Photosynthetic aliens or humans?

[Ambri]

I've been toying with the idea of creating a character who is either an alien or modified human who is capable of deriving part or all of her "food/ energy" from photosynthesis. Would this be believable/ feasible? I don't have enough of a hard science background to really figure out how long such a person would have to "sunbathe" each day to stay healthy or well fed, or whether such a person would have green skin, whether they'd have to eat regular food in addition to this, or could "tap into" the nutrients of the soil to get minerals, and stuff. Thoughts? Ideas?

Kinda gives a whole new twist to the "green skinned aliens" thing, huh? ;)

[Julie Worth]

I've been toying with the idea of creating a character who is either an alien or modified human who is capable of deriving part or all of her "food/ energy" from photosynthesis. Would this be believable/ feasible?

It's been done in SF, I'm pretty sure. And there's at least one real animal that can do this--a photosynthetic sea slug (http://www.newscientist.com/article/dn16124-solarpowered-sea-slug-harnesses-stolen-plant-genes-.html).

[Liosse de Velishaf]

Two words: John Scalzi

[Tanydwr]

Regarding the skin colour - chlorophyll's green because it reflects green light and absorbs the rest. There are other chemicals in plants that can absorb light which are different colours - I think. (I'm basing this on vague memories of GCSE and A-Level Biology.) I mean, you get brown and black seaweed. Do a bit of research into photosynthesis before you do anything else. You can always invent a chlorophyll alternative.

Otherwise, looks like an interesting idea, just be aware that it does put limits on your characters if they need sunlight to function - does it mean they have a 'deciduous' or 'hibernation' period? Do they flag without enough sunlight, or can they store up energy? As for water and minerals, that depends on if they have a digestive system or not. Plants get water and minerals through their roots, via osmosis and diffusion. Water is then taken through the plant via tubes called xylem, which open at the end in the leaves to release water (keeping a constant flow of water through the plant called transpiration). Minerals, as well as the sugars created in the plant, are moved around via tubes called phloem. Obviously, if your person has a form of digestive system or blood circulatory system, I would suggest that eating ordinary food would be a good idea. Plus an active, moving body is going to need a hell of a lot more glucose (i.e. sugars, bringing about energy) than a plant. And that's ignoring the potential effects of child-birth.

Still, it sounds like a very interesting idea.

[SPMiller]

Part, maybe. Human bodies have a low surface-area-to-mass ratio.

[Lhun]

While certainly possible, humans have nowhere near the surface to mass ratio to produce any significant portion of their food via photosynthesis. So i'd be pretty pointless. Now, if there's a lot of sun input, and not a lot of energy used, it might make a difference. But we're talking hibernation sleep out in the open in the middle of summer here. Sunlight has an intensity of about 1kW per square meter, measured at a surface perpendicular to the rays. Now, photosynthesis can use around 10% of that, at best. (hard limit)
Which means, that with a surface area of about 2m² (big person), and really great efficiency, sunbathing could generate about 180 calories with an hour of full light. Now, in the Sahara, you could power a sedentary lifestyle with photosynthesis, but anywhere with clouds, or during winter, or where you're not in full sunlight, and it hardly matters.
On another note, photosynthesis could provide calories (starch and sugar) but it can not provide other required substances, so unless there is some really great closed-loop system in the body recycling those, some food would still be required.
As an aside, a person generating all their energy from photosynthesis wouldn't need to breathe. Shouldn't breathe in fact, since exhaling CO² means losing carbon, and photosynthesis can't generate that from nothing.
Alternatively, they could do like plants do, and have two different metabolic processes running, in daylight exhaling net oxygen and absorbing carbon dioxide from the air, and without light, exhaling CO² like everything else.

Roots have nothing to do with photosynthesis, they're just big webs which absorb various substances from the ground. No reason why a person or some exotic animal couldn't use them, besides the obvious: those roots need to be in the ground, which gives one the mobility of a tree. But something like a bunch of root-like appendages which are used to absorb various substances from bodies of water would be plausible. Or at least possible.

Skin color would be relatively arbitrary. Whatever portion of light doesn't get absorbed determines colour. Photosynthesis absorbs very narrow lines of radiation so the color still mostly depends on pigment. What part of the light gets actually absorbed is determined by the exact chemical photosynthesis process that takes place (there's more than one possible).
Even white (as in paper-white, not the pinkish skin-white) is theoretically possible, if the photosynthesis works at a very low efficiency, and enough light is still reflected to make it look white. That's counter-productive of course.

[Canotila]

Darker pigment would likely be better for a mobile organism. If you look at desert plants with access to plenty of light, even in cold high desert areas, they are often a pale or yellowish green. Plants that grow in darker forest understory, and underwater where light is limited tend to be much darker in pigment. Either from high chlorophyll concentrations or from a different substance like the brown and red algae that was mentioned earlier.

I would go with an organism that could also ingest things in addition to photosynthesizing. It's even necessary for some plants who can't get the minerals they need from the soil.

[third person]

Damn, you guys are smart. I need to start hanging around these parts to soak up some knowledge.

[defcon6000]

Perhaps don't have your hybrid human solely depend on photosynthesis for food, since it's clear they could never survive on it. Like, they would still need protein for tissue repair. Maybe have it more as a supplement, like they won't have a need to eat veggies (would be funny if they thought eating veggies was a sort of cannibalism :tongue) and etc.

[SPMiller]

Perhaps don't have your hybrid human solely depend on photosynthesis for food, since it's clear they could never survive on it. Like, they would still need protein for tissue repair. Maybe have it more as a supplement, like they won't have a need to eat veggies (would be funny if they thought eating veggies was a sort of cannibalism :tongue) and etc.Oh, wow. That would enrage vegan organizations the world over. What a great damn idea.

[kposa]

Hmmm, upon reading the OP, I thought of Aprilynne Pike's debut novel "Wings". I don't want to spoil the book for those who haven't read it, but from what I remember, her main character (Laurel) has a little bit of this quality (photosynthetic). It wasn't a big factor in the story, but it was simply part of who Laurel was (or what she was).

[waylander]

Colin Harvey also uses the idea of a modified human who can supplement his eating with photosynthesis in his recent book 'Winter Song'

[Julie Worth]

Part, maybe. Human bodies have a low surface-area-to-mass ratio.

That's why little green men are little.

[shaldna]

Ooh, Ooh, Farscape.

Zaahn was a plant, and she needed light etc to live. And once, she bloomed, it was all a bit chaotic.

Her body structure was plant based too - that is , a seried of fibres rather than bones, so she healed differently than others.

She did eat food though, but I would check her out.

[Liosse de Velishaf]

There's an anime, Wolf's Rain, where one of the characters is a plant, and she needs sunlight and water to live. Not very scientific, though, and no green skin.

[Ambri]

Oh, wow. Thanks, guys, for all the replies, especially the bits about science. SPMiller and Lhun--thanks for the reminder of the surface-area-to-mass ratio; that's a rather significant factor I didn't take into consideration! After all, unlike humans, trees can have big ol' crowns/ canopies of leaves . . .

Hey . . . how about photosynthetic HAIR? Then I would have an "excuse" for having a "flower child" with crazy long hair in some wild color . . . it could even change colors with the seasons!

Oh, wow. That would enrage vegan organizations the world over. What a great damn idea.

LOL that would be highly amusing. "Lettuce live; eat beef." ;)

[Canotila]

Oh, wow. Thanks, guys, for all the replies, especially the bits about science. SPMiller and Lhun--thanks for the reminder of the surface-area-to-mass ratio; that's a rather significant factor I didn't take into consideration! After all, unlike humans, trees can have big ol' crowns/ canopies of leaves . . .

Hey . . . how about photosynthetic HAIR? Then I would have an "excuse" for having a "flower child" with crazy long hair in some wild color . . . it could even change colors with the seasons!



LOL that would be highly amusing. "Lettuce live; eat beef." ;)

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[Liosse de Velishaf]

Oh, wow. Thanks, guys, for all the replies, especially the bits about science. SPMiller and Lhun--thanks for the reminder of the surface-area-to-mass ratio; that's a rather significant factor I didn't take into consideration! After all, unlike humans, trees can have big ol' crowns/ canopies of leaves . . .

Hey . . . how about photosynthetic HAIR? Then I would have an "excuse" for having a "flower child" with crazy long hair in some wild color . . . it could even change colors with the seasons!



LOL that would be highly amusing. "Lettuce live; eat beef." ;)


Well, it wouldn't really be "hair"...

[Ambri]

Well, no, it wouldn't really be "hair" . . . but it would serve a similar look and function. I might end up with a version of this in fantasy rather than SF; maybe a somewhat modified version of wood nyphs or naiads . . .

Does anyone ever wonder how single-sexed fantastical creatures, like naiads, fauns, and centaurs, reproduce? I guess maybe that would account for the centaurs' always running off with human women . . . although the mechanics of such things might make for some awkward scenarios, lol . . .

Am I allowed to derail (or unravel) my own thread? ;)

[Maxx]

Well, no, it wouldn't really be "hair" . . . but it would serve a similar look and function. I might end up with a version of this in fantasy rather than SF; maybe a somewhat modified version of wood nyphs or naiads . . .

Does anyone ever wonder how single-sexed fantastical creatures, like naiads, fauns, and centaurs, reproduce? I guess maybe that would account for the centaurs' always running off with human women . . . although the mechanics of such things might make for some awkward scenarios, lol . . .

Am I allowed to derail (or unravel) my own thread? ;)

You could have a two-part organism: a personable part who can go sleep in a shrub-part and get food from the shrub and give the shrub nutrients it lacks. The person part could of course call their in-the-shrub "vegetating" or something. The Shrub would be less articulate, maybe.

[Ardent Kat]

Humans have tried it. Check out Inedia/Breatharianism (http://en.wikipedia.org/wiki/Inedia):

Breatharianism is a related concept, in which believers claim food and possibly water are not necessary, and that humans can be sustained solely by prana (the vital life force in Hinduism), or according to some, by the energy in sunlight (according to Ayurveda, sunlight is one of the main sources of prana). The terms breatharianism or inedia may also refer to this philosophy practised as a lifestyle in place of the usual diet.

[RemusShepherd]

Poul Anderson did this in his novel "Fire Time". He had a centauroid race whose manes were formed out of a symbiotic plant. They ate normal food most of the time, but the mane plants aided their metabolism and could serve as a substinence-level food in an emergency.

[sunandshadow]

One of Nancy Kress's novels in the Beggars series has the people of near-future earth being converted to be photosynthetic. I'm thinking it happens right at the end of the third one, but not sure. The overall idea was that people hate working, and a group of working people makes a huge ecological footprint, so making sunlight take care of part of their nutritional needs was good for both people and the environment. Also the plant-ified people quit wearing clothes, at least when it wasn't snowing, so that also made there be less working and ecological footprint necessary to produce clothing.

[dclary]

Triffids?

[Smiling Ted]

"Desolation Road," by Ian McDonald.

[MGraybosch]

I've been toying with the idea of creating a character who is either an alien or modified human who is capable of deriving part or all of her "food/ energy" from photosynthesis. Would this be believable/ feasible? I don't have enough of a hard science background to really figure out how long such a person would have to "sunbathe" each day to stay healthy or well fed, or whether such a person would have green skin, whether they'd have to eat regular food in addition to this, or could "tap into" the nutrients of the soil to get minerals, and stuff. Thoughts? Ideas

John Scalzi flirted with this idea in Old Man's War. It's plausible, given that humans already synthesize Vitamin D through exposure to sunlight.

[yttar]

Personally, I think photosynthetic humans are about as plausible as giant insectoids. But big name authors, who I'm pretty sure write hard SF have gotten away with both. So I guess it comes down to how plausible you make it in your writing.

As mentioned Scalzi did photosynthetic humans and both Heinlein (Starship Troopers) and Card (Ender's Game) had giant insects.

Yttar

[rmgil04]

Personally, I think photosynthetic humans are about as plausible as giant insectoids. But big name authors, who I'm pretty sure write hard SF have gotten away with both.


Old Man's War was Scalzi's first novel, so he made it work before he was a big name author. OMW is one of my favorite books also.

[MGraybosch]

Personally, I think photosynthetic humans are about as plausible as giant insectoids.

IIRC, the enhanced soldiers in Scalzi's novel didn't depend solely on photosynthesis for sustenance. It was an enhancement meant to help them stretch their rations should they be stranded in hostile territory.

[M.C.BROWN]

First thing I asked myself when I read your post was about the solar cycle or if the planet this is happening on is earth. If not, I would do this some how changing the orbital patterns of earth. I would find it easier to do on another planet where the plant life varied in a manner to enable the human/alien to photosynthesize. Like the plants and human/alien in a symbiot type relationship.
This is a huge topic. can be very detailed. let me know if this helps. I've been working on a novel with alot of exotic plant life. I really have been meaning to do some reseach, but usually get caught up in writing.

[blacbird]

It's been done in SF, I'm pretty sure.

A classic: The Day of the Triffids, John Wyndham.

caw

[Thomas_Anderson]

Even if our bodies were designed to do it, I don't think photosynthesis produces enough energy to do things like move around. Hence why plants never evolved to do so (except a handful of flytrap plants, but that's just one jaw).

I'm no expert, but it seems if photosynthesis could produce enough energy for locomotion, we'd animals with it. Not having to search for food is an extremely useful trait, and if it were possible to use photosynthesis for sentient beings, evolution would've already produced it.

[Aservan]

Remember the other law of the universe all changes have consequences. If you want to make people photosynthetic then they have to give something up. You can't just keep adding things to an organism willy-nilly.

It's one of those things that's a neat idea, but you have to ask why? If you still need to eat is photosynthesis really necessary? What does it get you? Why would an organism evolve this trait?

Maybe on the world this creature evolved solar radiation is through the roof and the organisms evolved a way to "eat the sunlight" rather then have it burn them. In this case using sunlight to move becomes a survival mechanism. Then they fought over rich places to get nutrients. Remember plants are not peaceful fluffy trees.

Plants fight with nasty biological warfare against each other. They drown out sunlight. They poison soil. They soak up all the water. They choke competitors roots.

They look peaceful to us, but are anything but.

[Polenth]

I'm no expert, but it seems if photosynthesis could produce enough energy for locomotion, we'd animals with it. Not having to search for food is an extremely useful trait, and if it were possible to use photosynthesis for sentient beings, evolution would've already produced it.

There are animals who can photosynthesise - green sea slugs. The fact it's not more common is due to how evolution went on Earth, not because it's impossible for it to evolve. Basically, the original ancestor of animals couldn't photosynthesise, which makes it less likely for animals to evolve the ability. On another planet, perhaps the original ancestor could photosynthesise.... which would mean more moving plant-animals critters.

(I wrote a blog post awhile back about some of the basic issues of being a photosynthetic animal, which might interest people: http://polenth.blogspot.com/2010/03/inspired-by-nature-rise-of-planimals.html )

[Canotila]

I'm no expert, but it seems if photosynthesis could produce enough energy for locomotion, we'd animals with it. Not having to search for food is an extremely useful trait, and if it were possible to use photosynthesis for sentient beings, evolution would've already produced it.

Photosynthetic flagellates prey upon other single celled organisms, and photosynthesize when light is available. They are also highly mobile.

Even plants that live in nutrient poor environments need to supplement their diets by consuming other organisms. The problem with having a completely photosynthetic mobile organism is they don't have root structures to absorb minerals like plants do. They have to supplement to obtain those minerals.