Growth and turnover rates can be determined by incorporating tracer isotopes of C into precursors of cytoplasmic constituents, membranes, or cell wall components. The increase in the tracer over short periods yields estimates of growth. For example, a technique to estimate relative fungal growth rates was based on the addition of [14C]acetate to a soil slurry and measurement of the subsequent uptake and incorporation of the labeled acetate into the fungus-specific sterol, ergosterol (Newell and Fallon, 1991). The specificity of this incorporation was shown by using fungal and bacterial inhibitors. Incorporation rates were linear up to 18 h after the acetate was added, but absolute growth rates could not be calculated due to the uncertainty of conversion factors and problems associated with saturation of the incorporation of the added acetate. Similar techniques have also been developed for C isotopes in microbial lipids. Fatty acids may also be used in combination with their role as biomarkers to monitor the C flux in a bacterial community by measuring the ratios of their C isotopes. Before fatty acids can be used as chemo-taxonomic markers and as indicators of substrate use in microbial communities, calibration studies on the degree and strain specificity of isotopic 13C fractionation with regard to the growth substrate are necessary.
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