Reduction spheroids preserve a uranium isotope record of the ancient deep continental biosphere

Sean McMahon (Corresponding Author), Ashleigh v. S. Hood, John Parnell, Stephen Bowden

Research output: Contribution to journalArticlepeer-review

6 Citations (Scopus)
8 Downloads (Pure)


Life on Earth extends to several kilometres below the land surface and seafloor. This deep biosphere is second only to plants in its total biomass, is metabolically active and diverse, and is likely to have played critical roles over geological time in the evolution of microbial diversity, diagenetic processes and biogeochemical cycles. However, these roles are obscured by a paucity of fossil and geochemical evidence. Here we apply the recently developed uranium-isotope proxy for biological uranium reduction to reduction spheroids in continental rocks (red beds). Although these common palaeo-redox features have previously been suggested to reflect deep bacterial activity, unequivocal evidence for biogenicity has been lacking. Our analyses reveal that the uranium present in reduction spheroids is isotopically heavy, which is most parsimoniously explained as a signal of ancient bacterial uranium reduction, revealing a compelling record of Earth’s deep biosphere.
Original languageEnglish
Article number4505
Number of pages6
JournalNature Communications
Early online date29 Oct 2018
Publication statusPublished - 29 Oct 2018

Bibliographical note

S.M. acknowledges the support of the NASA Astrobiology Institute grant NNA13AA90A, Foundations of Complex Life, Evolution, Preservation and Detection on Earth and Beyond, and the European Union’s Horizon 2020 Research and Innovation Programme under Marie Skłodowska-Curie grant agreement 747877. Av.S.H. was supported by a NASA Astrobiology Institute Postdoctoral Fellowship and acknowledges the support of Xiangli Wang and Devon Cole for lab assistance. S.M. and Av.S.H. thank Noah Planavsky for technical advice, lab support, and comments on an early draft. J.P. was supported by NERC under grant number NE/L001764/1. The isotope facility at SUERC is supported by NERC. The authors thank the two anonymous referees for constructive criticisms that improved the manuscript.


  • astrobiology
  • element cycles
  • environmental microbiology
  • palaeontology


Dive into the research topics of 'Reduction spheroids preserve a uranium isotope record of the ancient deep continental biosphere'. Together they form a unique fingerprint.

Cite this