A study by Lawrence Berkeley National Laboratory and California Institute of Technology, or Caltech, debunks a common misconception about the origin of oxygen-producing photosynthesis.
Berkeley Lab postdoctoral fellow Patrick Shih, along with Woodward Fischer and Lewis Ward from Caltech, published the study online Sept. 18 and in the science journal Proceedings of the National Academy of Sciences on Oct. 3. The study disproved the idea that Chloroflexi bacteria were the precursors to oxygen-producing photosynthesis on Earth.
There are two types of photosynthesis: oxygenic, which is used by plants and algae and produces oxygen as a byproduct, and anoxygenic, which does not produce oxygen.
Cyanobacteria are theorized to be the evolutionary origin of oxygenic photosynthesis, according to Fischer.
“Before there’s cyanobacteria, there’s no oxygen on the planet,” Fischer said.
The study specifically looks at what came after the development of anoxygenic photosynthesis and before the speciation of cyanobacteria — prior to the study, many researchers generally believed that Chloroflexi bacteria did.
Often, Shih said in evolutionary biology, there is so little data that researchers “build stories” that align with their observations. This study was an opportunity to test the particular theory that Chloroflexi provided the evolutionary groundwork that allowed cyanobacteria to invent oxygenic photosynthesis — a theory that was rejected.
According to Shih, the study found that Chloroflexi bacteria were too evolutionarily young to have been this missing link. Shih said the missing link would be 2.5 billion years old, but Chloroflexi bacteria is only about 867 million years old.
The study involved molecular clock analysis, which is used to reconstruct evolutionary history. Using this method, Fischer, Shih and Ward could determine how many mutations occur in a genome over a period of time, and they mapped where organisms lie on a phylogenetic tree based on similarities and differences between their genomes. After this molecular analysis, researchers determined that Chloroflexi were much younger than they expected.
Going forward, Fischer describes two options: Whatever came before cyanobacteria is either alive or extinct.
If it is extinct, he said it may be difficult for researchers to find in fossil records. If it is alive, however, there isn’t a targeted way for researchers to look for it. For now, Fischer said researchers will continue to sequence different samples from different environments, and whenever a new photosynthetic organism is discovered, they will determine where it sits in the evolutionary tree of life.
