Photo of War in a water droplet

War in a water droplet

by Stephanie Orford News Integrated Microbial Biodiversity 13.04.2017

In oceans around the world, billions of microscopic soldiers are waging war using complex weapons that operate like harpoons and Gatling guns.

The soldiers are single-celled phytoplankton called Polykrikos kofoidii and Nematodinium. A team of researchers has just revealed the inner workings of these organisms in detail for the first time, showing microscopic weaponry and hunting techniques that are surprisingly intricate — and violent.

“You can tell when Polykrikos knows the prey is there because they sort of stiffen up and start swimming in circles, like a little shark, because they can smell the prey in the water,” says Greg Gavelis, lead author of the paper and now a post-doctoral researcher at Arizona State University.

When Polykrikos bumps into the prey it shoots its harpoon, a syringe-like tubule topped with a sharp stylet and attached to a long rope-like filament. The team believes this pierces the prey’s armour and injects venom “like a spider or a snake would.” Then Polykrikos tows the prey away to consume it. The whole process takes about 15 to 30 seconds.

Gavelis’ paper, published in Science Advances, was co-authored by IMB Program Director Patrick Keeling and Senior Fellow Brian Leander.

Nematodinium hunts in a similar way, launching 11 to 15 projectiles at once from cellular machinery that works in a similar way to a Gatling gun, shooting in rapid fire with  pressurized capsules.

Both single-celled hunters accomplish these feats without a brain. Their “guns” are specialized organelles that function together to carry out every step of the hunting process.

A series of images of the nematocysts in the dinoflagellate Nematodinium, showing details of how the harpoon-like nematocysts work. Image courtesy of Science Advances.

The entire process evolved under evolutionary pressure over millions of years. As its prey evolved armour to protect itself, Polykrikos evolved an armour-piercing harpoon. As its prey developed ways to swim away quickly, Polykrikos evolved a tow line to shackle it.

“It’s a clear arms race,” says Gavelis. “It’s a tit-for-tat, one-for-one escalation of adaptations that seems to have led to this really complicated microscopic weapon.”

Although researchers had seen these weapon organelles in 2D cross-section before, prior to this study they didn’t have a clear sense of how they worked. To get a clearer picture of what was happening, the team used focused ion beam scanning electron microscopy to create a 3D digital model of the cellular machinery.

“It was only through this 3D approach that we could see how everything fit together,” Gavelis says.

The team also captured high-resolution video of Polykrikos in action.

These dinoflagellate phytoplankton aren’t the only organisms to evolve harpoon-like weaponry. Jellyfish stingers contain similar harpoon-like structures.

But when the researchers compared the DNA of dinoflagellates, including Polykrikos, to that of jellyfish, they found that the harpoon structures hadn’t originated from a common ancestor. They had evolved independently.

“Nature has invented a harpoon gun at least twice, independently in jellyfish and again in dinoflagellates,” says Gavelis. “What it tells us is that, for whatever reason, microscopic ballistics are a really useful thing.”

Other organisms, including parasites and fungi, have evolved similar ballistic machinery with many different applications.

Similar structures could eventually be useful in medicine and engineering. For instance, medical nanotechnology might use microscopic, syringe-like structures to inject materials into individual cells.

“Single cells can do a lot more than we thought they could,” says Gavelis. “Understanding how they do that is going to be important to understanding how life works.”

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