A 3.45-Million-Year-Old Diet
Australia is famous for its ancient, rugged landscapes of arid deserts and windswept rock formations. Some regions of the continent have remained relatively untouched by geology for over billions of years. Because of this, scientists have tramped all across the “sunburnt country” in search of clues about Earth’s past biosphere and environment. Now, with some clever isotopic study, a team of scientists is using ancient Australian rocks to reveal information about the metabolic pathways used by microorganisms billions of years ago.
The stromatolites from the Strelley Pool Formation contain what some scientists believe is direct evidence of possible microbial communities. Sandwiched in the rock are thin layers of sediment, known to geologists as ‘laminae,’ which are rich in organic carbon in the form of kerogen. It’s these organic-rich laminae that make Strelly Pool stromatolites unique among other Archaen stromatolites, such as those found in the Barberton Greenstone Belt region, in South Africa. Bontognali’s team wanted to see if they could add to the story, and have found some interesting evidence of what ancient microbes in the mat may have eaten.
Studying the diet of ancient microbes isn’t as simple as with large, multi-cellular organisms. For instance, paleontologists can look at the teeth of dinosaurs and, based on the scratches and scrapes left by years of chewing, make a good guess at the types of plants and animals they ate. Microbes have no teeth, but they do leave a lasting mark on what they eat. Namely, their metabolism prefers specific isotopes of the same chemical element. Certain ratios (or fractionations) of isotopes can be linked to biological reactions, showing that the elements of interest were processed by microbes and not non-biological reactions or weathering.
For Strelley Pool, the team focused on isotopes of sulfur found in the kerogen laminae. They discovered fractionations consistent with an Archean sulfur cycle, indicating that sulfate-respiring microbes were at least living there when the mats were formed. This is also true for modern stromatolites – meaning that ancient and modern stromatolites may have more in common than their looks alone.
“Our results do not prove that sulfate-respiring microbes produced the stromatolitic laminations—they just indicate that sulfate-respiring microbes were present within the microbial mat,” explains Bontognali. “This finding highlights important similarities between these early ecosystems and their modern counterparts. Sulfur-respiring microbes are commonly found at depth within modern microbial mats, and—in some lithifying mats—they play an important role in forming laminations. Thus, it is legitimate to hypothesize that similar biological processes might have contributed to the formation of the Strelley Pool stromatolites.”
The results could provide valuable insight into biological processes used by Earth’s ancient biosphere, and the biosphere’s ultimate connection to the planet’s environment.
According to Bontognali, “By measuring sulfur isotopes in kerogen, the geochemical evidence for metabolism can be directly linked to fossil material. The evidence is chemically encoded in the organic remains of the organisms themselves. This association reinforces any biological interpretation from previous studies and represents one of the main advantages of our new approach with respect to the classic measurements targeting sulfur minerals.”
It is key that the numbers from the kerogen match previous studies by researchers like David Wacey of the University of Western Australia (2). “[Our work] measured sulfur isotopes in pyrite, whereas this new paper measured sulfur isotopes in organic material, and the data all matches,” explains Wacey. “This is strong evidence that these sulfur metabolisms were active 3.4 billion years ago, and that they were active in more than one environment.“
“This new research shows that the organic matter was probably biological and that sulfate-reducing and/or sulfur disproportionating bacteria were probably part of the biota,” Wacey continued. “When put with data obtained from previous studies of the Strelley Pool material it certainly strengthens the case for these structures being biological stromatolites.”
Sulfur is essential for life on Earth because it is used in things like amino acids and proteins that keep cells alive. Today, microorganisms help to keep sulfur cycling on Earth so that it can be used by the rest of the biosphere. Understanding how and when sulfur metabolism developed on Earth can thereby help astrobiologists understand the origin, history and evolution of Earth’s biosphere.
The strange thing is, previous studies on sulfur isotope ratios in sediments have limited ancient sulfur metabolism to only a few occurrences during the early Archean period on Earth. After this, they seem to disappear until the Paleoproterozoic, more than a billion years later. This has led some scientists to believe that the isotopic fractionation of sulfur in these ancient rocks is not actually evidence of Archean microbial activity. Instead, the ratios of sulfur isotopes were caused by other abiotic processes.
“The fact that there is a gap between the first isotopic evidence for sulfur metabolisms (i.e. ~3.5 Gyr ago) and the time when evidence for such metabolism is clearly and continuously recorded in the rock record (i.e. ~2.5 Gyr ago) raised doubts about the biological origin of the oldest pyrites,” says Bontognali. “Our study provides an independent evidence for the early onset of sulfur metabolisms, indicating that the gap, although unexplained, may be real.”
Building toward the future
The methods used by the team could also have further application in studying other ancient fossils.
“The same approach that we used for investigating the Strelley Pool stromatolites can surely be applied to assess the biogenicity of other putative fossil organisms present in the geological record,” explains Bontognali. “But more studies on the effect of secondary processes are required to conclude whether organic sulfur or sulfur minerals provide the most reliable signal of a primary microbial process.” However, when thinking about using similar isotopic studies beyond Earth in the hunt for biosignatures in our solar system, there are still too many technological barriers.
“The technique of measuring multiple sulfur isotopes from small amounts of organic material will undoubtedly be useful for further studies of early life on Earth,” says David Wacey. “However, one needs to be careful in drawing conclusions from such isotopic data. A firm understanding of the age, geological setting, and post-depositional history of the organic matter being studied is needed before the sulfur isotope data can be interpreted correctly. The technique is not suited for remote studies on Mars for example. Samples would have to be returned to Earth for this technique to be applied. “
The work was supported in part by the NASA Exobiology and Evolutionary Biology program. Further support came from the Caltech Center for Microanalysis and the Swiss National Science Foundation.
The paper, “Sulfur isotopes of organic matter preserved in 3.45-billion-year-old stromatolites reveal microbial metabolism,” was published in the Proceedings of the National Academy of Sciences (PNAS) under lead author Tomaso R.R. Bontognali.