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.
FOOTNOTES
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This work was funded by National Science Foundation grants (OCE 0851043 and 0526502 to B.A.S.). Lastly, I thank R. Suarez, H. Hoppeler, M. Handel, The Company of Biologists and The Journal of Experimental Biology for the kind invitation and hospitality during the symposium ‘The Biology of Energy Expenditure’ in Mürren, Switzerland.
List of abbreviations
- ETP
- Eastern Tropical Pacific
- oxygen consumption rate in molar units
- OMZ
- oxygen minimum zone
- P50
- oxygen partial pressure resulting in 50% saturation of respiratory proteins
- P90
- oxygen partial pressure resulting in 90% saturation of respiratory proteins
- Pcrit
- critical oxygen partial pressure
- PO2
- oxygen partial pressure
- ΔG
- free energy
- © 2011.