Cyanobacterial Calcite Precipitation - Laboratory Study on Different Spatial Scales

Lacustrine calcite precipitation with sedimentation rates up to 1 mm per year can result in large carbonate deposits. Varved lake sediments high in calcite content are now intensively studied as high-resolution continental archives for environmental change. Especially in oligotrophic hardwater lakes, eukaryotic and prokaryotic picoplankton was found to be important in the overall process of calcite precipitation. Rates and mechanisms of bacteria-surface mediated precipitation reactions, however, remain poorly understood. For interpreting geochemical and isotopic information stored in sediments, it is essential to know the carbonate precipitation mechanisms and the governing environmental factors. Thus, laboratory experiments with picoplankton under controlled conditions were combined with electron and atomic force microscopy in order to obtain insights into the details of the calcite precipitation mechanisms of cyanobacteria. This study aimed at evaluating the potential of such laboratory studies and investigating the influence of environmental parameters as a first step. Under carefully controlled chemical and physical conditions, precipitation of calcite was induced by adding cultures of picocyanobacteria to supersaturated solutions of CaCO₃ at different CaCl₂/NaHCO₃ ratios. The cell suspensions were purged with an artificial atmosphere at constant CO₂ concentration. Abiotic solutions were used as reference systems. Chemical conditions such as pH, calcium and carbonate concentrations were continuously monitored by ion selective electrodes. The influence of supersaturation in the range of Ω = 2 - 20 on biogenic precipitation was quantified at different ratios of calcium (0.7 - 48 mM) /carbonate (6 μ M - 35 μ M). In a series of bulk experiments it was possible to detect critical values of supersaturation for the onset of precipitation both with and without cells, both in the range of Ω = 5 - 10. The morphology of the precipitates was analyzed by scanning electron microscopy. Both, initial crystals (<500 nm) and larger grown crystals (>10 μ m) showed rhombohedral and hexagonal-prism shapes characteristic for calcite. Ultrathin-sections ( ∼100nm) of the cyanobacteria cells and the attached calcite crystals were prepared by Focused Ion Beam. This technique allowed us to preserve the cell wall - crystal interface as intact as possible in order to perform Transmission Electron Microscopy (TEM) analysis of the sections. Using TEM-Electron Energy Loss Spectroscopy, carbon bonds in both calcite crystals and cyanobacteria cells were detected and identified. The carbon absorption spectra are significantly different to discriminate the organic and inorganic carbon. Thus, the absorption peaks are the fingerprints of the analyzed material and can be used to analyze the transition zone between the cell and the particle.