Feasibility of photosynthesis in mammals/(trans/post/humans)

[AvatarIII]

basically i was thinking about how possible it would be to supplement energy intake in humans by putting chloroplasts in mammalian cells, either all the cells of the body, or just the skin cells, whichever would make more sense. the idea has been used in several SF books i've read, but none of them really went into detail about the science of it.
i had a look in a few other places, but other than just conjecture about chloroplasts being too big for mammalian cells, the surface area of animals being to low to have any noticable increase in energy intake, and the fact that cellular physiology might be wrong to sustain chloroplasts (which for the sake of a SF story, could be handwaved through genetic engineering) I was wondering about the actual science, not just conjecture.

[Soralin]

http://en.wikipedia.org/wiki/Elysia_chlorotica

It's not a mammal, but it's an animal that can photosynthesize. It's a sea slug, and it eats algae, and strips out their chloroplasts and incorporates them into it's own cells.

Elysia chlorotica feeds on the intertidal algae Vaucheria litorea by puncturing the algal cell wall with its radula. The slug then holds the algal strand firmly in its mouth and, as though it were a straw, sucks out the contents.[3] Instead of digesting the entire cell contents, or passing the contents through its gut unscathed, it retains only the algal chloroplasts, by storing them within its own cells throughout its extensive digestive system. The acquisition of chloroplasts begins immediately following metamorphosis from the veliger stage when the juvenile sea slugs begin to feed on the Vaucheria litorea cells.[4] Juvenile slugs are brown with red pigment spots until they feed upon the algae, at which point they become green. This is caused by the distribution of the chloroplasts throughout the extensively branched gut.[3] Initially, the slug needs to continually feed upon algae to retain the chloroplasts, but over time the chloroplasts become more stably incorporated into the cells of the gut enabling the slug to remain green without further feeding.

The incorporation of chloroplasts within the cells of Elysia chlorotica allow the slug to capture energy directly from light, as most plants do, through the process known as photosynthesis. This is significantly beneficial for Elysia chlorotica because during time periods where algae is not readily available as a food supply, the Elysia chlorotica can survive for months on the sugars produced through photosynthesis performed by their own chloroplasts. Kept within the slug's own cells, it has been found that the chloroplasts can survive and function for up to nine or even 10 months.[5] In one study Elysia chlorotica were deprived of alga ingestion for a period of eight months. After the eight month period, despite the fact that the Elysia chlorotica were less green and more yellowish in colour, the majority of the chloroplasts within the slugs appeared to have remained intact while also maintaining their fine structure.[4] Although Elysia chlorotica are unable to synthesize their own chloroplasts, the ability to maintain the chloroplasts acquired from Vaucheria litorea in a functional state indicates that Elysia chlorotica must possess photosynthesis-supporting genes within its own nuclear genome; most likely acquired through horizontal gene transfer.[5] Since chloroplast DNA alone encodes for just 10% of the proteins required for proper photosynthesis, scientists investigated the Elysia chlorotica genome for potential genes that could support chloroplast survival and photosynthesis. The researchers found a vital algal gene, psbO (a nuclear gene encoding for a manganese-stabilizing protein within the photosystem II complex[5]) in the sea slug's DNA, identical to the algal version. They concluded that the gene was likely to have been acquired through horizontal gene transfer, as it was already present in the eggs and sex cells of Elysia chlorotica.

[idobox]

The main issue here would be power density. Humans are endotherm animals that consume a lot of power, even at rest. Photosynthesis could be useful in times of food shortage, but I doubt it could sustain a human alone.

[Meteorswarm]

It would also be interesting to transfer plants' UV-resistance to humans.

[AvatarIII]

The main issue here would be power density. Humans are endotherm animals that consume a lot of power, even at rest. Photosynthesis could be useful in times of food shortage, but I doubt it could sustain a human alone.

yeah, i figured that, part of the reason why i said suppliment energy intake, i know there is no way something like a mammal could exist on photosynthesis alone.

http://en.wikipedia.org/wiki/Elysia_chlorotica

It's not a mammal, but it's an animal that can photosynthesize. It's a sea slug, and it eats algae, and strips out their chloroplasts and incorporates them into it's own cells.

Elysia chlorotica feeds on the intertidal algae Vaucheria litorea by puncturing the algal cell wall with its radula. The slug then holds the algal strand firmly in its mouth and, as though it were a straw, sucks out the contents.[3] Instead of digesting the entire cell contents, or passing the contents through its gut unscathed, it retains only the algal chloroplasts, by storing them within its own cells throughout its extensive digestive system. The acquisition of chloroplasts begins immediately following metamorphosis from the veliger stage when the juvenile sea slugs begin to feed on the Vaucheria litorea cells.[4] Juvenile slugs are brown with red pigment spots until they feed upon the algae, at which point they become green. This is caused by the distribution of the chloroplasts throughout the extensively branched gut.[3] Initially, the slug needs to continually feed upon algae to retain the chloroplasts, but over time the chloroplasts become more stably incorporated into the cells of the gut enabling the slug to remain green without further feeding.

The incorporation of chloroplasts within the cells of Elysia chlorotica allow the slug to capture energy directly from light, as most plants do, through the process known as photosynthesis. This is significantly beneficial for Elysia chlorotica because during time periods where algae is not readily available as a food supply, the Elysia chlorotica can survive for months on the sugars produced through photosynthesis performed by their own chloroplasts. Kept within the slug's own cells, it has been found that the chloroplasts can survive and function for up to nine or even 10 months.[5] In one study Elysia chlorotica were deprived of alga ingestion for a period of eight months. After the eight month period, despite the fact that the Elysia chlorotica were less green and more yellowish in colour, the majority of the chloroplasts within the slugs appeared to have remained intact while also maintaining their fine structure.[4] Although Elysia chlorotica are unable to synthesize their own chloroplasts, the ability to maintain the chloroplasts acquired from Vaucheria litorea in a functional state indicates that Elysia chlorotica must possess photosynthesis-supporting genes within its own nuclear genome; most likely acquired through horizontal gene transfer.[5] Since chloroplast DNA alone encodes for just 10% of the proteins required for proper photosynthesis, scientists investigated the Elysia chlorotica genome for potential genes that could support chloroplast survival and photosynthesis. The researchers found a vital algal gene, psbO (a nuclear gene encoding for a manganese-stabilizing protein within the photosystem II complex[5]) in the sea slug's DNA, identical to the algal version. They concluded that the gene was likely to have been acquired through horizontal gene transfer, as it was already present in the eggs and sex cells of Elysia chlorotica.

hmm interesting. i'm sure any important genes for being able to support chloroplasts could be aquired from them little things. mentioned towards the end of the quote.

[grythyttan]

The average Body Surface Area for an adult is 1.73 m². At a sunny day you can get 1 kW/m² (I think, correct me if I'm horribly off my mark here). However, that is only at a 100% conversion rate. A Sugar cane used for biofuel has a 7-8% conversion rate at peak efficiency.

Let's cover a human in whatever makes a sugar cane so damn efficient. 1.73*0.08=0.1384.

0.1384 kWh isn't so little considering that a human uses something like 2kWh each day.

Now, obviously all of a humans body wouldn't be usable at the same time, and clouds and nighttime and clothes get in the way. But still, lying naked in the sun for an hour would probably be the equivalent of a light snack.

(unless, of course, I have miscalculated everything. Always a possibility.)

[AvatarIII]

thaks for the calculation, that's pretty interesting, although you should have probably halved the body surface area right?
unless everything is mirrored, you can't have sunlight falling on 100% of your body. although i suppose reflected sunlight would photosynthesise too.

bring in the possibility of a culture change to allow for minimal clothing, and/or clothing that lets through as much UV as possible. with your calculation, i could see people in high sunlight areas shaving of a good few hundred calories of required intake per day.

[Thirty-one]

The average Body Surface Area for an adult is 1.73 m². At a sunny day you can get 1 kWh/m² (I think, correct me if I'm horribly off my mark here). However, that is only at a 100% conversion rate. A Sugar cane used for biofuel has a 7-8% conversion rate at peak efficiency.

It's stupid of me to argue when I don't for sure recall the correct number myself, but I believe you're only half right on the energy density. As I recall it is 1 kW/m² on a sunny day.
Agreed with the rest of your post though, it'd still likely end up as a negligible amount of energy anyway.

[Cobramaster]

Here is a link to an article on related science.
http://www.technologyreview.com/blog/editors/25139/

[idobox]

The kW/m2 figure is about right.
Of course, you have to add a cos(alpha) term to account for position of the sun, and you have to use the area of the projection of the human body, which should be a bit less than half the skin surface.

So 1kW/m2, 0.8m2 and 8% efficiency give us 64W.
I think the basal human metabolism is about 100W, but I got the figure from a poster's signature, not the most reliable source.

It would be much more useful for a cold blooded animal.

[AvatarIII]

Here is a link to an article on related science.
http://www.technologyreview.com/blog/editors/25139/

hmm interesting stuff,

[Thirty-one]

..but I got the figure from a poster's signature, not the most reliable source.

Argh! I'll have to completely rewrite my bachelor thesis now.

[Soralin]

Specifically: http://en.wikipedia.org/wiki/Solar_constant

The solar constant, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit (AU) (roughly the mean distance from the Sun to the Earth).

The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1.366 kilowatts per square meter (kW/m²).[1][3][4] The actual direct solar irradiance at the top of the atmosphere fluctuates by about 6.9% during a year (from 1.412 kW/m² in early January to 1.321 kW/m² in early July) due to the Earth's varying distance from the Sun, and typically by much less than one part per thousand from day to day. Thus, for the whole Earth (which has a cross section of 127,400,000 km²), the power is 1.740×1017 W, plus or minus 3.5%. The solar constant does not remain constant over long periods of time (see Solar variation), but over a year varies much less than the variation of direct solar irradiance at the top of the atmosphere arising from the ellipticity of the Earth's orbit.

Not all of that is in wavelengths that are commonly used for photosynthesis though, and some of it doesn't reach the ground, and if your location happens to be anywhere the sun is not directly overhead (or at least where whatever is absorbing is not perpendicular to the sunlight), what you can actually get will be less than that.

bring in the possibility of a culture change to allow for minimal clothing, and/or clothing that lets through as much UV as possible. with your calculation, i could see people in high sunlight areas shaving of a good few hundred calories of required intake per day.

Chlorophyll has peaks in absorption in blue and red light, so letting through UV wouldn't do much, it would have to let through visible light, at least in the blue and red parts of the spectrum.

[gmalivuk]

I think the basal human metabolism is about 100W, but I got the figure from a poster's signature, not the most reliable source.

Yeah, but it's not as if the math is even a little bit difficult. Hell, Google will do it for you directly

[idobox]

I admit I was too lazy to look for a reliable source of what the basal metabolism is, and then how much a kcal is in a "real" unit.

[Izawwlgood]

Oh this again.

Why not add chloroplasts to your skin, and then give yourself a low level GI tract yeast infection. Viola! Alcohol from the sun!

[Meteorswarm]

Oh this again.

Why not add chloroplasts to your skin, and then give yourself a low level GI tract yeast infection. Viola! Alcohol from the sun!

You don't need the chloroplasts, we have plenty of blood sugar derived from food.

The problem would be fermenting without dying from anaerobic bacterial infection.

[Izawwlgood]

The point isn't to get drunk from a pasta platter, but from sugar derived from the sun. And people are fine with these infections, they just can't eat carbs.

[AvatarIII]

Oh this again.

Why not add chloroplasts to your skin, and then give yourself a low level GI tract yeast infection. Viola! Alcohol from the sun!

You don't need the chloroplasts, we have plenty of blood sugar derived from food.

The problem would be fermenting without dying from anaerobic bacterial infection.

maybe, the human immune system?

although this is exactly why i asked, what hurdles would be in the way of photosythesis in humans?

[idobox]

what hurdles would be in the way of photosythesis in humans?

Human cells are unable to support chloroplasts as is. One species of sea slug is able to do that, although it cannot transmit them to its progeny.
You would need to alter the biochemistery of skin cells to make them suitable for chloroplasts. I don't think current understanding is enough to modify humans that way, but in a sci-fi seting, even in the near future, it seems plausible.

[Izawwlgood]

maybe, the human immune system?

Maybe the color purple!
Be specific; what do you mean the human immune system prevents anything we've discussed?

[Meteorswarm]

Human cells are unable to support chloroplasts as is. One species of sea slug is able to do that, although it cannot transmit them to its progeny.
You would need to alter the biochemistery of skin cells to make them suitable for chloroplasts. I don't think current understanding is enough to modify humans that way, but in a sci-fi seting, even in the near future, it seems plausible.

If I understand things correctly, the main problem with supporting chloroplasts is that, in plants, a fair amount of chloroplast genes have migrated to the nucleus, so in order to successfully host chloroplasts, you need to have and express those genes, which is a bit tricky.

However, it's probably entirely doable with today's technology. Who wants to experiment on naked mice?

[grythyttan]

How about using something like tattoos or skin grafts instead? Something that isn't integrated in your genes. Then it just has to avoid being rejected by the body.

[Meteorswarm]

How about using something like tattoos or skin grafts instead? Something that isn't integrated in your genes. Then it just has to avoid being rejected by the body.

Chloroplasts need nuclear support to keep functioning. Unless you go all the way to injecting algal cells (which might work; there are cases of secondary endosymbiosis in some algae) into you, the chloroplasts would at best be useless, and at worst, just die.

[AvatarIII]

maybe, the human immune system?

Maybe the color purple!
Be specific; what do you mean the human immune system prevents anything we've discussed?

sorry, i meant the human immune system would prevent bacterial infections,

[idobox]

How about using something like tattoos or skin grafts instead? Something that isn't integrated in your genes. Then it just has to avoid being rejected by the body.

Yeah! let's grow transgenic moss on our skin

[darthchazza]

Could it also be used to augment breathing? (converting some of the carbon dioxide we would breathe out into oxygen)

[Mokele]

Could it also be used to augment breathing? (converting some of the carbon dioxide we would breathe out into oxygen)

Not substantially - the human metabolic demand for oxygen is just way too high. It's the same reason you can't have a mammal with gills - even if countercurrent exchangers could prevent heat loss at the gills, there simply isn't enough oxygen in water to supply the needs of a fully endothermic animal with a 37C body temperature.

[MHD]

If your gene-tech muscles are this big, you could just add a set of retractable, winglike surface enhancers.

1. Lie down on a sunny day.
2. Unfold sunwings.
3. ???
4. Profit.