"False" Biosignatures May Complicate Search for Ancient Life on Earth, Other Planets
Self-assembling carbon microstructures created in a lab by University of Colorado Boulder researchers could provide new clues – and new cautions – in efforts to identify microbial life preserved in the fossil record, both on Earth and elsewhere in the solar system.
The geological search for ancient life frequently zeroes in on fossilized organic structures or biominerals that can serve as “biosignatures” that survive in the rock record over extremely long time scales. Mineral elements such as sulfur are often formed through biological activity. Microbes can also produce a variety of telltale extracellular structures that resemble sheaths and stalks.
However, according to new findings published in the journal Nature Communications, carbon-sulfur microstructures that would be recognized today by some experts as biomaterials are capable of self-assembling under certain conditions, even without direct biological activity. These “false” biosignatures could potentially be misinterpreted as signs of biological activity due to their strong resemblance to microbial structures.
“Surprisingly, we found that we could create all sorts of biogenic-like materials that have the right shape, structure and chemistry to match natural materials we assume are produced biologically,” said Associate Professor Alexis Templeton of CU Boulder’s Department of Geological Sciences and senior author of the new study.
The study arose from field research in the Canadian High Arctic, where a team of scientists working with Templeton had identified sulfur-metabolizing organisms that live in shopping mall-sized mineral deposits that form on ice surfaces. Some of these sulfur deposits were returned to CU-Boulder to determine whether they contained “biosignatures” that could be relevant to the search for life on Mars or Europa, one of Jupiter’s moons.
Templeton and CU-Boulder Research Associate Julie Cosmidis then set out to study the underlying mechanisms of biological sulfur mineral formation before realizing that some of the “extracellular structures” and associated sulfur minerals could be reproduced in the lab without any organisms present.“It was very disconcerting- at first to see that the carbon-sulfur structures appear in our tests without biological activity, as they looked very microbial,” said Cosmidis, the lead study author.
“But the fact that these structures self-assemble makes their discovery even more exciting. They challenge our conception of what a biosignature is, and they can teach us about unexpected interactions between carbon and sulfur,” said Cosmidis.
The research was supported by a NASAExobiology grant; the NASA Astrobiology Institute Cooperative Agreement; the Canada Foundation for Innovation, Natural Sciences and Engineering Research; Council of Canada; The University of Saskatchewan; the Government of Saskatchewan; Western Economic Diversification Canada; the National Research Council Canada, and the Canadian Institutes of Health Research.