Emergent behaviour in a chlorophenol-mineralising three-tiered microbial `food web'

TL;DR

This study simplifies complex anaerobic digestion models to analyze stability and emergent behaviour in microbial food webs, revealing counterintuitive interactions and stability under various substrate conditions.

Contribution

It introduces a simplified three-tiered microbial food web model for anaerobic digestion, highlighting emergent phenomena and stability analysis not previously detailed.

Findings

Steady-states are stable and non-oscillatory.

Low chlorophenol input maintains populations but reduces hydrogen flux.

Adding hydrogen and phenol stimulates all microbial populations.

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 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.