November 25, 2008

Sea Slug Harvests Solar Energy

Researchers have reportedly discovered the method by which sea slugs are able to operate on solar power.

"Elysia chlorotica," lives along the Atlantic seaboard of the United States, and although it may not seem to be anything different from other common slugs, it has the ability to behave like a plant and harness energy from the sun.

James Manhart, a biologist at Texas A&M University has been studying these tiny creatures for the past decade and, along with collaborators from several universities, has identified a possible cause of their ability to behave like plants.

Plants are much like solar-powered machines, Manhart reports in the current issue of Proceedings of National Academy of Sciences. The cells of plants contain tiny organelles called plastids that trap sunlight and convert it into energy by a process known as photosynthesis.

The sea slug primarily feeds off of a specific type of alga.

"It makes a cut in the alga, sucks out the cytoplasm [the material inside the alga] and digests most of it," said Manhart.

But the sea slug also does something interesting, it retains the plastids that trap the solar energy. Young E. chlorotica fed with algae for two weeks, could survive for the rest of their year-long lives without eating, said Mary Rumpho, an expert on the species from the University of Maine.

Rumpho and colleagues sequenced the chloroplast genes of Vaucheria litorea,. They confirmed that if the sea slug used the algal chloroplasts alone, it would not have all the genes needed to photosynthesize.

Her team then noticed that the sea slug's own DNA contained one of the vital algal genes, indicating that the slug had probably stolen the gene from its food.

"We do not know how this is possible and can only postulate on it," said Rumpho.

Manhart says the slug needs the alga to mature and complete its life cycle.

"It is totally dependent on the alga to survive. Once the slug has acquired a sufficient amount of plastids it can survive, like plants, for at least nine months by trapping solar energy and converting it into food," he said.

Researchers also found the algal gene in E. chlorotica's sex cells, meaning the ability to maintain functional chloroplasts could be passed to the next generation.

"Here we have something going across and working in an entirely different context, which is altogether more interesting," said Greg Hurst of Liverpool University in the UK.

"There was an example recently of a whole bacterial genome that ended up in a fruit fly species, but no-one knows if it functions," he says. "What is really unique here is the fact that the gene is transferred and appears to function."


Image Caption: The photosynthetic sea slug Elysia chlorotica appears like a dark green leaf as a result of retaining chloroplasts from its algal prey, Vaucheria litorea, in cells lining its digestive tract. The sea slug has acquired photosynthesis-supporting genes by horizontal gene transfer and can use the chloroplasts to carry out photosynthesis for several months. See the article by Mary E. Rumpho et al. on pages 17867"“17871. Photo courtesy of Mary S. Tyler. (PNAS) 


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