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One Variety of Sea Slugs Cuts Out the Energy Middleman
dragonturtle69 writes with this story, short on details but interesting: "These sea slugs, Elysia chlorotica, have evolved the ability to gain energy via photosynthesis. Forget about genetic modifications for sports enhancements. I want to be able to never need to eat again — or do I?"
No, the statement is essentially correct. The slugs harvest (i.e. gain) energy via trapping photons with chlorophyll. They store that energy as chemical bonds in sugar molecules. They then release the energy as needed by metabolizing the sugar. Photosynthesis is the coupled capturing and storing of energy so saying that it "gains" energy via photosynthesis is a reasonable simplification.
I just did a few quick calculations. Assuming humans have 2 square meters of skin, and stood naked in direct sunlight in the best conditions for 8 hours per day, and assuming 5% efficiency for photosynthesis, we would only get enough energy to provide for 11 hours of sleep (250 BTU/hr), 7 hours of sitting still (400 BTU), 4 hours of light work (650 BTU) or 1 hour of heavy work (2400 BTU). We'd still probably need to consume 2/3 or so of our normal caloric intake from food.
Sources:
http://www.solarexpert.com/Heat-theory.html
http://hypertextbook.com/facts/2001/IgorFridman.shtml
http://www.answerbag.com/q_view/514275
http://www.ccmr.cornell.edu/education/ask/index.html?quid=1021
Chloroplasts, just as with mitochondria, have a small DNA genome of their own. Due to the endosymbiotic relationship that has formed between chloroplasts and their photosynthetic hosts, chloroplasts have found it convenient to offload the majority of their genes to the nucleus. It is estimated that about 90% of the genes necessary for photosynthesis are nuclear, with the rest in chloroplasts, so these sea slugs appear to have acquired the nuclear genes, but not the chloroplast genes.
Chlorophyll itself is made in the cytoplasm, and actually requires relatively few new genes for an animal to be able to produce it, since the complicated steps of its biosynthesis are identical to the heme structures it is already able to make. The real difficulty, and one that this sea slug seems to have been able to surmount according to the Wikipedia page, is the production of the "oxygen-evolving complex," a metalloenzyme with a manganese-calcium core which transfers absorbed energy to a bound water molecule to break it into electrons, protons, and molecular oxygen. Heterotrophic organisms don't produce anything like it.
"FDA staff reviewers expressed concern about the number of patients who were left out of the study because they died."