ABSTRACT
Freshwater is scarce on Earth, unevenly distributed, and sometimes too heavily loaded with salts and minerals to sustain land life (Table 1.1). Water is mostly present in the oceans where salt involves a large osmotic pressure (high salt contents inside living cells), an additional pressure of one atmosphere for every 10 meters of additional depth, and a rapid dimming of solar energy. Deserts have vast water reserves accumulated over millennia, which are seldom accessible to life. Freshwater is found beyond the polar circles where cold imposes adaptation constraints on living organisms (such as elevated metabolisms to avoid water from freezing inside tissues). Finally, water vapor in the atmosphere is scarce and only available to life in land ecosystems (Figure 1.1) as a result of precipitation.
