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1: Introduction
ing. Most halophilic bacteria in culture have apparently been isolated from
solar salt, or from food products preserved with solar salt. Thus, it seems
reasonable to conclude that the natural habitats of these extreme halophilic
bacteria are salt pans and salt lakes.
Dry Environments
The work in this laboratory on water potential arose out of work David
Smith did to explain the presence and activity of the acidophilic, thermo-
philic alga Cyanidium caldarium in Yellowstone soils. In some ways saline
and dry environments are similar, since both have low water activities.
However, there are some major differences (Griffin, 1972). In saline envi-
ronments, ions are present that organisms might pump inside so that the
osmotic pressure of the environment can be balanced. In dry environ-
ments, on the other hand, water activity is reduced by way of adsorption
phenomena, the remaining water molecules being held tightly to clay or
other particles (so-called matric phenomena). In dry environments, no
possibility exists for organisms to pump in ions to balance the matrically
controlled water activity, so that generally organisms have more difficulty
coping with dry than with saline environments.
  The organisms found in dry environments are predominantly fungi.
Blue-green algae form crusts in dry deserts, but they don't really grow
when it is dry, merely waiting for the occasional brief rains, at which time
they quickly flair up (Brock, 1976).
Other Environmental Extremes
Space does not permit more than a brief mention of some other kinds of
extreme environments that have been studied microbiologically. Environ-
ments of high hydrostatic pressure in the depths of the oceans yield not
only microbes but higher animals, including vertebrates. Thus, by our
definition such environments are not extreme, although they may be biolog-
ically interesting. Surprisingly, although bacteria capable of growing at
high
hydrostatic pressure have been isolated, it is not certain that true barophilic
bacteria exist.
  Anaerobic environments including environments high in H2S concentra-
tion are undoubtedly extreme. Of the animals, only certain protozoa, and
a
few other invertebrates, can live anaerobically and of the plants probably
only the blue-green algae. Many bacteria are of course obligately anaero-
bic, although the reasons for this are still obscure (Morris, 1975). Highly
anaerobic environments are found in lake and estuarine sediments, animal
intestinal tracts, the rumen, sewage plants, and a few other areas. Although
H2S is more toxic than cyanide to most aerobes, many microorganisms
tolerate high amounts. Lackey et al. (1965) have discussed this problem in
an interesting article that deserves wider attention. They provide an exten-
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