Dynamics of bacteria-substrate stable isotope separation: dependence on substrate availability and implications for aquatic food web studies

Heterotrophic bacteria growing without substrate limitation discriminate against the heavier stable isotopes of C and N, which is reflected in the isotopic signatures of consumers of bacteria. We measured bacteria and substrate isotopic separation (C and N) during glucose and ammonium uptake by three strains of bacteria (Pseudomonas putida, Bacillus megaterium, and Enterobacter aerogenes) grown in batch culture at 4 and 30 degrees C. Isotopic separation between bacteria and substrate was dependent on substrate availability. Higher discrimination against the heavier isotopes, and therefore more depleted delta(13)C and delta(15)N values of bacterial biomass, were observed during the exponential growth phase when the nutrient supply was in excess of the demand. We also compared the isotopic ratios of Chironomus tentans grown on aged macrophyte detritus and commercial fish food. Isotopic signatures of Chironomus larvae grown on fish food were within the ranges typical of one trophic step, whereas larvae grown on detritus were strongly depleted in (13)C, suggesting assimilation of bacteria that had grown on the C-rich detritus. Our results are consistent with the very few other studies of bacterial fractionation of nonlimiting nutrients and may also explain the frequent observation of unexpectedly depleted isotopic signatures in aquatic food web studies.