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A Sea Slug Powered by the Sun

ORIGINAL: BiologyBiozine
December 1, 2008

Imagine that after eating a big salad, you were able to use the photosynthetic pigments in the lettuce to your advantage. Such is the case with Elysia chlorotica, and a few other unusual species of sea slugs. These unique animals are able to incorporate the photosynthetic components from the algae they eat into their own bodies—effectively becoming partially solar-powered.
The sea slug Elysia chlorotica uses the chloroplasts from the algae it eats to photosynthesize. (Photo Credit: Mary S.Tyler)
An Introduction to Elysia chlorotica

Elysia chlorotica is a species of sea slug that is found along the eastern coast of North America, from Nova Scotia to Florida. The sea slugs live in coastal salt marshes, tidal marshes, tidal pools, and shallow creeks. As a juvenile, E. chlorotica is reddish-brown in color. After feeding on the alga Vaucheria litorea, the sea slug turns a brilliant shade of green. Why does this dramatic change happen? As the sea slug digests the algae, it is able to retain the algae’s chloroplasts, or photosynthetic structures. These chloroplasts are sent to the surface of the sea slug’s body—where, curiously, they continue to photosynthesize.
Harnessing the Power of the Sun

Dr. Mary Rumpho, a professor of biochemistry at the University of Maine, studies E. chlorotica. Her research interests include learning how the sea slugs are able to photosynthesize by studying the relationship between the sea slugs and their algal food source. A second research project is focused on determining whether the large amount of mucous the soft-bodied sea slug produces as an anti-predator defense mechanism has any potential as an anti-cancer or anti-microbial medium.

Rumpho and her colleagues collected sea slugs from an intertidal marsh on Martha’s Vineyard (a large island located off the coast of Massachusetts). Although she had earlier found that the sea slugs were able to incorporate chloroplasts from the algae they ate into their bodies, it was not yet understood how the sea slugs were able to use the chloroplasts to photosynthesize on their own. The algal chloroplasts only contain enough DNA to encode about 10 percent of the proteins needed to photosynthesize; the remaining 90 percent of proteins are found within the algae’s nuclear DNA.
The alga Vaucheria litorea is the preferred food source of E. chlorotica.
(Photo Credit: Mary Rumpho-Kennedy)
In their experiment, Rumpho’s team first sequenced the DNA of the alga Vaucheria litorea. Their results confirmed that the alga’s chloroplasts would not be able to function properly on their own. Next, the scientists analyzed the DNA of the sea slugs. The scientists discovered that the sea slug’s DNA contained one of the vital algal genes necessary for photosynthesis to occur. The sequence of the gene was exactly the same as the algal version—evidence that the sea slug most likely “stole” the gene from its algal food source.

Mysteries Remain
Rumpho’s research showed that the sea slugs are able to use the chloroplasts because some of the alga’s genes become a part of the sea slug’s genome. How the genes are transferred from an alga to a sea slug is not entirely understood. One hypothesis is that genes from the alga are incorporated into the sea slug’s genome through a process called horizontal gene transfer. Although such a transfer of genes is common in bacteria (it’s the method behind bacterial antibiotic resistance), horizontal gene transfer is much less common in multicellular organisms. Horizontal gene transfer is an important mechanism of evolution. According to the theory of endosymbiosis, organelles such as mitochondria and chloroplasts began as separate prokaryotic organisms that were swallowed by different types of bacteria. Mitochondria developed from proteobacteria; chloroplasts developed from cyanobacteria (blue-green algae).

Another hypothesis is that the algal genes are transferred into the sea slug’s genome via a virus. However, though the scientists have identified viruses in the sea slugs, they have yet to find evidence that a virus is actually involved in gene transfer. As Rumpho and her colleagues learn more about the sea slug’s ability to photosynthesize, more questions arise. Currently, the scientists are studying how genes are expressed in specific cells, and they hope that these studies will help shine a light on the exact mechanism behind the gene transfer.

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