If you’re in the northern hemisphere, it’s that time of year when you start to get cozy, pull those sweaters out of the closet, sip your pumpkin spice lattes, and stare out the window, waiting for the leaves to start turning orange.

Well, over at microcosmos.store we have the perfect companion for this time of year and that is our Journey To The Microcosmos sweatshirt. It’s soft, it’s cozy, and it confirms to those around you that you are in fact human.

And speaking of that pumpkin spice latte, we sell a lovely tardigrade mug that is perfect for your warm beverage of choice, or if you’ve got 2 billion tardigrades that you need to re-home. So, check out microcosmos.store, where you can find our sweatshirt, mug, and even our very own Microcosmos microscope. James, our master of microscopes, is always on the hunt for rare ciliates that live in areas low in oxygen.

But when he gathers those samples and puts them under a growth light, his tubes quickly turn the color of the green sulfur bacteria that thrive in those anaerobic conditions. These organisms are not particularly hard to come by, especially if you know the kinds of environments that meet their basic needs. But they can still manage to surprise us, like in 2005, when scientists reported that they had found green sulfur bacteria living in a place that seemed impossible for them to survive in: a hydrothermal vent.

Now it doesn’t sound particularly comfortable to me, but life around a hydrothermal vent is not impossible. These structures tend to form in places where tectonic plates are shifting away from each other, creating cracks that heated water from under the crust can then erupt through. And despite the incredible heat within the vent and the lack of light around it, there is life— an ecosystem built in part on bacteria that are able to turn chemicals from the vent into food.

These bacteria do not need sunlight to feed themselves and the organisms that surround them. But green sulfur bacteria are not like those chemosynthetic residents that we know to expect in hydrothermal vents. Green sulfur bacteria rely entirely on photosynthesis for their food needs, which means they need light.

Now to understand the secrets behind the green sulfur bacteria’s deep sea survival, let’s start by talking about what a green sulfur bacteria even is. These bacteria are known more formally as the family Chlorobiaceae, and their distinguishing feature is that they abhor oxygen. To power their photosynthesis, they rely on reduced sulfur instead, which means that green sulfur bacteria prefer to live in areas of water deep enough to favor sulfur over oxygen but still allow enough light for the bacteria to create their food.

Most green sulfur species cannot swim, which is why they’re so still in our footage. There are a few species that have vacuoles that can fill or release gas to help them move up and down the water column. But overall, they're not really movers.

And you also may have noticed that not all of the bacteria in our footage is green, and that some of them are in fact, purple. At times they’re quite a contrast to the green sulfur bacteria, not just because of their color, but because they are able to swim around. These purple neighbors do share one common trait with green sulfur bacteria though: they also use sulfur to carry out photosynthesis.

And so they are known as, wait for it, purple sulfur bacteria. We would call this an uncreative rip-off, but they were actually described about a century before their green counterparts. As a family, the purple sulfur bacteria are known as Chromatiaceae, which does feel a bit more distinct as far as names go.

While the chemical reactions underpinning their photosynthesis are not identical, purple and green sulfur bacteria have similar requirements for where they live to make that photosynthesis happen. They need light and they need sulfur. So it isn’t strange to find both organisms living on our slides together.

But that doesn’t do their proximity enough justice. These two bacteria don’t just tend to show up in the same areas of water. Green sulfur bacteria like to live under purple sulfur bacteria, providing sulfur to their purple upstairs neighbors.

In exchange, the purple sulfur bacteria shield the green sulfur bacteria from oxygen. But of course, life isn’t necessarily easy just because you’ve buried yourself away from anything vaguely threatening. The green sulfur bacteria still has to be able to survive underneath its Chromatiaceae comforter.

It helps that green sulfur bacteria have a larger affinity for hydrogen sulfide, ensuring they still get the chemicals they need to power their reactions. But they still need light. And for that, they rely on a powerful organelle called the chlorosome.

The chlorosome is unique to green sulfur bacteria, and its entire purpose is to help the bacteria get light. It’s the shape of an oval and stuck to the inside of the plasma membrane. And inside the chlorosome are around 100,000 of the green sulfur bacteria’s photosynthetic pigments.

Those pigment are compounds that aggregate together, creating the largest known light-harvesting complex in nature. And this arrangement is different from other familiar complexes, where pigment compounds are arranged with proteins, and in much smaller numbers. In the green sulfur bacteria, the interactions between these pigments make the bacteria very efficient at gathering light, and then transferring that energy to a protein called the Fenna-Matthews Olson protein, which will help carry the energy to the areas of the cell that need it for photosynthesis.

So that even when they’re in the dark, the green sulfur bacteria can get what they need to survive. Of course, sometimes your surroundings are dark because you’re covered in purple sulfur bacteria. And sometimes they are dark because you are in a hydrothermal vent.

And for the scientists behind the 2005 paper we mentioned at the beginning of the episode, the chlorosome would end up being a very useful organelle to know about. They had gone to the East Pacific Rise, gathering samples from plumes above the vents. And in their samples, they found a small, green, non-motile phototrophic sulfur bacteria.

It looked a lot like a green sulfur bacteria. To confirm that it actually was a species of green sulfur bacteria, they looked for that Fenna-Matthews Olson protein that transfers energy from the chlorosome to the photosynthesis reaction centers. And there it was: a protein unique to green sulfur bacteria, a part of its ingenious methods for harvesting sunlight.

But where was the sunlight? This is the weird part. The bacteria weren’t actually harvesting sunlight.

They were absorbing light from the hydrothermal vent itself. The scalding temperatures of the vent produces thermal radiation, also know as infrared light. And the green sulfur bacteria’s chlorosomes were able to make enough use of that light to carry out photosynthesis.

They weren’t, like thriving under these circumstances. But they were able to survive, and that is no small feat. This actually wasn’t the first time scientists had found photosynthetic bacteria in hydrothermal vents, but that previous find appeared to only be getting extra energy to supplement its primary chemosynthetic source of food.

This was the first time scientists had found a species entirely dependent on photosynthesis at a hydrothermal vents. A species whose family is thought to survive only in places with sun, and it had turned up deep in the ocean thanks to thousands of pigment molecules that could take in geothermal radiation. In their paper, the scientists were quick to caution that the discovery of green sulfur bacteria in the hydrothermal vents doesn’t necessarily tell us much about the evolution of life in those environments.

The ecosystems in these types of places are always shifting around, and so we can’t know much from this one discovery about how the presence of green sulfur bacteria fits within the history of the landscape and the life within it. But even if it cannot tell us the entirety of the area’s past, the discovery of green sulfur bacteria in a hydrothermal vent shows us just how broad nature can be in applying the structures it creates, transforming an ancient process whose requirements we thought we understood into something entirely different. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

The names on the screen right now, they are names of our Patreon patrons, people who help us continue exploring this really marvelous planet that we all get to inhabit together. If you want to become one of them, you can go to Patreon.com/JourneytoMicro. And we very much will appreciate it if you do, it will allow us to keep making more and better videos like this one.

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 probably a subscribe button somewhere nearby.