Chemosynthesis Oxidation Reduction Reactions

To generate the vital cellular energy currency, ATP, nonphotosynthetic microorganisms use cellular processes to oxidize inorganic compounds (e.g., hydrogen gas, elemental sulfur, ammonia - to water, sulfate, and nitrate, respectively) or organic compounds (to CO2) and deliver the released electrons to the respiratory chain. The particular ecological context (habitat) of a microorganism determines which of a variety of possible final electron acceptors will be available for terminating its electron-transport chain. If the habitat is aerobic (rich in O2) then oxygen will be the dominant final electron acceptor, and microorganisms that carry out aerobic physiological reactions will predominate.

Aerobic respiration is, by definition, the use of molecular oxygen as a final electron acceptor. Biochemically, aerobic respiration is the stepwise passage of electrons through a series of membrane-bound molecular carriers of increasing redox potential - these constitute a respiratory chain. Aerobic respiration reactions are highly favorable, thermodynamically. The energy released by oxidation via oxygen (oxygen terminates the electron-transport chain and is reduced to H2O) provides a tremendous advantage to the microorganism carrying out aerobic respiration. For example, the oxidation of glucose by oxygen (yielding CO2 + H2O) has a free energy (ag° ) value of—2880kJmol—1 glucose. The actual ag in vivo may be even more negative due to the low concentration of the product CO2. Comparing this to ag0 = —200kJmol-1 for anaerobic glycolysis (to lac-tate) demonstrates that oxygen greatly increases the energy available from each glucose molecule respired by aerobic microorganisms.

Although oxygen is a widely used electron acceptor in energy-yielding metabolism, a number of other compounds can be used as electron acceptors. By definition, anaerobic respiration is the use of alternative electron acceptors (besides O2) to generate ATP via electron transport in habitats that lack molecular oxygen. In anaerobic respiration, electrons released from an electron donor are passed through the electron-transport systems containing cytochromes, quinones, iron-sulfur proteins and other electron-transport molecules analogous to those of aerobes. The dominant differences between aerobic and anaerobic respiration are (1) rather than molecular oxygen, the latter process uses largely inorganic ions (though organic anaerobic electron acceptors, such as fumarate, are known) and (2) anaerobic respiration features smaller free-energy yields than aerobic respiration.

A Thermodynamic Hierarchy that Systematically Predicts Microbial Physiological Reactions

Using oxidation-reduction half reactions, we can sort through and make sense of the many thermodynamically unstable resources that occur in natural systems such as waters, sediment, and soils. Figure 2 graphically depicts the relationship between reduced and oxidized substrates as a vertically arranged hierarchy of oxidation-reduction half reactions. The vertical axes in Figure 2 are Eh and, equivalently, pE. Compounds on the left of the half-reaction hierarchy (left-hand side of Figure 2) are in an oxidized state while those on the right are in the reduced form. Furthermore, the transition from oxidized to reduced forms is governed by the redox status of the system of interest and by the catalytic mechanisms of microbially produced enzyme systems. Highly oxidizing conditions appear in the upper portion of the hierarchy in Figure 2 while highly reducing conditions are listed in

Half-reaction hierarchy

Electron-accepting regimes

Oxidation-reduction status of field or laboratory system

Oxidized

MnO2

Halogenated organics

FeOOH

CH2O

so2-4

Reduced

MnCO3

Dehalogenated organics

FeCO3

CH3OH HS-H4

CH2O (glucose)

1

_

ró 0

.0 c

I

I

if

0

V

V

Figure 2 The hierarchy of final electron acceptors provides a simple means to integrate the thermodynamics, microbiology, and physiology of biogeochemical oxidation-reduction reaction. Redrawn from Madsen E L (1997) Methods for determining biodegradability. In: Hurst C J, Knudsen G R, McInerney M J, Stetzenbach L D, and Walter M V (eds.) Manual of Environmental Microbiology, 710pp. Washington: ASM Press.

the lower portion. Figure 2 can be used to predict which combination of half-reaction pairs are thermodynamically possible because under standard conditions, the lower reactions proceed leftward (electron producing) and the upper reactions proceed rightward (electron accepting). Graphically, pairs of thermodynamically favorable half-reactions can be linked simply by drawing arrows diagonally from the lower right to the upper left portion ofthe hierarchy. Fundamental reactions of the carbon cycle tie the oxidation of photosynthetically produced organic carbon (e.g., CH2O; lower right of the hierarchy in Figure 2) to the variety of final electron acceptors that may be present in natural habitats (O2, NO33, Mn4+, Fe3+, SO2~, CO2). Each of these coupled half-reactions is mediated by microorganisms. Moreover, when the diagonal arrows directing carbohydrate oxidation to the reduction of these electron acceptors are drawn, the length of each arrow is proportional to the free energy gained by the microorganisms. Thus, microorganisms metabolizing carbohydrates with O2 as a final electron acceptor are able to generate more ATP than those carrying out nitrate respiration. These microorganisms in turn gain more energy than those using Mn4+ and Fe3+ as final electron acceptors. This pattern continues down the hierarchy of electron-acceptors regimes until methanogenesis (CO2 as the final electron acceptor) is reached. There is a three-way convergence between the thermodynamics of half-reactions, the physiology of microorganisms, and the presence of geochemical constituents in naturally occurring waters, sediments, and soils. The final electron acceptors that dominate the physiological reactions of microorganisms provide useful criteria for categorizing the biogeochemical regimes that occur in habitats within ecosystems.

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Responses

  • Grossman Grubb
    Is chemosynthesis a reduction reaction?
    2 years ago
  • Futsum
    Is Chemosynthesis a noxidation reaction?
    7 months ago

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