Abstract
Climate change intensifies drought in several regions. Here, we assess how water availability and air humidity regulate photosynthesis in epiphytic cyanobacterial biofilms on Jacaranda mimosifolia . We combined field measurements with controlled laboratory experiments, exposing biofilms to hydration–dehydration cycles and assessing, by chlorophyll fluorescence: potential quantum yield (Fv/Fm), relative electron transport rate (rETR max ), and photosynthetic photon flux density at saturation (PPFD sat ). Historical precipitation and relative humidity (RH) data from the last six decades were analysed. Biofilms were completely inactive under dry conditions (Fv/Fm = 0), but rapidly recovered photosynthesis (Fv/Fm ≈ 0.5) within two hours of rehydration. A bark water content of approximately 16% was sufficient to restore 50% of Fv/Fm, whereas higher water availability (> 30%) was required to recover rETRmax and PPFDsat. Air humidity influences the duration of water availability, whereas photosynthetic reactivation depends primarily on liquid water. Climate analyses (1961–2024) revealed an increase in the local frequency of drier days (RH 1; rainfall = 0 mm). This trend suggests that these biofilms are increasingly exposed to prolonged water stress, potentially compromising their ecological functioning and associated ecosystem services. Overall, our findings highlight the vulnerability of epiphytic cyanobacterial biofilms to ongoing climatic drying.