MOLECULAR CHARACTERIZATION OF CYANOBACTERIAL SILICIFICATION USING SYNCHROTRON INFRARED MICRO-SPECTROSCOPY

Abstract

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.