The spotted salamander (Ambystoma maculatum) is found throughout the eastern USA and parts of southern Canada. While a number of animals, including corals, sponges, sea slugs and one species of hornet have algae living in them that use sunlight to make sugar, no backboned animal has been found that can harness the sun – until now. It has long been suspected, and now there is hard evidence: the spotted salamander is solar-powered.
Plants make food using photosynthesis, absorbing light to power a chemical reaction that converts carbon dioxide and water into glucose and releases oxygen. Corals profit from this reaction by housing photosynthetic algae inside their shells.
Spotted salamanders, too, are in a long-term relationship with photosynthetic algae. In 1888, biologist Henry Orr reported that their eggs often contain single-celled green algae called Oophila amblystomatis. The salamanders lay the eggs in pools of water, and the algae colonise them within hours.
By the 1940s, biologists strongly suspected it was a symbiotic relationship, beneficial to both the salamander embryos and the algae. The embryos release waste material, which the algae feed on. In turn the algae photosynthesise and release oxygen, which the embryos take in. Embryos that have more algae are more likely to survive and develop faster than embryos with few or none.
Then in 2011 the story gained an additional twist. A close examination of the eggs revealed that some of the algae were living within the embryos themselves, and in some cases were actually inside embryonic cells. That suggested the embryos weren’t just taking oxygen from the algae: they might be taking glucose too. In other words, the algae were acting as internal power stations, generating fuel for the salamanders.
To find out if that was happening, Erin Graham of Temple University in Philadelphia, Pennsylvania and colleagues incubated salamander eggs in water containing radioactive carbon-14. Algae take up the isotope in the form of carbon dioxide, producing radioactive glucose. Graham found that the embryos became mildly radioactive – unless kept in the dark. That showed that the embryos could only take in the carbon-14 via photosynthesis in the algae.
The algae do not seem to be essential to the embryos, but they are very helpful: embryos deprived of algae struggle.
“Their survival rate is much lower and their growth is slowed,” says Graham.