Microbial Habitats

Microorganisms are ubiquitous, occupying the entire biosphere.
Biosphere: The layer of Earth that supports life, including water, land, and a thin layer of air, extending from about 10 km in the atmosphere to the bottom of the oceans.
Lithosphere: The soil, containing both autochthonous (native) and allochthonous (non-native) microorganisms.
Hydrosphere: Bodies of water, also containing autochthonous and allochthonous microorganisms.
Atmosphere: The air, containing only allochthonous microorganisms.

Microorganisms thrive not only in common habitats like soil and water but also in extreme environments, on, and inside other organisms' cells.
A microbial community's habitat is governed by physicochemical conditions, which are influenced by the community's metabolic activity (Abiotic and Biotic Factors).
Microorganisms, being very small, directly experience a small, localized environment called a Microenvironment.
For example, for a bacterium of $3 \,\mu m$ , a distance of $3 \, mm$ is equivalent to a human experiencing $2 \, km$ .
Due to the small size of microorganisms, the variable metabolic activities of neighboring microorganisms, and changes in physicochemical conditions over small spatial and temporal scales, numerous microenvironments can exist within a given habitat.

Ecological theory states that every organism (including microorganisms) has at least one niche, the Effective Niche (or primary niche), in which it will be most successful.
The organism dominates in its effective niche but can also occupy other niches, where it may be less ecologically competent but still able to compete.

An important consequence of the small size of microorganisms is that diffusion often determines the availability of resources.
Consider the distribution of an important microbial nutrient like oxygen in a small soil particle as an example.

Physicochemical conditions in a microenvironment are subject to rapid changes in both space and time.
For example, oxygen concentrations in a soil particle represent