Life in the Open Ocean. Joseph J. Torres

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СКАЧАТЬ include the decapods, mysids, and copepods and even large populations of midwater (mesopelagic) fishes (Gjøsaeter 1984).

      Organisms that inhabit oxygen minima exhibit a range of adaptations for dealing with hypoxic conditions. These adaptations include behavior, such as diel vertical migration out of the layer, in addition to unusual morphological and physiological characteristics (Wishner et al. 2000). Childress and Seibel (1998) have proposed three modes of adaptation to the oxygen minimum: (i) development of mechanisms for efficient removal of oxygen from water; (ii) reduction of metabolic rates; and (iii) use of anaerobic metabolism to compensate for the difference between aerobic metabolism and total metabolic needs. The use of anaerobic metabolism may occur on a sustained basis, during periods of high metabolic demand, or during transient periods spent in the oxygen minimum layer by vertical migrators.

The decreased food energy available in the deep sea (Vinogradov 1970) favors an aerobic existence where there is sufficient oxygen to be utilized. However, an oxygen minimum such as that in the Arabian Sea, where oxygen drops to zero, constrains species to an anaerobic existence while resident within it. Anaerobiosis poses a special problem to vertebrates. With the exception of breath‐hold divers such as marine mammals and turtles, most vertebrates, including fish, rely exclusively on aerobic metabolism and only switch to locally active anaerobic pathways during periods of increased activity or ambient low oxygen levels. For example, tuna white muscle is very well adapted for anaerobiosis, which is used during burst swimming (Hochachka 1980).

      The main anaerobic pathway used by vertebrates is glycolysis, resulting in lactate production (Withers 1992). However, two major problems arise with use of the glycolytic pathway. First, the energy yield is very low, only 2 ATP per glucose as opposed to 38 ATP per glucose from aerobic metabolism. Second, the accumulation of lactate is potentially harmful to the organism. Build‐up of lactate can lead to osmotic imbalance, acidosis, and, ultimately, inhibition of glycolysis (Withers 1992).

      Box 2.1 An Aside on Units

      Oxygen is expressed in a variety of (nearly) interchangeable ways and those most commonly encountered in the literature have changed over the years. A few basics will help.

      1. The total pressure of a gas mixture such as atmospheric air equals the sum of pressures exerted by each constituent gas. This is Dalton’s law.

      2. Total (standard) atmospheric pressure is expressed as760 mm Hg (also known as “Torr”) or as 101 kilopascals (kPa), which is the SI unit for pressure.

      3. The oxygen partial pressure in a standard atmosphere is the product of the mole fraction of oxygen in air × the barometric pressure:

       PO2 = (0.2095) × (760 mm Hg) = 159.2 mm Hg (usually rounded up to 160 mm Hg)

       PO2 = (0.2095) × (101 kPa) = 21.2 kPa (usually rounded down to 21 kPa)

      4. Water in equilibrium with a gas mixture will have the same partial pressures as the gas mixture above it. This is Henry’s law. The snag is that the molar concentrations are a function of the partial pressures and an additional factor, the solubility coefficient. Thus: [A] = pA · α_A

      where [A] = mol l⁻¹ of A, pA = partial pressure of A (kPa), α _A = solubility coefficient (mol l⁻¹ kPa⁻¹).

      5. Solubilities of oxygen in seawater decrease with increasing temperature and salinity. They are normally determined empirically (with exacting care!) at air saturation (e.g. Murray and Riley 1969) and compiled in a table for researchers to use. For Oregon waters (Chapter 11), the oxygen concentration at 12 °C and 33‰ was used. It was reported as 6.14 ml l⁻¹ in Murray and Riley (1969). Knowing that a mole of oxygen is 22.4 l or 32 g, we can convert 6.14 ml l⁻¹ to 8.77 mg l⁻¹ (6.14/22.4) × 32. For future reference, to interchange ml l⁻¹ with mg l⁻¹ simply divide by 0.7. From there, it is a simple matter to convert to millimoles: 8.77/32 = 0.272 millimoles or 272 μmol kg⁻¹, since the mass of a liter is a kg.

      6. When comparing P_cs across pelagic systems with widely varying temperatures and oxygen concentrations, it is easiest to use PO₂ because the mole fractions do not change, but relative solubilities do.

      Severity of Oxygen Minima, “Dead Zones,” and the Intertidal

      It is important to appreciate the difference between oceanic oxygen minima, which are persistent, year‐round features located predictably in certain regions of the global ocean for millennia, and the seasonally appearing regions of hypoxia or “dead zones” like that at the mouth of the Mississippi River in the Gulf of Mexico (Rabalais et al. 1994) or in the deeper water of some Scandinavian fjords during summer (Diaz and Rosenberg 1995). The persistence and predictability of oceanic oxygen minima have allowed for adaptations in their resident fauna that enable a primarily aerobic existence. In contrast, the seasonally or episodically appearing anoxia of dead zones, as well as their duration and severity, precludes such adaptation. Infaunal species and any others with limited mobility experience 100% mortality in dead zones (Diaz and Rosenberg 1995). Life in both the oceanic oxygen minima and the episodic dead zones differs from the situation in the intertidal, where high intertidal species may experience anoxia twice daily but a return to normoxia is assured with the incoming tide.

      At what concentration does oxygen become limiting in oxygen minima? A good indication is a reduction in the biomass or the diversity of the fauna inhabiting them. Childress and Seibel (1998) observed that oxygen has little influence on the biomass or species composition of midwater organisms inhabiting minima down to a level of 0.20 ml l⁻¹ or 0.63 kPa. At oxygen concentrations of 0.15 ml l⁻¹ and below, such as that in the Eastern Tropical Pacific or Arabian Sea, oxygen minimum zones (OMZ’s) exhibit reduced biomass and diversity. In those cases, anaerobiosis while resident in the minimum accompanied by a vertical migration out of the layer to more highly oxygenated waters at night is the most likely strategy (Childress and Seibel 1998).

      Intertidal species, including those that dwell in burrows as well as epifaunal species such as bivalves that are exposed for extended periods, are usually highly competent anaerobes (Hochachka 1980). Their situation as intertidal dwellers is fundamentally different from that of oxygen‐minimum‐layer species. Their anoxia and normoxia are cyclic, varying with tidal exposure. Thus, it is adaptive to be able to extract oxygen efficiently, but only down to the point where a large investment in the systems involved in oxygen uptake and transport is unnecessary. For most species, that point lies in the partial pressure range of 20–30 mm Hg oxygen or 2.7–4.0 kPa (Torres et al. 1979, 1994; Childress and Seibel 1998). For intertidal species, oxygen availability will continue to decline to or near zero oxygen as the tide recedes and anaerobiosis inevitably will become necessary.

      In the deep sea, it is the stability of oxygen minima in concert with the limited food resources that have allowed the highly efficient respiratory systems of oxygen‐minimum‐layer fauna to arise.

      Adaptations to Oxygen Minima

      The Aerobic Strategy

Species that are able to maintain an aerobic existence in oxygen minima do so by having a highly effective system for removing oxygen from seawater, allowing them to consume oxygen in sufficient quantities to sustain life at very low oxygen concentrations. The ability to regulate oxygen consumption down to very low levels of external oxygen is defined in physiological terms as having a СКАЧАТЬ