Cyanobacteria have been recognized as key players in the precipitation of calcium carbonate in marine and freshwater systems. These bacteria increase pH, (as a result of photosynthetic activity) and also produce extracellular polysaccharides, which act as binding sites for Ca2+ and CO32−. Both processes influence the morphology and the mineralogy of the carbonate minerals. In order to clarify the role of polysaccharides of picocyanobacteria upon calcium carbonate precipitation, both their buffering capacity and ability to induce precipitation need to be investigated. In this experimental study, we characterized the polysaccharides of three unicellular autotrophic picocyanobacterial Synechococcus-type strains by potentiometric titration and infrared spectroscopy. Potentiometric titrations were conducted to determine the total buffering capacity. The nature and concentration of active sites of the polysaccharides was clarified with the aid of potentiometric titration and spectral analysis of an aqueous cellular suspension. Precipitation experiments with polysaccharides of different strains allowed an estimation of their potential to precipitate calcium carbonate. The results presented here indicate that polysaccharides from cyanobacteria have a strong potential to exchange protons with their surrounding environment. Precipitation experiments demonstrated that extracellular polysaccharides of all the strains studied able to precipitate calcium carbonate.
The chemistry of the surface functional groups of picocyanobacteria Synechococcus PCC 7942 cells was examined as a function of H+ and calcium concentrations. Titration experiments, infrared spectroscopy, biosorption experiments, and chemical modeling were used to gain insight into the mechanisms of biosorption. The pKa and concentration of active sites on the cell wall were clarified with the aid of potentiometric titration. Modeling calculations and infrared spectra are consistent with pKa's values of 4.3, 5.2, 6.9, 9.1, and 10.0 and a total concentration of 7.8 x 10(-4) mol g(-1). Spectral analysis of an aqueous cellular suspension revealed a presence of carboxyl, amide, phosphate, hydroxyl, and carbohydrate moieties. Correspondence between spectral data and potentiometric titration curves supported the hypothesis that carboxylate groups and phosphodiester groups mediate calcium adsorption to bacterial cells. This process is strongly pH dependent. In the second part of the experimental work, Synechococcus cells were suspended in the presence of different calcium concentrations. Mechanistic modeling demonstrated that the calcium adsorption phenomenon can be described taking into account only two mechanisms: ion exchange and complexation.
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