They grow in temperate marine and terrestrial settings, within other microbial or multicellular organisms, in deep subsurface niches, and in extreme environments that would be lethal for other life forms. They often influence geochemical reactions within the biosphere and frequently play key roles in food webs and complex ecosystems. Communities of metabolically active microbes may increase in biomass or simply maintain a constant density by replacing dead cells with new ones. Sometimes metabolic activity is restricted to the repair of macromolecular machinery without cell division. Examples of metabolically active microorganisms include exponentially growing cells, cultures that have reached a stationary phase during which cell division occurs either very slowly or at a rate equivalent to that associated with cell death, and organisms that have shifted from exogenous to endogenous metabolism to survive starvation conditions.
Dormant cells are metabolically inactive but are capable of returning to an active state.
The SSB, which has served the space science community since 1958, is a program office within the Commission of Physical Sciences, Mathematics, and Applications of the National Research Council, which is in turn the operational arm of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The SSB operates a number of standing committees, task groups, and workshops that perform studies in space science and policy for the federal government.