New twist on nitrogen cycling in oceanic oxygen minimum zones

Abstract

Large uncertainties in the future of Earth's climate have intensified research on the global biogeochemical cycles on land and in the sea. The ocean ecosystem is critical in the carbon balance between the atmosphere and deep ocean, but also plays a large role in chemical and nutrient fluxes that maintain oceanic productivity and greenhouse gas fluxes to and from the atmosphere. Particularly relevant to ocean–atmosphere gas fluxes and control of nitrogen (N) availability are the ocean regions called oxygen minimum zones (OMZs). A few years ago, we believed that we knew the major components of the N cycle and were only challenged by how to extrapolate these processes to global scales. A new perspective on N cycling was initiated with the finding that a relatively newly discovered microbial transformation, anaerobic ammonia oxidation (anammox) (1) occurred in oceanic OMZs (2, 3) and was the major pathway for formation of N2 in the Peruvian OMZ (4). This finding meant that previous notions of how, where, and when nitrogen was lost from the oceanic ecosystem had new uncertainties, not uncertainties of time and space, but of biological control of multiple competing biogeochemical pathways. In this issue of PNAS, Lam et al. (5) used a suite of approaches to tackle anammox in the Peruvian OMZ. They not only discovered how anammox requirements could be supported by other nitrogen transformations, but they added yet another nitrogen transformation to the oceanic nitrogen cycle mix: dissimilatory nitrate reduction to ammonia.