Emergent behaviour in a chlorophenol-mineralising three-tiered microbial ‘food web’

Abstract

Anaerobic digestion enables the water industry to treat wastewater as a resource for generating energy and recovering valuable by-products. The complexity of the anaerobic digestion process has motivated the development of complex models. However, this complexity makes it intractable to pin-point stability and emergent behaviour. Here, the widely used Anaerobic Digestion Model No. 1 (ADM1) has been reduced to its very backbone, a syntrophic two-tiered microbial ‘food chain’ and a slightly more complex three-tiered microbial ‘food web’, with their stability analysed as a function of the inflowing substrate concentration and dilution rate. Parameterised for phenol and chlorophenol degradation, steady-states were always stable and non-oscillatory. Low input concentrations of chlorophenol were sufficient to maintain chlorophenol- and phenol-degrading populations but resulted in poor conversion and a hydrogen flux that was too low to sustain hydrogenotrophic methanogens. The addition of hydrogen and phenol boosted the populations of all three organisms, resulting in the counterintuitive phenomena that (i) the phenol degraders were stimulated by adding hydrogen, even though hydrogen inhibits phenol degradation, and (ii) the dechlorinators indirectly benefitted from measures that stimulated their hydrogenotrophic competitors; both phenomena hint at emergent behaviour.

Generalised approach to modelling a three-tiered microbial food-web

“…More recently, the model described by [28] was extended by the addition of a third organism and substrate to create a three-tiered ‘food-web’ [24] , as shown in Fig. 1 .…”

“…Here, we pursue a generalised description and analysis of the model given by [24] . Chlorophenols are chemicals of importance due to their impact on the environment and to public health, their recalcitrance in food-webs and resistance to aerobic biodegradation via the oxygenase enzyme [21] .…”

A density-dependent model of competition for one resource in the chemostat

“…On the other hand, our study describes the operating diagram which shows the stability regions, in dependence of the operating parameters D and S in , when all biological parameters are fixed. This bifurcation diagram is an important tool for the experimentation as discussed in [1,12,42,48,49].…”

MI-Sim: A MATLAB package for the numerical analysis of microbial ecological interactions

“…Here, we have taken six common ecological motifs describing interactions between two distinct species, plus one extended motif that consists of three interacting species. The seven motifs, described in Table 1, are simple networks commonly observed at both micro-and macro-scales, and provide a theoretical basis by which scientists can test hypotheses in suitably sized community networks [15,20,30,31].…”

“…1, and described by Eqs (10)- (19), with units expressed in COD. To parameterise the model, we took the values given in a previous analysis performed independently of MI-Sim [31], with a revised and more realistic value for the substrate affinity constant, K S,1 , shown in Table 2. The model is described by the six ODEs: Fig 1. A three species food-web with competitive and syntrophic interactions.…”