Oxygen

Oxygen is a resource for both animals and plants. Only a few prokaryotes can do without it. Its diffusibility and solubility in water are very low and so it becomes limiting most quickly in aquatic and waterlogged environments. Its solubility in water also decreases rapidly with increasing temperature. When organic matter decomposes in an aquatic environment, microbial respiration makes a demand for oxygen and this 'biological oxygen demand' may constrain the types of higher animal that can persist. High biological oxygen demands are particularly characteristic of still waters into which leaf litter or organic pollutants are deposited and they become most acute during periods of high temperature.

Because oxygen diffuses so slowly in water, aquatic animals must either maintain a continual flow of water over their respiratory surfaces (e.g. the gills of fish), or have very large surface areas relative to body volume (e.g. many aquatic crustacea have large feathery appendages), or have specialized respiratory pigments or a slow respiration rate (e.g. the midge larvae that live in still and nutrient-rich waters), or continually return to the surface to breathe (e.g. whales, dolphins, turtles and newts).

The roots of many higher plants fail to grow into waterlogged soil, or die if the water table rises after they have penetrated deeply. These reactions may be direct responses to oxygen deficiency or responses to the accumulation of gases such as hydrogen sulfide, methane and ethylene, which are produced by microorganisms engaged in anaerobic decomposition. Even if roots do not die when starved of oxygen, they may cease to absorb mineral nutrients so that the plants suffer from mineral deficiencies.

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