Effect of photosynthesis on pH variation in cyanobacterial biofilms from Roman catacombs

Cyanobacterial biofilms present on stone surfaces in Roman hypogea were studied with the aim of assessing their deteriogenic activity on the colonised substrata. In order to achieve this, non-destructive methods were developed and applied to measure pH variation induced via photosynthesis and respiration in representative cyanobacteria from Roman catacombs. Amperometric and potentiometric microsensors were also used on Scytonema biofilms in culture in order to measure photosynthesis and assess pH decreases and increases during dark-light periods. Measurements of pH showed that, starting with values slightly below neutral, the pH in Scytonema biofilms increased by 0.24-0.77 units in the transition from dark to 1000 mu mol photon m(-2) s(-1) irradiance. Comparison of photosynthesis and pH curves recorded simultaneously on the same artificial biofilm showed a maximum increase in pH value at irradiances higher than those saturating photosynthesis. Alkalinisation of the substrate during illumination occurred to a sufficient extent to induce precipitation of mineral compounds, especially on calcareous substrates such as those present in Roman hypogea.