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.