In
hot spring in Yellowstone National Park, a microorganism does something that shouldn't be possible for living things: it breathes oxygen and sulfur at the same time.
With a deep breath, the airflow rushes into the lungs, where oxygen enters the bloodstream, fueling metabolic processes in cells throughout the body. Humans, as aerobic organisms, use oxygen as a cellular spark that releases molecular energy from the food they eat.
But not all organisms on the planet live or breathe this way. Instead of using oxygen for energy, many single-celled life forms living in environments far from the reach of oxygen, such as deep-sea hydrothermal vents, use other elements to breathe and release energy.
This physical separation of oxygen-rich and oxygen-deprived worlds is not simply a matter of utilizing available resources; it is a biochemical necessity. Oxygen does not combine with metabolic pathways that allow breathing using other elements such as sulfur or manganese.
It gives life to aerobes such as humans, but for many anaerobes, or creatures that breathe without oxygen, oxygen is a toxin that reacts with and damages their specialized molecular mechanisms.
"Oxygen - we love it, of course," says Courtney Steirs, an evolutionary biologist at Lund University in Sweden. "But it's actually a pretty harmful molecule to most life forms on our planet, and even to ourselves. We have ways to mitigate the negative effects of oxygen. So we can't imagine life without it, but actually living with it is quite hard."
For the first few billion years of life
on Earth, organisms completely avoided this problem. At that time, the air and oceans were virtually devoid of oxygen, so life was almost entirely anaerobic.
Then, about 2.7 billion years ago, the seas filled with hard-working photosynthesizing cyanobacteria. They invented a way to turn sunlight into carbohydrates and oxygen and thrived. Over hundreds of millions of years, their accumulated breath filled the atmosphere and oceans with oxygen.
This so-called Oxygen Catastrophe (the Great Oxidation) was a key transformation in the biosphere and the physical chemistry of Earth's atmosphere and oceans. In this new environment, aerobic respiration evolved to become the world's dominant respiration.
It remains a mystery to researchers how life moved from anaerobic to aerobic respiration; such diverse microorganisms must have adapted to a world filled with what was once a biochemical curse.
Researchers now have a new idea of what the transition might have looked like billions of years ago, derived from studying an organism living today.
The bacterium, which researchers collected from a hot spring cauldron in Yellowstone National Park, does something that life really shouldn't be able to do: it simultaneously performs aerobic and anaerobic metabolism - breathing both oxygen and sulfur.
The findings "remind us once again how much we have to learn about microbial diversity and metabolism," says Natalia Mrnjavac, a graduate student in evolutionary microbiology at Heinrich Heine University in Dusseldorf, Germany, who was not involved in the study. "And for those who love microbes, this is very exciting."
The findings, published in Nature Communications in early 2025, challenge assumptions about the limits of cellular respiration and may provide researchers with a model for understanding how life balances on the edge of heaven and hell.
Metabolic tricks
It has long been known that living organisms have evolved ways to alternate between aerobic and anaerobic respiration, for example, as a last resort when oxygen levels are low. But because oxygen disrupts anaerobic respiration, many researchers have hypothesized that cells cannot grow using both processes at the same time.
So when Eric Boyd, a microbiologist at Montana State University in Bozeman, and his colleagues found reports from the late 1990s and early 2000s suggesting that some bacteria could do just that, they were intrigued.
Photo: quantamagazine.org
Specifically, the bacteria were observed to produce sulfide, a product of anaerobic respiration, even in the presence of oxygen in the environment. "It's strange to read something like that because it contradicts textbooks - what you know about microbial metabolism," Boyd recalls.
Boyd is interested in how life evolves and persists in some of the most chemically and thermally hostile places on Earth. He and his team study the mix of hardy microbes that inhabit the junctions between the surface and the underworld, including the volcanic vents and thermal pools of Yellowstone National Park, near his university in Montana.
The strange microbe, which apparently used anaerobic respiration even in the presence of oxygen, was just his flavor. To learn more about it, Boyd and his team had to investigate the kinds of turbulent springs favored by such a microorganism, where volcanic bubbles mix with an oxygen-rich atmosphere and oxygen-free groundwater.
From a roadside thermal spring near Nymph Lake in the northwestern part of the park, they collected and isolated a strain called Hydrogenobacter RSW1. RSW1 seemed a natural candidate to investigate unusual respiration. This bacterium is widespread in volcanic hot springs around the world, from
Iceland to New Zealand, and can grow with very limited oxygen.
It also belongs to the same order Aquificales as the curious microbes from earlier reports. The researchers brought it to the laboratory to grow it and study its metabolism.
Team members went through a process of gradually identifying elements and molecules on which the bacterial strain could grow. They already knew it could use oxygen, so they tested other combinations in the lab.
In the absence of oxygen, RSW1 could process hydrogen and elemental sulfur - chemicals it could find in an erupting volcanic vent - and create hydrogen sulfide as a product.
However, while the cells in this state were technically alive, they were not growing or multiplying, but were producing a small amount of energy - just enough to stay alive, nothing more. "The cell was just sitting there spinning its wheels, getting no real benefit from it in terms of metabolism or biomass," Boyd says.
Photo: quantamagazine.org
The team then added oxygen back into the mixture. As expected, the bacteria grew faster. But to the researchers' surprise, RSW1 still produced hydrogen sulfide as if it were breathing anaerobically. In fact, the bacteria seemed to be breathing aerobically and anaerobically at the same time, capitalizing on the energy of both processes.
This double respiration was different from that described in previous reports: the cell not only produced sulfide in the presence of oxygen, but also performed both contradictory processes simultaneously. Bacteria simply shouldn't have been able to do this. "This made us wonder, 'What's really going on here?"," Boyd says.
Two ways of breathing
RSW1 appears to have a hybrid metabolism, simultaneously exercising a sulfur-based anaerobic mode and an oxygen-based aerobic mode.
"The ability of the organism to combine both of these metabolisms is unique," says Ranjani Murali, an environmental microbiologist at the University of Nevada, Las Vegas, who was not involved in the study. "Normally, when anaerobic organisms are exposed to oxygen, damaging molecules known as reactive oxygen species compounds create stress. The fact that this doesn't happen is really interesting."
Boyd's team observed that the bacteria grew best when they used both types of metabolism simultaneously. This may be an advantage in their unique environment: oxygen is not evenly distributed in hot springs such as those where RSW1 lives.
It has been observed that other microbes breathe in two ways at the same time: anaerobically using nitrate and aerobically using oxygen. But these processes use completely different chemical pathways, and when they are combined, they tend to require microbes to expend energy. In contrast, RSW1's hybrid metabolism, based on sulfur and oxygen, strengthens cells rather than weakening them.
This dual type of respiration may have gone unnoticed until now because it was thought to be impossible. "Oxygen and sulfide react quickly with each other; if you didn't observe sulfide as a byproduct, you might have missed it completely," Boyd says.
According to Murali, it is possible that microbes with dual metabolism are widespread. She pointed out the many habitats and organisms that exist with little transition between oxygen-rich and oxygen-deprived zones.
One example is submerged sediments in which cable bacteria can live. These elongated microbes are oriented so that one end of their body can use aerobic respiration in oxygenated water, while the other end is buried deep in oxygen-free sediments and uses anaerobic respiration.
Photo: quantamagazine.org
Cable bacteria thrive in their unstable state, physically separating their aerobic and anaerobic processes. But RSW1 seems to be performing several tasks at once, tumbling around in a bubbling spring.
It is still unknown how RSW1 bacteria manage to defend their anaerobic mechanisms against oxygen. Murali hypothesized that the cells may create chemical supercomplexes within themselves that can surround, isolate, and "absorb" oxygen, quickly consuming it as soon as it is encountered so that the gas does not interfere with sulfur-based respiration.
RSW1 and any other microbes with dual metabolism are interesting models of how microbial life may have evolved during the Great Oxidation.
"It must have been a pretty chaotic time for microbes on the planet," Boyd says.
As oxygen slowly made its way into the atmosphere and the sea, any life form that could handle the occasional contact with the new poisonous gas - or even use it to its energetic advantage - might have found itself in an advantageous position. In that transitional period, two metabolisms may have been better than one.