High biomass density promotes density-dependent microbial growth rate

Describing the interactions between a population and its resources is a research topic in both microbiology and population ecology. When there are fewer resources for the individuals in a large population, the overcrowding can lead to a density-dependent effect which is reflected by a negative feedback of' the organism density on the consumption process. In this paper, we investigate the growth rate of an aerobic microbial ecosystem by two series of experiments performed in continuous agitated cultures. Using a constant dilution rate, but different input substrate concentrations in each experiment, the biomass and substrate concentration were measured at steady state to confront their values with those obtained theoretically from the well-known mathematical model of the chemostat using either resource or density-dependent kinetics. The structures of both flocs and microbial communities were monitored in order to interpret the results. The experiments confirm that density-dependent growth-rate can result either from a high concentration of biomass or from the structuration of this biomass into flocs and we have shown that a new parametrized family of growth functions, that we proposed in this paper, suits better the experimental data than Monod or Contois growth functions.