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This study investigated the role of the solution composition on calcite precipitation induced by cyanobacteria. The precipitation of calcium carbonate was induced by addition of cyanobacterium cells Synechococcus strain PCC 7942 in two artificial solutions with a different composition at similar saturation states in respect to calcite. Ion-selective electrodes for pH, Ca 2+ , and CO 3 2-monitored the experiments, and the morphology of precipitated crystals was analysed by scanning electron microscopy. The calcite precipitation was observed in all experiments after the addition of the cells. The composition of solution (the ratio of dissolved inorganic carbon to dissolved calcium) strongly influenced the calcite precipitation. Based on laboratory experiment results, a possible mechanism for precipitation induced by Synechococcus is proposed linking precipitation with the conditions near to cell walls rather than with the saturation conditions in the bulk solution Durch das Cyanobakterium Synechococcus induzierte CalcitfällungDie durch Zugabe von Zellen des Cyanobakteriums Synechococcus (Stamm PCC 7942) induzierte Ausfällung von Calciumcarbonat wurde an zwei ähnlich stark übersättigten wässri-gen Calcitlösungen unterschiedlicher Zusammensetzung untersucht. Während des Experiments wurden mit ionenselektiven Elektroden der pH-Wert sowie die Konzentrationen von Ca 2+ und CO 3 2-verfolgt. Außerdem wurde die Morphologie der ausgefällten Kristalle mittels Rasterelektronenmikroskopie untersucht. In allen Versuchen wurde nach Zugabe der Zellen Calcitfällung beobachtet, dabei erwies sich das Verhältnis des gelösten anorganisch gebundenen Kohlenstoffs zum gelösten Calcium als die entscheidende Einflussgröße für die Calcitfällung. Aufgrund der erhaltenen Ergebnisse wird ein Mechanismus vorgeschlagen, bei dem die induzierte Calcitfällung eher von den Verhältnissen nahe der Zellwand als vom Sättigungszustand der Lösung abhängt.

Picocyanobacteria were found to play an important role in calcite precipitation in oligotrophic lakes. In this study, investigations on the interface between cyanobacteria and attached biogenic calcite crystals have been performed to gain further insights into the mechanisms of nucleation of these precipitates. Ultramicrotomy, the conventional preparation technique of thin sections for Transmission Electron Microscopy (TEM) investigations, often fails when working on heterogeneous samples containing soft organic material and hard minerals. Thus, in this study the thin sections were prepared using Focused Ion Beam (FIB) milling. This approach is usually applied in material sciences but until recently was not very common in environmental research. Different analytical TEM methods like Electron Spectroscopic Imaging (ESI) and Electron Energy Loss Spectrometry (EELS) were used to test the suitability of FIB-milling for the preparation of organic/inorganic interface specimens. With this approach we were able to analyze both organic and the inorganic phases of the same sample. Elemental maps of the samples were also calculated. By analyzing the structure of the C K-absorption edge, the different bonding forms of the organic carbon cell and the inorganic carbon of the crystal could be clearly distinguished.

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