MICRO-FTIR SPECTROSCOPIC CHARACTERIZATION OF FOSSIL AND EXTANT MICROORGANISMS

Precambrian microfossils sometimes retain carbonaceous cellular structures that resemble modern cyanobacteria and other prokaryotes in shape and size (e.g. Schopf, 1992; Knoll, 2003). Morphological taxonomy of these minute, simple, and more or less degraded fossils, however, is not established well enough to give their precise phylogenetic positions. We apply microscopic Fourier Transform Infrared (micro-FTIR) spectroscopy to extant and fossil microorganisms in order to find taxon-specific chemical signature from Precambrian microfossils. We (Igisu et al., 2006, 2009) showed the presence of ~2925 and ~2960 cm^-1 bands (aliphatic CH₂ and CH₃, respectively) in IR absorbance spectra of well-preserved Proterozoic prokaryotic fossils in stromatolitic cherts. The same bands around 2925 and 2960 cm^-1 are also confirmed in some biomolecules of extant microorganisms, such as lipid and protein. These indicate the evaluation of aliphatic CH moieties is effective in chemical characterization/comparison of ancient and extant microorganisms. The ratios between aliphatic CH₃/CH₂ absorbance (R_3/2) of microfossils resemble those of extant bacterial lipid, suggesting the possible preservation of fossilized precursor membrane lipid. R_3/2 values for n-alkane standard samples are positively correlated with the ratio of their CH₃/CH₂ number, thus R_3/2 value of organic matter with strait chain hydrocarbon (bacterial lipid) can be lower than that with highly branched hydrocarbon (archaeal lipid). These indicate that R_3/2 value is a promising new criterion for distinguishing the two domains; ancient-modern Bacteria and Archaea.