Guns, Germs, and Guano

mjs · June 6, 2020

What do caves, microbes, and bat poop have in common? Guns.

I promise this isn’t the setup to a bad “So a microbe walks into a cave…” joke.

Not only is Mammoth Cave the largest cave system on Earth, it has also long been a productive source of saltpeter, a major component of gunpowder. In fact, over 1,800 tons of cave sediments were mined for this explosive oxidizer in Mammoth during the War of 1812 alone!

An interesting tidbit, but what do microbes and bat poop (guano) have to do with that? Well, saltpeter is a nitrate-based salt; in other words, it contains an oxidized form of nitrogen, a crucial element for life. Nitrogen is often a limiting element in low-nutrient systems like caves, so it is surprising that there is enough of it in Mammoth Cave to have sustained large-scale saltpeter mining for decades. Where could all this nitrogen come from?

Bats roosting in caves need to “drop the pups off at the pool,” so to speak, regularly spritzing the cave floors with fresh droppings. Guano, as it turns out, is loaded with nitrogen. One school of thought is that the gobs of saltpeter found in Mammoth Cave are the result of degraded guano. In this view, blackpowder rifles couldn’t fire without guano - no guano, no gunpowder, no victory! Did America win the Battle of New Orleans because of bat poop? Maybe, but there is a problem with this bat s@#^-crazy theory: many saltpeter deposits in Mammoth Cave are found deeper than bats tend to travel.

That’s where the microbes come in. Back in 1977, scientists proposed that certain bacteria living in the cave, like Nitrobacter, are responsible for forming saltpeter. These “nitrifying” bacteria get their energy by oxidizing more reduced forms of nitrogen to nitrate, which then goes on to react with calcium in the sediments to form saltpeter. Those scientists noted that there were a lot more Nitrobacter cells in cave sediments compared to the soils above Mammoth Cave. They also observed that nitrate levels in the sediments would return to normal levels after they extracted it, implying that the Nitrobacter are actively producing nitrate.

So, gunpowder exists because of bacteria, not bats. Case closed, right? Well, not so fast. We still don’t know where the reduced forms of nitrogen come from in caves like Mammoth. They could still ultimately come from guano, with cave microbes involved in cycling the nitrogen through different forms. Nitrogen could also get washed in from the surface or surrounding rocks, entering the cave through water drips or underground rivers. Cave microbes themselves could also be involved in fixing atmospheric nitrogen into other forms that are usable by other life! Long story short, we are still a long way from understanding where all this cave nitrogen originates and where it eventually ends up. That’s why I think it is so important for the next generation of scientists to get their hands dirty and study cave microbes - it may not be as crappy of a job as you might think!

You can read more about the sometimes strange interactions between Mammoth Cave microbes and their environment in a MicrobeWiki page I authored for a microbial ecology class I recently took.

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