The ocean is home to a diverse and metabolically versatile microbial community that performs the complex biochemical transformations that drive the nitrogen cycle, including nitrogen fixation, assimilation, nitrification and nitrogen loss processes. In this Review, we discuss the wealth of new ocean nitrogen cycle research in disciplines from metaproteomics to global biogeochemical modelling and in environments from productive estuaries to the abyssal deep sea. Influential recent discoveries include new microbial functional groups, novel metabolic pathways, original conceptual perspectives and ground-breaking analytical capabilities. These emerging research directions are already contributing to urgent efforts to address the primary challenge facing marine microbiologists today: the unprecedented onslaught of anthropogenic environmental change on marine ecosystems. Ocean warming, acidification, nutrient enrichment and seawater stratification have major effects on the microbial nitrogen cycle, but widespread ocean deoxygenation is perhaps the most consequential for the microorganisms involved in both aerobic and anaerobic nitrogen transformation pathways. In turn, these changes feed back to the global cycles of greenhouse gases such as carbon dioxide and nitrous oxide. At a time when our species casts a lengthening shadow across all marine ecosystems, timely new advances offer us unique opportunities to understand and better predict human impacts on nitrogen biogeochemistry in the changing ocean of the Anthropocene.
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The authors thank J. Brown and USC Wrigley Institute for assistance with graphics. Grant support was provided by US National Science Foundation grants OCE 1638804 and OCE 1851222, and a USC Sea Grant-administered California Proposition 84 grant to D.A.H., and sustained NSF support to D.G.C.
The authors declare no competing interests.
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- Haber–Bosch process
The industrial chemical process whereby ammonia for agricultural fertilizer is produced directly from hydrogen and nitrogen gas.
An anion consisting of a C atom tripled-bonded to an N atom and single-bonded to an O atom in a linear configuration.
Organisms that are capable of fixing atmospheric N2 gas into bioavailable ammonia.
A descriptor for a group of organisms that is universally distributed across a range of environments.
A group of unicellular, photosynthetic phytoplankton in the division Haptophyta, with cells that are covered by overlapping calcium carbonate plates or ‘coccoliths’.
- Combined nitrogen
Inorganic forms of soluble nitrogen that are covalently bonded to other elements (typically O or H), including the highly bioavailable compounds nitrate, nitrite and ammonia.
An oceanographic method whereby a pumped seawater sampling intake system is towed behind a research vessel at a controlled depth and speed.
Anaerobic ammonium oxidation by chemoautotrophic bacteria of the phylum Planctomycetes, whereby nitrite and ammonium react directly to produce N2 gas and water.
- Autochthonous organic production
Primary production that occurs locally, within the environment being considered.
- El Niño Southern Oscillation cycle
La Niña and El Niño are alternate phases of the natural global climate cycle known as the El Niño Southern Oscillation, with contrasting global wind, precipitation and temperature trends.
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Hutchins, D.A., Capone, D.G. The marine nitrogen cycle: new developments and global change. Nat Rev Microbiol 20, 401–414 (2022). https://doi.org/10.1038/s41579-022-00687-z