Microbially-meediated oxidation and/or reduction of sulphur and iron emanating at deep-sea hydrothermal vents are known to be important globally. Because of their morphological similarities with modern S- and Fe-oxidising bacteria producing filamentous sulphur and hydrous ferric oxides, a putative microbial origin has been proposed for filamentous microfossil remains in fossil hydrothermal vent sites as well as other ancient environments. The oldest record is that of Rasmussen [1], who reported pyritic filaments in deep-sea volcanic rocks that are some 3,200 million years old, thus providing support for proposals in favour of a thermophilic origin of life (i.e., non-photosynthetic microbes utilising inorganic matter in their environment as the energy source and which lived at or near 100°C at depths far below the light-penetration zone on the sea floor). Morphological resemblance, however, is not sufficient for discriminating between true microbial fossils and nonbiogenic microstructures, specifically considering their minute size and incomplete preservation in geological material. Hence, an important challenge for microbiologists, geochemists and exobiologists is to extract unequivocal biogenic information from individual microfossils using high-resolution, non-destructive and sensitive techniques.

Here, we use combined synchrotron micro-XANES (ID21) and X-ray micro-fluorescence (SXRF; ID22) analyses to illustrate the spatial distribution on a µm-scale of a variety of potential biogenic markers (major and trace elements and sulphur-oxidation states) in individual filaments. Two types of filaments were analysed. 1) Sulphur- or iron-rich filaments of most likely Epsilon-Proteobacteria organisms collected from a micro-coloniser device exposed for 15 days to a fluid source vent of the Mid-Atlantic Ridge [2] (Figure 105a) and 2) fossilised iron-oxide filaments of putative biogenic origin encapsulated with amorphous silica from a fragment of an inactive chimney of the East Pacific Rise [3] (Figure 105b).


Fig. 105: X-ray microfluorescence spectra of (a) individual filamentous bacteria from the Mid-Atlantic Ridge (the delimited area corresponds to the zone analyzed for its sulphur redox distribution (Figure 106b) and (b) iron-oxide filament (arrowed) of putative biogenic origin encapsulated with amorphous silica from the East Pacific Rise.

SXRF spectra of filamentous bacteria and microfossils contain X-ray peaks of Ca, Ba, Mn, Fe, Cu, Zn, Pb, Br and Sr. In addition to these, Cr and Ni has been detected in the fossil microfilament and S, K, Tl, Se and Rb in the microbial filament. Owing to the high absorbance of the siliceous envelope, only a weak X-ray peak of sulphur was identified in the fossil filament. The origin of S, Tl and Se and possibly that of Mn, Fe, Cu and Zn, may be attributed to the microbe metabolic activity. In contrast, Cl, K, Ca, Ba, Br, Rb and Sr are most likely derived from seawater.

Micro-XANES spectra (Figure 106) show three main types of sulphur with X-ray peaks at 2,471, 2,478 and 2,482 eV. The marked peak at 2,482 eV is characteristic of sulphate. The peaks at 2,471 and 2,478 eV are typical of sulphide or ­SH amino acid groups, or both. Mapping of the different forms of sulphur performed at energies of 2,471, 2,478 and 2,482 eV revealed different distributions of sulphates, sulphides and amino acid-SH groups. Recognition of a heterogeneous overlap of sulphides and SH-radicals strongly suggests that the two sulphur components are present in the same filament. In both types of filaments, the occurrence of three main sulphur species showing heterogeneous distribution on a filament scale and locally underlining the shape of individual bacteria (microbial filament), strongly suggests that the original microorganisms were actively metabolising sulphur. Despite uncertainties on the metabolic capability of the bacteria that produced the filaments studied here, these results show the potential of combining high-resolution SXRF and micro-XANES techniques for extracting unequivocal biogenic information from fossilised hydrothermal systems or extraterrestrial material.


Fig. 106: Micro-XANES spectrum of sulphur in individual filaments showing three peaks at 2,482 eV (sulphate), 2,471 and 2,478 eV (sulphide and ­SH radicals). Associated maps show the distribution of sulphate, sulphide and SH-radicals within (a) a fragment of filamentous microfossil from the EPR (the arrow points to the trace of the filament embedded in the siliceous envelope) and (b) a microbial filament from MAR (note that sulphur distribution mimics the shape of individual microbes).

[1] B. Rasmussen, Nature 405, 676-679 (2000).
[2] P. Lopez-Garcia, H. Philip, F. Gaill, D. Moreira, PNAS, in press.
[3] S.K. Juniper, Y. Fouquet, Can. Mineral. 26, 859-869 (1988).

Principal Publication and Authors
P. Philippot (a), J. Foriel (a), J. Cauzid (a, b), J. Susini (b), B. Ménez (a), A. Somogyi (b), submitted to PNAS.
(a) Laboratoire de Géosciences Marines, Institut de Physique du Globe, CNRS, Paris (France)
(b) ESRF