Synchrotron-based Fourier-transform infrared (SR-FTIR) micro-spectroscopy was used to determine the concentration-dependent response of the organic structure of live cyanobacterial cells to silicification. Mid-infrared (4000-600 cm-1) measurements carried out on single filaments and sheaths of the cyanobacteria Calothrix sp. (strain KC97) were used to monitor the interaction between a polymerizing silica solution and the organic functional groups of the cells during progressive silicification. Spectra of whole-cells and sheaths were analyzed and the spectral features were assigned to specific functional groups related to the cell: lipids (-CH2 and -CH3; at 2870-2960 cm-1), fatty acids (>C=O at 1740 cm-1), proteins (amides I and II at 1650 and 1540 cm-1), nucleic acids (>P=O 1240 cm-1), carboxylic acids (C-O at 1392 cm-1), and polysaccharides (C-O between 1165 and 1030 cm-1). These vibrations and the characteristic vibrations for silica (Si-O between 1190 and 1060 cm-1; to some extent overlapping with the C-O frequencies of polysaccharides and Si-O at 800 cm-1) were used to follow the progress of silicification. Relative to unsilicified samples, the intensity of the combined C-O/Si-O vibration band increased considerably over the course of the silicification (whole-cells by >90% and sheath by ∼75%). This increase is a consequence of (1) extensive growth of the sheath in response to the silicification, and (2) the formation of thin amorphous silica layers on the sheath. The formation of a silica specific band (∼800 cm-1) indicates, however, that the precipitation of amorphous silica is controlled by the dehydroxylation of abiotically formed silanol groups.
- infrared spectroscopy