Sunlight-free microbes may explain Earth's massive iron deposits

Iron is everywhere on Earth's surface—in soil, in rock, in the rust that blooms on forgotten metal. Yet for decades, geologists have puzzled over how the planet accumulated such staggering quantities of it, locked away in massive ore deposits that formed billions of years ago. The standard answer has been straightforward: ancient bacteria that fed on sunlight oxidized dissolved iron in the oceans, causing it to sink and pile up on the seafloor in thick, orderly layers. This theory held until researchers confronted a problem it could not solve.

During a period called Snowball Earth, roughly 700 million years ago, the planet froze nearly solid—ice sheets hundreds of meters thick covered the oceans. In such darkness and cold, photosynthetic bacteria could not have survived. Yet iron deposits from that era exist, layered and vast, as if life had continued its work beneath the ice. A team of Korean scientists has now proposed an answer: microbes that needed no sunlight at all.

In 2013, an icebreaker called the Araon drilled into the seafloor near Antarctica's Larsen C Ice Shelf and pulled up a sediment core 2.4 meters long from a depth of 324 meters. The researchers, led by Jihyun F. Kim at Yonsei University and including colleagues from Sungkyunkwan University, Seoul National University, and other institutions, extracted DNA from the ancient mud. What they found was a bacterium unlike any previously linked to iron formation—a chemolithotrophic organism that oxidized iron not through photosynthesis but through a chemical process that required only the iron itself and the right enzymes.

The team named this microbe Candidatus Mariimononas ferrooxydans. Within its genome lay a gene encoding a protein called Cyc2, a fusion protein embedded in the bacterium's outer membrane. To test whether this protein actually oxidized iron, the researchers cloned the gene into laboratory E. coli and watched what happened. Iron in its dissolved form, iron(II), was rapidly converted to iron(III)—a form so insoluble in water that it precipitates out and sinks. Over millions of years, such precipitation could build the massive deposits geologists observe in the rock record.

What made this discovery particularly striking was where the bacteria lived. The Antarctic sediments where they were found receive no sunlight and contain almost no oxygen—conditions that would have been common in ice-covered oceans during Snowball Earth. The researchers also noticed something else: in the sediment layers, two distinct microbial groups alternated in dominance, creating a cyclical pattern. This biological rhythm might explain the banded structure of ancient iron formations, those striking layers of iron and silica that have puzzled geologists for generations.

The implications reach far beyond Antarctica. If chemolithotrophic bacteria could oxidize iron in sunless, oxygen-poor environments, they might have done so throughout Earth's deep past—not just during Snowball Earth, but in the Archean and early Proterozoic eons, long before photosynthesis itself evolved. This suggests a metabolic pathway so fundamental, so independent of the sun's energy, that it may have shaped the planet's chemistry for billions of years.

The research represents a methodological shift as well. Rather than simply sequencing DNA from ancient sediments, the team reconstructed entire genomes and then validated their findings through experiment—proving that the genes they identified actually produced proteins that did what the theory predicted. Yoon, the first author of the study, called this approach a major achievement: connecting genome reconstruction with functional validation to solve puzzles that sequence data alone could not answer.

The discovery does not overturn the photosynthesis model entirely. Sunlight-powered bacteria likely did contribute to iron formation once they emerged. But it expands the picture considerably, suggesting that Earth's iron deposits are the work of multiple metabolic strategies operating across different eras and environments. For fields ranging from geology to microbiology to biogeochemistry, the implications are still unfolding—a reminder that the story of how Earth's surface formed is more complex, and more microbial, than previously understood.