A Mathematical Model of Diel Activity and Long Time Survival in Phototrophic Mixed-Species Subaerial Biofilms

Subaerial biofilms (SAB) are intricate microbial communities living on terrestrial surfaces, of interest in a variety of contexts including cultural heritage preservation, microbial ecology, biogeochemical cycling, and biotechnology. Here we propose a mathematical model aimed at better understanding the interplay between cyanobacteria and heterotrophic bacteria, common microbial SAB constituents, and their mutual dependence on local environmental conditions. SABs are modeled as thin mixed biofilm-liquid water layers sitting on stone. A system of ordinary differential equations regulates the dynamics of key SAB components: cyanobacteria, heterotrophs, polysaccharides and decayed biomass, as well as cellular levels of organic carbon, nitrogen and energy. These components are interconnected through a network of energetically dominant metabolic pathways, modeled with limitation terms reflecting the impact of biotic and abiotic factors. Daily cylces of temperature, humidity, and light intensity are considered as input model variables that regulate microbial activity by influencing water availability and metabolic kinetics. Relevant physico-chemical processes, including pH regulation, further contribute to a description of the SAB ecology. Numerical simulations explore the dynamics of SABs in a real-world context, revealing distinct daily activity periods shaped by water activity and light availability, as well as longer time scale survivability conditions. Results also suggest that heterotrophs could play a substantial role in decomposing non-volatile carbon compounds and regulating pH, thus influencing the overall composition and stability of the biofilm.

Water activity in subaerial microbial biofilms on stone monuments

Stone monuments can be difficult environments for life, particularly with respect to liquid water access. Nevertheless, microbial communities are found on them with apparent ubiquity. A variety of strategies for access to liquid water have been proposed. Regardless of their water-retention mechanisms details, though, we argue that water activity (a key indicator for cell viability) is constrained by environmental conditions, largely independently of community structure, and is predicted by the local temperature and relative humidity. However, direct measurement of water activity in SABs, particularly those growing on stone surfaces, is difficult. A method for estimating water activity within SABs is presented that uses a minimally invasive combination of conservative sampling, weather data, confocal imaging, and mathematical modeling. Applying the methodology to measurements from the marble roofs of the Federal Hall National Memorial and of the Thomas Jefferson Memorial, estimations are made for water activity in their subaerial stone communities over the course of an approximately one year period.

The Ecology of Subaerial Biofilms in Dry and Inhospitable Terrestrial Environments

The ecological relationship between minerals and microorganisms arguably represents one of the most important associations in dry terrestrial environments, since it strongly influences major biochemical cycles and regulates the productivity and stability of the Earth’s food webs. Despite being inhospitable ecosystems, mineral substrata exposed to air harbor form complex and self-sustaining communities called subaerial biofilms (SABs). Using life on air-exposed minerals as a model and taking inspiration from the mechanisms of some microorganisms that have adapted to inhospitable conditions, we illustrate the ecology of SABs inhabiting natural and built environments. Finally, we advocate the need for the convergence between the experimental and theoretical approaches that might be used to characterize and simulate the development of SABs on mineral substrates and SABs’ broader impacts on the dry terrestrial environment.