Most organisms need oxygen for aerobic respiration, which releases far more ATP than anaerobic respiration. Oxygen is plentiful in air (about 21%), so it rarely limits land animals except at high altitude, where the partial pressure of oxygen is lower. It is much more often limiting in water and in soil.Oxygen in water. Water holds far less oxygen than the same volume of air, and its oxygen content depends on:
• temperature: cold water holds more dissolved oxygen than warm water
• movement: fast, turbulent water is mixed with air, so it is well oxygenated, while still water may be low in oxygen
• photosynthesis and respiration: plants and algae add oxygen by day, but at night respiration uses it up
• organic pollution: sewage feeds bacteria whose respiration removes oxygen, so fish and many invertebrates dieSo species with high oxygen needs, such as trout and stonefly nymphs, live only in cool, fast, clean streams, while species that tolerate low oxygen, such as bloodworms (midge larvae with haemoglobin) and sludge worms, live in polluted or stagnant water. These are used as indicator species.Oxygen in soil. Roots need oxygen for respiration to provide ATP for active uptake of mineral ions. In waterlogged soil the air spaces fill with water, so oxygen is scarce. Plants adapted to these conditions, such as rice, have aerenchyma (tissue with large air spaces) that carries air to the roots.
• temperature: cold water holds more dissolved oxygen than warm water
• movement: fast, turbulent water is mixed with air, so it is well oxygenated, while still water may be low in oxygen
• photosynthesis and respiration: plants and algae add oxygen by day, but at night respiration uses it up
• organic pollution: sewage feeds bacteria whose respiration removes oxygen, so fish and many invertebrates dieSo species with high oxygen needs, such as trout and stonefly nymphs, live only in cool, fast, clean streams, while species that tolerate low oxygen, such as bloodworms (midge larvae with haemoglobin) and sludge worms, live in polluted or stagnant water. These are used as indicator species.Oxygen in soil. Roots need oxygen for respiration to provide ATP for active uptake of mineral ions. In waterlogged soil the air spaces fill with water, so oxygen is scarce. Plants adapted to these conditions, such as rice, have aerenchyma (tissue with large air spaces) that carries air to the roots.