In using water as a metabolic raw material, organisms withdraw it mainly from the hydrosphere.
Aquatic animals absorb water directly from their liquid environment. They excrete some of it back while alive and after death the remaining water is returned through decay.
Terrestrial animals absorb water from the reservoir present in soil and in bodies of fresh water. Animals and plants move such water through their bodies and in the process retain required amounts.
The rest is excreted, partly as liquid water but more particularly as water vapor, which raises the moisture content of the atmosphere. Thus, a given amount of environmental water goes from hydrosphere to atmosphere faster through the metabolic processes of living organisms than if that water were simply allowed to evaporate directly from the hydrosphere.
In other words, the metabolism of terrestrial organisms actively contributes toward the maintenance of the global water cycle. Sometimes this may have an effect on the climate. For example, the tropical forest frees release so much water vapour that the surrounding air remains permanently saturated with moisture. After terrestrial organisms die, any liquid water in their bodies again returns to the hydrosphere through decay.
Water influences metabolism not only as the most important nutrient but also through its effect on all aspects of climate and weather, both in the sea and on land. In the sea, water warmed in the tropics becomes light and rises to the surface, whereas cool polar water sinks.
These up-down displacements bring about massive horizontal shifts of water between equator and poles. The rotation of the earth introduces east- west displacements. These effects, reinforced substantially by similar wind-producing air movements result in oceanic currents. The latter influence climatic conditions not only within the seas, but also in the air and on land.
Another climatic effect is the result of the thermal properties of water. Among all liquids, water is one the slowest to heat or cool, it stores a very large amount of thermal energy. The oceans thus become huge reservoirs of solar heat. The result is that sea air chilled by night becomes less cold because of heat radiation from water warmed by day. Conversely, sea air warmed by day becomes less hot because of heat absorption by water cooled by night. Warm or cool onshore winds then moderate the inland climate in daily patterns.
Thirdly, global climate over long period of time is determined by the relative amount of water locked into polar ice. As polar ice is melting, water levels are now rising and coast lines are gradually being submerged. If trends during the past fifty years are reliable indications, the earth appears to be warming up generally. Deserts are presently expanding; snow-lines on mountains are receding to higher attitudes. In given localities more days of the year are snow-free, and the flora and fauna native to given latitudes are slowly spreading pole-ward.
It is difficult to be sure whether these changes are merely part of a short warm cycle or are really indicative of a long-range trend. But global climate change is a major perturbation to the hydrological cycle. Large- scale deforestation usually to create new agricultural land also causes perturbation of the water cycle resulting in drought or flooding.
All these various cyclic changes in the hydrosphere have profound impact on metabolism. By influencing temperature, humidity, amount of precipitation, winds, waves, currents, and indeed the very presence or absence of water in given localities, they play a major role in determining what kinds and amounts of metabolic processes are possible in such localities, hence what kinds and numbers of organisms may live there.