Metabolic microenvironmental control by photosynthetic biofilms under changing macroenvironmental temperature and pH conditions.

Ex situ microelectrode experiments, using cyanobacterial biofilms from karst water creeks, were conducted under various pH, temperature, and constant-alkalinity conditions to investigate the effects of changing environmental parameters on cyanobacterial photosynthesis-induced calcification. Microenvironmental chemical conditions around calcifying sites were controlled by metabolic activity over a wide range of photosynthesis and respiration rates, with little influence from overlying water conditions. Regardless of overlying water pH levels (from 7.8 to 8.9), pH at the biofilm surface was approximately 9.4 in the light and 7.8 in the dark. The same trend was observed at various temperatures (4 degrees C and 17 degrees C). Biological processes control the calcium carbonate saturation state (Omega) in these and similar systems and are able to maintain Omega at approximately constant levels over relatively wide environmental fluctuations. Temperature did, however, have an effect on calcification rate. Calcium flux in this system is limited by its diffusion coefficient, resulting in a higher calcium flux (calcification and dissolution) at higher temperatures, despite the constant, biologically mediated pH. The ability of biological systems to mitigate the effects of environmental perturbation is an important factor that must be considered when attempting to predict the effects of increased atmospheric partial CO(2) pressure on processes such as calcification and in interpreting microfossils in the fossil record.