High-resolution imaging of sulfur oxidation states, trace elements, and organic molecules distribution in individual microfossils and contempo rary microbial filaments
Résumé
Owing to the delicate nature of fossil microorganisms and inherent difficulties for discriminating true fossils from artifacts, an important challenge is to extract unequivocal biogenic information from individual microfossils using high-resolution, nondestructive and sensitive techniques. Here, we use combined synchrotron (X-ray microfluorescence, X-ray absorption near-edge structure and infrared microspectroscopies) and particle-induced X-ray emission analyses to image the spatial distribution at a μm-scale of a variety of potential biogenic markers (major and trace elements, C-H bonds, and sulfur-oxidation states) in individual prokaryotic microfossils. In particular, we analyzed iron-oxide fossil filaments of putative biogenic origin encapsulated with amorphous silica from a fragment of an inactive hydrothermal chimney of the East Pacific Rise. In order to test the biogenic origin of the markers studied, we performed the same analyses on filamentous bacteria corresponding most likely to the ɛ-Proteobacteria, and collected from substrates exposed to a hydrothermal fluid vent at the Mid-Atlantic Ridge. In both types of fossil and contemporary filaments, the occurrence of CH2 groups and of three sulfur species (sulfate, sulfite, organic S) showing heterogeneous distribution that underline the cytoplasm of individual cells in the case of the present-day filament, suggests that the original microorganisms were actively metabolizing sulfur. These results show the large potential of combining high-resolution synchrotron techniques to analyze individual microfossils for extracting unequivocal biogenic information. Furthermore, they also suggest that cooccurrence of different sulfur oxidation states within single microfossils could constitute a biogenic metabolic marker indicating S-metabolizing activities.