'False' biosignatures may complicate search for ancient life on Earth, other planets

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 findings indicate that carbon-sulfur microstructures may no longer be surefire microbial indicators, but they are still useful for reconstructing environmental processes anywhere there is active sulfur cycling.

"We're interested to learn how organisms mediate mineralization and commonly it is challenging to demonstrate that a mineral was produced by living organism," said Templeton. "This research is another step forward in understanding fundamental self-assembly processes that are important to materials scientists, biologists and chemists alike."

But while carbon-sulfur microstructures could confound efforts to identify ancient life, they may provide a roadmap to an entirely different innovation: Next-generation lithium-sulfur (Li-S) batteries.

Rechargeable Li-S batteries are considered to be a promising successor to the lithium-ion batteries that power most of today's consumer electronics. Li-S batteries can contain up to five times the energy of lithium-ion batteries, but present a number of manufacturing hurdles that have yet to be overcome on a commercial scale.

The carbon-sulfur microstructures created in the new study, however, may solve one of the key challenges by encasing the sulfur in conductive carbon, potentially creating more electrically efficient Li-S batteries.

"We're making materials that have the desired properties and we're doing it by mimicking a natural environmental process," said Templeton. "It's a promising new pathway to battery design."