A green sea slug found off North America's east coast not only looks like a leaf, but can also make food out of sunlight, just like a plant.

The solar-powered sea slug Elysia chlorotica can live entirely from energy from the sun. It's about three centimetres long. ((PNAS))

U.S. researchers have found that the sea slug Elysia chlorotica can photosynthesize, using energy from light to convert carbon dioxide into sugars.

"If you shine light on these slugs, they fix carbon dioxide and make oxygen just like a plant," Sidney Pierce of the University of South Florida told CBC Radio's Quirks & Quarks.

Pierce reported his findings Jan. 7 at the annual meeting of the Society for Integrative and Comparative Biology, and has submitted his research to the journal Symbiosis.

The slugs look just like a leaf, green and about three centimetres long, and are found off the east coast of North America from Nova Scotia to Florida.

They acquire the ability to photosynthesize by eating algae and incorporating the plants' tiny chlorophyll-containing structures, called chloroplasts, into their own cells.

"Certain cells that line the walls of [the slugs'] digestive tubules are able to actually take up the chloroplasts from the algae, and they sequester them in the cells for very long periods of time," said Pierce.

Pierce said the chloroplasts can live inside the slugs' cells for nine or 10 months, nearly the entire lifetime of the slug, and can provide the animal with all the energy it needs.

"It can complete its entire life cycle, including reproduction, just on photosynthesis," he said.

With the help of radioactive trackers, Pierce found that the slugs are using the chloroplasts to make new chlorophyll, the green pigment that makes photosynthesis possible.

Gene transfer a revelation

The chloroplasts from algae aren't enough in themselves to allow the slugs to live off the sun. The process of photosynthesis requires enzymes and other proteins to keep the chloroplasts working.

"That requires a lot of genes that are present in the algal cell, and so those have been transferred as well," said Pierce.

At some point in the evolution of these sea slugs, genes from the algae transferred over and now reside in the genome of the slug.

"We found 14 or 15 algal genes so far and I'm pretty confident that we're going to find dozens more," said Pierce.

Gene transfers are common in single-celled organisms, but Pierce said this is the first time it has been described in multicellular organisms.

Exactly how the genes got from the algae to the slug isn't clear, but Pierce said research into this mechanism could lead to advances in gene therapy and genetic engineering.

Pierce said gene therapists are "trying to insert genes from one organism to another to fix genetically based diseases, and honestly that doesn't work very well yet, but these slugs have figured that out."

The mechanism could also be important in the study of evolution, giving biologists a new way to explain how organisms acquired certain genes.

"You don't have to sit around waiting for a random mutation to occur. You can take a gene or a group of genes, as is the case with the slugs, and really give yourself a real evolutionary boost," said Pierce.