Description: Two different cyanobacterial biofilms from German karstwater creeks were investigated with respect to their photosynthetic effect on Ca2+ removal and potential CaCO3 precipitation in artificial creek waters of different CO2 partial pressures at a given, constant calcite supersaturation. CO2 partial pressures were adjusted to 350 ppmV, 2200 ppmV and 8700 ppmV respectively, covering the range of Phanerozoic atmospheric CO2 partial pressures inferred from palaeosoils, stomatal indices and model calculations. Microsensor measurements of calcium, pH and oxygen revealed differences in the potential to precipitate CaCO3 between the two model organisms Tychonema-relative strain SAG 2388 and Synechococcus sp. strain SAG 2387. Whereas a strong removal of Ca2+ from the solution was measured at Tychonema-relative biofilm, the Synechococcus sp. biofilm exercised a much lower Ca2+ removal during photosynthesis. Photosynthesis was enhanced in both organisms with increasing CO2 and HCO3−, as indicated by enhanced O2 production, but only for the motile filamentous taxon Tychonema-relative a concomitantly increasing calcium removal was measured. However, model calculations indicate that this short-term Ca2+ binding in the Tychonema-relative is due to complexation to exopolymers or oscillin, with no immediate CaCO3 precipitation. In contrast, Ca2+ and pH measurements at Synechococcus sp. biofilm could be consistent with immediate CaCO3 precipitation at the cells. In both biofilms, pH gradients increase with increasing pCO2 from 350 to 2200 ppmV due to enhanced photosynthesis, but decrease at a pCO2 of 8700 ppmV despite of further enhanced photosynthesis. This observation, regardless whether CO2 or HCO3− is used by the cyanobacteria, is in accordance with hydrochemical modeling demonstrating an increased DIC buffering at high pCO2 conditions. These results indicate that the potential of cyanobacteria to form spatially defined calcification pattern via pH gradients at their cell envelopes ('calcified cyanobacteria') increases at elevated pCO2, while at high pCO2 conditions Ca2+ binding and lowered pH microgradients lead to spatially diffuse calcification without defined cell envelope precipitates.