When you think about it, animals are weird. They ignore the abundant source of energy above their heads – the sun – and choose instead to invest vast amounts of energy in cumbersome equipment for eating and digesting food. Why don’t they do what plants do, and get their energy straight from sunlight?
The short answer is that many do. Corals are animals but have algae living in them that use sunlight to make sugar. Many other animals, from sponges to sea slugs, pull the same trick. One species of hornet can convert sunlight into electricity. There are also suggestions that aphids can harness sunlight, although most biologists are unconvinced.
But all these creatures are only distantly related to us. 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.
It’s less clear how well the algae get on without the embryos. In the lab, they transform into dormant cysts. The salamander eggs are only around in spring, suggesting that in the wild, the algae spend the rest of the year as cysts. The ponds they live in dry up in summer, so the algae may sit out the rest of the year in the sediment.
Now that one vertebrate has been shown to use photosynthesis, Graham says there could well be others. “Anything that lays eggs in water would be a good candidate,” she says, as algae would have easy access to the eggs. So other amphibians, and fish, could be doing it. It’s much less likely that a mammal or bird could photosynthesise, as their developing young are sealed off from the outside world.