A pair of new papers outlines unique microbes that have evolved to harvest energy from iron in the same way that humans do with oxygen, suggesting that oxygen might not be as essential to biological life as we think.
The NASA-funded research examines tiny creatures that have existed in the cracks of the Earth for millions of years and could not only help scientists better understand their origins, but help them discover alien life on other planets.
The findings reveal the possibility that life on other planets might not require oxygen, expanding the number of potential locations that we have to explore in our search.
"These are fundamental studies, but these chemical transformations are at the heart of all kinds of environmental systems, related to soil, sediment, groundwater and waste water," said Eric Roden, senior author of the study and a professor of geoscience at the University of Wisconsin (UW), Madison.
Amazingly, the organisms not only evolved a complex system to gather energy from iron, they continue to use this system even today, where oxygen has dominated the atmosphere.
The unique bacteria "eat organic matter like we do," Roden said. "We pass electrons from organic matter to oxygen. Some of these bacteria use iron oxide as their electron acceptor. On the flip side, some other microbes receive electrons donated by other irons compounds. In both cases, the electron transfer is essential to their energy cycles."
The team obtained genetic data that revealed a unique capability of these bacteria: the transportation of electrons both ways across the cell's outer membrane.
"Bacteria have not only evolved a metabolism that opens niches to use iron as an energy, but these new electron transport mechanisms give them a way to use forms of iron that can't be brought inside the cell," said Shaomei He, a research scientist from UW Madison.
The results suggest that there could be other iron-metabolizing bacteria and alien life on planets like Mars that mirror those in the current study.
"A fundamental approach in astrobiology is to use terrestrial sites as analogs, where we look for insight into the possibilities on other worlds," Roden concluded. "Some people believe that use of iron oxide as an electron acceptor could have been the first, or one of the first, forms of respiration on Earth. And there's so much iron around on the rocky planets."
