Geoökologisches Kolloquium SS 2005

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Dr. Maria Dittrich
Forschungszentrum für Limnologie, EAWAG,Kastanienbaum, Schweiz
Tuesday, 03.05.2005 17:00 H6

CaCO₃ precipitation by cyanobacteria: insights from field, laboratory and microscopic studies.

Cyanobacteria are photosynthetic prokaryotes commonly found in soils and freshwater. They are also an important component of marine phytoplankton and contribute significantly to the overall primary production in ecosystems of all climatic zones. Cyanobacteria often dominate the picoplankton (cell diameter 0.2 -2 um) community in oligotrophic systems. Picocyanobacteria are increasingly investigated due to their ecological significance. Recent studies indicate that picoplankton plays an important role in calcite precipitation in oligotrophic lakes. However, the mechanism of calcite precipitation on picocyanobacteria is still unclear. In this presentation I will show the results of our studies on the role of picoplankton for CaCO3 precipitation. Our studies include field observations and different kinds of laboratory experiments.
We provide field observations in order to correlate spatial and temporal patterns of picoplankton abundance and calcite precipitation in Lake Lucerne. Seasonal calcite concentration peaks in oligotrophic lakes may be explained by the activity of unicellular autotrophic picocyanobacteria. In a pre-study we investigated the cell mineral interface by analytical Transmission Microscopy (TEM). For this analysis samples were prepared using a Focused Ion Beam (FIB)-milling. From these experiments we concluded that the composition of the cell surface seems to play a crucial role in the nucleation process. In order to investigate the nucleation of calcite on the cell surface, experiments under controlled conditions were performed in a flow-through design of an AFM as well.
In the most recent study, the surface properties of two picocyanobacteria strains were examined by measurements of zeta potential, potentiomentric titration, and infrared spectroscopy (IR). The results demonstrated that the deprotonation of picocyanobacterial cells surfaces is a reversible process. Modelling results, data on zeta potential and IR spectra are consistent with the presence of three surface sites, corresponding to carboxyl, phosphate, and amine groups. The carboxyl group slightly dominates the surface sites in both strains with 37% and 44%, the amine groups represent 36% and 29%. The smaller fraction of phosphate was similar in both strains with 27% and 26%.
During the picocyanobacterial bloom in oligotrophic lakes the concentration of functional groups amounts to ca. 0.35 mmol/ml. The zeta potential measurements have shown that picocyanobacteria are negatively charged at pH between 6 and 7, which is typical for natural surface water. Calcium cations can therefore be easily attracted. On the other hand, the presence of amine groups on the cell surface influences the carbonate anions. Both reactions are important in the nucleation of calcite on the cell and may be key processes for the calcite precipitation in lakes.


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