Although oxygen is an essential compound in life, excess
oxygen becomes toxic to the organisms. Oxygen is a one of strong oxidation
agent in the word. Oxidation of oxygen is able to produce highly toxic forms of
oxygen like super oxide and hydroxyl. They kill the living cell oxidizing
chemicals in the cell.
In
human excess oxygen is not a serious problem because oxygen is transported into
cells by the blood. Pigments in the
human blood, hemoglobin is mainly responsible for oxygen transportation. It is
only have ability to transport 4 oxygen molecules once so cells do not
experience high concentrations of oxygen. But single celled tiny microorganisms
always exposure to the environment and oxygen go through their cell. Some
organisms have enzymes those are able to destroy the toxic forms of
oxygen. The enzymes are catalase, peroxidase and super oxide
dismutase. So they can live under even high concentration of oxygen due to
the presence of those enzymes. They are unable to live under anaerobic
conditions. So they refer to as aerobic organisms. They can only
perform aerobic respiration to produce ATP.
Some
of microorganisms are unable to live in aerobic environment due to the absence
of above enzymes. So they have to remain in anaerobic environment like deep mud
layer in sea and intestine of animals. Those organisms refer to as anaerobic microorganisms. They perform
both fermentation and aerobic respiration for their ATP production. They use
inorganic substances like ferrous, nitrate and sulfate as their terminal
electron acceptor in their electron transport chain instead of oxygen in
anaerobic respiration. Some
microorganisms can perform both mechanisms to synthesis ATP while some only
perform fermentation due to the absence of electron transport chain. Those who
can perform both prefer anaerobic respiration than fermentation because
efficiency and number of ATP synthesis of anaerobic respiration is higher than
that of fermentation. When there is no terminal electron acceptor is available
they perform fermentation.
Another
group of microorganisms are able to live under both aerobic and anaerobic
conditions. Because they have enzymes to destroy the toxic forms of oxygen so
they can live in aerobic environment. They can perform fermentation and/or
anaerobic respiration under anaerobic condition so they are able to live under
even anaerobic conditions. Those
microorganisms refer to as facultative microorganisms. Thiobacillus dentrificans is a facultative
microorganism. It performs aerobic respiration to produce ATP under aerobic
conditions. If the environment suddenly
becomes anaerobic, they shift into anaerobic respiration. They use nitrate ion
in the soil as their terminal electron acceptor. So it convert nitrate into atmospheric
nitrogen. It cause releasing nitrogen from the soil and decrease the soil
fertility. If aeration is return, then again they start to perform aerobic
respiration because efficiency and number of ATP synthesis of aerobic
respiration is little bit higher than that of anaerobic respiration. Escherichia
coli are another example for facultative microorganisms. It lives in the
human intestine where the conditions are anaerobic. So they perform anaerobic
respiration for their ATP production. Somehow they enter into free environment
through the human faces. They can
survive 2-3 days in the free environment and produce ATP aid in aerobic
respirations.
Some
microorganisms are able to live under little concentration of oxygen. They
refer to as microaerophiles. They
have little amount of enzymes those destroy the toxic forms of oxygen or not.
So if the environment suddenly becomes highly oxygenated, they form spores in
order to survive. Spores are a domal structure. Metabolic actives do not happen
within it. The tick wall of spores is not allowed oxygen to penetrate through
it. If they suddenly fallen into
environment with anaerobic condition like deep wound in animals or oxygen
concentration become vanish, they become active again. Clostridum is an example
for microaerophiles.
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