The Evolution of Symbiosis: Species Always Take What They Can Get

It turns out that ancient symbiosis between species wasn't perfect by a longshot. Instead, it was founded entirely on exploitation rather than mutual benefit.

For decades, researchers have debated whether symbioses, like the Paramecium-Chlorella association, are based on mutual benefit or exploitation. In the case of this symbiosis, the single celled protozoa called Paramecium bursaria benefits from a green algae that lives inside it, which provides its host with sugar and oxygen from photosynthesis.

In this latest study, the researchers tested the symbiotic relationship of the protozoa and the algae across gradients of different light intensity.

"We found that for the host the benefits of being in symbiosis increased with light," said Mike Brockhurst, one of the researchers, in a news release. "Although symbiosis is very costly in the dark for the hosts, because the algae are useless, when you increase the light intensity then it becomes very beneficial to have algae because they give you lots of sugar. Across all the environments that we tested we never found any conditions where both species benefitted. For the algae it is always costly to be in symbiosis."

The findings suggest that what researchers considered to be mutualism might actually be based on exploitation where one species gains by capturing and then taking resources from another. The findings may have other implications for other species in symbiosis.

"The big one is corals, where climate change related bleaching results from loss of photosynthetic microbial symbionts," said Brockhurst. "I suspect in a lot of cases where we assume mutualism we might find that isn't the case, which has important implications for understanding symbioses in nature. Because symbioses are so common, understanding how symbiotic species interact and how they evolve will tell us a lot about ecosystems and how they will respond to climate change."

The findings are published in the journal Current Biology.

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