GROWTH MODELS IN MICROBIAL ECOSYSTEMS - RESOURCE OR DENSITY DEPENDENCE?

Jérôme Harmand

doi:10.32006/eeep.2021.1.6167

Abstract

This paper aims at discussing the two main modeling schemes that are used to describe dynamically the growth of microbial ecosystems, that are resource and density-dependent growth functions, respectively. Monod has been the first to hypothesize that this growth is, before all, an increasing saturated function of the main limiting substrate concentration. Contois assumed that the growth is not only a function of the substrate but also of the biomass density-itself, and thus the name « density-dependent ». In re examining their respective experiments (species used, conditions of experiments, mode of reactor functioning, measurement techniques), we try to understand the engines for a density-dependent phenomenon to appear. In particular, we refer to recent experiments where it was shown that density-dependent appeared as soon as the biomass structures into flocs or in the presence of filamentous bacteria even at relatively low concentrations. Based on this historical review of data, it is shown that density-dependent kinetics is not systematically a question of biomass density but rather related to its structure within the medium and to the mobility of microbial cells.

Highlights

Modeling growth rates of a microbial ecosystem using either resource or density-dependent function is an old question
E. coli and A. aerogenes are two species belonging to the Enterobacteriaceae, a family characterized by a Gram-negative stain and their ability to reduce nitrates to nitrites
B. subtilis belongs to the Bacillaceae family, which has the property of producing spores and having ellipsoidal endospores located in the center of the cell

Summary

Introduction

Modeling growth rates of a microbial ecosystem using either resource or density-dependent function is an old question. The most common models of biotechnology all use combinations of Monod functions and almost never the contois function. It is for instance the case of models of the International Water Association successfully used to model complex wastewater treatment plants, cf for instance IWA TG on ASM modeling, 2000. While the «Monod function» is used in biotechnology and microbiology, ecologists rather term this model «Holling Type II». While microbiologists use the term «Contois function», ecologists call the usual density-dependent function the «Arditi-Ginzburg functional response»

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Discussion

Conclusion