Water

Because life originated in the seas and roughly three-quarters of Earth's surface is covered by water, water availability and quality remain fundamental abiotic constraints on every organism. Water occurs on Earth in fresh form (rivers, lakes, ponds) and salt form (seas and oceans), and can further be classed as hard water, carrying dissolved calcium or magnesium salts (temporarily hard if boiling removes the hardness, permanently hard if it does not), or soft water. Several of water's unusual physical properties make it uniquely important ecologically: it is a key agent of soil formation (pedogenesis); it acts as the medium for entire ecosystems; it is present as atmospheric moisture, unevenly distributed across the globe; being denser than air, it imparts buoyancy that lets organisms float at different depths; its high heat capacity and latent heat let large water bodies absorb and release heat slowly, keeping oceans, lakes and, by extension, the whole biosphere relatively thermostable; it is less dense as ice than as liquid, and reaches its maximum density at 4°C rather than at its freezing point — which is why ice floats and only the very surface of a cold water body freezes, letting life continue in the liquid water below; it functions as the 'universal solvent', transporting chemical n …