A Scorching Symbiosis

One special type of bacteria could be the key to dragonfire.

A Scorching Symbiosis

One special type of bacteria could be the key to dragonfire.

How do dragons breathe fire? The Skyflame from our previous article is a very well-engineered beast. It collects methane from the environment — or from captive cows, as the case may be — and synthesises it into methanol for better storage. To conserve space, that methanol is stored under pressure using special graphene-strengthened muscles. An extra fuel-tank for carbon dioxide serves to set its breath alight, and given enough time to topup it can rival the X15 flamethrower.

If we’re trying for a biologically feasible dragon, you could say that problem is now, to some degree, solved. But there are still some problems to address.

Unlike other Snipette pieces, this one involves quite a bit of math and biochemical equations. If you want to skip these sections, feel free to use the "math-free" toggle below or watch out for icons on the sides of the paragraphs:

This is the second of a five-part series on the evolution and biology of (hypothetical) dragons. Each part is a self-contained unit, so you could either read them all or simply dive in where you like.

Methanol burns slowly, steadily, and with a faint blue colour. Not quite what you’d expect from a dragon. What’s more, our dragon would need to constantly exert force on its gas sac using its highly efficient muscles, possibly to the point of exhaustion. Finally, we don’t know of any organisms, besides humans, that can produce graphene — an essential material for the dragon’s methanol production — from scratch. It looks like we’ll need another fuel source, and thankfully we might have one: acetic acid.

Acetic acid is a naturally occurring volatile liquid. Acetic acid releases about 14.6 megajoules of energy per kilogram burned. So our acetic-acid dragon could output the same maximum firepower as our methanol-spewing dragon using 31.6 kilograms of acetic acid. The space it would take up in our dragon’s body is about 30.1 litres—a little under half the size of a human. An acetic acid fuel sac would be a little bigger and heavier than a methanol fuel sac, but it would eliminate the need for extraordinary muscles.

Acetic acid is a naturally occurring volatile liquid. A dragon fuelled by acetic acid would only need about 30 litres of space to store its 31.6 kilograms of acetic acid. An acetic acid fuel sac would be a little bigger and heavier than a methanol fuel sac, but it would eliminate the need for extraordinary muscles.

This dragon would need to be very careful not to use too much acetic acid at once, seeing as acetic acid is fairly explosive. However, the increased ease with which it could produce fuel may be worth it — not to mention the fact that it wouldn’t have to devote energy to compressing its fuel constantly.

Acetic acid also has a low flash point. A flash point is the temperature at which a flammable compound in the presence of a catalyst decomposes to form a mixture of flammable gases. For acetic acid, this mixture is composed of ketene and our old friend methane. A dragon could ignite these gases and use their heat to ignite the acetic acid itself. However, the decomposition of acetic acid also creates carbon dioxide and water vapour. These gases are problematic because they take up space and dilute oxygen enough that the methane and ketene can’t burn. To make things worse, what little gas does burn produces even more carbon dioxide and water vapour.

To solve this dilemma, our dragon could rapidly exhale oxygen stored in its body. This flow of oxygen would drive away the fire-suppressing gases while also allowing the flammable gases to burn brighter. The dragon would also need to produce some sort of catalyst to help the acid decompose, likely an enzyme.

Now, above its flash point, the acetic acid could produce flames, but we still need an initial spark. Our dragon could use a natural flint in its throat or mouth to spark the mix of methane and ketene. Then, it could ignite the acetic acid by shooting it through the burning mixture.

Body temperature is an important factor to consider here. As stated in the last installment, a reasonable dragon body temperature would be 38°C (100°F). The flash point of acetic acid is a comfortable 39°C (102°F). That’s just 1°C above the temperature of our dragon’s body. A dragon could easily heat itself to this temperature...if it were warm-blooded.

Body temperature is an important factor to consider here. It just so happens that the flash point of acetic acid is 39°C (102°F), which is just 1°C above the body temperature we used for our last two dragons. A dragon could easily heat itself to this temperature…if it were warm-blooded.

Warm-blooded animals, also known as endotherms, can regulate their body temperatures without relying on external sources of heat. Most mammals are warm-blooded and most reptiles aren’t, which is why lizards and snakes have to bask in the sun to warm themselves up. Right now you might be thinking: “Hang on. Aren’t dragons reptiles, too?”

We based our dragons off pterosaurs, which were indeed reptiles. However, many scientists think pterosaurs were actually warm-blooded. If they were, then by the same evolutionary logic, dragons could also be warm-blooded.

Warm-blooded animals can suffer health consequences if their body temperatures fluctuate too much, so our dragon would likely keep its body temperature just below 39°C and only increase it when attacking. It would do this by increasing its metabolic rate.

Now, where would our dragon get all this acetic acid in the first place?

Remember how I said that acetic acid is naturally occurring? It’s created by acetic acid bacteria when they oxidize ethanol. These microbes are quite widespread. You can find them within flowers, sugary foods, and alcoholic beverages.

Our dragon could make its body a breeding ground for acetic acid bacteria by maintaining a diet of sugary, acidic, and fermented foods. It could also prey on animals that ate a lot of these foods. It would then enter into a mutualistic relationship with the bacteria, where they produce the acetic acid necessary for its firebreath while it provides nutrients for them within its body.

Because its diet would include flowers, I say we dub this dragon the Acidic Wildflower.

In addition to creating fuel for its firebreath, the Wildflower’s relationship with acetic acid bacteria could protect it from the chemical burns acetic acid typically causes. Acetic acid bacteria have a natural resistance to the acid they produce, and it’s possible the cells layering the Wildflower’s mouth, throat, and acetic acid chamber could inherit that resistance through a process called horizontal gene transfer.

Horizontal gene transfer is when one organism receives DNA or RNA from another organism that isn’t one of its ancestors. Scientists have found that horizontal gene transfer between eukaryotes, like animals, and bacteria is actually common, and it might be a major contributor to the evolution of eukaryotic species.

So a mutualistic relationship with acetic acid bacteria could be the bread and butter of an efficient firebreather. It could even  protect said firebreather from the corrosiveness of its own fuel. The idea of acid-proofing leads us to another important thing our dragon needs protection from: its own fire.

What’s next? This is the third of a five-part series on dragons. Watch out for the next instalment on Thursday, where we’ll talk all about organic fireproofing and how dragons could safely use their fire to hunt.