Solar power is a relatively new development for humans but, of course, many living things have been exploiting the power of the sun for millions of years, through the process of photosynthesis. This ability is usually limited to plants, algae and bacteria, but one unique animal can do it too – the emerald green sea slug Elysia chlorotica. This remarkable creature steals the genes and photosynthetic factories of a type of algae that it eats (Vaucheria littorea), so that it can independently draw energy from the sun. Through genetic thievery, it has become a solar-powered animal and a beautifully green one at that.
The cells of algae, like those of plants, contain small compartments called chloroplasts that are its engines of photosynthesis. As the Elysia munches on algae, it takes their chloroplasts into the cells of its own digestive system, where they provide it with energy and sugars. It’s a nifty trick that provides the sea slug with an extra energy source, but the problem is that it shouldn’t work.
Chloroplasts are not independent modules that can be easily separated from their host cell and implanted into another. They are the remnants of once-independent bacteria that formed such a strong alliance with the cells of ancient plants and algae, that they eventually lost their autonomy and became an integral part of their partner. In doing so, they transferred the majority of their own genes to their host so that today, chloroplasts only have a tiny and depleted genome of their own, containing just 10% of the genes it needs for a free-living existence.
So, shoving a chloroplast from an algal cell into an animal one should be about as effective as installing a piece of specialised Mac software on a PC. The two simply shouldn’t be compatible, and yet Elysia and its chloroplasts clearly are. Mary Rumpho from the University of Maine discovered the key to the partnership – the sea slug has also stolen vital genes from the algae that allows it to use the borrowed chloroplast. It has found a way to patch its own genome to make it photosynthesis-compatible.