Symbiosis is exploitation not mutual benefit?

Biologists have discovered that an ancient symbiosis is founded entirely on exploitation, not mutual benefit.

A new study shows that a single-celled protozoa called Paramecium bursaria benefits from exploiting a green algae that lives inside it, providing its host with sugar and oxygen from photosynthesis.

Scientists have debated for decades whether symbioses, like the Paramecium-Chlorellaassociation, are based on mutual benefit or exploitation.

The common belief has been that both the protozoa and algae benefit.

“This research suggests that what we have always thought of as mutualism—where species gain mutual benefit from interacting with each other—might actually be based on exploitation where one species gains by capturing and then taking resources from another,” says lead author Chris Lowe, lecturer in evolutionary ecology at the University of Exeter.

“This new research has turned the assumption that symbiosis is mutually beneficial on its head,” says Mike Brockhurst, professor of biology at the University of York.

The researchers tested the symbiotic relationship of the protozoa and algae across gradients of different light intensity.

“We found that for the host the benefits of being in symbiosis increased with light. 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,” Brockhurst says.

“Across all of the environments that we tested we never found any conditions where both species benefited. For the algae it is always costly to be in symbiosis.”

The findings, published in the journal Current Biology, suggest there is likely to be an exploitative component in other symbiotic relationships.

“The big one is corals, where climate change related bleaching results from loss of photosynthetic microbial symbionts,” Brockhurst says.

“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 and conserving 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.”

Researchers from the University of Sheffield contributed to the study. The Natural Environment Research Council (NERC) funded the work.

Source: University of York

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