Résumé
The ability of microbes to metabolize arsenic may have emerged more than 3.4 billion years ago(1,2). Some of the modern environments in which prominent arsenic metabolism occurs are anoxic(3,4), as were the Precambrian oceans. Early oceans may also have had a relatively high abundance of arsenic(5). However, it is unclear whether arsenic cycling occurred in ancient environments. Here we assess the chemistry and nature of cell-like globules identified in salt-encrusted portions of 2.72-billion-year-old fossil stromatolites from Western Australia. We use Raman spectroscopy and X-ray fluorescence to show that the globules are composed of organic carbon and arsenic (As). We argue that our data are best explained by the occurrence of a complete arsenic cycle at this site, with As(III) oxidation and As(V) reduction by microbes living in permanently anoxic conditions. We therefore suggest that arsenic cycling could have occurred more widely in marine environments in the several hundred million years before the Earth's atmosphere and shallow oceans were oxygenated.