Geared Toward the Glow
How did green bacteria get so good at catching light?
Green bacteria, that hugely abundant group of micro-organisms that perform nearly half the planet’s photosynthesis, live in murky environments from hot springs to ocean depths. Of necessity, they are extremely efficient at harvesting light. Until recently, however, nobody knew exactly how they got that way. That’s because no one had figured out the unique structure of their light-absorbing chlorophyll molecules.
Credit David M. WardOctopus Spring in Yellowstone Park, site of
Don Bryant's discovery of the Cab. thermophilium green bacterium
Green bacteria, as Don Bryant explains, are outfitted with antenna-like sacs called chlorosomes, each of which contains up to 250,000 chlorophylls. “Each chlorosome has a unique organization," says Bryant, Ernest C. Pollard professor of biotechnology at Penn State. "They are like little andouille sausages. When you take cross-sections of andouille sausages, you see different patterns of meat and fat; no two sausages are alike in size or content.” This variability had prevented scientists from characterizing the internal structure of chlorosomes by traditional means.
To get around the problem, Bryant and an international team of colleagues created a mutant bacterium, inactivating three genes that green bacteria acquired late in their evolution, genes the scientists suspected were keyed to better light-gathering. "Essentially, we went backward in evolutionary time in order to understand why green bacteria acquired these genes," Bryant says. Sure enough, they found, the more evolved, wild-type bacteria grew faster in response to light than the mutant form did. "Indeed, the reason that chlorophylls became more complex was to increase light-harvesting efficiency," Bryant concludes.
Next, the team used advanced imaging techniques to compare chlorosome structures. The pictures revealed that chlorophyll molecules have a “nanotube” shape, Bryant says: "They are like Russian dolls, with one concentric tube fitting inside the next." Again, there were differences between the wild and mutant types. "The mutant bacterium's chlorosomes contain only one set of tubes,” Bryant explains, “whereas the wild-type chlorosomes contain many tubes, each arranged in a unique pattern." NMR spectroscopy confirmed that the orientation of chlorophyll molecules in the wild-type chlorosomes was less uniform than that in their mutant cousins.
"At first it seems counterintuitive that green bacteria have managed to evolve a better light-harvesting system by increasing disorder in the chlorosome structure," says Bryant. "But if all of the chlorophylls are identically arranged, then the energy from the photon, once it is absorbed, is going to wander around over all of those chlorophylls, which could take a long time. In the wild-type form, you have these different domains and the ability of photon energy to migrate becomes restricted. That's an advantage to the organism because the energy can get to where it needs to go faster.
“Speed is the name of the game that green bacteria play with light,” he continues. “These organisms have only a couple of nanoseconds for the energy to get someplace useful or else it is going to be lost. The ability to capture light energy and rapidly deliver it to where it needs to go is essential."
The team's results, he adds, may one day be adapted to build artificial photosynthetic systems for converting solar energy into electricity. "The interactions that lead to the assembly of the chlorophylls in chlorosomes are rather simple, so they are good models for artificial systems," he explains. "You can make structures out of these chlorophylls in solution just by having the right solution conditions. In fact, people have done this for many years; however, they haven't really understood the biological rules for building larger structures.
“I won't say that we completely understand the rules yet, but at least we know what two of the structures are now and how they relate to the biological system as a whole, which is a huge advance."
Donald A. Bryant, Ph.D., is Ernest C. Pollard professor of biotechnology and professor of biochemistry and molecular biology in the Eberly College of Science; dab14@psu.edu. The team he worked with on this project also includes researchers from the Leiden Institute of Chemistry and the Groningen Biomolecular Sciences and Biotechnology Institute in the Netherlands, and the Max Planck Institute in Germany. This research was supported by the United States Department of Energy, and the results were published in the May 4, 2009 issue of the Proceedings of the National Academy of Sciences.
