Another extremely limiting factor for silicon as a biotrophic substance is that it cannot easily take part in biogeochemical cycles. Such cycles are increasingly seen as important to life on earth; indeed, the complex, interconnected cycles of elements on earth are regarded by many daring scientists as constituting a "physiology of Gaia." Look at carbon: it occupies huge reservoirs in the crust (as carbonates and dead organic matter), in the oceans (as dissolved CO₂) in living systems (vast stores of carbon in forests, grasslands, plankton and other ecosystems) and in the atmosphere (as CO₂). Carbon is continuously cycled to and from these vast reservoirs, primarily as CO₂ expired by animals and inspired by plants and released from the oceans. SiO₂, silicon dioxide, is also abundant on the earth—as sand. Sand, as you might imagine, doesn’t cycle much. So silicon can’t participate in the biogeochemical cycles that form the lifeblood of large interlocked ecologies, at least not at terrestrial temperatures and pressures.