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I just read, or re-read, a story by Alan Dean Foster from 1983, "Village of the Chosen."  A married couple genetically engineer an alga called Chlorella  to provide photosynthesis for humans.  It's already symbiotic with a hydra, possible is this?  We are of course, already symbiotic with plenty of other organisms...
This is mentioned in the Fasting Lifestyle page, and is impractical for a human or near-human because they don't have enough surface area to capture enough sunlight for their energy needs. Though it might be useful for a tweak with increased surface area (e.g. in the form of wings) and/or decreased energy needs. Note that if you want to get useful amounts of energy from photosynthesis, it would also be necessary to spend the day exposed to the sun and either go naked or wear transparent clothing.
The idea of humans being modified in some way to photosynthesize has been around for decades in SF. As Tardigrada indicates, it consistently runs up against issues of insufficient surface area and the need to be exposed (in both senses of the word) to sunlight for long periods to produce any significant effect.

An alternative might be a system where the modification uses sunlight to power the generation of some nutrient or hormone or something that humans need for good health, but which isn't needed to the degree that humans need food to keep their bodies going.

Another option might be photosynthesis as an emergency backup system to keep body and mind together when all other sources of nutrition are unavailable. You're not going to have the energy to do much more than lie in the sun and just keep breathing, but at least you'll still BE breathing. Not sure if this would work, but throwing it out there.

Finally, using a thin film solar collector that can safely cover your skin (or is incorporated into your skin) might be a much better option. IIRC photosynthesis is not very efficient and even existing solar cells do a much better job at converting sunlight to useful energy. Whether this would be paired with a system to convert the energy into something biology can use, be limited to powering cyborg components, or be intended to power a purely cybernetic being (a vec) is an open question.

Olaf Stapledon was aware of the fact that humans do not have enough surface area to support their metabolism via photosynthesis. His Plant-men were sessile during the day, and were connected to a much larger rosette of light-collecting leaves, from which they could detach and wander about at night.

Plant-derived sophont clades like Keruing also rely for most of their life on a much larger tree-like structure, which has a larger light-collecting surface and supports the mobile morph by secreting milk-like sap. For a short period of time the Keruing live independently, until they run out of stored energy and are forced to become rooted once more.
As others said, photosynthesis is not enough to sustain an animal due to the much higher metabolic energy requirementes.
Still, I'd like to point that the photosynthesis of natural plants goes up to just a mere 4% in the C4 plants,which employs a biological shuttle to concentrate the CO2 near the pockets that contain the CO2 fixing enzymes. A solar panel is up to 20%.
The efficiency of the parts of the photosynthetic systems is quite high but you have a lot of loss here and there, here a list:

Overall, even mantaining a humanoid form, the surface area in particula, I presume that a good hyperturing total reworking of a modosophont could somewhat allow that, if it's possible to both increase brutally the photosynthetic efficiency and reduce energy consumption (e.g.: increase the efficiency of the muscle and nerve transmission). Still good luck staing alive during the night...

Whitout hyperturing I guess there is no way to achieve it.

Anyway, if you need to do this in any period of OA you are in big troubles or you have been memed to hell Tongue
Using vegetal resources as a source of nutrients and energy seems quite feasible to me if the vegetal resources are a large enough fraction of the total. We do that already, after all. They’re just not a very intimate part of us.

I read a story recently about the end of the world. It described a very large plant which had people (not much like us) as mobile fruit. They could detach themselves to wander after sleeping for long periods of time. Eventually the available resources became too limited and the wanderers were recalled to be absorbed to nourish the plant. The story described the experiences of one such “fruit”. Sorry, I don’t recall the author. It was the final chapter of a much longer novel describing the future of mankind.
The winning combination for plants is photosynthesis combined with low-energy nutrients available for not too much expenditure of energy.  IOW, leaves and roots. 

The winning combination for animals is oxidation of much higher-energy food in much greater quantities.  And access to nutrients suitable to release energy by oxidation requires mobility (instead of roots) and decisions about how to use energy.

It's not very easy to find a good combination or compromise between these strategies. 

We can take the example of lifeforms very efficient in their use of energy.  Cold-blooded reptiles get along on less than a tenth of the food mammals need for similar body mass.

Even so, if there are photosynthetic people, even highly efficient ones, I'd still expect it to be very minor as a source of energy.  If they are acting like people - moving around and thinking, etc - I would expect that they still need to eat higher energy nutrients.
Of note is that photosynthesis has been found in quite a wide range of animals, like sea slugs, salamanders, hornets and aphids: So it seems that at least a salamander-like metabolism is possible with photosynthesis (though I'm not sure how much energy these salamanders get from photosynthesis).
Quote:Of note is that photosynthesis has been found in quite a wide range of animals, like sea slugs, salamanders, hornets and aphids: So it seems that at least a salamander-like metabolism is possible with photosynthesis (though I'm not sure how much energy these salamanders get from photosynthesis).

The interesting one here is the photosynthetic sea slug, as the article correctly say it harvest chloroplast eating algae. The part on the horizontal gene transfer is important but not well explained: what make this seal slug stand out from the rest is the fact that it is capable to integrate and mantain the chloroplast in its body for a very long time and this is possible only due to the transfer of genes that are responsable for the chloroplast manteinance.
As wrote, horizontal gene transfer is the exception among animals (preservation of your own genetic code) but much more common in bacterias (at times, usually when food is not abundant, some bacteria actively scavenge DNA from their surronding to "fish" new metabolic pathways).
What the slug pass to the offspring are the genes for the chloroplast manteinance, not the chloroplast themselves.

The slug can indeed survive months only on the chloroplast action... but it is a slug: flat, with a very large surface area compared to the body.
Also, slug are not that active metabolically, they don't have to mantain a certain body temperature and you have to factor that in water you don't have to support your body against gravity.

The sea slug is the only know animal, IIRC, that get a significant part of its energy intake from photosyntesis.
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