F

QX 1GX

+ (...) H2O + (...) HCO3 + (...) H Many different possibilities

Donor

Organic

Inorganic

Acceptor

Organic

Inorganic

C-source Organic Inorganic

Figure 2. (a) System definition of microbial growth. (b) Macro-chemical reaction equation of microbial growth.

known that to convert the five compounds into biomass, microorganisms use a large amount of biochemical energy in the form of ATP (10). Clearly the production of biomass from the five building compounds requires input of large quantities of Gibbs energy. The amount of energy needed to make biomass depends on the type of C source used. Intuitively, one expects that making 1 C-mol biomass from CO2 requires more Gibbs energy than making 1 C-mol biomass from an organic compound. A quantitative relation for this energy need is presented later (equations 2, 3a, and 3b). The required energy, which must be taken as Gibbs energy and not as enthalpy, is delivered by a redox reaction between an electron donor and an electron acceptor. This redox reaction is called catabolism (Fig. 2a). Examples are the aerobic combustion of glucose (C6H12O6 + 6O2 r 6HCO— + 6H+) and the anaerobic formation of ethanol from glucose (C6H12O6 + 2H2O r 2HCO3— + 2H+ + 2C2H5OH). Obtaining the required Gibbs energy is as essential for micro-organisms as it is for higher organisms, and even for human society. Therefore it should not be surprising that during evolution a wide diversity of microor ganisms developed that are mainly different in the applied redox reaction for catabolism to obtain Gibbs energy (Fig. 2b). Electron donor or acceptor couples can be organic and inorganic compounds. This microbial variety in catabolic possibilities for generating Gibbs energy has led to the use of a classification system for naming microorganisms (Table 1). This system is understandably based on the source of Gibbs energy (light or chemical energy), the source of electron donor (inorganic or organic), and the source ofbio-mass carbon (CO2 or organic).

In addition, microorganisms may employ a wide variety of electron acceptors, as reflected in their class names. These class names are related to the electron acceptor used in catabolism (O2, aerobic; NO3, denitrification; SO4 —, sulphate reduction) fermentation (absence of external electron acceptor), or to the product of the catabolic reaction (CH4, methanogenic; acetate, acetogenic; H2S, sulphido-genic, etc.). The C source also functions often as electron donor, except in autotrophic microorganisms, where the C source is CO2. For example, a microorganism growing aerobically in the dark on H2S as the electron donor (inorganic compound) using CO2 as the C source is called an aerobic chemolithoautotrophic organism. In summary, in each realistic chemotrophic microbial growth system there must be present the five compounds of anabolism and an electron donor/acceptor combination for catabolism.

These considerations bring us then to Figure 2b, which shows the macrochemical reaction equation containing all the stoichiometric information of the growth process. The macrochemical equation of Figure 2b should not be considered a mathematical equation but is a chemical reaction where substrates and products have negative and positive stoichiometric coefficients, respectively. Therefore for a = 0, sign is absent. In addition, the stoichiometric involvement of enthalpy and Gibbs energy is expressed in their respective stoichiometric coefficients ( Fqx, and FGX, which have units of C-mol X/kJ). The macrochemical reaction equation is therefore a compact, but exact, form of notation of the relevant stoichiometry of growth. This macrochem-ical equation shows that for the formation of + 1 C-mol of biomass an amount of — 1/FDX of electron donor is required. The minus sign shows that the electron donor is consumed. FDX is in C-mol biomass per C-mol electron donor (in case of an organic donor) or per mol donor (in case of an inorganic donor). Its units are written as C-mol X/(C) mol D. An amount of — 1/FAX mol electron acceptor is consumed (minus sign) per 1 C-mol biomass produced and, in addition, 1/FQX kJ of heat and 1/FGX kJ of Gibbs energy are involved in the production of 1 C-mol of biomass. 1/Fqx and 1/Fgx are found as the conventionally calculated enthalpy of reaction and Gibbs energy of reaction of the mac-rochemical reaction equation, which produces 1 C-mol of biomass. Finally, certain amounts of H2O, CO2, (or HCO—), H +, and N source are involved. It is important that in each macrochemical reaction equation in which biomass is grown, HCO—, H2O, H + , and N source be present. The differences between different organisms occur mostly in the electron acceptor/donor combinations used. The N source is often NH4 and sometimes NO—, N2, or something else. The most important point in stoichiometry is to recognize that it is nearly always sufficient to measure one stoichio-

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