Several animals can already do photosynthesis. Will humans follow suit?
The sun is a nearly endless source of fuel, for those who can tap it. Image: Yoshikazu Takada / Flickr
Imagine if we could do as the plants do, and feed directly off the sun's energy. It would certainly make our lives easier: The countless hours spent purchasing, preparing, and eating food could be redirected elsewhere (and we wouldn't even need Soylent). Over-exploited agricultural lands would revert to natural ecosystems. Rates of starvation, malnutrition, and food borne illness would plummet.
But humans and plants haven't shared a common ancestor for hundreds of millions of years. Nearly everything about our biology is fundamentally different. So it would seem like there's no way we could actually engineer ourselves to do photosynthesis—or is there?
Synthetic biologists like Christina Agapakis have actually been exploring this possibility in depth, and have even tried to create plant-animal hybrids of their own. While we're far from building a photosynthetic human, new research reveals an intriguing biological mechanism that could advance this nascent field.
Woods Hole's Marine Biological Laboratory reported last week that scientists have unlocked the secret behind Elysia chlorotica, the brilliant green sea slug that looks like a leaf, eats sunlight like a leaf, but is, in fact, an animal. Turns out E. chlorotica maintains its vivid hue by consuming algae and pirating their photosynthesis genes. It's the only known instance of a multicellular organism co-opting DNA from another.
"There is no way on Earth that genes from an algae should work inside an animal cell," study co-author Sidney K. Pierce, an emeritus professor at University of South Florida, said in a statement. "And yet here, they do. They allow the animal to rely on sunshine for its nutrition." If humans wanted to hack our own cells to do photosynthesis, researchers say, we might use a similar mechanism.
When it comes to tapping the sun's energy, humans have spent a billion years moving in the wrong evolutionary direction. As plants became paper thin and transparent, animals became thick and opaque. Plants conserve their slow but constant drip of sun juice by remaining still, but we like to move about—and require energy-dense food to do so.
When you get down to biology's smallest scales—our cells and our genetic code—it turns out, we're not so very different. It's the astounding convergence of life at its most fundamental levels that enables extraordinary feats, like an animal stealing photosynthesis, to be possible. Now, through the burgeoning field of synthetic biology, we may be able to recreate such events in an evolutionary blink, making biopunk ideas like photosynthetic skin patches seem a little less science fictional.
"Usually, when you take genes from one organism and insert them into the cells of another organism, it doesn't work," Pierce told me. "But when it does work, it can make a huge difference overnight. It's like rapid evolution."
For all their exquisiteness, sea slugs are not the only animals that have stolen solar secrets from the plant kingdom. There's the pea aphid, which, as Nature documented in 2012, charges up a solar-powered backpack using light-harvesting pigments called carotenoids. And the oriental hornet might use a similar trick, utilizing a pigment called xanthopterin to convert light energy to electricity. But neither of these creatures are truly photosynthetic—both lack the critical ability to turn carbon dioxide into sugar.
Still, there are other animals that really do photosynthesize, through symbiotic relationships. The classic example is coral; colonies of hundreds of soft-bodied animals that harbor photosynthetic dinoflagellates within their cells. Likewise, the spotted salamander uses algae to solar-power its embryos as they develop inside eggs.
Among the handful of animals who've gone green, however, solar-powered sea slugs are special. They've figured out how to cut out the middleman and do photosynthesis entirely for themselves, by slurping up algal chloroplasts—jelly bean-shaped photosynthetic organelles—and plastering them along the walls of their digestive tracts. Thereafter, the plant-animal hybrids can live for months eating nothing but sunlight. But exactly how slugs maintain their stolen solar factories has been a mystery.
Now Pierce and his co-authors have an explanation. The slugs, it seems, not only steal chloroplasts from algae, they swipe critical DNA blueprints as well. In a paper which appears in The Biological Bulletin, the authors use an advanced imaging technique to confirm that a gene from the alga V. litorea is present on E. chlorotica's chromosome. The gene, which encodes an enzyme used for chloroplast repair, probably helps the slug maintain its sunlight machines long after the algae have been devoured.
Genetic expropriation may be rare in nature, but scientists have been tinkering with it in the lab for decades. By hacking genes from one organism into another, we've created scores of new life forms, from corn that makes its own pesticide to glow-in-the-dark plants. Is it crazy to suppose, then, that we might follow in nature's example, and hack photosynthesis into animals—perhaps even humans?
"I tried to sell that idea to the Navy years ago," Pierce told me over the phone. "My thinking was, if we could get chloroplasts into human skin, well, maybe we could keep a person underwater forever." (Photosynthesis produces oxygen, in addition to sugar.)
The Navy never went for it, but other researchers have given the matter serious consideration. Biologist, designer and writer Christina Agapakis, who holds a PhD in synthetic biology from Harvard, has spent a lot of time thinking about how to engineer new symbioses, including animal cells that can do photosynthesis. As Agapakis pointed out when I spoke with her over the phone, plants' predecessors co-opted chloroplasts—which were once free-living bacteria—eons ago.
"This is an evolutionary story that happened billions of years ago," Agapakis told me. "We asked what would happen on a shorter timescale." She went on to recount an experiment wherein zebrafish embryos were injected with photosynthetic bacteria. Astonishingly, the fish didn't die, and neither did the bacteria.
"If we injected E. coli—even dead ones—the embryos would die within an hour," Agapakis said. "But injecting photosynthetic bacteria, the fish would still grow. From an evolutionary point of view, it seems like there's something special about photosynthesis."
It was a fascinating demonstration of biological versatility. But it's a far cry from creating, de novo, an organism that lives off the sun. The trouble, Agapakis explained, is it takes a heck of a lot of surface area to capture enough sunlight to make a meal. With leaves, plants are able to harness an enormous amount of solar energy relative to their size. Thick, fleshy humans, with our low surface-area-to-volume ratios, probably don't have the necessary bandwidth.
"If you're asking, can I make myself entirely photosynthetic, I'd say that first, you'd have to stop moving entirely, second, you'd have to become see-through," said Agapakis. "We did a calculation for the zebrafish and realized we'd need thousands of algae per cell to make it work."
Indeed, sunlight-hungry E. chlorotica might be the exception that proves the rule. The slug has evolved to look and act so much like a leaf that, in many ways, it's arguably more plant than animal.
But even if we couldn't subsist entirely off the sun, who's to say we can't supplement our diets with a little solar snack every now and again? Indeed, most animals that do photosynthesis, including a few ofE. chlorotica's cousins, don't rely entirely on sun-fuel. They use their photosynthetic machinery as more of a back-up generator, for times when food is otherwise scarce.
"It's a hedge against starvation," Pierce told me. "In an ecological sense, photosynthesis can provide animals considerable freedom."
Or, perhaps we could design a new use for photosynthesis entirely.
"Maybe you'd just have a green patch on your skin, a light-activated wound healing system, for instance," Agapakis said. (It's an awesome idea that scientists have tossed around over beers, but, apparently, is still waiting to be tackled.) "Something that doesn't require as much energy as a human."
So, while we might not go full solar anytime soon—not before we decide to make some more dramatic body modifications—we can still let nature's example inspire us. At the very least, celebrities sporting algal hair implants, or distance runners whose greenish hues allow them to stretch a little further on a sunny day, don't seem totally outside the realm of possibility.