Geoscience Reference
In-Depth Information
izing bacteria which obtain energy from the oxidation of hydrogen, and carbon from
carbon dioxide during autotrophic growth. However, some of these bacteria can util-
ize simple organic compounds instead and so they can be regarded as facultative auto-
trophs.
Like green plants, many microorganisms can grow photosynthetically, using car-
bon dioxide as a carbon source and light for energy. Examples of these are the blue-
green Cyanobacteria and green and purple sulphur bacteria. Photosynthetic bacteria
contain a kind of chlorophyll known as bacte-riochlorophyll, but they differ some-
what as regards photosynthesis from green plants. For instance, in the light, bacteria
do not liberate free oxygen; moreover, they can utilize hydrogen or hydrogen sulph-
ide as electron acceptors in place of water; they can also convert hydrogen sulphide to
free sulphur. Some photosynthetic microorganisms can grow heterotrophically, being
able to use simple organic compounds as the carbon source in place of carbon dioxide
and with light as energy source. In some lakes in Libya, sulphur deposits occur which
have been produced by microbial activity.
The great majority of microorganisms, however, are heterotrophic, that is, they
require preformed organic substances for their nutrition. The range of such organisms
is indeed very extensive, including bacteria of all kinds, actinomycetes, fungi,
myxobacteria, myxomycetes, acrasiae, protozoa and others.
Members of the genus
Arthrobacter
are characteristic soil bacteria which are of
particular interest because they exhibit a distinct developmental cycle. Under normal
conditions, they are coryneform or club-shaped bacteria (koryne = Greek for club)
and so resemble the true
Corynebacterium
species in form. In the course of their life
cycle, the club-shaped cells give place to small, spherical cells or cocci. Probably the
coccal forms are the ones that survive in soil under poor nutritive conditions; although
they are not spore-forming organisms, they can resist desiccation in soil. These in-
digenous soil bacteria are versatile in their metabolism and activities and some are
known to participate in degrading various synthetic chemicals including some herbi-
cides. Occasionally, arthrobacters may become the dominant bacteria in a soil.
Attempts have been made to classify soil microorganisms on a nutritional basis.
For example, some bacteria can thrive on quite simple organic compounds as sources
of carbon and energy, and simple nitrogen compounds, usually ammonia, to supply
nitrogen. Then there are organisms which require nitrogen-containing organic com-
pounds, such as amino acids (i.e. breakdown products of proteins) to supply both car-
bon and nitrogen. Other microorganisms are still more exacting and need, in addition,
certain growth factors or vitamins in small amounts. Finally, there are microorgan-
isms with even more complex nutritional requirements, which include unidentified
