I was reading a recent article – “Changing the nature of human beings” – by Julian Savulescu in the Sydney Morning Herald, and he mentions this:

So one day we could have people with sonar like bats, or people with the ability to create their own energy by photo-synthesising sunlight like plants.

At first I was dismissive of the idea of solar-powered people, but then I remembered reading in a marine biology pamphlet that certain sea slugs are ‘solar-powered‘. I investigated that some more, and it does turn out that certain molluscs have a symbiotic relationship with chloroplasts that they steal from the algae they eat, which – like plants -are organisms that normally utilise chlorplasts. (Rumpho et al, 2000). One molluscan slug species, Elysia chlorotica, can survive for up to nine months without eating: just on light and carbon dioxide (Green et al, 2000), and even then the slugs die of old age not hunger. Still, the chloroplasts die after about six to ten months, and need to be replenished by eating more algae.

Chloroplasts are solar-power plants of the plant cell, just like the mitochondria that animals and fungi rely on (plants have mitochondria too though). Just as mitochondria were once proteobacteria, plastids (of which chloroplasts are the most noteworthy) were once cyanobacteria, and both still have their own DNA and a very bacteria-like membrane. They have evolved to get very comfortable with the relationship, offloading much of their essential genes to the host nucleus, and now they can’t live without their hosts (then again, we can’t live without our endosymbionts).

This means, however, that if we humans wanted chloroplasts for ourselves, or our livestock or pets, we would need to genetically modify the host animal to express proteins required for chloroplast function. It has been estimated that about 70-90% of the genes required for chloroplast function are provided by the plant’s genome (Martin et al, 1998). In the case of the sea slugs, some of these genes appear to exist in the animal’s genome, although probably not enough for the chloroplasts to be able to reproduce. Which is why the slugs use kleptoplasty – removing the chloroplasts from their food.

It would probably be most feasible for chloroplasts, along with the required genes, to be added to skin stem cells and applied as a skin graft, as there is a lot of research in this area for burns victims. This approach has been used to produce proteins in mice (Larcher et al, 2001), and so should be feasible for producing sugar by photosynthesis in humans. At first this graft may require regular replacement, but eventually the chloroplasts will be sustainable within the skin.

There are a few problems (the 5th problem is, in my mind, the biggest too).

1. The immune system may attack the chloroplasts, but maybe they will be safe from antibodies if they are inside the cell (the immune system will attack mitochondria, but only if they are present in the blood).

2. The photosynthesising skin would necessarily be green as that is the colour of chlorophyll. I suppose the melanocytes of human skin could be engineered to produce another pigment, causing the skin to take on a different colour, but then again it might not be such a big problem to be green skinned…unless you are sensitive to Bruce Banner jokes.

3. People may get sunburn and skin cancer when they are out ‘feeding’ on sunlight, as while the red and blue parts of light will be used, the ultraviolet component of sunlight causes damage to living cells. To absorb this before it causes damage, vertebrates have melanins (and humans augment this with sunscreen), and plants/algae (which don’t use UV light) produce screening compounds. It is likely that a derivable sunscreen pigment, which does not darken the skin like melanin, could be produced by melanocytes and absorb the UV-B light. But the idea of endogenous sunscreen is beside the point of this post (to be dealt with another time).

4. The reaction of photosynthesis can be simplified as the following:

6 CO 2(g) + 12 H 2 O (l) + light → C 6 H 12 O 6 (aq) + 6 O 2(g) + 6 H 2 O (l)

It is now obvious why plants need to be watered – there is a net loss of six water molecules for every glucose molecule produced. This would mean that the plant-person (or algae-person) would also need a lot more water than a normal human, which would be a disadvantage in a desert environment.

5. It wouldn’t produce much energy for an active organism like a human. The average human being has 1.8m2 of skin, approximately half of which would be exposed to the sun (if naked and lying as you would if tanning). The Earth is bathed in much energy from the sun, but of that solar radiation only the wavelengths from 400-700nm are usable by plants (termed photosynthetically active radiation, or PAR). Even at midday on a very sunny day, the PAR energy flux density (or, the amount of plant-usable light energy per unit area of ground per second) is only 400W/m2 (Warrington, 1978). Photosynthetic efficiency (amount of light energy converted into usable chemical potential energy) typically is about 3-6%, so let’s assume 5. So the energy produced by a human being lying in the sun for an hour (3600 seconds) at midday would be:

400 J/s/m2 x (0.5 x 1.8m2) x 0.05 x 3600s = 64800J = 64.8kJ (or 15.43 kcal)

By comparison, an apple has about 400kJ of usable food energy. So an hour in the sun is about the same as a sixth of an apple. The daily energy requirements for a human being sit around 10,000 kJ per day, so that’s going to require 150 hours per day of sitting in the sun. Needless to say, that’s impossible.

So, although photosynthetic humans would need less food, it wouldn’t be substantially less. Still, over a large population, it could slightly reduce the need for farmland. In addition, as I alluded to earlier, this could be done to livestock too, and with a large number of livestock that could noticeably reduce the area of land required to feed cattle or horses (hairy animals like sheep or sensitive-skinned animals like pigs may be more difficult, as the hair would reduce the light available for photosynthesis).

So, solar-powered photosynthetic people are possible, but it wouldn’t significantly alleviate food requirements…but it might make a little bit of a difference, until the sun burns out or is clouded out by pollution or something.

Image credit:

The image, of the sea slug, is of the sacoglossan slug Elysia ornata. It was taken by Flickr user budak, and released under Creative Commons BY-NC-SA license.