Critical oxygen levels and metabolic suppression in oceanic oxygen minimum zones

Abstract

SummaryThe 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.

The Relationship between Mollusks and Oxygen Concentrations in Todos Santos Bay, Baja California, Mexico

“…Another work found two groups of species but only gastropods were related to a DO gradient in Guanabara Bay, Rio de Janeiro, Brazil; in this case, the first group was conformed by Natica pusilla, Acteocina bidentate, Olivella minuta, and Turbonilla sp., while the second group was conformed by Anachis isabellei, Heleobia australis, and Nassarius vibex [41]. Species inhabiting oxygen minimum zones (OMZ) require specific behavioral adaptations; for example, zooplankton migrates to well oxygenated waters and the pteropod Diacria quadridentata reduces oxygen consumption and ammonia excretion in OMZ [45,46]. Taxa most tolerant to severe oxygen depletion (0.2 mL L −1 ) in seafloor include calcareous foraminiferans, nematodes, and annelids; calcified invertebrates are usually less tolerant [47].…”

Expansion of oxygen minimum zones may reduce available habitat for tropical pelagic fishes

“…This threshold has been reported 8,9,16,17 as a plausible lower habitat boundary for billfishes, tropical tunas and other tropical pelagic fishes, but occasional short-duration deeper dives occur 9 . Although dissolved oxygen requirements of individual species vary depending on their mode of respiration, metabolic and physiological requirements 18 , dissolved oxygen levels ≤3.5 ml l −1 may induce stress symptoms reaching lethality over prolonged exposure for high-oxygen-demand billfishes and tunas 15,19 , thus potentially restricting their depth distribution to the oxygenated near-surface layer 8,9 . Tropical pelagic tunas and billfishes exhibit a high-performance physiology 17 , including exceptionally high rates of somatic and gonadal growth, digestion, and rapid recovery from exhaustive exercise.…”

“…Hence, these pelagic predators and their preferred prey tend to be compressed together in the oxygenated narrow surface mixed layer habitat above the thermocline 8,9 . Oceanic hypoxia can impact food pathways within the pelagic ecosystem 18 by decoupling predators from their prey, or putting them in closer proximity to each other as reported in the ETA and eastern tropical Pacific (ETP) OMZs (refs 8,26). For example, the average size of sailfish landed in the ETA and ETP has been consistently larger when compared with those caught in non-compression areas, a result attributed to increased proximity to prey 8,9 .…”

Sensitivity to ocean acidification parallels natural pCO 2 gradients experienced by Arctic copepods under winter sea ice

“…18,19), with the hypothesis being that organisms will have more effective mechanisms to cope with stress if they frequently experience a more variable environment. One example of this hypothesis for pelagic zooplankton comes from knowledge of mesozooplankton (20) distributions in relation to oxygen minimum zones (OMZs), and thus their sensitivity to low oxygen (20).…”