Billions of years ago, Earth’s atmosphere looked very different from today. Oxygen levels were about a million times lower, forests and animals did not exist, and for many organisms, oxygen itself was toxic. A key question for scientists has long been: how did life survive and adapt during this oxygen-poor period?

A team led by Fatima Li-Hau, then a graduate student at the Earth-Life Science Institute (ELSI), Institute of Science Tokyo, along with her supervisor Associate Professor Shawn McGlynn, explored this puzzle by studying modern hot springs that resemble Earth’s primordial oceans. Their work sheds light on the microbial strategies that bridged the gap between oxygen-free and oxygen-rich worlds.

The Great Oxygenation Event

Around 2.3 billion years ago, the Great Oxygenation Event (GOE) transformed Earth’s atmosphere. Cyanobacteria began splitting water using sunlight, releasing oxygen as a by-product. Over time, this process altered the composition of the air, eventually making it about 21% oxygen as we breathe today.

While this oxygen boom enabled animals like us to evolve, it posed major challenges for early microbes that had never encountered O₂ before. Understanding how they managed to adapt is central to piecing together the story of life’s evolution.

Iron-Rich Hot Springs as Natural Laboratories

To investigate, the researchers studied five hot springs in Japan — one in Tokyo, two in Akita, and two in Aomori Prefecture. These waters are naturally rich in ferrous iron (Fe²⁺), low in oxygen, and near neutral in pH — conditions that closely mirror those of Earth’s ancient oceans.

“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. They help us understand how primitive microbial ecosystems may have been structured before the rise of plants, animals, or significant atmospheric oxygen,” explains McGlynn.

Microbes That Turn Waste Into Energy

In four out of five springs, the dominant organisms were microaerophilic iron-oxidising bacteria. These microbes thrive in low-oxygen conditions, using ferrous iron as fuel and converting it into ferric iron. Cyanobacteria — oxygen-producing microbes — were also present but in smaller numbers. Interestingly, one Akita spring showed a very different microbial setup, with non-iron metabolisms dominating.

Through metagenomic analysis, the team reconstructed over 200 microbial genomes, uncovering how these organisms functioned together. Microbes that linked iron and oxygen metabolism played a crucial role by transforming what would otherwise be a toxic by-product into usable energy, while also maintaining conditions that allowed oxygen-sensitive anaerobes to survive.

Surprisingly, the researchers also found signs of a cryptic sulfur cycle, even though sulfur compounds were scarce in the springs. Genes linked to sulfide oxidation and sulfate assimilation suggest microbes may have recycled sulfur in hidden, complex ways.

Coexistence of Diverse Communities

“Despite differences in geochemistry and microbial composition across sites, our results show that in the presence of ferrous iron and limited oxygen, communities of microaerophilic iron oxidisers, oxygenic phototrophs, and anaerobes consistently coexist and sustain remarkably similar and complete biogeochemical cycles,” says Li-Hau.

Their findings suggest that early Earth ecosystems may have resembled these modern hot springs — diverse communities of microbes sharing resources, balancing oxygen levels, and supporting complex cycles long before plants and animals appeared.

Implications for Early Earth and Beyond

“This paper expands our understanding of microbial ecosystem function during a crucial period in Earth’s history, the transition from an anoxic, iron-rich ocean to an oxygenated biosphere at the onset of the GOE. By understanding modern analogue environments, we provide a detailed view of metabolic potentials and community composition relevant to early Earth’s conditions,” adds Li-Hau.

These insights not only illuminate Earth’s evolutionary past but also guide the search for life on other planets with geochemical environments similar to early Earth.

Summary of Key Findings

• Japanese hot springs mimic conditions of Earth’s ancient oceans.
• Dominant microbes were iron-oxidising bacteria, supported by smaller populations of Cyanobacteria and anaerobes.
• Microbes transformed toxic compounds into energy, enabling coexistence in low-oxygen environments.
• Evidence of a cryptic sulfur cycle suggests complex unseen microbial processes.
• The study provides a new perspective on early microbial ecosystems and their role in Earth’s oxygen transition.

Findings also inform the search for extraterrestrial life in Earth-like conditions.

Original Publication
Authors: Fatima Li-Hau, Mayuko Nakagawa, Takeshi Kakegawa, L.M. Ward, Yuichiro Ueno and Shawn Erin McGlynn.
Journal: Microbes and Environments
DOI: 10.1264/jsme2.ME24067
Method of Research: Observational study
Subject of Research: Not applicable
Article Title: Metabolic Potential and Microbial Diversity of Late Archean to Early Proterozoic Ocean Analog Hot Springs of Japan
Article Publication Date: 23-Jul-2025

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