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Life doesn’t always play by the tidy rules in textbooks. Most organisms use oxygen to produce ATP, which is energy used by cells. Some life forms, especially microbes, tap other chemicals when oxygen is scarce. The usual explanation says it’s one mode or the other.

A team studying a microbe from a Yellowstone hot spring found something different. This bacterium can use oxygen and sulfur at the same time to produce energy. That mixed strategy gives it an edge when oxygen levels fluctuate.

Lisa Keller of Montana State University is the lead author of this research that describes her work with bacterial samples from a group called Aquificales.

Along with her adviser and mentor Eric Boyd, professor in the College of Agriculture’s Department of Microbiology and Cell Biology, they published their fascinating work in the journal Nature Communications.

Microbes that breathe oxygen and sulfur

Respiration is how a cell converts food into usable energy (ATP). In oxygen-based respiration, cells move electrons through a chain of reactions and pass them to oxygen at the end.

Anaerobic respiration does a similar job but transfers electrons to other acceptors, such as sulfur, nitrate, or iron. Both strategies work; they are just different.

The hot-spring bacterium, Aquificales, challenged the usual either-or assumption. Under the right conditions, it kept both systems running.

That meant that while the bacteria were producing sulfide – an anaerobic process – they were using oxygen, meaning that both metabolisms were occurring.

“There’s no explanation other than that these cells are breathing oxygen at the same time that they are breathing elemental sulfur,” Keller said.

Keller explained that the bacterium’s ability to conduct both processes at once challenges our understanding of how microbes survive, especially in dynamic, low-oxygen environments such as hot springs. 

Oxygen and sulfur in hot springs

Hot springs are tough places to live. Temperatures run high. Minerals dissolve into the water. Gases bubble in and out.

Oxygen dissolves less in hot water than in cool water and escapes more easily, so levels change from moment to moment. In that kind of environment, a flexible energy strategy goes a long way.

The bacterium in this study thrives at high temperatures and feeds on simple molecules, including hydrogen gas. It can use oxygen when it’s available and elemental sulfur when oxygen dips.

How the study was done

Keller and her team isolated the microbe, then grew it in the lab at high temperatures with three ingredients: hydrogen gas as the energy source, elemental sulfur, and oxygen. They then tracked the cells’ chemical reactions and which genes were switched on.

Next, the team measured oxygen levels directly using gas chromatography. They also watched for the conversion of sulfur to sulfide, a clear sign of anaerobic sulfur respiration.

Gene expression data aligned with the chemistry: enzymes for both oxygen use and sulfur processing were active simultaneously.

Microbe’s oxygen-sulfur strategy

Cultures given hydrogen, sulfur, and oxygen grew faster and reached higher cell counts than cultures that had to use only oxygen or only sulfur.

That growth boost points to a simple payoff: more net energy when both pathways run together under low or unstable oxygen.

One detail matters for interpreting the results. The sulfide produced doesn’t persist in a mixed setup. Oxygen and certain metal ions in the broth can quickly consume it.

Without careful controls, that can hide the microbe’s dual strategy. This study accounted for that factor, which helps explain why this behavior may have been missed in past experiments.

Widespread pattern in nature

Genes and enzymes similar to those involved here are found in many microbes.

That suggests this hybrid mode could be more common than we realized, especially in places where conditions shift minute to minute. Hot springs and deep-sea vents contain fuels and oxidants that rise and fall.

Microbes that can keep multiple electron acceptors available may outgrow neighbors that wait for a single, ideal condition.

Flexibility like this also fits the story of early Earth. Oxygen didn’t flood the oceans all at once. It rose in patchy, inconsistent ways.

Microbes that could sense tiny amounts of oxygen while still relying on older, oxygen-free reactions likely had an advantage.

The results of this study may explain how ancient lifeforms adapted to the progressive oxygenation of Earth that began around 2.8 billion years ago – the Great Oxidation Event.

“This is really interesting, and it creates so many more questions,” Keller said. “We don’t know how widespread this is, but it opens the door for a lot of exploring.”

The Yellowstone bacterium isn’t ancient, but it shows a strategy that would have made sense when oxygen first began to matter.

How oxygen-sulfur combo works

Oxygen sits at the top of the energy ladder because it accepts electrons strongly, which usually means more energy per unit of fuel.

Sulfur compounds accept electrons too, though the energy yield is lower. When oxygen is scarce or fluctuating, using sulfur in parallel keeps the energy flowing rather than stalling.

Temperature and chemistry help set the stage. High heat speeds reactions and lowers oxygen solubility. Hydrogen gas, common around hydrothermal systems, supplies a steady stream of electrons.

Elemental sulfur is abundant in many volcanic and geothermal settings. Together, these conditions make simultaneous oxygen and sulfur respiration advantageous.

Real-world implications

Mixed respiration hints at new ways to run bioreactors and environmental cleanup efforts.

If microbes can be encouraged to keep more than one pathway active, engineers may squeeze extra efficiency from waste-to-energy systems, or keep pollutant breakdown steady when oxygen supply is uneven.

The same thinking applies to managing the sulfur and carbon cycles in complex settings where oxygen isn’t easy to control.

The work also urges careful experimental design. Testing a microbe in a strict “oxygen-only” or “no-oxygen” setup can miss behaviors that only appear when both are present.

Real environments rarely offer neat categories. Lab protocols that match those complex realities reveal strategies that would otherwise stay hidden.

To sum it all up, this heat-loving bacterium, Aquificales, broadens how we think about life’s energy playbook. It’s messy, adaptive, and full of clever workarounds that let microbes, and maybe eventually us, survive in a changing world.

The full study was published in the journal Nature Communications.

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Around 11,300 years ago, a massive star teetered on the precipice of annihilation. It pulsed with energy as it expelled its outer layers, shedding the material into space. Eventually it exploded as a supernova, and its remnant is one of the most studied supernova remnants (SNR). It’s called Cassiopeia A (Cas A) and new observations with the Chandra X-ray telescope are revealing more details about its demise.

Cas A’s progenitor star had between about 15 to 20 solar masses, though some estimates range as high as 30 solar masses. It was likely a red supergiant, though there’s debate about its nature and the path it followed to exploding as a supernova. Some astrophysicists think it may have been a Wolf-Rayet star.

In any case, it eventually exploded as a core-collapse supernova. Once it built up an iron core, the star could no longer support itself and exploded. The light from Cas A’s demise reached Earth around the 1660s.

There are no definitive records of observers seeing the supernova explosion in the sky, but astronomers have studied the Cas A SNR in great detail in modern times and across multiple wavelengths.

This is a composite false colour image of Cassiopeia A. It contains data from the Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray telescope. Image Credit: NASA/JPL-Caltech

New research in The Astrophysical Journal explains Chandra’s new findings. It’s titled “Inhomogeneous Stellar Mixing in the Final Hours before the Cassiopeia A Supernova.” The lead author is Toshiki Sato of Meiji University in Japan.

“It seems like each time we closely look at Chandra data of Cas A, we learn something new and exciting,” said lead author Sato in a press release. “Now we’ve taken that invaluable X-ray data, combined it with powerful computer models, and found something extraordinary.”

One of the problems with studying supernovae is that their eventual explosions are what trigger our observations. A detailed understanding of the final moments before a supernova explodes is difficult to obtain. “In recent years, theorists have paid much attention to the final interior processes within massive stars, as they can be essential for revealing neutrino-driven supernova mechanisms and other potential transients of massive star collapse,” the authors write in their paper. “However, it is challenging to observe directly the last hours of a massive star before explosion, since it is the supernova event that triggers the start of intense observational study.”

The lead up to the SN explosion of a massive star involves the nucleosynthesis of increasingly heavy elements deeper into its interior. The surface layer is hydrogen, then helium is next, then carbon and even heavier elements under the outer layers. Eventually, the star creates iron. But iron is a barrier to this process, because while lighter elements release energy when they fuse, iron requires more energy to undergo further fusion. The iron builds up in the core, and once the core reaches about 1.4 solar masses, there’s not enough outward pressure to prevent collapse. Gravity wins, the core collapses, and the star explodes.

This high-definition image from NASA’s James Webb Space Telescope’s NIRCam (Near-Infrared Camera) unveils intricate details of supernova remnant Cassiopeia A (Cas A), and shows the expanding shell of material slamming into the gas shed by the star before it exploded. Image Credit: NASA, ESA, CSA, STScI, Danny Milisavljevic (Purdue University), Ilse De Looze (UGhent), Tea Temim (Princeton University)

Chandra’s observations, combined with modelling, are giving astrophysicists a look inside the star during its final moments before collapse.

“Our research shows that just before the star in Cas A collapsed, part of an inner layer with large amounts of silicon traveled outwards and broke into a neighboring layer with lots of neon,” said co-author Kai Matsunaga of Kyoto University in Japan. “This is a violent event where the barrier between these two layers disappears.”

The results were two-fold. Silicon-rich material travelled outward, while neon-rich material travelled inward. This created inhomogeneous mixing of the elements, and small regions rich in silicon were found near small regions rich in neon.

Inhomogeneous elemental distribution in Cas A observed by Chandra. The difference in the mixing ratio of blue and green colors clearly shows the different composition in the O-rich ejecta. The red, green, and blue include emission within energy bands of 6.54–6.92 keV (Fe Heα), 1.76–1.94 keV (Si Heα), and 0.60–0.85 keV (O lines), respectively. The ejecta highlighted in red and green are products of explosive nucleosynthesis, while the ejecta in blue and emerald green reflect stellar nucleosynthesis. The circles in the small panels are O-rich regions used for spectral analysis. The regions of high and low X-ray intensity in the Si band are indicated by the magenta and cyan circles, respectively. Image Credit: Toshiki Sato et al 2025 ApJ 990 103

This is part of what the researchers call a ‘shell merger’. They say it’s the final phase of stellar activity. It’s an intense burning where the oxygen burning shell swallows the outer Carbon and Neon burning shell deep inside the star’s interior. This happens only moments before the star explodes as a supernova. “In the violent convective layer created by the shell merger, Ne, which is abundant in the stellar O-rich layer, is burned as it is pulled inward, and Si, which is synthesized inside, is transported outward,” the authors explain in their research.

This schematic shows the interior of a massive star in the process of a ‘shell merger.’ It shows both the downward plumes of Neon-rich material and the upward plumes of silicon-rich material. Image Credit: Toshiki Sato et al 2025 ApJ 990 103

The intermingled silicon-rich and neon-rich regions are evidence of this process. The authors explain that the the silicon and neon did not mix with the other elements either immediately before or immediately after the explosion. Though astrophysical models have predicted this, it’s never been observed before. “Our results provide the first observational evidence that the final stellar burning process rapidly alters the internal structure, leaving a pre-supernova asymmetry,” the researchers explain in their paper.

For decades, astrophysicists thought that SN explosions were symmetrical. Early observations supported the idea, and the basic idea behind core-collapse supernovae also supported symmetry. But this research changes the fundamental understanding of supernova explosions as asymmetrical. “The coexistence of compact ejecta regions in both the “O-/Ne-rich” and “O-/Si-rich” regimes implies that the merger did not fully homogenize the O-rich layer prior to collapse, leaving behind multiscale compositional inhomogeneities and asymmetric velocity fields,” the researchers write in their conclusion.

This asymmetry can also explain how the neutron stars left behind get their acceleration kick and lead to high-velocity neutron stars.

These final moments in a supernova’s life may also trigger the explosion itself, according to the authors. The turbulence created by the inner turmoil may have aided the star’s explosion.

“Perhaps the most important effect of this change in the star’s structure is that it may have helped trigger the explosion itself,” said co-author Hiroyuki Uchida also of Kyoto University. “Such final internal activity of a star may change its fate—whether it will shine as a supernova or not.”

“For a long time in the history of astronomy, it has been a dream to study the internal structure of stars,” the researchers write in their paper’s conclusion. This research has given astrophysicists a critical glimpse into a progenitor star’s final moments before explosion. “This moment not only has a significant impact on the fate of a star, but also creates a more asymmetric supernova explosion,” they conclude.