Photosynthesis, respiration and exopolymer calcium-binding in biofilm calcification (Westerhfer and deinschwanger creek, germany)

The impact of microbial activity on biofilm calcification in aquatic environments is still a matter of debate, especially in settings where ambient water has high CaCO3 mineral supersaturation. In this study, biofilms of two CO(2-)degassing karst-water creeks in Germany, which attain high calcite supersaturation during their course downstream, were investigated with regard to water chemistry of the biofilm microenvironment. The biofilms mainly consisted of filamentous cyanobacteria (Phormidium morphotype) and heterotrophic bacteria (including sulfate-reducing bacteria), which affect the microenvironment and produce acidic exopolymers. In situ and ex situ microelectrode measurements showed that a strong pH increase, coupled with Ca2+ consumption, occurred in light conditions at the biofilm surface, while the opposite occurred in the dark. Calcite supersaturation at the biofilm surface, calculated from ex situ Ca2+ and CO32- microelectrode measurements, showed that photosynthesis resulted in high omega values during illumination, while respiration slightly lowered supersaturation values in the dark, compared to values in the water column. Dissociation calculation demonstrated that the potential amount of Ca2+ binding by exopolymers would be insufficient to explain the Ca2+ loss observed, although Ca2+ complexation to exopolymers might be crucial for calcite nucleation. No spontaneous precipitation occurred on biofilm-free limestone substrates under the same condition, regardless of high supersaturation. These facts indicate that photosynthesis is a crucial mechanism to overcome the kinetic barrier for CaCO3 precipitation, even in highly supersaturated settings.