Microbial metabolisms and calcification in freshwater biofilms

Abstract

In this study, biofilms of two CO2-degassing karst-water creeks in Germany, which attain high calcite supersaturation during their course downstream, were investigated with regard to the effects of microbial activity on CaCO3 precipitation, water chemistry of micro- and macroenvironment, stable isotopic records, and tufa fabric formation. In situ and ex situ microelectrode measurements of annually laminated calcified biofilms composed mainly of filamentous cyanobacteria revealed that they strongly induced CaCO3 precipitation by photosynthesis under illumination, and inhibited precipitation by respiration in the dark. Photosynthesis-induced CaCO3 precipitation was also confirmed by radioactive isotope (45Ca2+) uptake studies as well as mass balance calculations. Oxygen and carbon stable isotopic records of the tufa stromatolites did confirm photosynthetic effects despite the evident photosynthesis-induced calcite precipitation, and therefore, the absence of photosynthetic effect in the isotopic records of carbonate minerals (e.g., heavier d13C) does not indicate the absence of photosynthetic effect on the carbonate precipitation. Similarly, fabrics of calcified cyanobacteria cannot be used to distinguish photosynthesis-induced from physicochemically-induced CaCO3 precipitation because encrusted cyanobacterial sheaths, that was previously suggested as an indicator of physicochemically-forced precipitation, was observed in tufa stromatolite instead of sheath impregnation, that was previously suggested as an indicator of photosynthesis-induced precipitation. Although tufa stromatolites are formed by photosynthesis-induced calcite precipitation, mass balance calculations demonstrated that biofilm photosynthesis was responsible for only 10–20% of Ca2+ loss in the creek, while remaining Ca2+ loss derived from physicochemical precipitation on branches, leaves and fine-grained calcite particles. Therefore, the effects of photosynthesis-induced precipitation are diluted, and undetectable by conventional water analysis except for the period of low flow rate. In contrast, endolithic cyanobacterial biofilms and mosses, both can also perform photosynthesis, did not cause photosynthesis-induced precipitation under experimental conditions of ex situ microelectrode measurements because of their lower photosynthetic activity. No spontaneous precipitation occurred on biofilm-free limestone substrates under the ex situ measurements, despite of high supersaturation, while tufa stromatolites could induce precipitation in the same condition. This fact indicates that photosynthesis is a crucial mechanism to overcome the kinetic barrier for CaCO3 precipitation, even in highly supersaturated settings. The simulations of photosynthetic effects in various pH, DIC and Ca2+ concentration revealed the preconditions of photosynthesis-induced carbonate precipitation are 1) optimum pH–DIC condition where low DIC effect, CO2 and CO32– buffering are not severe, 2) sufficient initial saturation state, 3) Ca2+ concentration is not extremely low. In addition, high ionic strength weakens and/or inhibits photosynthesis-induced precipitation. Of course, photosynthetic activity of biofilms must be high enough to shift carbonate system on and in the biofilms. Most of these preconditions are also applicable for the carbonate precipitation induced by other types of microbial metabolisms such as sulfate reduction. Therefore, it is concluded that these preconditions would be the important keys to understand the formation and distribution of carbonate microbialite through the geologic time.