Microbial ecology (or environmental microbiology) is the ecology of microorganisms: their relationship with each other and with their environment. These are the three main domains of life-Eukaryota, Archaea and Bacteria-as well as viruses.
Microorganisms, by their omnipresence, affect the entire biosphere. Microbial life plays a key role in regulating biogeochemical systems in virtually every environment on our planet, including some of the most extreme, from frozen environments and acidic lacquers, to hydro-thermal vents at the bottom of deeper oceans and some of the more familiar as the human small intestine. As a consequence of the quantitative magnitude of microbial life, microbes, by virtue of their biomass alone, constitute an important carbon sink (Whitman et al., Calculated at 5.0 × 1030 cells, eight orders of magnitude higher than the number of stars in the observable universe). Apart from carbon sequestration, key collective metabolic processes of microorganisms (including nitrogen fixation, methane metabolism, and sulfur metabolism) control the global biogeochemical cycle. The immensity of the production of microorganisms is such that, even in the absence of total eukaryotic life, these processes are likely to continue unchanged.