Microbial oceanography of anoxic oxygen minimum zones.

Vast expanses of oxygen-deficient and nitrite-rich water define the major oxygen minimum zones (OMZs) of the global ocean. They support
diverse microbial communities that influence the nitrogen economy of the oceans, contributing to major losses of fixed nitrogen as
dinitrogen (N2) and nitrous oxide (N2O) gases. Anaerobic microbial processes, including the two pathways of N2 production, denitrification
and anaerobic ammonium oxidation, are oxygen-sensitive, with some occurring only under strictly anoxic conditions. The detection limit
of the usual method (Winkler titrations) for measuring dissolved oxygen in seawater, however, is much too high to distinguish low
oxygen conditions from true anoxia. However, new analytical technologies are revealing vanishingly low oxygen concentrations in nitriterich
OMZs, indicating that these OMZs are essentially anoxic marine zones (AMZs). Autonomous monitoring platforms also reveal
previously unrecognized episodic intrusions of oxygen into the AMZ core, which could periodically support aerobic metabolisms in
a typically anoxic environment. Although nitrogen cycling is considered to dominate the microbial ecology and biogeochemistry of AMZs,
recent environmental genomics and geochemical studies show the presence of other relevant processes, particularly those associated with
the sulfur and carbon cycles. AMZs correspond to an intermediate state between two “end points” represented by fully oxic systems
and fully sulfidic systems. Modern and ancient AMZs and sulfidic basins are chemically and functionally related. Global change is affecting
the magnitude of biogeochemical fluxes and ocean chemical inventories, leading to shifts in AMZ chemistry and biology that are likely to
continue well into the future.