Synergy between CO₂ and water activity in subaerial biofilms in varying environments

Subaerial biofilms (SABs) are microbial communities that colonize exposed surfaces, playing a role in biogeochemical cycles and the deterioration of built heritage. Their functioning is tightly coupled to environmental conditions, particularly moisture and carbon availability. This work presents a predictive framework to assess the long-term stability of SABs in response to future environmental changes - specifically, variations in temperature (T), air relative humidity (RH), and atmospheric CO₂ partial pressure. The approach is based on a system of ordinary differential equations that describe the dynamics of key SAB components such as cyanobacteria, heterotrophs and polysaccharides. Using daily environmental profiles representative of summer and winter conditions, the model explores both the individual and combined impacts of T, RH and CO₂ on microbial metabolism, productivity, and ecosystem composition. Results suggest that while temperature increases negatively affect inorganic carbon availability, they do not cause substantial shifts in SAB structure - consistent with the wide thermal tolerance typical of these communities. In contrast, elevated atmospheric CO₂ may enhance carbon fixation and overall productivity. However, air relative humidity emerges as the primary regulator of microbial viability: even small fluctuations significantly alter the duration of metabolically active periods, with declines potentially leading to ecosystem collapse. Notably, the interaction between CO₂ and water activity is synergistic: increases in atmospheric CO₂ together with temperature-driven changes in relative humidity - either upward or downward - can in combination significantly influence SAB dynamics. These findings highlight the central role of water activity in maintaining SAB viability and suggest that even moderate shifts in microclimatic moisture availability could have profound impacts on these microbial communities.