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When we hear the term symbiosis, we tend to think about a simple partnership between two biological organisms. But in many cases, there are more than two parties involved and it's way more complicated.

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Go to to learn more. [ ♪INTRO ]. When we hear the term symbiosis, we tend to think about a partnership.

Like bees and flowers. The bees get nectar. The flowers get pollinated.

It's a biological win-win. But when we dive into the science of symbioses, it turns out there are a couple caveats. First off, though the term symbiosis is usually thought of as good-for-everyone, technically, it just means a close ecological relationship.

It can be an everyone-wins-type situation, or a mutualism, but it definitely doesn't have to be. For example, Parasitism is also a symbiosis. So is commensalism — where one side is unaffected by the relationship.

Also, and most importantly for this episode, in many cases, there are more than two parties involved. And things get really interesting when you look at symbiotic threeways. Take lichens, for instance.

Often seen growing on rocks or trees, lichens are one of the classic examples of symbiosis. For 140 years, we've known that they contain both a fungus and a photosynthetic partner, like algae or bacteria. Sometimes both.

But in 2016, a team of scientists announced they'd found a new partner in the mix. They were looking at messenger RNA — the chemical “scripts” which dictate protein synthesis — from beard-like lichens. The idea was to sequence those and then work backwards to pin them to the different branches of the tree of life they came from.

You see, there had been some hints in previous studies that researchers were missing something about lichens. For instance, when they tried to grow them in sterile labs, they didn't really look right, even when both the fungus and algae were present. In particular, a part called the cortex — a structural layer which helps transfer nutrients and water — often didn't form as expected.

Some researchers suggested that could be because there was yet another partner in the mix — something missing from those sterile environments — so the scientists hoped looking at messenger. RNA could help them find it. A lot of what they got seemed to belong to the expected partners, but a few results suggested there was another fungus present.

Upon further investigation, they decided it was probably a yeast. So they took to the microscope, and sure enough, they found teeny tiny cells inside the lichen. And when they checked with other scientists, it turned out that lichens all around the world also have this yeasty partner.

The yeast is found in the cortex, that nutrient-transfering structure that was so hard to grow in the lab. It might even help build it. It may also help produce compounds like vulpinic acid, a greenish-yellow toxic pigment that may help protect the photosynthetic microbes living inside the lichen from radiation.

And the story doesn't end there. In 2019, scientists found another yeast that may be a natural component in some lichens. Making them a symbiotic 4-way!

But, for another example of a symbiotic trio, let's look at panic grass. Panic grasses are common throughout the world. But one especially hardy type grows near the hot springs in Yellowstone National Park, where soil temperatures can get up to 50 degrees C or more.

We've known for some time that this heat-tolerant panic grass has a fungal partner, and that if you break up that partnership, the grass loses its ability to grow in such hot soils. But it turns out this extreme heat resistance isn't wholly from the fungus. It's thanks to a third symbiotic partner: a virus.

Scientists were investigating this grass back in the late 2000s. They wanted to know how viruses might affect plant-fungus mutualisms, so they used a technique to look for viral genetic material inside the fungus. They detected an unknown virus infecting the fungus, and singled it out.

Normally, we'd think of a viral infection as a bad thing, but in this case, it actually seems to be an important part of mutualistic relationship between the plant and its fungus. When scientists cured the fungus of its infection and then put the newly virus-free plants in 2-week-long heat tolerance tests, they shrivelled and died, while the still-infected plants did just fine. And when fungus was re-infected, the plants regained their heat resistance.

The scientists weren't quite able to figure out how this works, but they said that it seems like the virus somehow affects the plant's stress-response system, maybe by helping the fungus eliminate damaging chemicals generated by the plant's defense mechanisms. So those are some examples of symbiosis where everyone benefits from the relationship. But remember how symbioses can be a lot more complicated?

Well, consider Bryopsis, a kind of marine algae, and their weird relationship with certain bacteria and voracious, inch-long sea slugs. Bryopsis grows in the Pacific in places like Hawai`i, and both it and its predator — a kind of sea slug — use chemicals called kahalalides to defend themselves. The slugs gets these kahalalides from eating the algae.

But it turns out that the algae gets them from somewhere else, too. Specifically, symbiotic bacteria. These bacteria are found exclusively inside the algae, where they synthesize the algae's toxic defenses from compounds they get from their hosts.

These bacteria are so specialized that they can't live on their own, and about a quarter of all their messenger RNAs — those protein “scripts” we mentioned earlier — are dedicated to kahalalide synthesis. The kahalalides they produce would normally keep the algae safe, but the slugs have evolved an immunity to their effects. In fact, they actually hijack these molecules for their own defenses, as well as the algae's chloroplasts.

So by eating these algae, the little grazers can transform themselves from unassuming mollusks into toxic, solar-powered slugs. Life is beautiful, man! And even though the bacteria and algae don't benefit from this because they die — they're so closely linked to the slugs that it's still considered a symbiosis.

The fact is, life is built on relationships and on webs of relationships. And these aren't even the most complex ones around. The gardens of fungus-growing ants may have five symbionts.

Even humans have symbiotic relationships with the myriad of species in our guts. So, symbiosis is complicated. It's not limited to two organisms, and not everyone wins.

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