Are bacteria more like plants or animals? Growth rate and resource dependence of bacterial C:N:P stoichiometry

1. We examined the relative importance of resource composition (carbon:phosphorus molar ratios which varied between 9 and 933) and growth rate (0.5-1.5 h^-1) to biomass carbon:nitrogen:phosphorus stoichiometry and nucleic acid content in Escherichia coli grown in chemostats, and in other heterotrophic prokaryotes using published literature. 2. Escherichia coli RNA content and the contribution of RNA-P to total cellular P increased with increasing growth rate at all supply C:P ratios. Growth rate had a much stronger effect on biomass C:P than did supply C:P, and increased RNA content resulted in low biomass C:P and N:P ratios. 3. However, we observed only twofold variations in biomass C:P and N:P ratios in the experiments, despite a difference of two orders of magnitude in C:P and N:P supply. The response of biomass C:P and N:P ratios to alteration of the supply C:P and N:P ratios revealed that E. coli was strongly homeostatic in its elemental composition. 4. This result, and a literature survey, suggest that each heterotrophic bacterial strain regulates its elemental composition homeostatically within a relatively narrow range of characteristic biomass C:P and N:P ratios. 5. Thus shifts in the dominance of different bacterial strains in the environment are probably responsible for the large variation in bacterial biomass C:P, as has been suggested for crustacean zooplankton. These findings indicate that bacteria are more like animals than plants in terms of biomass C:P and N:P homeostasis.