Metals are widely used by all microorganisms and could act as indicators of past microbial activity in the rock record but only if abiotic processes (passive metal absorption on biofilms or metal remobilization by diagenesis) can be excluded. Stromatolites ubiquitous in the rock record for the last 3.5 Ga, are excellent targets for investigating the behaviour of trace metals during these processes.
Here I present the results of the study of the distribution, inter-element correlation and speciation of trace metals and metalloids in modern (The Bahamas) and ancient stromatolites (2.7 Ga, Australia) at different scales, from the cm- to the nm-scale using a combination of Synchrotron Radiation X-ray Microfluorecence (SR-µXRF), Synchrotron Radiation X-ray Absorption Near-Edge Structure (SR-µXANES), Raman spectroscopy and Confocal Laser Scanning Microscopy (CLSM).
Results show that the distribution of metals in stromatolite is mainly governed by passive absorption in the living mats and remobilization and concentration through diagenesis and metamorphism. Both in the modern and Archean stromatolites, cell-like organic globules were identified, which significant enrichments in arsenic, with no, or only negligible contributions from other metals (specifically Fe). These As-bearing organic globules were interpreted as biological in origin and it is the first time that a link between As-based metabolism and the primitive Earth is identified. These findings suggest that arsenate was available in the environment at least 2.7 Ga ago, thus providing niches for As(V)-respiring prokaryotes several hundred millions of years before the great oxygenation event.