Micro-FTIR spectroscopic signatures of Bacterial lipids in Proterozoic microfossils

Precambrian microbial fossils show carbonaceous cellular structure, which often resemble in shape and size cyanobacteria and other prokaryotes. Morphological taxonomy of these minute, simple, and more or less degraded fossils is, however, often not enough to determine their precise phylogenetic positions. Here we report the results of micro-FTIR spectroscopic analyses of well-preserved microfossils in 850 Ma and 1900 Ma stromatolites, together with those of 8 species of extant prokaryotes and 5 of eukaryotes for comparison. These Proterozoic fossils have low CH₃/CH₂ absorbance ratios (R_3/2 < 0.5) of aliphatic CH moieties, suggesting selective preservation of long, straight, aliphatic carbon chains probably derived from bacterial membrane lipids. All the observed R_3/2 values of coccoids, filaments and amorphous organic matter resemble lipid fractions of extant Bacteria including cyanobacteria, but not Archaea. The results indicate that Proterozoic microfossils belong to Bacteria, which is consistent with the cyanobacterial origin inferred from morphology. Moreover, the R_3/2 value of fossilized cell would reflect chemical composition of its precursor membrane lipid, thus could be a useful new tracer for distinguishing Archaea, Bacteria and possibly Eucarya for fossilized and extant microorganisms.