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New Study Establishes Evolutionary Origin of Photosynthesis in Cyanobacteria

The ability to generate oxygen through photosynthesis--that helpful service performed by plants and algae, making life possible for humans and animals on Earth--evolved just once, roughly 2.3 billion years ago, in certain types of cyanobacteria. This planet-changing biological invention has never been duplicated, as far as anyone can tell. Instead, according to “endosymbiotic theory,” all the "green" oxygen-producing organisms (plants and algae) simply subsumed cyanobacteria as organelles in their cells at some point during their evolution. "Oxygenic photosynthesis was an evolutionary singularity," says Woodward Fischer, Ph.D., Professor of Geobiology at Caltech, referring to the process by which certain organisms use the energy of sunlight to convert carbon dioxide and water into sugar for food, with oxygen as a by-product. "Cyanobacteria invented it, and then ultimately become the chloroplasts of algae. Plants are just a group of algae that moved on land." Yet, as world-shaping as cyanobacteria are, relatively little is known about them. Until a couple of decades ago, they were called "blue-green algae" by taxonomists, though it was later revealed that they are not algae at all, but rather a completely different type of organism. That lack of taxonomic understanding made deciphering the riddle of their evolution all but impossible, Dr. Fischer says. "For the longest time, they were just their own group. We had no answer about where they came from, or what other organisms they were related to," Dr. Fischer says. "Imagine trying to understand something about human evolution without knowledge of the great apes." Publishing in the March 31, 2017 issue of Science, Dr. Fischer and colleagues from Caltech and the University of Queensland in Australia have finally fleshed out cyanobacteria's family tree. They added the genomes of 41 uncultured microorganisms, which helped to pin down the precise point in the evolution of cyanobacteria at which oxygenic photosynthesis arose. The 41 species are all types of cyanobacteria but none carry genes for photosynthesis, and therefore they don't produce organic matter, as algae and plants do. Rather, they consume it. The Science article is titled “On the Origins of Oxygenic Photosynthesis and Aerobic Respiration in Cyanobacteria."


"This nails down that Oxyphobacteria were really the only ones to ever invent this globe-shaping chemical process," Dr. Fischer says.

The 41 new species fall into both Melainabacteria and Sericytochromatia, the latter of which had not been described before this paper. All names of these organisms are subject to change, as taxonomists catch up with the team's discoveries. "We know they're there, and we know their gene repertoire. Now we can start putting them into evolutionary trees, and begin efforts to isolate them and study their physiology and ecology," says James Hemp, Ph.D., an Agouron Postdoctoral Scholar at Caltech when the research was conducted, and co-author of the Science article.


"Now we have culture-independent ways of assessing microbial diversity," says Dr. Rochelle M. Soo, postdoctoral researcher at the University of Queensland in Australia and co-author of the Science article.






In addition to Dr. Fisher and Dr. Soo, co-authors of this report include Dr. Donovan H. Parks and Dr. Philip Hugenholtz of the University of Queensland in Australia.

[Press release] [Science abstract]