Oxidation Of Reduced C

Aerobic oxidation of CH2O to CO2 is an important source of energy for chemoorganotrophs. In its simplest form this process is the reverse of the overall oxygenic photosynthesis reaction (Table 9.12, Eq. [20]). This group of transformations

TABLE 9.10 Estimate of Overall Stoichiometry of NH4 Oxidation for Energy Production and C Reduction by Species such as Nitrosomonas spp.

36(ADP + Pj) + 10NH+ + 15O2 ^ IONO3 + IOHOH + 20H+ +36ATP

36ATP + 12CO2 + 48e~ + 48H+ ^ 12CH2O + 12HOH + 36(ADP + Pj)

Overall: 18NH+ + 12CO2 + 15O2 ^ 18NO2 + 12CH2O + 6HOH + 36H+ Which can be divided by 3:

6NH+ + 4CO2 + 5O2 ^ 6NO2 + 4CH2O + 2HOH + 12H+ [18]

(Reproduced from McGill (1996), with permission from SBCS and SLCS.)

TABLE 9.11 Estimate of Overall Stoichiometry of NO2 Oxidation for Energy Production and C Reduction by Species such as Nitrobacter spp.

3ATP + CO2 + 4e3 + 4H+ ^ CH2O + HOH + 3(ADP + Pi) Overall: 5NO3 + CO2 + 1.5O2 + HOH ^ 5NO3 + CH2O [19]

(Reproduced from McGill (1996), with permission from SBCS and SLCS.)

TABLE 9.12 Oxidation of Reduced C (Represented as CH2O) using O2 as Terminal Electron Acceptor

TABLE 9.13 Oxidation of N-Containing Organic Molecules to CO2 as a Way to Obtain Energy

Overall: 2(C5NH8O4) + 8.5O2 ^ 2NH3 + 10CO2 + 5HOH + energy [21]

L-Glutamic acid oxidation via a-ketoglutaric acid is used as an example.

(Reproduced from McGill (1996), with permission from SBCS and SLCS.)

also includes oxidation of N- or S-containing organic molecules as a source of energy. Oxidation of l-glutamic acid is one example. Such oxidations to yield energy are responsible for mineralization reactions that release plant nutrients such as NH+ (Table 9.13). In Table 9.12, oxidation of 1 mol of C as CH2O generates 4e-, whereas in Table 9.13, oxidation of 1 mol of C as l-glutamic acid generates only 3.4e-. Consequently the amount of ATP synthesized from the oxidation of 1 mol of C from l-glutamate would be expected to be less than during the oxidation of 1 mol of C from glucose. Such N-containing energy sources are less energetically favorable than pure carbohydrates or lipids. In turn amino acids are best reserved for protein biosynthesis. The search for energy at the expense of N-containing organic molecules (Eq. [21]) in the absence of more energetically favorable alternatives is the basis for much of the N supply to crops from decomposition of humus, plant residues, and manures.

A wide array of substrates is metabolized under anaerobic conditions: sugars; organic, fatty, and amino acids; purine and pyrimidine bases; heterocyclic compounds; and polymers such as polysaccharides, proteins, and lipids. Lignin and saturated hydrocarbons are more recalcitrant under anaerobic conditions but ring oxygenation can be achieved by some anaerobes using O from HOH. Under anaerobic conditions, mechanisms to generate energy vary from use of external electron acceptors other than O2 to using no external electron acceptors.

Use of oxidized minerals as alternate external electron acceptors leads to reduction of oxidized elements. O2 prevents reduction of NO-, SO4-, and N2; however, it is a major product from oxygenic photosynthesis. Hence environments must exist from which O2 is excluded or in which it is consumed faster than it can diffuse into them. Soils are uniquely suited to provide such microsites within aggregates even under well-aerated conditions (see Chaps. 2 and 8). Consequently, anaerobic-aerobic environments must function in sequence forming a syntrophic system. A syntrophic system may be defined as one in which two or more species of organisms with contrasting characteristics require each other in order to function or survive. For example, one species may produce a product or environmental condition that is essential for a second species, which in turn produces a product or condition essential for the first. In the case of the above soil situation, the anaerobic portion of the environment and the organisms in it generates reduced minerals from oxidized forms; the aerobic portion generates O2 and

TABLE 9.14 C Oxidation Using NO3 or SO4 as Terminal Electron Acceptor

Overall:

Overall:

NO3 accepts e 5CH2O + 5HOH ^ 5CO2 + 20H+ + 20e3 4NO3 + 20H+ +20e3 + 4H+ ^ 2N2 + 12HOH + energy

SO4 accepts e

2CH2O + 2HOH

(Reproduced from McGill (1996), with permission from SBCS and SLCS.)

oxidized minerals from reduced forms. The sequence of aerobic-anaerobic environments may be organized either temporally or spatially. Oxidized products are typical of the oxygenic cycle. Without a way to reduce such materials, the system would stop. The anaerobic phase, which shares elements with the anoxygenic cycle, reduces oxidized minerals to allow completion of the cycle (Table 9.14). A vast array of elements is reduced this way (Ehrlich, 1993) with N as NO3, S as SO43, and C as CO2 as frequent examples.

Reduction of oxidized forms of elements serves diverse purposes designated by specific terms. For example NO3 may be reduced for two distinctly different reasons. The first is for incorporation into monomers such as amino acids and assimilation into polymers such as protein, which is called assimilatory NO33 reduction. The second is to use NO33 as a terminal electron acceptor in electron transport phosphorylation, which is termed dissimilatory NO33 reduction because the N is not assimilated. Alternatively it is called nitrate respiration because of its parallel to oxygen respiration. When dissimilatory NO33 reduction leads to N2 production it is called denitrification (see Chap. 13 for further coverage of denitrifi-cation). When it leads to NH3, it is called dissimilatory NO33 reduction to NH3. Similar distinctions apply to reduction of oxidized forms of S, such that assimila-tory S reduction leads to incorporation of S into amino acids or other monomeric building blocks and eventually into biomass. On the other hand, S may be reduced as a terminal electron acceptor during oxidation of C, in which case it is called dissimilatory S reduction.

Two important distinctions exist between organisms responsible for oxidizing reduced C using NO3 as a terminal electron acceptor (Table 9.14, Eq. [22]) and those that oxidize C using SO43 as an electron acceptor (termed dissimilatory SO43 reduction or sometimes SO43 respiration; Eq. [23]). First, NO3 respiration is mediated by facultative anaerobes (e.g., Pseudomonas denitrificans), whereas SO423 respiration is mediated by strict anaerobes (e.g., Desulfovibrio desulfuricans). Second, the free energy change and ATP/2e3 ratio is greater for NO3 respiration than for SO43 respiration (Gottschalk, 1986). Because of these differences some people prefer to restrict the term anaerobic respiration to denitrification. This metabolic distinction and the associated greater energy yield from NO3 respiration (Eq. [22])

compared to dissimilatory SO4~ reduction (Eq. [23]) suggest that one would not expect to find dissimilatory SO4~ reduction in a soil well supplied with NO3.

The elemental cycling implications should be noted. For example, production of

1 mol of NO3 from NH+ consumes 2 mol of O2. Subsequent reduction of that mole of NO3 back to NH+ will support as much oxidation of C as would the original

2 mol of O2. This is because O accepts 8 mol of electrons from a mole of N and in turn a mole of N gains 8 mol of electrons from C. Similarly using O2 to oxidize S2~ to SO4~ and then using SO4~ to oxidize C yields the same amount of CO2 as if the C had been oxidized directly with O2. So where is the distinction between anaerobic oxidation by anaerobic chemoorganotrophs using NO3 or SO4~ and aerobic chemoorganotrophs using O2? The difference is in the yield of ATP and hence potential for growth of biomass, which subsequently controls the rate of the process. The ATP yield is much lower under NO3 respiration or SO4~ respiration (for those who accept that term for S reduction during C oxidation) than when using O2.

What about the NO3 or SO4~ generated during reduction of CO2 to CH2O for growth? The treatment above considers only the production of NO3 or SO4~ associated with O2 reduction. But more biogenic NO3 and SO4~ are present in ecosystems than are generated by O2 reduction. For example, from Eq. [12] about 1.8 times as much SO4~ is generated in reducing C with S0 as is used to generate energy by reducing O2. Similarly, from Tables 9.10 and 9.11, about 42% of the NO3 generated from NH+ originates through production of reducing equivalents to reduce CO2 to CH2O. Consequently the potential for oxidation of C using NO3 or SO4~ in ecosystems is greater than the consumption of O2 for their production would indicate.

From a practical environmental perspective, dissimilatory NO3 reduction, or the organisms involved in it, may be helpful in facilitating oxidation of persistent organic pollutants under anaerobic conditions. For example, Fig. 9.6 shows the oxidation of aniline by mixed cultures including denitrifiers and/or methanogens under anaerobic conditions. The initial attack is by CO2, followed by investment of ATP for attachment of coenzyme A (HSCoA) and a series of reduction steps prior to oxidations and actual ring opening by insertion of water.

Is it possible to generate ATP in the absence of external electron acceptors? To do so would require an internal electron acceptor and a net H balance of 0. Further, without an external electron acceptor, there would be no need to transport electrons through a respiratory chain and hence no opportunity for electron transport phosphorylation. Consequently ATP generation would be by substrate-level phosphorylation. Fermentation is the mechanism by which ATP is generated in organisms without access to external electron acceptors. A wide range of substrates is fermented, including carbohydrates, organic acids, amino acids, and purine and pyrimidine bases. Chemically fermentation can be treated as a dispro-portionation reaction (sometimes called dismutation; an oxidation-reduction reaction in which a reactant or element is both oxidized and reduced leading to two different products) in which both the source of and the acceptor is an organic molecule. A divergence is seen: one product of such reactions is more oxidized than the parent molecule, and one is more reduced (Ehrlich, 1993). Ethanol

FIGURE 9.6 An example of anaerobic metabolism of aniline (denitrifiers, methanogens, or cocultures) (from Schink et al., 1992, with permission from Wiley-VCH).
TABLE 9.15 An Example of Fermentation: Anaerobic Oxidation of Glucose to Form Ethanol and ATP"

C6Hi2O6 + 2ATP

—>

2C3H5O3-

PO3H2 + 2ADP

2C3H5O3-PO3H2 + 2NAD+ +2H3PO4

—>

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