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Index >> Microbial Metabolism >> Autotrophy

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Autotrophy

	Autotrophy - Autotrophic organisms use CO2 as the sole source of carbon. Microbial autotrophic processes can be divided into two classes: (i) Photosynthetic, in which the organisms derive energy from light as is found in photosynthetic algae and photosynthetic bacteria, and (ii) chemoautrophic, in which the energy is derived by the oxidation of an inorganic substrate.
In some of these. organisms, oxygen serves as a terminal electron acceptor while in others, such as Desulfovibrio NO3', or SO4' are used as electron acceptors. Chemoautotrophy: Chemoautotrophs can grow in a mineral medium, deriving carbon from CO2 and energy from the oxidation of inorganic compounds. These organisms frequently lack the ability to use an organic carbon source and have a high specificity to inorganic energy sources.
The concept of chemoautotrophy has been linked with the autotrophic assimilation of CO2 and to the early concept of the toxicity of organic nutrients. This concept is now invalid since bacteria which are facultatively chemolithotrophic, that is which are able to grow on organic media and are also able to obtain energy or carbon from both inorganic as well as organic sources have been identified.
Reactions which yield energy in chemoautotrophs are the oxidation of Hz, NH4+, NO2', S and reduced S compounds and Fe++. All these oxidations, except H2 oxidation, couple electron transport to the cytochrome system and NAD+ reduction occurs by energy dependent reverse electron flow. The assimilation of CO2 in these organisms occurs through the reactions of the Calvin cycle. When grown chemoautotrophically cells contain high levels of the two key enzymes of this pathway, namely carboxydismutase and phosphoribulokinase.
	Most chemoautotrophs accumulate poly-β-hydroxybutyrate as a reserve material which serves as endogenous carbon and energy reserve. Hence, there is good reason to believe that chemoautotrophs possess the enzymes necessary for the metabolism of this reserve material. One possible explanation for the failure of these organisms to use exogenous organic compounds may be a generalized impermeability to organic compounds, but experiments with obligate chemoautotrophs have shown that 14C- labelled organic substrates are taken up by the cells. Biochemical studies have shown that many obligate autotrophs do not utilize acetate and this is attributed to the absence of a functional TCA cycle. These organisms have been found to lack α-ketoglutarate dehydrogenase and have very low levels of succinic and malic dehydrogenases.

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Propionic Acid Fermentation Mixed Acid Fermentation Butyric-Butylic Fermentation Mixed Amino Acid Fermentation Respiration Metabolism on C₁ Compounds Autotrophy Bacterial Photosynthesis

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