In the northeast Atlantic Ocean, plankton populations aren’t looking like they used to.

These tiny marine organisms have included an array of diatoms and dinoflagellates and copepods, riding the currents to food and survival. But as ocean waters have gotten warmer, the populations of copepods and krill have declined in the summer.

And at the center of it all are tiny, photosynthetic bacteria called picocyanobacteria who may just outlast us all. Cyanobacteria come in many different shapes and sizes. Some are simple, some are long, and some are just odd to look at.

And if we needed just one word to define picocyanobacteria, we would probably go with “small.” They’re generally 0.2 to around 2 micrometers in length, which makes them small even by bacterial standards. Piled together like this, they look like those optical illusion paintings you’re supposed to squint at to find the hidden image. But don’t waste your time pushing your face closer and closer to the screen.

There is no picture hiding amongst these bacteria…. Well, or is there? There are two genera of picocyanobacteria: Synechococcus and Prochlorococcus.

And together, they kind of rule the world. At the very least, they are considered to be the most abundant photosynthetic organisms on our planet. The abundance of picocyanobacteria is matched by just how geographically widespread they are.

That's made them fascinating for scientists who want to understand how they managed to spread so far and how their prevalence has shaped our planet. Assembling the history of picocyanobacteria is a complex undertaking, but it has some delightful theories. Like recently, scientists came up with one potential explanation for how picocyanobacteria were able to spread to new parts of the ocean hundreds or millions of years ago.

I'm going to bet that you can’t guess how that happened… unless you watched the video our sister channel Eons did on this topic recently. Here’s what you need to know first. For a long time, picocyanobacteria were thought to be autotrophic, meaning they could only get their food by making it— in their case, through photosynthesis.

But it turns out, they’re mixotrophs, able to take in elements like carbon through other sources— including, it seems, a material found in arthropod exoskeletons called chitin. And scientists looking at the evolution of picocyanobacteria came to an interesting conclusion: that the picocyanobacteria might not just have been eating chitin from ancient bug-like creatures. They might have actually been setting sail on them, cruising around on an edible boat made of discarded body parts as they began their slow but immense takeover of the ocean… an expansion that would see them become responsible today for around 25% of all photosynthesis in the ocean.

This chitin raft hypothesis provides one possible story for how picocyanobacteria spread around the world. But Prochlorococcus and Synechococcus did not accomplish what they have accomplished just by spreading. They also had to adapt.

At any given point in time, their survival is linked to a myriad of factors: the availability of nutrients, the amount of light they’re getting, the temperature of the water, and the fact that every organism around them is also attempting to survive the same conditions. Adaptation isn’t a challenge unique to picocyanobacteria after all. But they are very, very good at it.

They’re so good that it’s difficult to summarize their abilities because they have adapted to so many different conditions in so many different ways. So let’s just focus on one example. Synechococcus are found in warm and cold waters alike, which is challenging for organisms like picocyanobacteria because photosynthesis is a very temperature-sensitive process.

And yet Synechococcus have managed to be found in waters as cold as 2°C by altering the behavior of their phycobilisome, a protein complex that acts like an antenna to harvest the sunlight. Part of the job of the phycobilisome is to regulate the amount of light that the cell receives. It makes sure that the cell gets enough for photosynthesis, but not so much that the organism will accumulate a dangerous amount of reactive oxygen species as byproducts.

One of the way that organisms like Synechococcus limit the amount of light transmitted by the phycobilisome is through a molecule called orange carotenoid pigment, which turns excess light into heat. The orange carotenoid protein isn’t limited to picocyanobacteria. And in fact, not all Synechococcus species produce it.

Many of the warmer-weather species don’t even have the gene for it. But for those species living in colder waters, the protein is part of their survival, allowing them to adapt to a niche that would otherwise be unwelcoming. So we saw one way that picocyanobacteria have spread, and one way that they have adapted.

But that isn't insufficient to describe their history. Because not only do they travel and adjust. They also compete, putting their adaptations to use quickly when conditions change around them with consequences for the species that share their home.

Synechococcus can rapidly allocate resources towards photosynthesis as waters warm, allowing them to potentially outcompete other organisms. So as climate change continues to see the warming of oceans, perhaps picocyanobacteria will continue their dominance. In fact, we’re seeing it already— in the krill and copepod populations that have declined over the years in the northeast Atlantic Ocean, where rising temperatures have been affecting the ecosystems in a number of ways… including through picocyanobacteria.

Because along with their talent for harvesting light, one of other advantages that picocyanobacteria is their small size. It makes them able to bring in so many more nutrients relative to their tiny bodies compared to other plankton. And in turn that helps them reproduce and survive, especially as temperatures change and affect those nutrient levels.

That’s great news for picocyanobacteria, and terrible news for copepods. Because those other plankton— the larger ones that struggle to get as many nutrients— those are what the copepods like to eat, not picocyanobacteria, which are just too small and lacking in nutrients. So as the population of non-picocyanobacteria plankton goes down, so too does the population of copepods and krill.

And this in turn affects us in a lot of ways, through the basic biogeochemical cycles that serve as the foundation for our planet to the fishing industries that provide people’s livelihoods, and food. And scientists predict that the global population of Synechococcus will go up around 14% by the end of this century. Does that mean picocyanobacteria will win?

Who knows. There are other organisms, and not to mention entire ecosystems that surround the picocyanobacteria and that shape their success. And besides, what does it mean to win, in a world where the demands of survival are always in flux?

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And remember, if you’d like to learn about picocyanobacteria and their chitin raft adventures, you can head over to youtube.com/eons to see their episode about it! The folks on the screen right now, they are our Patreon patrons.

They’re a bunch of people who individually could not make this show happen, but when they all work together, like maybe a bunch of bacteria in the ocean doing 25% of the world's photosynthesis, they can make really amazing and big things happen, like the existence of this entire channel and its entire multi-year history. Thank you so much to all of them. If you would like to become a Patreon patron, you can go to Patreon.com/JourneytoMicro.

If you want to see more from our Master of Microscopes, James Weiss, you can check out Jam and Germs on Instagram. And if you want to see more from us, there's always a subscribe button somewhere nearby.