## Measurement Of Growth Stoichiometry

As shown earlier, the measurement of one stoichiometric coefficient suffices, in general, to calculate all the other stoichiometric coefficients using the conservation relations. This measured stoichiometric coefficient requires the measurement of two conversion rates because, by definition, a stoichiometric coefficient is the ratio of two conversion rates. For example, YDX = rx/ —rD. The most simple growth system contains eight conversion rates (biomass, N source, H + , H2O, CO2, electron donor, electron acceptor, heat production) and six conservation equations (C, H, O, N, enthalpy, charge). Measurement of two conversion rates is then sufficient to calculate all other rates and, hence, the complete growth stoichiometry. Currently, the most common measurements are biomass production and substrate (equal to electron donor) consumption. For aerobic growth the on-line measurement of O2 consumption and CO2 production by the analysis of O2 and CO2 in the off gas in air-sparged fermentors is becoming more and more routine. Especially for autotrophic growth, the online measurement of CO2 consumption by off-gas analysis gives direct and highly accurate information on microbial growth (because all consumed CO2 appears as biomass). This method was very successfully applied to study the growth stoichiometry and kinetics of solid pyrite oxidation by Fe2 +-oxidizing bacteria (12,13) and of Methanobacter-ium thermoautotrophicum on H2/CO2 (14).

Most recently, it was also shown that on-line measurement of heat production during microbial growth can be used to explore growth stoichiometry and kinetics (15-17).

However, such a simple approach of measuring only two conversion rates often makes certain assumptions:

• Each chosen pair of measured conversion rates will allow the complete calculation of all other conversion rates.

• All measurements are reliable within a certain statistical error but without a systematic deviation.

• The assumed description of the growth system is correct, which means that by-products or additional substrates are assumed to be absent.

All these assumptions are subject to critical considerations, which are dealt with extensively in a recent series of publications (18-21). Here, simple examples are provided to illustrate the points of interest. The reader is referred to Refs. 18-21 for a more elaborate introduction, including the full mathematical and statistical aspects.

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