It's no secret that in the process of evolution green plants, algae and some bacteria have 'learned' to use the energy of the sunlight to produce glucose. To do that they only need water (H₂O) and CO₂. The energy from the sun is absorbed by the pigment chlorophyll and then used to build the molecules ATP and NADPH/H⁺ which later deliver the energy to build glucose (6 H₂O + 6 CO₂ ---> C₆H₁₂O₆ + 6 O₂).

Are animals able to do that as well? Not really ... But amazingly several kinds of animals eat algae without to digest them. As a consequence these algae survive, get into the cells of the animals, keep doing photosynthesis and produce glucose and oxygen.

The 'solar salamander'

By D. Gordon E. Robertson - Own work, CC BY-SA 3.0

Even vertebrates can have these skills: It was already well known before that embryos in the eggs of the salamander Ambystoma maculatum develop slower if there are not enough algae (Oophila amblystomatis) in their surrounding jelly.^[1] At first scientists concluded that there should be a kind of symbiosis whereby the embryos of the salamander profited from the produced oxygen of the algae and the algae could use the nitrogen-rich waste of the embryos. However using the method of transmission electron microscopy Ryan Kerney of Dalhousie University in Halifax found out that other than expected algae lived not only in close proximity of the eggs but actually inside the embryonic cells. Two arising questions come in mind:

How could the algae come into the embryo?
Why doesn't the immune system of the embryonic cells attack the algae (which normally happens with 'unknown' material in cells of vertebrates)?

One possible explanation beside others is that symbiotic algae in the oviducts of adult female salamanders could enter the passing germ cells. Also nearly all salamander cells beware a certain degree of pluripotency. That means even specialized cells can still change into other cell types. Therefore salamanders are able to substitute for example lost extremities. This flexibility could allow salamander cells also to 'learn' to accept algae inside themselves in a process of 'self-recognition' which differs from other vertebrates.
Another interesting observation was that within the salamander cells mitochondria were placed around the ingested algae. Mitochondria are the power plants of the cells and obviously profit from the oxygen and the glucose released by the algae.

Sea slugs: the real 'masters of photosynthesis'

By Karen N. Pelletreau et al., CC BY 4.0, source: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0097477

If they knew about it, maybe some sea slugs would name the salamanders above as "beginners only". :) While salamander cells incorporate complete algal cells, some slug species like Elysia chlorotica only suck the liquid out of the cells of the algae Vaucheria litorea. Hereby they also 'steal' their chloroplasts which is called kleptoplasty. :) Thus the algal chloroplasts get into the cells of the slugs where they still keep working and doing photosynthesis.^[2] Amazingly the lifespan of the chloroplasts within the slugs lasts longer than it does within the algal cells. According to a recent study published in The Biological Bulletin^[3] the slugs do not only incorporate the algal chloroplasts but in addition there can actually happen a transfer of nuclear algal genes into the genome of the slugs! These genes code for chloroplast proteins and chlorophyll synthesis and are active within the slugs. At least one of the genes, prk, has also been verified to exist in larval slugs, that means it becomes transmitted into the next generation of slugs. It plays an important role by maintaining the chloroplasts and allowing for carbon fixation.^[4] This also helps to understand why the absorbed chloroplasts survive so long within the slugs.

By the way chloroplasts contain their own genes (like mitochondria as well) and their predecessors have been independent organisms. This is explained by the endosymbiotic theory. You can read more about that in one of my other steemit articles.^[5]

Will genetic engineering help us one day to do the same like the slugs? To lie on the beach, reading a book and doing some photosynthesis at the same time? Some scientists do believe exactly that ...