Autotrophic and Chemotrophic Bacteria
1: Chemolithotrophs utilize the
energy present in chemical compounds and carbon in the form of carbon dioxide.
Chemolithotrophic bacteria are very important to the biogeochemical cycling of various
elements. These include organisms like Beggiatoa (utilizing hydrogen sulfide as
energy source), Nitrosomonas (utilizing ammonia as energy source), Thiobacillus
(utilizing iron), Nitrobacter (using nitries) etc.Chemolithotrophs are unique
in their ability to use reduced inorganic chemicals such as hydrogen sulfide
(~S) and ammonia (NH:J as a source of energy. These are the compounds produced
as a result of anaerobic respiration, when inorganic, molecules such as sulfate
and nitrate serve as terminal electron acceptors. This is one important example
of how nutrients are cycled; the waste products of one organism serve as an
energy source for another.
Chemolithotrophs fall into four
groups with respect to their energy source:
(A) Hydrogen Bacteria: Oxidize
hydrogen gas.
(B) Sulphur Bacteria: Oxidize
hydrogen sulfide.
(C) Iron Bacteria: Oxidize
reduced forms of iron.
2. Anoxygenic Phototrophs: The
photosynthetic systems of the anoxygenic phototrophs are fundamentally different
from those of plants, algae, and cyanobacteria. They have a unique type of chlorophyll
called bacteriochlorophyll. This pigment absorbs wave lengths of light that penetrate
to greater depths and are not used by other photosynthetic organisms. By producing
elemental sulphur as a by-product of their photosynthesis, anoxygenic phototrophic
bacteria probably caused the huge deposits of elemental sulphur that occur in
various parts of the world. Certain lakes in North Africa are now to be in the
process of forming new sulphur deposits. They contain large populations of
anoxygenic phototrophs, and their bottoms are covered with elemental sulphur. These
bacteria are deeply coloured- red, orange, purple and bright green, because of
the chlorophyll and accessory photosynthetic pigments they contain. Water
samples from deep regions of lakes with an abundance of these organisms are
also intensely colored.
Physiologically, these bacteria
carry out photosynthesis anaerobically. The anaerobic photosynthetic bacteria
typically occur in aquatic habitats, often growing at the sedimentwater interface
of shallow lakes where there is sufficient light penetration to permit photosynthetic
activity, anaerobic conditions are sufficient to permit the existence of these'
organisms, and there is a source of reduced sulphur or organic compounds to act
as electron donors for the generation of reduced coenzymes.
The anoxygenic phototrophic
bacteria include the Rhodospirillaceae (purple nonsulphur bacteria),
Chromatiaceae (purple sulphur bacteria), Chlorobiaceae (green sulphur
bacteria), and Cloroflexaceae (green flexibacteria).
In the light under anaerobic
conditions, typical members of the Rhodospirillaceae use molecular hydrogen or
sulfide as an electron donor and can grow without organic compounds. As such,
these organisms may be viewed as photoheterotrophs or as photoautotrophs,
generally requiring organic growth factor compounds. Indeed, most strains in
the Rhodospirillaceae require one or more vitamins. Clearly, the
Rhodospirillaceae occupy a boundary position between autotrophs and
heterotrophs. The basic metabolic pathways of the Rhodospirillaceae are the
same as those of other autotrophic microorganisms. Their ability to assimilate
organic compounds and the requirement of many members of the Rhodospirillaceae
for such compounds establish the resemblance of these organisms to
heterotrophs.
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