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People first noticed green algae in salamander eggs more than a century ago. ((Roger Hangarter/Indiana University Bloomington))

Algae have been found in a very unusual place — inside the cells of salamander embryos.

The discovery was "a bit of a shock," said Ryan Kerney, a biologist at Dalhousie University in Halifax who led the study that uncovered the algae, Tuesday.

This appears to be the first time algae have been found living in such a close relationship with a vertebrate, he and his collaborators reported in Proceedings of the National Academy of Sciences this week.

People first noticed more than a century ago that green algae were often found inside the eggs of the spotted salamander. Previous experiments have shown that salamander embryos grow better in the presence of algae and algae grow better in the presence of the salamander embryos.

What researchers didn't know is exactly how close the relationship between the two was.

Algae have previously been found living inside the cells of invertebrates such as sea slugs, hydras and sponges. This kind of cell-within-a-cell relationship is called endosymbiosis, but it is very rare in vertebrates.

That's because animals with a backbone, like amphibians, reptiles, birds and mammals, have immune systems that typically destroy foreign cells that try to enter their own cells.

Vertebrates such as sloths, turtles and tadpoles often have algae growing on their bodies but not inside. That was what scientists thought may have been happening with spotted salamanders, amphibians that are common in eastern North America, Kerney said.

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Spotted salamanders are a common amphibian in eastern North America. ((Ryan Kerney/Dalhousie University))

What Kerney originally wanted to know was how the algae got into the salamander eggs. He looked at the eggs using a technique called fluorescence microscopy that makes the algae cells light up and therefore easier to see. He decided to examine the tissues of the salamander embryos themselves "just out of curiosity."

He cut out the parts that glowed under the fluorescence microscope and zoomed in to a much higher magnification using an electron microscope to pinpoint the location of the algae.

"When we saw they were embedded inside cells, it was a real surprise," Kerney said.

He ran further tests that confirmed the presence of algae genetic material in the tissues of the salamander.

Kerney said the salamanders likely get extra oxygen from the algae, and the algae probably get extra carbon dioxide and nitrogen from the salamanders.

So far, there is no evidence that the algae provide any food to the salamanders via photosynthesis. Kerney thinks that is possible, but it's probably not significant because the skin of the salamander isn't very transparent, and it lives underground during most of its adult life.

It is not clear exactly how the algae get inside the eggs. But Kerney did find algae genetic material in the reproductive tracts of adult salamanders, suggesting that algae might live inside the cells of salamanders their whole lives. That may allow salamanders to transfer algae to their young and pass on the benefits of the salamander-algae relationship.
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By zooming in using an electron microscope, Kerney discovered that the algae cells were inside the salamander cells. ((Ryan Kerney/Dalhousie University))