There’s something wrong with mitochondria.

These tiny powerhouses of the cell – little membrane-bound organelles – are responsible for producing adenosine triphosphate (ATP), which transports chemical energy to and fro. Without them, all life with complex cells would grind to a halt, so it’s vital we understand as much about them as possible.

Clearly, we’ve got a long way to go: A brand-new PLOS Biology study has revealed that our mitochondria are, well, surprisingly hot – at 50°C, they’re hotter than Death Valley in the height of summer.

A pre-print published on bioRxiv back in May of last year caused quite a buzz in the biology world when it was released, and judging by the accompanying cautious editorials, its official publication is set to do the same again.

Dr Ben Libberton, a microbiologist at the MAX IV Laboratory in Lund, Sweden, who was not involved in the paper, told IFLScience that this paper is “very controversial,” explaining that it could upend what we understand about mitochondria – a story of cohabitation more than a billion years in the making.

“This study could be game-changing, but it’s not easy to say for sure just now.”

Our body temperature clocks in at around 37.5°C (99.5°F), and it has to stay within a narrow range – from around 35°C (95°F) to 40°C (104°F) – for us to remain healthy.

Our body’s ability to keep to around 37°C is down to homeostasis, a system of thermoregulation built into our biology. Much of our internal heat happens to come from our mitochondria. When they convert ATP into energy, a “significant fraction [about 60 percent] of the released energy is dissipated as heat,” the authors of the new study explain.

The team, led by the France-based research group INSERM, was comprised of five international research groups, from Finland to Lebanon, Korea to Germany. They were all deeply curious as to where all this excess heat goes and, perhaps more importantly, how hot the mitochondria get during this somewhat inefficient conversion process.

In order to complete this curious quest, the team had to get creative.

First, in a laboratory, they grew human skin and embryonic kidney cells, making sure to keep them at or very close to the ideal internal body temperature. Then, using a temperature-sensitive fluorescent dye, they painted the mitochondria with incredible precision.

They found that their temperature was always 7°C (12.6°F) to 12°C (21.6°F) higher than the ambient temperature, or about 10°C (18°F) higher on average. Although shocking, the team were forced to conclude that “mitochondria are physiologically maintained at close to 50°C (122°F)” in almost all warm-blooded animals. Indeed, it appears that the enzymes they use have specifically evolved to work best at that extreme temperature, at least in humans.

Co-author Dr Pierre Rustin, a mitochondrial expert at INSERM, told IFLScience that the team reacted with “surprise, followed by doubt” upon making their discovery. “It took us more than two years of continuous work to convince ourselves and to make our results public in a scientific journal.”

Rustin added that “although we’re perfectly confident in our data, we’re very cautious ourselves in their interpretation,” explaining that independent methods that could confirm their findings would be ideal, but at present “do not exist, unfortunately.”

This isn’t just the discovery of a number; it potentially revolutionizes how we understand how cells operate. After all, the temperature differences between the mitochondria and the rest of the cell “can accelerate or slow down all kinds of chemical reactions taking place in the cells of the body.”

They aren’t alone in their finding. Back in February of 2017, a separate research team found that the mitochondria in human cancer cells can be around 9°C (48.2°F) hotter than the rest of the cell.

Regardless, this paper caused a bit of a stir in the community. An accompanying editorial in PLOS One starts by noting that “self-respecting scientists might be inclined to back away from the claims made by Chrétien and colleagues in PLOS Biology this week.”

Describing it as a “radical claim,” it wonders why such a fundamentally odd characteristic has evaded identification for so long. Despite being somewhat concerned at the temperature sensitivity of the fluorescent dye, the editorial concludes that this study challenges key beliefs, and brings “this important subject back to centre stage, which is exactly where it should be.”