The ability to generate oxygen through photosynthesis—that helpful service that plants and algae perform, 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, all the “green” organisms that we know of as oxygen-producers (plants and algae) simply subsumed cyanobacteria as organelles in their cells.
"Oxygenic photosynthesis was an evolutionary singularity," says Woodward Fischer, professor of geobiology at the California Institute of Technology.
"Cyanobacteria invented it, and then ultimately become the chloroplasts of algae. Then plants are just a group of algae that went on land. It's endosymbiotic theory."
Yet as world-shaping as Cyanobacteria are, comparatively 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 bacteria.
Recent advances in genomics now allow researchers to sequence genomes without having to culture an organism first.
"Now we have culture-independent ways of assessing microbial diversity, Rochelle Soo, a postdoctoral research fellow in the Australian Centre for Ecogenomics at The University of Queensland says.
“We can go into any environment more or less agnostically, remove DNA from the environment, sequence it, and recover genomes of microbes living in that environment.
“We don't have to ask anything to grow – but rather let the environment do the work and just sequence what's already there."
Publishing in the journal Science on March 31, the researchers from The University of Queensland and Caltech have fleshed out Cyanobacteria's family tree – adding the genomes of 41 uncultured microorganisms and helping to pin down the point in cyanobacteria's evolution that oxygenic photosynthesis arose.
These 41 microbes are close living relatives of oxygen-producing Cyanobacteria, however none of these organisms have genes for photosynthesis—rather they make their living eating organic matter instead of making it.
Dr Soo and her colleagues found that one branch of the cyanobacterial tree—called Oxyphobacteria for the moment—were likely the first and only ones to evolve oxygenic photosynthesis.
Their closest relatives, Melainabacteria, live in the guts of animals (including humans) among other environments, and do not produce oxygen.
And while one might suggest that Melainabacteria simply lost the ability to produce oxygen over time, the next most closely related Cyanobacteria after those, described in the paper as Sericytochromatia, also do not engage in oxygenic photosynthesis.
Unraveling the evolutionary mystery of photosynthesis and its genesis could shed light on everything from sustainable energy sources to the potential for life to exist on other planets.
The study is titled "On the origins of oxygenic photosynthesis and aerobic respiration in Cyanobacteria."
Authors include Rochelle Soo, Donovan Parks, and Philip Hugenholtz of The University of Queensland, and James Hemp and Woodward Fischer at California Institute of Technology.
This research was funded by the Australian Research Council, the Agouron Institute, NASA, and the David and Lucile Packard Foundation. Sequencing data have been deposited at the National Center for Biotechnology Information.
