38):10601-6. doi: 10.1073/pnas.1600359113. Epub 2016 Sep 6.
A major percentage of fixed nitrogen (N) loss in the oceans occurs within nitrite-rich oxygen minimum zones (OMZs) via denitrification and anammox. It remains unclear to what extent ammonium and nitrite oxidation co-occur, either supplying or competing for substrates involved in nitrogen loss in the OMZ core. Assessment of the oxygen (O2) sensitivity of these processes down to the O2 concentrations present in the OMZ core (<10 nmol⋅L(-1)) is therefore essential for understanding and modeling nitrogen loss in OMZs. We determined rates of ammonium and nitrite oxidation in the seasonal OMZ off Concepcion, Chile at manipulated O2 levels between 5 nmol⋅L(-1) and 20 μmol⋅L(-1) Rates of both processes were detectable in the low nanomolar range (5-33 nmol⋅L(-1) O2), but demonstrated a strong dependence on O2 concentrations with apparent half-saturation constants (Kms) of 333 ± 130 nmol⋅L(-1) O2 for ammonium oxidation and 778 ± 168 nmol⋅L(-1) O2 for nitrite oxidation assuming one-component Michaelis-Menten kinetics. Nitrite oxidation rates, however, were better described with a two-component Michaelis-Menten model, indicating a high-affinity component with a Km of just a few nanomolar. As the communities of ammonium and nitrite oxidizers were similar to other OMZs, these kinetics should apply across OMZ systems. The high O2 affinities imply that ammonium and nitrite oxidation can occur within the OMZ core whenever O2 is supplied, for example, by episodic intrusions. These processes therefore compete with anammox and denitrification for ammonium and nitrite, thereby exerting an important control over nitrogen loss.
海洋中大部分固定氮（N）的损失是通过反硝化作用和厌氧氨氧化在富含亚硝酸盐的氧含量极低区（OMZ）内发生的。目前尚不清楚铵氧化和亚硝酸盐氧化在多大程度上同时发生，是为OMZ核心区氮损失过程提供底物还是与之竞争。因此，评估这些过程对低至OMZ核心区存在的氧（O2）浓度（<10 nmol·L⁻¹）的敏感性，对于理解和模拟OMZ中的氮损失至关重要。我们在智利康塞普西翁附近季节性OMZ中，在5 nmol·L⁻¹至20 μmol·L⁻¹的受控O2水平下，测定了铵氧化和亚硝酸盐氧化速率。在低纳摩尔范围（5 - 33 nmol·L⁻¹ O2）内，这两个过程的速率均可检测到，但表现出对O2浓度的强烈依赖性，假设为单组分米氏动力学，铵氧化的表观半饱和常数（Km）为333 ± 130 nmol·L⁻¹ O2，亚硝酸盐氧化的表观半饱和常数（Km）为778 ± 168 nmol·L⁻¹ O2。然而，亚硝酸盐氧化速率用双组分米氏模型能更好地描述，表明存在一个Km仅为几纳摩尔的高亲和力组分。由于铵氧化菌和亚硝酸盐氧化菌群落与其他OMZ相似，这些动力学应适用于整个OMZ系统。高O2亲和力意味着只要有O2供应，例如通过间歇性入侵，铵氧化和亚硝酸盐氧化就可以在OMZ核心区内发生。因此，这些过程与厌氧氨氧化和反硝化作用竞争铵和亚硝酸盐，从而对氮损失发挥重要控制作用。
