Humans have to grow, hunt, and gather food, but many living things aren’t so constrained. Plants, algae and many species of bacteria can make their own sustenance through the process of photosynthesis. They harness sunlight to drive the chemical reactions in their bodies that produce sugars. Could humans ever do something similar? Could our bodies ever be altered to feed off the Sun’s energy in the same way as a plant?
As a rule, animals cannot photosynthesise, but all rules have exceptions. The latest potential deviant is the pea aphid, a foe to farmers and a friend to geneticists. Last month, Alain Robichon at the Sophia Agrobiotech Institute in France reported that the aphids use pigments called carotenoids to harvest the sun’s energy and make ATP, a molecule that acts as a store of chemical energy. The aphids are among the very few animals that can make these pigments for themselves, using genes that they stole from fungi. Green aphids (with lots of carotenoids) produced more ATP than white aphids (with almost none), and orange aphids (with intermediate levels) made more ATP in sunlight than in darkness.
Another insect, the Oriental hornet, might have a similar trick, using a different pigment called xanthopterin to convert light to electrical energy. Both insects could be using their ability as a back-up generator, to provide energy when supplies are low or demand is high. But both cases are controversial, and the details of what the pigments are actually doing are unclear. And neither example is true photosynthesis, which also involves transforming carbon dioxide into sugars and other such compounds. Using solar energy is just part of the full conversion process.
There are, however, animals that photosynthesise in the fullest sense of the word. All of them do so by forming partnerships. Corals are the classic example. They’re a collection of hundreds and thousands of soft-bodied animals that resemble sea anemones, living in huge rocky reefs of their own making. They depend upon microscopic algae called dinoflagellates that live in special compartments within their cells. These residents, or endosymbionts, can photosynthesise and they provide the corals with nutrients.
Some sea anemones, clams, sponges, and worms also have photosynthetic endosymbionts, and they’re joined by at least one back-boned example: the spotted salamander. Its green-tinged eggs are loaded with algae, which actually invade the cells of the embryos within, turning them into solar-powered animals. The algae die as the salamanders turn into adults, but not before providing them with a useful source of energy in the earliest parts of their lives.
Despite these varied examples, photosynthetic symbionts are again the exception rather than the rule. In a classic paper, botanist David Smith and entomologist Elizabeth Bernays explain why: such partnerships are more complicated than they seem. The host needs to “pay” its symbionts in nutrients. They need ways of persuading the symbionts to release their manufactured nutrients, rather than hoarding it for themselves. They need to control the symbionts’ growth, so their populations don’t run amok. They need to transfer their partners to the next generation (corals do it by releasing the symbionts into the surrounding water).
But maybe the seeds of such relationships aren’t as difficult to plant as they might seem. In 2011, Christina Agapakis, a synthetic biologist from the University of California, Los Angeles got baby zebrafish to accept photosynthetic bacteria, simply by injecting them into the fish when they were embryos. As she wrote on her blog, “The biggest surprise was that nothing happened.” The fish cannot photosynthesise, but they didn’t reject the bacteria either. Agapakis’ experiment showed that back-boned animals can, at the very least, tolerate the presence of photosynthetic microbes, or the type that fuels the baby salamanders. And with a little tweak, she even persuaded the bacteria to invade mammalian cells.