Thanks to Brilliant for supporting this episode of SciShow.

Go to Brilliant.org/SciShow to learn how you can take your STEM skills to the next level this year. [♪ INTRO]. Most of us don’t think of algae as being particularly aggressive.

I mean, that’s the fuzzy green stuff that accumulates in your fish tank, right? Not exactly scary. But sixty-six million years ago, marine algae were suddenly forced to learn how to live by their wits.

Which is how these innocent photosynthetic organisms turned... to murder. Or at least, to a weird, 50/50 way of getting energy that we almost never see. You see, way back in the day all these marine algae were happily floating around in the ocean, just minding their own business and making their own food.

No need to swim when there’s plenty of sun. See, like plants and some microbes, these algae were autotrophs: organisms that produce their own energy from what they have at hand. In this case, they did it through photosynthesis -- using light to convert water and carbon dioxide into food.

Everything was fine for these tiny marine algae, until an asteroid came along. Actually, the asteroid. The one that killed most of the dinosaurs.

But they weren’t the only ones who suffered during the aftermath of that impact. It was everyone, from the biggest T. rex all the way down to these tiny, unimposing ocean-dwellers. After the collision, the sun disappeared behind a cloud of dust and debris.

And it stayed dark for a long time -- some scientists think as many as two years. A lot of marine organisms, down to the algae, couldn’t cope with the lack of light and ended up dying out. And that’s left scientists to wonder how marine ecosystems survived at all, since those algae are pretty fundamental to the oceanic food web.

Some oceanic algae did manage to hang on. But to do that they had to find another source of food. And when you no longer have the ingredients to make your own food, well, sometimes the best alternative is the guy who lives next door.

That’s right: some of these once-gentle marine algae adapted to their new circumstances by learning how to kill. The opposite of an autotroph is a heterotroph — an organism that gets its energy from other life forms, rather than making its own food. And the fossil record contains evidence that marine algae — many belonging to a group called the coccolithophores — suddenly became mobile.

In other words, they no longer had to float around. They could swim. They also became predators.

Researchers can tell because the fossils include tiny structures called flagella and haptonema. Flagella are tentacle-like appendages that helped the algae swim. And we’re not totally sure yet, but we think haptonema were used to capture food.

Basically, it meant they were able to dart around and catch things -- something you see in predatory organisms, not floating photosynthesizers. Algae with these features suddenly became more dominant in the fossil record during the time that coincided with the extinction event 66 million years ago. And these predatory algae persisted long after the skies cleared, for the next million years.

But here’s the really interesting thing: in general, predatory marine algae didn’t entirely throw photosynthesis out the window. Many of them have gone extinct in the millions of years that have passed, but there are still plenty of marine algae that can hunt and photosynthesize. With coccolithophores, we’re not sure if they ever retired from predation and returned to photosynthesis.

They might still do both. We often think that organisms are either heterotrophs or autotrophs, but these algae don’t care about our boundaries — they’re mixotrophs, meaning they’ve got one foot in each world. Or they would, if they had feet.

Which reminds us not to think of the natural world in black and white terms, because it’s just going to make us look silly when it proves us wrong. That’s all the more reason to learn about how the world really works -- and a course from Brilliant can help. Brilliant is an online learning platform with courses about science, engineering, computer science and math.

Like their course on Special Relativity, which will get you up to speed on your Einstein -- light speed, that is. And the first 200 people to sign up at Brilliant.org/SciShow will get 20% off an annual Premium subscription -- so if you’re interested, check out the link in the description to learn more. [♪ OUTRO].