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Microorganisms use C compounds for biosynthesis, forming new cellular or extracellular material, and as an energy supply. In the latter process, CO2, microbial cells, and waste products are produced. Under aerobic conditions, the amount of waste products produced is not usually high, and the amount of biosynthesis, or production of microbial cells, can be calculated from CO2 data. This requires knowledge of yield or efficiency of substrate conversion to microbial biomass,

where C is the substrate decomposed, Cj the CO2-C evolved, and Y the efficiency (yield, or sometimes CUE for C utilization efficiency) of the use of C for biosynthesis, expressed as a percentage of the total C utilized for production of microbial material. The decomposition rate constants (k), corrected for biosynthesis, differ significantly from the uncorrected ones (Table 16.2). Growth efficiencies of 40-60% are generally considered realistic for the decomposition of soluble constituents; other compounds, such as waxes and cellulose, result in lower efficiencies. Aromatics such as lignin appear to be largely cometabolized by fungi. This involves enzymatic degradation of the substrate but little uptake of the breakdown products. The fungi gain little, if any, energy for growth and incorporate little C during the decomposition of the aromatics. Therefore, aromatic decomposition occurs only in the presence of available substrate. Where data are available only over extended periods, it is not possible to calculate true decomposition values and microbial growth efficiency because CO2 is evolved from both the original substrate and the turnover of microbial cells.

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