Leslie_2009_Origin_of_photosynthesis - On the Origin of...

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Try to picture the world without photo- synthesis. Obviously, you’d have to strip away the greenery—not just the redwoods and sunflowers, but also the humble algae and the light-capturing bacteria that nourish many of the world’s ecosystems. Gone, too, would be everything that depends on photo- synthetic organisms, directly or indirectly, for sustenance—from leaf-munching bee- tles to meat-eating lions. Even corals, which play host to algal partners, would lose their main food source. Photosynthesis makes Earth congenial for life in other ways, too. Early photosynthesizers pumped up atmospheric oxygen concentra- tions, making way for complex multicellular life, including us. And water-dwellers were able to colonize the land only because the oxygen helped create the ozone layer that shields against the sun’s ultraviolet radiation. Oxygen-producing, or oxygenic, photo- synthesis “was the last of the great inventions of microbial metabolism, and it changed the planetary environment forever,” says geo- biologist Paul Falkowski of Rutgers Univer- sity in New Brunswick, New Jersey. Given its importance in making and keep- ing Earth lush, photosynthesis ranks high on the top-10 list of evolutionary milestones. By delving into ancient rocks and poring over DNA sequences, researchers are now trying to piece together how and when organisms first began to harness light’s energy. Although most modern photosynthesizers make oxygen from water, the earliest solar-powered bacteria relied on different ingredients, perhaps hydro- gen sulfide. Over time, the photosynthetic machinery became more sophisticated, even- tually leading to the green, well-oxygenated world that surrounds us today. In the lab, some biochemists are recapitulating the chemical steps that led to this increased complexity. Other researchers are locked in debates over just when this transition happened, 2.4 billion years ago or much earlier. Looking so far into the past is difficult. The geological record for that time is skimpy and tricky to interpret. Eons of evo- lution have blurred the molecular vestiges of the early events that remain in living organisms. But “it’s a terribly important problem,” says biochemist Carl Bauer of Indiana University, Bloomington, one well worth the travails. To catch a photon Over more than 200 years, researchers have ironed out most of the molecular details of how organisms turn carbon dioxide and water into food. Chlorophyll pigment and about 100 other proteins team up to put light to work. Plants, some protists, and cyano- bacteria embed their chlorophyll in two large protein clusters, photosystem I and photosystem II. And they need both systems to use water as an electron source. Light
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Leslie_2009_Origin_of_photosynthesis - On the Origin of...

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