Paleontologists trying to trace human evolution can’t scrutinize a living Neandertal or Homo erectus. But scientists hoping to piece together how photosynthesis began might have that luxury. Microbiologists suspect that bacteria that preserve stages in photosynthetic evolution—call them missing links if you like—are still with us. No one has found such a microbe—yet.
Bacteria are the Thomas Edisons of metabolism. They have “invented” myriad biochemical pathways that enable them to eke out a living from substrates as diverse as the oils on your skin, the tiny amounts of carbon monoxide in the atmosphere, and the hydrogen sulfide spewed by deep-sea volcanic vents. That metabolic diversity might include photosynthetic intermediates, scientists argue.
Researchers hope that such microbes will help them determine how early cells assembled the photosynthetic machinery, which involves more than 100 proteins working in concert to absorb light and make sugars. One of the most contentious questions in the field, as discussed in a recent Origins essay, is the origin of the photosystems, the molecular clusters that contain chlorophyll and other light-capturing proteins. Photosystems come in two flavors, I and II. Plants, algae, and primitive cyanobacteria all have both photosystems—and need both to exploit light energy. But other bacteria have only one molecular cluster—what scientists think are the ancestors of photosystem I or photosystem II. Microbial missing links might shed light on how the ancestors of today’s cyanobacteria ended up with two photosystems.
So far, researchers haven’t pinned down any of these missing links. But they take heart from a 2007 paper by microbial physiologist Donald Bryant of Pennsylvania State University, University Park, and colleagues that identified a new solar-powered bacterium. The researchers discovered the bug through metagenomics, which entails searching for organisms’ unique DNA strands rather than the creatures themselves. The bacterium’s DNA was lurking in samples from the microbial mats that grow in hot springs in Yellowstone National Park.
Subsequently, the researchers reared the bug in the lab (left, two jars of the bacteria) and sequenced itsgenome. The bacterium isn’t photosynthetic. It has the “photo” part down, absorbing light energy with chlorophyll to make the ATP necessary for living. But it hasn't mastered “synthesis.” Instead of using carbon dioxide to manufacture sugars, it depends on other bacteria for its carbon needs.
What makes the microbe noteworthy is that it belongs to a group, the Acidobacteria, that researchers had thought didn’t have light-harvesting ability. By expanding the range of bacteria that can use light, the discovery implies that “photosynthesis is more widely distributed than previously thought,” Bryant says. That means bacteria with other unknown metabolic styles, including possible photosynthetic missing links, are probably waiting for the scientists who are now looking for them.