Standard Description Of Microbial Growth Stoichiometry

The stoichiometry of microbial growth is most easily understood from Figure 2. Figure 2a introduces the biomass composition of 1 C-mol biomass (the ash-free organic fraction). The composition shown is fairly typical and is taken from Roels (1). One C-mol ash-free organic biomass is the amount of organic dry biomass that contains 12 g of carbon. The indicated biomass organic fraction corresponds to an elemental composition of 48.8% carbon, 7.3% hydrogen, 32.5% oxygen, and 11.4% nitrogen (w/w).

In practice, total dry biomass, which includes the organic fraction and the ash fraction (S, P, K, Mg, etc.), is measured. In general, the organic and ash fraction are obtained by combusting the organic biomass at 500 to 600 °C and weighing the ashes. Recently Battley (9) has indicated that this simple procedure underestimates the real organic biomass weight by 5 to 6%. This is due to the formation of P, S, and metal oxides in the ash during combustion, whereas such oxides are not present in the dry biomass. The composition formula follows directly from the elemental analysis of the biomass. In Figure 2a only the four major elements (C, H, O, N) are shown; however, it is straightforward to include P, S, and metals such as K or Mg in this composition formula, and also in the stoichiometric/ener-getic calculation. Figure 2a also shows that in the formation of biomass for all chemotrophic growth systems a C source, N source, H2O, CO2, and H+ are always involved. These five compounds provide the building elements for making biomass. This is also called anabolism. For hetero-trophic organisms the C source is organic; for autotrophic organisms the C source is CO2.

Although it is possible to establish a stoichiometrically correct description to make biomass from these five building compounds, it is easily shown that this is not acceptable from the point of view of the second law of thermodynamics. It has been calculated that the Gibbs energy of such a hypothetical reaction, depending on the C source used, is often positive (7), although sometimes small negative values can also be calculated (5). In addition, it is

Microbial growth system

Anabolism Biomass (X)

8O0.5N0.2

Energy

C source N source h2o

Catabolism Electron donor (D) Electron acceptor (4)

Oxidized donor Reduced acceptor

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