Several lines of evidence indicate that the stromatolites were formed by live organisms, according to Nutman and his team, who conducted the research with funding from the Australian Research Council. The evidence is based on prior knowledge of other existing stromatolites and their tell-tale characteristics.
For example, the structures have a conical shape and internal layering. Such contrasting composition and texture in the bounds of conical formations within rock "are fairly credible hallmarks of microbial activity," Abigail Allwood, a scientist at the Jet Propulsion Laboratory at the California Institute of Technology who did not work on the new study, wrote in an accompanying Nature article. She added that this type of layering, as well as the shape and texture of the structures, means that they are not just folded rock.
Concentrations of titanium and potassium are higher in the structures than between them, providing evidence that a different type of sediment had accumulated there. This is also a characteristic associated with other stromatolites.
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As for what type of microbe could have produced the stromatolites, its precise identity is unknown at this point.
"Beyond that stromatolites are microbial constructs, there is no evidence of what the culprits looked like beyond that they would have been single cellular," Nutman said, adding that he and his colleagues have already begun additional sulfur isotope testing to determine if there are signatures for metabolic (physical and chemical processes) present in the stromatolites. The results of those studies may reveal more about the likely microbes.