Labile substrate availability affects interactions and function in degrader communities: Insights from a combined experimental and modelling approach

Bacterial communities that degrade chitin in soils often exhibit “social” behavior, where different strains fulfill different functional roles. “Degraders” produce extracellular enzymes that attack and cleave the complex biopolymer, releasing the monomer N-acetylglucosamine (NAG) as a public good. This compound can be readily taken up by both degraders and “exploiters”. The latter do not contribute directly to the degradation process but might in turn produce different substances that can be utilized by other members. “Scavengers” do not utilize NAG themselves but live mostly off metabolites secreted by the other strains. Our work aimed to investigate what effect the addition of a readily available C compound, like NAG, has on these interactions in such a system. Based on the results of a wet-lab experiment using a model bacterial consortium, we hypothesized that adding labile C leads to domination of the exploiter-strain though competitive exclusion. This in turn results in the breakdown of positive interactions, and a loss of diversity and functionality of the community. To further investigate this, we designed an ordinary differential equation (ODE) consumer-resource model, consisting of different linked pools representing the experiment. By parametrizing this model based on our respiration measurements and simulating the system over time, we were able to reproduce most of the observed experimental patterns in silico. When there was no NAG added to the system, the model matched the measured respiration when we included several positive interactions (such as crossfeeding or division of labor). This resulted in a more diverse community that degraded chitin more efficiently than the degrader-strain in monoculture. When NAG was added, the exploiter-strain outcompeted the other strains quickly, resulting in a loss of their potential function for the community. Through this combined experimental and modelling approach, our work shows that the addition of excess labile C to degrader communities in soil can alter interactions between bacteria, possibly leading to a loss of biodiversity and function.