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Encyclopedia of Science   A unicellular organism so small that
it can only be seen with the aid of a microscope. The term
microbe is synonymous with microorganism.  Terrestrial microbes
fall into the major categories of bacteria, archaea, and
viruses. To a far greater extent than complex, multicellular
organisms, they are capable of coping with and even prospering
from environmental change and extremes. Their high surface-area-
to-volume ratio allows rapid rates of chemical exchange with the
surroundings, so that microbes can exploit favorable conditions.
Moreover, their diverse biochemistry enables them to survive and
flourish, as extremophiles, under what, to us, appear hostile
regimes including environments that would have been common on
the Earth at the time life first evolved (see Earth, early
conditions on).  The adaptability of microbes, their ability to
take advantage of a wide range of conditions, and their ability
to survive in unexpected places,