How Can Humans Survive Longer In Space? Photosynthetic Skin
If scientists could genetically engineer chloroplasts into human skin cells, could it give us the energy needed to live in space long-term?
The following is an excerpt from The Next 500 Years: Engineering Life to Reach New Worlds by Christopher Mason.
The Next 500 Years: Engineering Life to Reach New Worlds
Floating in the waters around Boston and New York is a strange, small green-hued hybrid sea slug: Elysia chlorotica. This unique species has the ability to become plantlike by stealing fully functional, photosynthesizing chloroplasts from the algae it eats—a process called “kleptoplasty,” which literally means “theft” of the plasmids (chloroplasts) or organelles. While absorbing DNA and moving mobile genetic plasmids in bacteria is common, moving entire systems is rare in larger organisms.
Sometimes called “solar-powered sea slugs,” the Elysian species uses the green chloroplast as camouflage against predators. Algae normally have a hard, thick plant-cell wall that prevents any breakage or invading species—so how do Elysians get the chloroplasts into their body?
With straws, of course! Elysians have built-in, molecular straws that enable them to pierce through the algae’s walls and suck out the chloroplastic goodness, turning bright green. If the slugs don’t eat enough of their “vegetables” (chloroplasts), they become brown with red pigment spots.
Surprisingly, the chloroplasts can survive for months, or even years, within the large branching digestive system of the (now green) sea slug. Much like the phagocytes of the human immune system, the Elysian phagocytes can engulf the algae easily, and then integrate the chloroplasts into their own biological systems. Even when embedded into their bodies, the chloroplasts still function, capture sunlight, create sugar, and exhale oxygen. Although it was first thought that the eerie green sea slugs needed the chloroplasts to survive, it turned out they did just fine without the light. A researcher named Sven Gould showed that even without light, the slugs’ survival and weight were about the same. So, this is to some degree a recreational feature of the slugs, as if their favorite way to spend their day is the cellular thieving of green, internal adornments.
But these little green thieves raise the question—how do the chloroplasts survive and function within their bodies? In normal plants, chloroplasts require 90 percent of their essential proteins to come from the nucleus of the plant host. They are basically mooching roommates. Clearly, these sea slugs can also accommodate the needs of their visiting chloroplasts, but how? When looking for possible genes that could support chloroplast survival and photosynthesis, James Manharte and other researchers noticed that a key algal gene, psbO, was in the sea slug’s DNA. psbO is a critical gene because it encodes for a manganese-stabilizing protein which is part of the photosystem II complex of the chloroplast.
Most importantly, the DNA sequence of the sea-slug gene and the algal gene were almost identical. It seems as if the sea slug had, long ago, borrowed the gene from the algae and never returned it. This opened up the exciting possibility of horizontal gene transfer (HGT), where a gene from an organism is “horizontally” moved from one species to another. This is in contrast to “vertical” gene transfer, where DNA moves between one generation and the next.
But how could these researchers be sure it was HGT? Initial evidence showed that the gene was already present in the eggs and sex cells of Elysia chlorotica. However, the genes did not seem to be active when subsequent work examined their RNA, with further analyses in 2017 indicating that there was actually little evidence of these genes in the egg (germline) DNA. Thus, while the mechanism of how chloroplasts captured by Elysia chlorotica can survive so long is still somewhat of a mystery, it is clearly possible, and it might have been helped by HGT.
Another example of HGT comes from tardigrades, which are the famed “water bears” that can survive in the vacuum of space (also featured in chapter 4). Dozens of tardigrades’ genes are likely to be derived from HGT and may also contribute to the biology of the organism. This process of “fluid genes” between species is a key driver of evolution because millions of years of selection pressure in one context can suddenly be positioned in an entirely new context for a new genetic enrichment of features and functions.
Could humans mimic our thieving friends, Elysia chlorotica, and photosynthesize instead of always having to eat with our mouths? To get chloroplasts to work in humans, we would have to make some big assumptions. The first assumption is that human skin cells would be capable of supporting the chloroplasts. This support would require our immune system to not reject them and that melanin (the pigment that gives skin its color) would not interfere with chloroplasts’ functions. Beyond this, the chloroplasts would need to survive and be functional, but the Elysia chlorotica system shows it is possible.
The next assumption we have to make concerns the chloroplast photon capture efficiency within its new, human host. No chemical reaction is ever 100 percent efficient, mostly because of the second law of thermodynamics, biophysical limits of efficiency, and other errors. So, what percent of the sun’s energy should we assume the new “green human” can capture? Estimates suggest that plant efficiency of capturing photon is only about 5 percent. So, we will assume that the new “chloroskin” cells would act similarly.
The next question is about how much energy we could get from our chloroskin. On average, each human has about 1.7 square meters of skin, but even if fully naked, only half of that skin would likely be exposed to the sun (e.g., when you’re lying on your stomach). On a bright day, sunlight energy levels are about 300 watts per square meter, which is enough to power a normal light bulb for about three hours. Finally, to be conservative, assume the photosynthesis biochemistry inside of the chloroplast is only 75 percent efficient. Given that input, the chloroskin would only collect about 34 kilojoules of energy per hour. An average-sized human needs about 10 million joules per day to survive.
Thus, for a human to function at normal energy levels, 290 hours of midday sunlight would be needed to collect enough energy to get through one day. However, to reach the required energy, more skin could do the trick. If a human epidermis was expanded 300-fold (1.7 m2 × 300), which is about the size of two tennis courts, a chlorohuman lying on their stomach would only need to sit in the sun for about one hour. Therefore, a chlorohuman could go on a lunch break, unfurl their newfound skin in a large empty field somewhere, get a meal while maybe taking a nap, and then close up their skin and head back inside, full and satiated.
Given that chloroplastic kleptomaniacs exist in the animal kingdom, it should come as no surprise that there are other small, mobile molecules moving between species too. In 2010, Alain Robichon discovered high levels of carotenoids in aphids, small insects that can be found in leaves around the world. This, on its own, isn’t too strange, considering animals need carotenoids for a variety of cellular functions including vision, coloration, and vitamin processing. The peculiarity comes from previous research by Nancy Moran and Tyler Jarvik, which showed carotenoids were not present in the diet of aphids. The orange and red organic pigments, which give the characteristic autumn colors to pumpkins and tomatoes, were thought to be made only by plants, algae, bacteria, and fungi—yet here is an insect that apparently could make them on its own.
Robichon and his team set out to discover what these small insects could possibly be doing with such high levels of, apparently, synthesized or stolen carotenoid. They first noticed that cells with high levels of carotenoids also had elevated levels of adenosine triphosphate (ATP)—essentially the gasoline of the cells. They then noticed that the levels of ATP would change depending on the insect’s exposure to light. Place the insect in light, ATP goes up; put it in the dark, ATP goes down. To further test their response to light, the team split the aphids into two teams: those with high levels of carotenoids and those with lower levels. As expected, the group with higher carotenoids was able to absorb more light. The team further showed that the carotenoids in the aphids were close to the surface (0–40 nm), exactly what one would expect if the carotenoids were being used to capture sunlight.
Then, in 2012, Moran and Jarvik completed a phylogenetic analysis that identified genes in the insects which were almost identical to those in the carotenoid pathway in fungi. They looked at thirty-four aphid species around the world and noted that all aphids had at least one copy of this gene (lycopene cyclase/phytoene synthase), and some aphid species even had seven. In contrast, all fungal genomes only have a single copy. The closest-living relatives of aphids, called adelgids, also showed evidence of possessing this pathway. Thus, given enough time, the genes from one entire kingdom of life can move into another and further provide entirely new functions.
Importantly, these are not the only examples of genes moving from one organism to another. HGT has been shown in bacteria to fungi (Saccharomyces cerevisiae), bacteria to plants (Agrobacterium), bacteria to insects (Wolbachia, in beetles and bed bugs), organelle to organelle (in parasites of the Rafflesiaceae), plant to plant (hornwort to ferns), fungi to insects (as above with pea aphids), human to parasite (Plasmodium vivax), virus to plant (tobacco mosaic virus), and possibly plants to animals (Elysia species, above). The most scandalous and extensive HGT that has ever been observed is a jump from bacteria to animals in small mites that live throughout the world’s oceans, bdelloid rotifers. An estimated 8 percent of their genes are derived from bacteria.
However, the most striking example of genes moving is when they do so en masse. The endosymbiosis theory of mitochondria and chloroplasts holds that, at some point, these “minibacteria” were ingested or merged with eukaryotic cells. Rather than die or split up, they decided to marry—and have been together in loving cellular matrimony ever since. This enabled not just one gene to get moved, but entire networks, membranes, and new abilities. For example, the ATP in human cells, on which human biology depends for sustenance and existence, is not even made by the human component of a cell; it is created in the mitochondria.
Notably, the transfer of genes from the mitochondria to the human genome, and vice versa, is a process that is still ongoing. Nuclear mitochondrial DNA segments (NUMTs) are a result of this engagement and exist where the mitochondrial genes have migrated, like nomads, from the mitochondria to the human nucleus. The DNA in our cells works regardless of where it came from, meaning that our gene networks do not choose their place in our cells with any regard to their history; rather, their place is determined based on what is needed. This same principle that applies to life on Earth can be readily applied beyond Earth as well. Given these widespread and pervasive examples of exchange of DNA between species, it is not unexpected, or even unnatural, to begin to think about doing so in human cells. Because our own human lineage only provides evolutionary lessons from the past few million years, we would be better served by taking the lessons from billions of years of evolution for us to survive on faraway worlds.
Excerpted from The Next 500 Years: Engineering Life to Reach New Worlds, by Christopher E. Mason, published by The MIT Press.
Christopher Mason is the author of The Next 500 Years: Engineering Life to Reach New Worlds (The MIT Press, 2021) and a professor of Physiology and Biophysics at Weill Cornell Medicine in New York, New York.