Did early life on Earth use sulfate ions to breathe?

What kind of environment did ancient life forms live in and what kind of mechanism did they use to obtain energy? An important clue to answer these questions has been found.

A research team led by Project Researcher Kohei Sasaki and Assistant Professor Naoto Takahata of the Atmosphere and Ocean Research Institute, the University of Tokyo, Associate Professor Akizumi Ishida of the Graduate School of Science, Chiba University, Professor Takeshi Kakegawa of the Graduate School of Science, Tohoku University, and Professor Kenichiro Sugitani of the Graduate School of Environmental Studies, Nagoya University, found traces of early life on Earth breathing sulfate ions in 3.4 billion-year-old rocks. Their findings were published in Geochimica et Cosmochimica Acta.

Regarding early life forms in the Archean eon (approximately 4.0 to 2.5 billion years ago), the existence of sulfur-metabolizing microorganisms, in addition to photosynthetic bacteria and methanogenic (methane-producing) bacteria, have been inferred mainly from isotopic data of sedimentary rocks.

Meanwhile, due to the lack of direct geological evidence depicting the living environment of sulfur-metabolizing microorganisms, how and where early life forms procured sulfur, an element essential for life, remained unknown.

The team took samples from rocks deposited in Strelley Pool Formation, Australia, which used to be a shallow-water environment 3.4 billion years ago. In these samples, they discovered pyrite grains with a concentric structure (colloform) about 0.01 mm in diameter. Since pyrite formed through inorganic processes in sediments is usually cubic, the colloform pyrite suggested that it had been formed through a different mechanism. The team also found micro-scale layers of organic matter inserted within this characteristic structure, which they assumed to be traces of microbial activity.

To verify this, in addition to conventional carbon isotope analysis, sulfur isotope ratio analysis was performed on colloform pyrite to track traces of sulfur metabolism. The analysis was performed using a nanoscale secondary ion mass spectrometer, and variation of the sulfur isotope ratio within the concentric structure and the corresponding distribution of organic matter were determined with a high-resolution power of less than 0.01 mm.

The results revealed that the colloform pyrite showed a systematic variation in sulfur isotope distribution, with more than 20% difference between the innermost and outermost layers. They also revealed for the first time that layered organic matters with biogenic carbon isotope ratios were present at the boundaries of this distribution.

These results indicated that the colloform pyrite found by the team was actually formed by the biological activity (dissimilatory sulfate reduction) of sulfate-breathing organisms, suggesting that an ecosystem centered on sulfate respiration had been established in the shallow-water environment 3.4 billion years ago.

This was the first report in which micro-scale sulfur isotope analysis of geological samples demonstrated that, despite the low oxygen level in the Archean eon, early life forms existed in "sulfur-enriched oases" locally formed in shallow waters as a result of hydrothermal supply and evaporitic conditions. Their findings represent a major advance in the understanding of the earliest element cycles and biological activities on Earth.

The study has provided clues to elucidate how early life developed its metabolic strategy under limited resource availability. The findings are also expected to serve as a powerful mineralogical biosignature in future life-origin studies as well as in detection of life on Mars, where the presence of sulfur-containing organic matter has recently been discovered in rover missions.

Journal Information
Publication: Geochimica et Cosmochimica Acta
Title: Microbial activity preserved in 3.4 Ga colloform pyrite: A micro-scale sulfur isotope analyses
DOI: 10.1016/j.gca.2026.03.005