Originally published: Visions: Literary and Arts Magazine by Tina Mai (April 2022)
Unlike plants, which use photosynthesis to turn sunlight, carbon dioxide, and water into food for themselves, humans have to use up tons of precious resources, labor, and time for producing food. What if, instead, humans could cut out this destructive agricultural middleman and create food for themselves by manipulating their own biology? It may seem like complete science fiction, but
scientists are looking into it because of the critical environmental and agricultural situation. Currently, about 70% of all global freshwater resources are withdrawn for use in agriculture, and approximately half of all Earth’s habitable land is devoted to farming. Livestock production is by far the most harmful of our current agricultural practices. Despite consuming all these resources, generating 1/5 of the world’s greenhouse gas emissions, and leaching toxic chemicals into natural ecosystems, over 600 million people globally still remain undernourished. And, the problem is getting worse. In fact, to meet the nutritional needs of the world’s population in 2050, global food production must increase by 60% and further tax the environment.
While this is all mainly speculation and type-III (super-advanced) civilization stuff at this point, if humans were photosynthetic, part of this increase in food production could come without more harm to the environment. Aside from the environmental benefits, humans would also have no need to purchase, prepare, and eat food, freeing up time for more productive activities. Not to mention, rates of starvation and food borne illness would plummet across the globe. Modern science already has considerable experience transplanting DNA from one organism to another (in the creation of GMOs), studied a rare cluster of photosynthetic animals, and have done experiments and calculations relating to this nascent field of photosynthetic humans. So, let’s take a close look at the research that has been done to get a better understanding of what it would take for humans to cut out the destructive agricultural middleman and “eat the sun.”
To jump right in, being photosynthetic would require vast biological changes on both the macroscale (human physiology) and microscale (the contents of our cells). For one, photosynthetic humans would have green skin, and considering Shrek and Gamora, we would all be a bit more attractive. This would be due to the presence of chlorophyll in our cells. Chlorophyll is a green pigment embedded in plant cell organelle called a chloroplast. (Organelles are cellular compartments with specific jobs, like a micro-version of an organ would be in the human body.) Chlorophyll appears green, and makes plants green, because it absorbs red and blue wavelengths while reflecting green and yellow light. When these electromagnetic solar waves (sunlight) hit the chlorophyll pigment, they displace electrons, setting off a chain of reactions in the chloroplast that create ATP and NADPH. Electrons are stolen off of water molecules (which is why plants need to be watered) during this set of reactions to resupply the chlorophyll. ATP and NADPH store chemical energy for the cell. A process called the Calvin Cycle utilizes the energy molecules produced in the earlier light-dependent reactions with chlorophyll to turn inorganic carbon-dioxide into useful organic compounds like glucose (sugar). And, voila, the plant has just produced food for itself using the essentially inexhaustible and easily-accessible materials of sunlight, water, and carbon-dioxide.
Speaking of carbon-dioxide, humans would need a way to intake enough of it for photosynthesis. Plants have pores on their leaves called stomata, which means that, in addition to being green, photosynthetic humans would also require weirdly porous skin (we may or may not leak moisture and fluids). Photosynthesizing humans would also need to spend a lot more time in the sun to generate the necessary energy. Even with that, however, photosynthesis alone would not be enough to supply the daily energy needs of a person like it can a plant. This is because, for one, plants do not move, so they require less energy. Additionally, humans do not have broad, flat, sun-collecting structures like leaves that would enable them to absorb adequate sunlight. Associate Professor Lindsay Turnbull, from the University of Oxford, estimated in a study that if the surface area of an adult woman’s skin contained chlorophyll, photosynthesis would only be able to meet 1% of her daily energy needs.
This all sounds pretty discouraging to anyone dreaming of being photosynthetic when they grow up (just me?), but never fear, synthetic biologists from renowned institutions have been vigorously studying a niche group of animals with semi-photosynthetic capabilities to evaluate the path to our potentially green-skinned future.
One of these animals is the sea slug Elysia chlorotica, which resembles a swimming leaf. When Elysia chlorotica eats algae, it pirates their chloroplasts and genetic code. This allows them to line their digestive tracts with chloroplasts and utilize photosynthesis to go for months without eating. This is the only known instance of a multicellular organism co-opting DNA from another organism, and, according to science, should be impossible. Sydney Pierce, an emeritus professor at the University of South Florida, explored the idea of lining human intestines with chloroplasts (this is not so far-fetched because of gene-editing technologies and GMOs). In his pitch to the Navy, Pierce explained how humans could use chloroplasts to remain underwater forever. (Since, in addition to energy and sugar, chloroplasts produce oxygen during photosynthesis.) Christina Agapakis, a synthetic biologist with a Ph.D. from Harvard, explained an experiment where scientists super-accelerated evolution by injecting growing zebrafish embryos with photosynthetic bacteria, and, surprisingly, neither the bacteria nor embryo died. She said that the embryo would have quickly died if another bacteria, like E. coli, had been used. This means that, according to Agapakis, “From an evolutionary point of view, it seems like there's something special about photosynthesis.”
The scientists note that becoming entirely photosynthetic is pretty far off, but these advances demonstrate that using at least some aspects of photosynthesis to benefit ourselves is not so far-fetched. Some animals demonstrate the evolutionary advantages that being semi-photosynthetic has.
The pea aphid (a strange little green bug), for instance, produces a chlorophyll-like pigment that they use to produce ATP using solar energy. The Oriental Hornet has a stripe of yellow-pigment on its back that allows it to make electricity from sunlight, making it more active in the middle of the day. A species of salamander uses algae to solar-power its eggs. And, coral (technically animals) form symbiotic relationships with algae to produce food for themselves.
Burton Nitta (an interdisciplinary arts-science organization) recently released designs for a photosynthetic helmet, which uses algae to produce food for the person via photosynthesis, forming an artificial-symbiotic relationship. There is also a potential for less-frightening wearables, like sunlight-activated skin patches that accelerate human cellular-regeneration (wound healing). Notably, humans have already evolved to do vitamin D photosynthesis (the use of UVB light to synthesize vitamin D compounds). According to a study from the National Library of Health, “at least 1,000 different genes governing virtually every tissue in the body are now thought to be regulated by [active vitamin D]... including several involved in calcium metabolism and neuromuscular and immune system functioning.” So, who knows what our evolutionary path could look like even without scientific intervention?
Okay, thank you for going on this weird semi-science fiction exploratory journey with me. Yet, a lot of scientific discoveries started with creative, seemingly impossible ideas. Perhaps nature has evolved the way it has for a reason, and may hold many of the answers to solving modern science problems. Photosynthesis was one of the evolutionary processes that survived. And, considering how close we are to the brink of environmental catastrophe, we should not disregard any solutions, no matter how much they seem like only the material of science-fiction novels. In reality, the future of agriculture will probably involve a myriad of sci-fi solutions like 3D-printed meat, a day’s worth of food in a pill, or even algae-based diets. Mobile phones used to seem impossible (they used to only exist in Star Trek), so who knows what’s next!
Sources:
https://www.vice.com/en/article/3dk4bv/human-photosynthesis-will-people-ever-be-able-to-eat-sunlight
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