Hot spring in Japan. Credit: Fatima Li-Hau, ELSI
New evidence suggests that early microbes living on Earth about 2.3 billion years ago didn’t use light from the sun, but rather iron and traces of oxygen, as their primary energy source.
A team from the Earth-Life Science Institute at the Institute of Science Tokyo studied iron-rich hot springs across Japan which have a similar chemistry to Earth’s ancient oceans.
The results help scientists understand how life on Earth may have evolved and may also assist in the search for life on other planets with low-oxygen atmospheres.
Currently, Earth’s atmosphere is made up of around 21% oxygen, but this was not always the case. The atmosphere in the planet’s early years contained approximately a million times lower oxygen levels than today.
However, around 2.3 billion years ago the atmosphere experienced a sudden rise in oxygen, likely due to cyanobacteria converting carbon dioxide into oxygen through photosynthesis.
Known as the Great Oxygenation Event (GOE), this completely changed the evolution of life on Earth with oxygen being a key ingredient for animals like humans.
However, ancient bacteria that existed before oxygenation would have had to adapt to this new atmosphere. How this adaptation occurred remains a mystery to scientists.
Sediment and rocks of one of 5 hot springs during low tide, showing iron oxide mineral precipitates. Credit: Fatima Li-Hau, ELSI
To get one step closer to answering this question, the research team studied 5 hot springs in Japan which are naturally rich in iron and low in oxygen like the earliest 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,” says the study’s supervisor, Shawn McGlynn.
“They help us understand how primitive microbial ecosystems may have been structured before the rise of plants, animals, or significant atmospheric oxygen.”
The researchers found that ‘microaerophilic iron-oxidising’ bacteria dominated 4 of the 5 hot springs. These use ferrous iron (Fe²⁺) as their main energy source, converting it into ferric iron (Fe³⁺). The team found these organisms thrive in low-oxygen conditions.
Oxygen producing cyanobacteria were only present in very small numbers. A full analysis of the results has been published in Microbes and Environments.
A hot spring during winter, showing the source water and CO2 bubbles. Credit: Fatima Li-Hau, ELSI
The team then analysed more than 200 microbial genomes and showed the microbes work together as an ecosystem to carry out essential biological processes like carbon and nitrogen cycling.
“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 Fatima Li-Hau, the study’s lead.
The researchers were surprised to find genes involved in sulphur oxidation, meaning the bacteria even have a partial sulphur cycle. They are hopeful that these results may have implications for the search for life on planets with similar geochemical conditions to ancient Earth.
“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,” says Li-Hau.
“By understanding modern analogue environments, we provide a detailed view of metabolic potentials and community composition relevant to early Earth’s conditions.”
AloJapan.com