Diffusion-based mechanism explains spatial organization in cross-feeding biofilms.

Complex symbiotic interactions were claimed for explaining spatial organization of microbial species in cross-feeding biofilms. Here however, a distinct mechanism is proposed, called diffusion-based enhanced microbial organization (DEMO). An accepted mathematical model based on one-dimensional balances with diffusion-reaction of substrates and convection of multiple microbial types in a cross-feeding biofilm was used to describe emerging microbial distributions. The model allowed isolation of the effects of diffusion from other factors (kinetics, stoichiometry, specific symbiotic interactions), pointing to a possible mechanism for stratification in anaerobic biofilms. The secondary degrader consuming waste metabolite from a primary degrader was retained in anaerobic biofilms in an apparent growth yield disproportion. However, diffusion of an intermediate substrate can be responsible for this disproportion, even in longer food chains. This microbial distribution was not observed in independent feeding. In aerobic biofilms, this mechanism remains inactive, explaining the preference for full oxidation of organic matter in aerobic degradation.