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Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia



Sulphur isotope data from early Archaean rocks suggest that microbes with metabolisms based on sulphur existed almost 3.5 billion years ago, leading to suggestions that the earliest microbial ecosystems were sulphur-based1,2,3,4,5. However, morphological evidence for these sulphur-metabolizing bacteria has been elusive. Here we report the presence of microstructures from the 3.4-billion-year-old Strelley Pool Formation in Western Australia that are associated with micrometre-sized pyrite crystals. The microstructures we identify exhibit indicators of biological affinity, including hollow cell lumens, carbonaceous cell walls enriched in nitrogen, taphonomic degradation, organization into chains and clusters, and δ13C values of −33 to −46‰ Vienna PeeDee Belemnite (VPDB). We therefore identify them as microfossils of spheroidal and ellipsoidal cells and tubular sheaths demonstrating the organization of multiple cells. The associated pyrite crystals have Δ33S values between −1.65 and +1.43‰ and δ34S values ranging from −12 to +6‰ Vienna Canyon Diablo Troilite (VCDT)5. We interpret the pyrite crystals as the metabolic by-products of these cells, which would have employed sulphate-reduction and sulphur-disproportionation pathways. These microfossils are about 200 million years older than previously described6 microfossils from Palaeoarchaean siliciclastic environments.

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Figure 1: Examples of spheroidal/ellipsoidal microfossils from the SPF (samples SP9D2, SPE1, SPV3a–c).
Figure 2: Examples of hollow, tubular, sheath-like microfossils from the SPF (samples SPV3a-c).
Figure 3: Microfossil walls and quartz grain boundaries (sample SPV3b).
Figure 4: Spatial relationships between microfossils and pyrite (SP9D2a–b, SPV3b).

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The authors acknowledge the facilities, scientific and technical assistance of the AMMRF at both the Centre for Microscopy Characterization and Analysis, The University of Western Australia, and Adelaide Microscopy, The University of Adelaide. These facilities are funded by the Universities, State and Commonwealth Governments. The Geological Survey of Western Australia, C. Stoakes, N. McLoughlin and O. Green are thanked for assistance with fieldwork, A. Steele for providing access to laser Raman facilities, and S. Menon and L. Green for assistance with FIB sample preparation. D.W. is supported by a postdoctoral fellowship from The University of Western Australia. M.D.B. and D.W. were funded for the initial stages of this research by a NERC grant to M.D.B. (NE/C510883/1) and by the support of the University of Oxford.

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D.W., M.R.K. and M.D.B. performed the field mapping and collected samples. M.D.B. and D.W. carried out the petrography. M.R.K. and D.W. performed the NanoSIMS analyses. J.C. performed the IMS 1280 carbon isotope analyses. M.S. performed the TEM analyses. D.W. performed the sample preparation and some of the FIB-SEM work. All authors helped to interpret the data. D.W. and M.D.B. wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to David Wacey or Matt R. Kilburn.

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Wacey, D., Kilburn, M., Saunders, M. et al. Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia. Nature Geosci 4, 698–702 (2011).

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