Earth was once an anoxic planet where oxygen was toxic to life, and researchers are turning to Japan's rare iron-rich hot springs to better understand how early microbes survived. A study led by Fatima Li-Hau, with supervision from Associate Professor Shawn McGlynn at the Earth-Life Science Institute (ELSI), shows that these springs mimic the chemistry of Earth's ancient oceans, offering a glimpse into microbial ecosystems that thrived before photosynthesis dominated.

The research focused on five springs across Tokyo, Akita, and Aomori prefectures. Their waters contain ferrous iron (Fe2+) and little oxygen, resembling conditions around the time of the Great Oxygenation Event (GOE) 2.3 billion years ago. In most springs, microaerophilic iron-oxidising bacteria were the leading organisms, using iron as an energy source in low-oxygen environments. Cyanobacteria, though present, were far less abundant, except at one Akita spring where other metabolisms dominated.

Metagenomic analysis yielded more than 200 high-quality microbial genomes, revealing how these communities processed carbon, nitrogen, and even sulfur despite the low availability of sulfur compounds. Evidence pointed to a cryptic sulfur cycle, where microbes recycle sulfur in subtle ways not easily detected. Crucially, microbes that coupled iron and oxygen metabolisms transformed toxic compounds into energy, sustaining oxygen-sensitive anaerobes and balancing ecosystem functions.

"These iron-rich hot springs provide a unique natural laboratory to study microbial metabolism under early Earth-like conditions during the late Archean to early Proterozoic transition, marked by the Great Oxidation Event," explained McGlynn. Li-Hau added that the consistency of these ecosystems across diverse geochemical settings suggests that iron oxidisers, oxygenic phototrophs, and anaerobes coexisted in early oceans, driving complete biogeochemical cycles.

The findings highlight how primitive microbes adapted to limited oxygen, laying the groundwork for Earth's eventual oxygen-rich biosphere. The work also informs astrobiology, suggesting that planets with geochemical conditions similar to early Earth might host comparable microbial communities.

Research Report:Metabolic Potential and Microbial Diversity of Late Archean to Early Proterozoic Ocean Analog Hot Springs of Japan