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Did you know ...? – Green sea slugs

Why don’t animals use photosynthesis?

The answer has always been that their ancestors never acquired the equipment the way plants did, and even if they had, their surface-area-to-volume ratio is too small to allow a surface of green tissue to feed the entire animal.

Now it turns out that at least one animal does photosynthesise.

A study published by the Proceedings of the National Academy of Sciences of the USA reports the discovery of photosynthesis by a sea slug, Elysia chlorotica (PNAS 105: 17867–17871; DOI: 10.1073/pnas.0804968105). Author Mary Rumpho and her colleagues discovered that when the sea slug ingests its food alga, Vaucheria litorea, instead of digesting everything, it takes the alga’s photosynthetic plastids and incorporates them into its own cells within the digestive tract.

On its own this is not enough, because the photosynthetic plastids, called chloroplasts in higher plants, depend for more than 90% of their proteins on the nuclear genome, not their own genome. So how do they do it?

Some background is in order here. The cells of plants and animals contain mini-cells called organelles, which carry out various functions. The chloroplast is one example; mitochondria, the cell’s energy factories, are another. Morphological study and genetic analysis show clearly that these organelles are descended from free-living bacteria that became ingested (but not digested) billions of years ago. In this example of symbiosis, it benefited both the host cell and the passenger cells to coexist, dividing the labour. Over evolutionary time, as the cells came to depend totally on one another, the organelles gradually lost many of their genes. If they had remained free-living, they would have died, but as organelles, they were able to take advantage of the host cell’s own genes and proteins. Hence the situation today that the photosynthetic plastids from Vaucheria litorea depend on the alga’s own genes and proteins.

So how could they continue to function when filched by the sea slug? Rumpho and colleagues had two ideas. The first was that the Vaucheria litorea plastids really can function on their own. The second was that the sea slug provided the necessary tools.

They sequenced the genes of the plastid and found that, as expected, it lacks the necessary components. To test the other idea, they looked for algal genes in the sea slug’s genome. And indeed they found one. It appears that somewhere in its past, the sea slug has acquired this gene (and probably others) from the alga and incorporated it into its own genome, where it continues to supply the needs of the photosynthetic plastids.

But don’t expect green self-nourishing cattle.

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