Pulling water from a stone: subaerial biofilm water dynamics on porous solid materials

Subaerial biofilms (SABs) are microbial communities that colonize air exposed surfaces in both natural and engineered environments, forming an interface between the atmosphere and the substrate. Often water availability is crucial for their metabolic activity and survival. While precipitation may contribute to SAB hydration, the dynamic processes of evaporation and condensation - closely linked to fluctuating temperature and humidity conditions - can be key determinants of water availability and they constitute the main focus of this study. We present a model connecting the water-related dynamics in an SAB, its surrounding ambient air and underlying stone/masonry. A central feature of the model is the increased water affinity of the SAB in comparison to the stone pore network. Based on thermodynamic principles, the model captures changes in liquid water content and availability by accounting for the evaporation and condensation at liquid-gas interfaces through gradients in water chemical potential. It accounts for the SAB's capacity to delay water loss by drawing water from substrate material pores to compensate for atmospheric evaporation. The proposed model is compared to prior experimental observations of water transport in limestone samples with and without SAB. Additional numerical simulations under variable environmental conditions demonstrate how permeability of the substrate layer may influence SAB water activity, and how the presence or absence of an SAB might affect water content within stone/masonry.