That hypothesis contradicts one of the widely held ideas about the origins of photosynthesis: that species incapable of photosynthesis suddenly obtained the capacity through genes passed laterally from other organisms. According to Cardona, in light of the new discoveries, horizontal gene transfer and gene loss may both have played a role in the diversification of reaction centers, although he suspects that the latter may have been responsible for the earliest events. The finding, he said, might suggest that “the balance skews toward the gene-loss hypothesis”—and toward the idea that photosynthesis was an ancestral characteristic that some groups of bacteria lost over time.
Cardona, who was not involved in the recent study but has begun interpreting its results, thinks he may have found a hint in the heliobacterium reaction center. According to him, the complex seems to have structural elements that would have later lent themselves to the production of oxygen during photosynthesis, even if that wasn’t their initial purpose. He found that a particular binding site for calcium in the heliobacteria’s structure was identical to the position of the manganese cluster in photosystem II, which made it possible to oxidize water and produce oxygen.
So it seemed as though there were two evolutionary trees to follow—that was, until the crystal structures of these reaction centers began to emerge in the early 1990s. Researchers then saw undeniable evidence that the reaction centers for photosystems I and II had a common origin. Specific working components of the centers seemed to have undergone some substitutions during evolution, but the overall structural motif at their cores was conserved. “It turned out that big structural features were retained, but sequence similarities were lost in the mists of time,” said , the chairman in biochemistry of solar energy at Imperial College London.
Both types of photosystem come together in green plants, algae and cyanobacteria to perform a particularly complex form of photosynthesis—oxygenic photosynthesis—that produces energy (in the form of ATP and carbohydrates) as well as oxygen, a byproduct toxic to many cells. The remaining photosynthetic organisms, all of which are bacteria, use only one type of reaction center or the other.
The latest important clue comes from Heliobacterium modesticaldum, which has the distinction of being the simplest known photosynthetic bacterium. Its reaction center, researchers think, is the closest thing available to the original complex. Ever since the biologists , and of Arizona State University, in collaboration with their colleagues at Penn State, published in a July edition of Science, experts have been unpacking exactly what it means for the evolution of photosynthesis. “It’s really a window into the past,” Gisriel said.
Scientists want to figure out what made that possible. In its current form, the machinery that converts light energy to chemical energy in photosynthesis—a protein complex called a reaction center—is incredibly sophisticated. The evidence suggests, however, that its design, which stretches back almost to the root of the tree of life, was once very simple. Researchers have been trying for decades to fill that enormous gap in their understanding of how (and why) photosynthesis evolved.
The process whereby plants make sugars is photosynthesis. The plant takes in carbon dioxide from the air though pores in its leaves and absorbs water through its roots. These are combined to make sugar using energy from the sun and with the help of a substance called chlorophyll. Chlorophyll is green which allows it to absorb the sun's energy more readily and which, of course, gives the plants' leaves their green colour. The reaction of photosynthesis can be written as the following chemical equation when sucrose is being made: