The dynamics of the macromolecular composition of biomass

The biomass composition of microorganisms depends on the growth conditions. This study explores whether a two-component model can explain how the elem ental and macromolecular composition of the biomass of bacteria varies with the specific growth rate. The model describes the rates at which microorga nisms assimilate substrates into reserves and utilize reserves for maintena nce and growth. Crucial model assumptions are that biomass consists of rese rves and structure and that each of these components has an invariant compo sition. The composition of biomass can vary when the ratio between reserves and structure varies. Literature data on the macromolecular composition of Escherichia coli, cultivated on various substrates, show that the protein, RNA and DNA content of biomass follow a distinctive trend when plotted as a function of the dry-weight-specific growth rate. This observation leads t o the proposition that the macromolecular composition of E. coli depends di rectly on the growth rate, and only indirectly on the carbon- and energy-so urce used as substrate. We show that the variation of the macromolecular co mposition of E. coli over its entire range of growth rates can be described with invariant macromolecular compositions of the reserve and structural c omponents of biomass. The model is also applied to our data on a succinate- limited continuous culture of Paracoccus denitrificans. (C) 2001 Academic P ress.