Tuesday, December 14, 2010

Animals that eat sunshine

This sea-slug steals the chloroplasts from algae it feeds on and lives of the sugar they produce.
[Edited from New Scientist article (see reference below)]

Did you think only plants and a few bacteria can photosynthesize? A surprising number of animals can get energy from sunlight too - by living in symbiosis with photosynthetic organisms: The list includes salamanders, sea slugs (see picture above), giant clams, sea squirts, jellyfish, corals, anemones, hydras and sponges.

For example, females of the spotted salamander Ambystoma maculatum store algal cells in their oviducts and somehow pass them on to their eggs, where the algae continue to live within the cells of the developing salamander embryos. Once the salamander are adults, their skin does not let enough light through and they live mostly underneath stones.
Another example are oriental hornet's whose shell can trap sunlight, while the pigment xanthopterin converts it to energy (Read more about oriental hornets here)

Actually, even plants are not capable of performing their own photosynthesis: Plants can only harvest light energy because they engulfed photosynthetic cyanobacteria around 2.5 billion years ago - which we now know as "chloroplasts". Plants live in endosymbiosis with the engulfed cyanobacteria.

The question is not whether animals can photosynthesise, but why not more do. To make photosynthesis effective, the animal needs to be exposed to a lot of light and have a large body surface. To sustain chloroplasts within their cells, animals would have to add several hundred genes to their genome. This method has been found in the sea slug Elysia chlorotica that are native to the salt marshes of New England and Canada (read more here: Bizarre sea slug is half animal and half plant) [See image above]. Most other organisms go the easier way by engulfing entire photosynthetic cells. Researchers from Harvard Medical School did just that - by artificially injecting photosynthetic cyanobacteria into the egg cells of a zebrafish.

So will we have solar-powered pet fish soon? That might take a while. Being a photosynthetic animal would need a change in behavior, as too much ultraviolet sun light can damage the chlorophyll (that's why all known photosynthetic animals live under water, which reduces the UV radiation). In addition, photosynthesis would provide animals with sugar, but animals would still require proteins, vitamins, and minerals from other sources. In theory, adding nitrogen-fixing cyanobacteria, as found in some sponges and corals, could provide animals with the protein they need. So far nobody managed to this in even plants, despite decades of effort.

Genetic engineering might be able achieve what evolution hasn't, but would the benefits outweigh the costs for any vertebrates, especially energy-hungry animals with active lifestyles? Maybe one day, humans will be able feed directly off the sun, and might look like this: 

Orion Woman from Star Trek
Read the original article her: Light diet: Animals that eat sunshine - environment - 13 December 2010 - New Scientist

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