Critical oxygen levels and metabolic suppression in oceanic oxygen minimum zones

TLDR

Oceanic oxygen minimum zones (OMZs) force organisms to balance oxygen demand with extraction capacity, relying on anaerobic ATP production and metabolic suppression, and critical oxygen partial pressures have evolved to match species’ minimum exposure levels, yet the physiological response to expanding hypoxia remains understudied. The study seeks to define low‑oxygen habitats by biological response rather than arbitrary concentrations and to identify two oxygen thresholds that can inform policy as OMZs expand. The author reviews hypoxia definitions, critical oxygen levels, animal adaptations to OMZs, and the prevalence of metabolic suppression during temporary OMZ residence, highlighting potential climate‑change impacts on OMZ ecology. Climate change that lowers oxygen concentrations below the lower threshold (~0.8 kPa) will shift ecosystems from diverse mid‑water fauna to diel migrant biota that must return to surface waters at night.

Abstract

Summary The survival of oceanic organisms in oxygen minimum zones (OMZs) depends on their total oxygen demand and the capacities for oxygen extraction and transport, anaerobic ATP production and metabolic suppression. Anaerobic metabolism and metabolic suppression are required for daytime forays into the most extreme OMZs. Critical oxygen partial pressures are, within a range, evolved to match the minimum oxygen level to which a species is exposed. This fact demands that low oxygen habitats be defined by the biological response to low oxygen rather than by some arbitrary oxygen concentration. A broad comparative analysis of oxygen tolerance facilitates the identification of two oxygen thresholds that may prove useful for policy makers as OMZs expand due to climate change. Between these thresholds, specific physiological adaptations to low oxygen are required of virtually all species. The lower threshold represents a limit to evolved oxygen extraction capacity. Climate change that pushes oxygen concentrations below the lower threshold (∼0.8 kPa) will certainly result in a transition from an ecosystem dominated by a diverse midwater fauna to one dominated by diel migrant biota that must return to surface waters at night. Animal physiology and, in particular, the response of animals to expanding hypoxia, is a critical, but understudied, component of biogeochemical cycles and oceanic ecology. Here, I discuss the definition of hypoxia and critical oxygen levels, review adaptations of animals to OMZs and discuss the capacity for, and prevalence of, metabolic suppression as a response to temporary residence in OMZs and the possible consequences of climate change on OMZ ecology.