Thanks to the Monterey Bay Aquarium for partnering with us on this episode of SciShow.

They hope you have a plank-ton of fun watching! [♪ INTRO]. Right now, there are incredible creatures in the ocean with the power to reshape the planet's atmosphere.

After all, they've done it before. And you've probably never even seen them, unless you've looked at seawater under a microscope. I'm talking, of course, about phytoplankton, the microscopic critters that almost all life in the oceans (and arguably, on land) depends upon.

Phytoplankton is a catch-all category for all the tiny aquatic organisms that can turn sunlight into food via photosynthesis. And there are all kinds of different organisms included in it, from cyanobacteria to whip-tailed dinoflagellates to armored diatoms—which look amazing under a microscope, by the way. Diatoms are essentially algae that build little houses of glass —or, silica, to be precise.

And they come in a stunning diversity of forms. In fact, if you've ever spread diatomaceous earth around your garden or your house, you were sprinkling fossilized diatoms! There are also coccolithophores, which can look kind of like a soccer ball covered in Frisbees thanks to their teeny-tiny shells made of calcium carbonate.

In fact, England's famous White Cliffs of Dover contain the fossilized remains of countless coccolithophores. And in today's ocean, the smelly compounds they produce help give the sea its unmistakable scent—a smell birds like albatrosses use to find their meals. The compounds that make those smells also can travel up high into the atmosphere to help clouds form and grow, making them more reflective and helping to keep us down here a lot cooler.

And dinoflagellates are also very cool - don't want to leave them out. They have two tail-like flagella that help them swim around, and many can produce their own light— which makes for some really magical effects. But, too many of them in one place can create a harmful algal bloom —a phenomenon you might have heard referred to as “red tide”.

Which is just one example of how these super teeny creatures can have a big impact on the rest of us—even us landlubbers. No matter what form phytoplankton take, they are unbelievably important to all life on Earth. First off, they're the foundation of the marine food web, underpinning everything from anchovies to whales.

In fact, with a wee bit of help from seaweed and aquatic plants, they manage to support life in the ocean despite being outweighed by roughly five to one by organisms that don't make their own food. And if that's not enough, they're also atmosphere-altering machines. They consume about as much carbon dioxide as all of the world's forests and other plants combined, pulling roughly eleven giga-tons of carbon from our atmosphere every year.

Which is especially impressive when you consider that by mass, there's about 450 times as much plant life on land. Plus, phytoplankton account for more than half of the planet's yearly oxygen production. But even more importantly, they're kind of the reason we have oxygen in the atmosphere at all.

Back about 2.5 billion years ago, near the boundary between the Archean and Proterozoic eons, the Earth was a very different place. There were green seas and orange skies. And the atmosphere was likely a mix of gases like ammonia and methane that would immediately kill us.

Then, cyanobacteria evolved and ran rampant, pumping out a huge amount of oxygen in what's now called. The Great Oxidation Event. All that oxygen helped turn the ocean and skies blue, and likely jump-started the evolution of complex life.

Then, around 540 million years ago, phytoplankton did it again, exploding in numbers and bringing us up to more or less the levels of oxygen in the atmosphere today. Paleontologists think the boom may have occurred thanks to nutrients weathered out of rocks by giant, continent-sized glaciers. Or, a more recent idea is that it happened because the plankton evolved a more efficient internal machinery related to photosynthesis.

Either way, this set the stage for life to fully spread throughout the ocean and onto the land. Since then, they've calmed down on the “reshaping the entire planet” front and adopted more of a stabilizing role. For example, paleontologists think the evolution of calcifying plankton balanced ocean chemistry and helped protect the seas from mass extinctions.

Before about 80 million years ago, the ocean was not a super stable place to be. Changes in the chemistry of the atmosphere from events like volcanic eruptions often caused deadly swings in seawater pH. Then, some lineages of plankton—including those ball-like coccolithophores—figured out how to make shells of calcium carbonate.

Soon, they took over the open ocean. And over a relatively short period of time, so many of their shells sank to the depths of the oceans that they created a vast, thick layer of calcium carbonate ooze on the seafloor. That ooze acts as a chemical buffer in the ocean —a natural braking system on runaway acidity.

See, the shells are solid at a more neutral pH, but as the water gets more acidic, they start to dissolve, releasing carbonate ions. Those chemically react with the surrounding water to push an acidic pH back towards neutral. Now, there had been a minor version of this in the shallows thanks to mollusks and other large shelled organisms.

But it took phytoplankton to make it ocean-wide. And the end result was more stable ocean chemistry, which meant marine life became less vulnerable to things like volcanic eruptions. Even today, this chalk layer is helping buffer the acidifying effects of all the carbon dioxide that we humans are pumping into the atmosphere—though, there's only so much it can do.

When we consider everything that lives in the ocean, from majestic whales to bountiful coral reefs, these little, unassuming phytoplankton are easy to overlook. But if you look at the history of the Earth, it doesn't take long to realize that though they are small, they are also mighty. And they deserve as much fascination, love, and protection as any other living thing on this planet.

Thank you again to the Monterey Bay Aquarium for partnering with us on this episode of SciShow. The Aquarium's mission is to inspire conservation of the ocean. So give them a follow on their social media accounts (they're very good at it), and visit their website at montereybayaquarium.org, or if you are nearby, swing on by - it is a beautiful and amazing place.

They're there to kelp, and hope to sea you soon. [♪ OUTRO].