Autotrophe chemoautotrophe

Prokaryotes Autotrophs Chemoautotrophs Photoautotrophs Aerobes Energy from energised minerals Energy from oxidised sources, Fe3+, S()42, NO s Energy from light, Mg, Mn Tolerate and use 02 from moderate to high atmospheric levels Use of Mg2+, Fe2+, Mo(W)... 

Jprgensen, B.B. (1989) Biogeochemistry of chemoautotrophic bacteria. In Autotrophic Bacteria (Shlegel, H.G, and Bowien, B., eds.), pp. 117-146, Science Technical Publishers, Madison, Wlp and Springer-Verlag, New York. 

When we subtract the costs of all other metabolic processes by the chemoautotrophs and photoautotrophs, the organic carbon that remains is available for the growth and metabolic costs of heterotrophs. This remaining carbon is called net primary production (NPP) (Lindeman, 1942). From biogeochemical and ecological perspectives, NPP provides an upper bound for all other metabolic demands in an ecosystem. If NPP is greater than all respiratory consumption of the ecosystem, the ecosystem is said to be net autotrophic. Conversely, if NPP is less than all respiratory consumption, the system must either import organic matter from outside its bounds, or it will slowly mn down—it is net heterotrophic. 

Any carbon-fixing process, photoauto trophic or chemoautotrophic, would remove CO2 from the ocean and hence the atmosphere. This would require that the CO2 concentration of the atmosphere before these autotrophic processes began was considerably higher than at present. For example, Garrels et al., (1973) have estimated that in order to produce all of the ferric iron, sulfate, and free O2 that has been formed since the evolution of the earth s crust, a total of 5.9 X 10 mol of CO2 must have been depleted from the original CO2 reservoir. 

Organisms that use CO2 as the principal carbon source are defined as autotrophic organisms that use organic compounds as the principal carbon source are defined as heterotrophic. A combination ofthese two criteria leads to the establishment of four principal categories (i) photoautotrophic, (ii) photoheterotrophic, (Hi) chemoautotrophic and (iv) chemoheterotrophic organisms. 

Chemoautotrophs are microbes which obtain energy by catalyzing an exergonic chemical reaction (commonly at interfaces between aerobic and anaerobic environments) and which produce biomass by fixing inorganic carbon. Nitrosomonas europea oxidizes ammonia and uses rubisco and the Calvin Cycle to fix carbon (the most common sulfide-oxidizing organisms also use the Calvin Cycle). Its isotopic characteristics could therefore, be expected to be similar to those of other aerobic, prokaryotic autotrophs such as Synechocystis. As shown in Table 5, this expectation is fulfilled. 

In addition to chemoautotrophs, methanotrophic bacteria are also capable of oxidizing ammonium to nitrate. Several similarities exist between methane oxidizers and autotrophic ammonium oxidizers (Figure 8.36). Numerous studies have shown that methane (CH4) oxidizers can cooxidize ammonium to nitrite, nitrate or both. Collectively, methylotrophs are bacteria that are capable of growth on one-carbon compounds as their sole carbon and energy source. Thus, methanotrophs are a group of methylotrophic bacteria that utilize methane as their sole source of carbon and energy. 

Chemoautotroph An autotroph that obtains energy by oxidizing simple inorganic substances such as sulfides and nitrates. 

The utilization of carbon dioxide by chemoautotrophic and hetero-trophic organisms for biosjmthetic purposes is, like photoc thesis, a reduction of carbon dioxide. In heterotrophic life this reduction is carried out at the expense of preformed organic matter while chemo-autotrophs utilize for the same purpose hydrogen made available through oxidation of inorganic compounds. On the other hand photosjmthesis, at least as carried out by green plants, depends on water as the ultimate hydrogen donor. 

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