Effects of combined increase in temperature and CO2 concentration on the weathering activity of phototrophic organisms inhabiting granitic rocks and its implications in terms of cultural heritage conservation

• The increase in temperature reduced phototrophic biomass and photosynthetic efficiency. • The increase in CO 2 concentration slightly improved photosynthetic efficiency. • Green algae proliferated preferentially at low temperatures. • Cyanobacteria proliferated preferentially at high temperatures. • The presence of organisms induced a biogeochemical assimilative degradation of granite. The increase in CO 2 concentration and temperature observed in the current context of climate change may cause changes in the behavior of phototrophic organisms colonising stone cultural heritage, leading to undesirable changes in terms of biodeterioration. In this study, we examined the combined effect of both parameters (18°C or 24°C and ambient or high CO 2 concentration) on the physiological state of organisms and on granite weathering through experiments involving a green alga ( Bracteacococcus minor ) and a cyanobacterium ( Synechocystis sp. PCC 6803) grown in medium containing granite. The results revealed an increase in the green algae population relative to the cyanobacteria at lower temperature and a reduction in biomass and efficiency during the early stages of culture development at the higher temperature. In addition, although rock weathering was favoured by modification of the solution equilibrium through cation uptake by the organisms no significant differences in response to environmental conditions were detected.