Bacteria
Bacteria are often maligned as the causes of human and animal disease
(like this one, Leptospira, which causes serious disease in livestock). However,
certain bacteria, the actinomycetes, produce antibiotics such as streptomycin
and nocardicin; others live symbiotically in the guts of animals (including
humans) or elsewhere in their bodies, or on the roots of certain plants,
converting nitrogen into a usable form. Bacteria put the tang in yogurt and the
sour in sourdough bread; bacteria help to break down dead organic matter;
bacteria make up the base of the food web in many environments. Bacteria are of
such immense importance because of their extreme flexibility, capacity for rapid
growth and reproduction, and great age - the oldest fossils known, nearly 3.5
billion years old, are fossils of bacteria-like organisms. Bacteria grow in a
wide variety of habitats and conditions. When most people think of
bacteria, they think of disease-causing organisms, like the Streptococcus
bacteria growing in culture in this picture, which were isolated from a man with
strep throat. While pathogenic bacteria are notorious for such diseases as
cholera, tuberculosis, and gonorrhea, such disease-causing species are a
comparatively tiny fraction of the bacteria as a whole. Bacteria are so
widespread that it is possible only to make the most general statements about
their life history and ecology. They may be found on the tops of mountains, the
bottom of the deepest oceans, in the guts of animals, and even in the frozen
rocks and ice of Antarctica. One feature that has enabled them to spread so far,
and last so long is their ability to go dormant for an extended period.
Bacteria have a wide range of envronmental and nutritive requirements.
Most bacteria may be placed into one of three groups based on their
response to gaseous oxygen. Aerobic bacteria thrive in the presence of oxygen
and require it for their continued growth and existence. Other bacteria are
anaerobic, and cannot tolerate gaseous oxygen, such as those bacteria which live
in deep underwater sediments, or those which cause bacterial food poisoning. The
third group are the facultative anaerobes, which prefer growing in the presence
of oxygen, but can continue to grow without it. Bacteria may also be
classified both by the mode by which they obtain their energy. Classified by the
source of their energy, bacteria fall into two categories: heterotrophs and
autotrophs. Heterotrophs derive energy from breaking down complex organic
compounds that they must take in from the environment -- this includes saprobic
bacteria found in decaying material, as well as those that rely on fermentation
or respiration. The other group, the autotrophs, fix carbon dioxide to
make their own food source; this may be fueled by light energy
(photoautotrophic), or by oxidation of nitrogen, sulfur, or other elements
(chemoautotrophic). While chemoautotrophs are uncommon, photoautotrophs are
common and quite diverse. They include the cyanobacteria, green sulfur bacteria,
purple sulfur bacteria, and purple nonsulfur bacteria. The sulfur bacteria are
particularly interesting, since they use hydrogen sulfide as hydrogen donor,
instead of water like most other photosynthetic organisms, including
cyanobacteria. Bacteria play important roles in the global
ecosystem. The ecosystem, both on land and in the water, depends heavily
upon the activity of bacteria. The cycling of nutrients such as carbon,
nitrogen, and sulfur is completed by their ceaseless labor. Organic
carbon, in the form of dead and rotting organisms, would quickly deplete the
carbon dioxide in the atmosphere if not for the activity of decomposers. This
may not sound too bad to you, but realize that without carbon dioxide, there
would be no photosynthesis in plants, and no food. When organisms die, the
carbon contained in their tissues becomes unavailble for most other living
things. Decomposition is the breakdown of these organisms, and the release of
nutrients back into the environment, and is one of the most important roles of
the bacteria. The cycling of nitrogen is another important activity of
bacteria. Plants rely on nitrogen from the soil for their health and growth, and
cannot acquire it from the gaseous nitrogen in the atmosphere. The primary way
in which nitrogen becomes available to them is through nitrogen fixation by
bacteria such as Rhizobium, and by cyanobacteria such as Anabaena, Nostoc, and
Spirulina, shown at right. These bacteria convert gaseous nitrogen into nitrates
or nitrites as part of their metabolism, and the resulting products are released
into the environment. Some plants, such as liverworts, cycads, and legumes have
taken special advantage of this process by modifying their structure to house
the basteria in their own tissues. Other denitrifying bacteria metabolize in the
reverse direction, turning nitrates into nitrogen gas or nitrous oxide. When
colonies of these bacteria occur on croplands, they may deplete the soil
nutrients, and make it difficult for crops to grow.