Surface-enhanced infrared absorption-reflectance (SEIRA) microspectroscopy for bacteria localization on geologic material surfaces
Surface-enhanced infrared absorption-reflectance (SEIRA) microspectroscopy is potentially a useful chemical/biological probe to provide insights into the localization of living endolithic bacteria on the surfaces of geologic materials. This hypothesis was tested by validating and demonstrating the use of SEIRA with a metal-overlayer configuration to identify qualitatively on vesicular basalt surfaces the highly localized differences in the chemical composition and in the structure of clusters of endolithic bacteria, vesicles, and minerals. The metal-overlayer configuration was achieved by evaporating a thin gold-film on basalt specimen surfaces. Fourier-transform SEIRA microspectra of the specimen surfaces were recorded in the 650–4000 cm?1 infrared region at a resolution of 4 cm?1 on a Fourier-transform infrared spectrometer coupled to an infrared microscope. All bacteria-inhabiting surfaces exhibited infrared absorption bands indicative of bacterial cells, bands that became ideal biomarkers by which to detect the presence of bacteria. All basalt surfaces exhibited infrared absorption bands indicative of silicates, bands that became ideal mineral markers by which to detect the presence of silicate-containing minerals and locations of vesicles (gas-bubble cavities). Comparative analysis of space-resolved microspectra suggested that bacteria in the vesicular basalt lived not only on the vesicle surface but that they also penetrated and lived beneath the vesicle surface. The penetration terminated when calcic-plagioclase feldspar became the dominant constituent mineral in the vesicular basalt. With this experimental effort, the practical aspects and the usefulness of SEIRA as a promising tool to complement existing techniques for studying the in-situ localization of living bacteria in geologic materials have been demonstrated.