 Because symbiotic microorganisms are tiny and pervade all ecosystems, and because such symbioses contribute so much metabolically to the overall biochemical activity on the Earth, some theorists have been moved to suggest what is known as the Gaia hypothesis of the living Earth. In this view, Gaia is the total complex of “atmosphere, biosphere, oceans and soils” that is regulated by “a feedback or cybernetic system which seeks an optimal physical and chemical environment for the biota” (Margulis &amp; Lovelock 1974: 473). Microorganisms are given primary mechanistic responsibility for such regulatory effects, even though all life participates in self-maintenance of the wider system. Many biologists and philosophers remain unpersuaded by the Gaia hypothesis (see Resnik 1992; Pigliucci 2014); nevertheless, it is worthy of mention as an instance in which the versatility, abundance, minuteness and pervasiveness of microbes lead to them being given a powerful ecological status for their roles in determining the way life works on Earth.