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.
There is another option to adding entire symbionts: steal their factories instead. Within the cells of plants and algae, photosynthesis takes place within tiny structures called chloroplasts. Chloroplasts are the remnants of a free-living photosynthetic bacterium that was swallowed by a larger microbe billions of years ago. Unlike many such events, this fateful encounter didn’t end with the engulfed bacterium being digested. Instead, the two cells formed a permanent partnership that fuels the cells of plants and algae to this day. So rather than teaming up with a symbiont, why not cut out the middle-man and take its chloroplasts for yourself?
At least one group of animals has done this – the Elysia sea slugs. These beautiful green creatures graze on algae, and co-opt their chloroplasts for themselves. The pilfered chloroplasts line the slug’s digestive tract, provide it with energy, and allow it to “live as a plant”, as Elysia expert Mary Rumpho describes it. This association is vital to the slug, which cannot reach adulthood without it.
Taking a leaf
It’s still unclear how the slugs maintain and use their chloroplasts. These structures aren’t green USB sticks. You cannot plug them into a fresh host cell and expect them to work normally, because many of the proteins that they use are encoded within the genome of their host cell. These proteins, which number in their hundreds, are made in the cell’s nucleus, and transported into the chloroplast. Elysia’s genome contains at least one algal gene, and while more could lie in wait, it’s unlikely to contain the hundreds necessary to sustain a functional chloroplast.
That’s a mystery for another time. For now, Chris Howe from the University of Cambridge says, “If you wanted to set up a relationship between a chloroplast and a new animal host, you’d need all that extra support machinery. You’d have to put those genes in the host’s genome.” And with hundreds of such genes, turning a human cell into a compatible home for chloroplasts would involve genetic engineering on a vast scale.
And to what end? Even if the symbionts took, even if the controlling genes were successfully added, would this make a difference to us? Probably not. Photosynthesis is a useless ability without some way of exposing yourself to as much of the Sun’s energy as possible. That requires a large surface area, relative to their volume. Plants achieve that with large, horizontal, light-capturing surfaces – leaves. Elysia, the sea slug, being flat and green, looks like a living leaf. It’s also translucent, so light can pass through its tissues to the chloroplasts within.
Humans, on the other hand, are pretty much opaque columns. Even if our skin was riddled with working chloroplasts, they would only manufacture a fraction of the nutrients we need to survive. “Animals need a lot of energy, and moving at all doesn’t really jive well with photosynthesis,” says Agapakis. “If you imagine a person who had to get all of their energy from the sun, they’d have to be very still. Then, they’d need a high surface area, with leafy protrusions. At that point, the person’s a tree.”
And why would be bother? Agapakis points out that by domesticating wild plants, and growing them for food, we have effectively outsourced the process of photosynthesis on a massive scale. Agriculture is a global symbiosis – our version of what the pea aphid does, without the faff of maintaining symbionts in our own bodies. We just plant them in fields.