RED IS THE NEW GREEN Researchers working with cyanobacteria may have extracted a new variety of chlorophyll that can use more near-infrared light than is typical for oxygenic photosynthetic organisms. The chloroplasts pictured here are in the cells of a moss. Image: Wikimedia Commons
Researchers may have found a new form of chlorophyll, the pigment that plants, algae and cyanobacteria use to obtain energy from light through photosynthesis. Preliminary findings published August 19 in Science suggest that the newly discovered molecule, dubbed chlorophyll f, has a distinct chemical composition when compared with the four known forms of chlorophyll and can absorb more near-infrared light than is typical for the photosynthetic pigments. Chlorophyll f, which was extracted from cultures of cyanobacteria and other oxygenic microorganisms, may allow certain photosynthetic life forms to harvest energy from wavelengths of light that many of their competitors cannot use.
"This is the most red-shifted chlorophyll we have found in nature," says Min Chen, a biologist at The University of Sydney in Australia and lead author of the study. "That means that organisms that have this chlorophyll inside can extend their photosynthetic range for maximum use of solar energy."
Some photosynthetic bacteria are known to use infrared light, but—in contrast to plants and cyanobacteria—these microorganisms do not produce oxygen. Instead, they rely on anoxygenic photosynthesis, which can function on the low-energy photons provided by infrared light. "Nobody thought that oxygen-generating organisms were capable of using infrared light, because the kind of photosynthesis that actually produces oxygen is thought to require a greater amount of photon energy from visible light," says Samuel Beale, a molecular biologist at Brown University whose work centers in part on chlorophylls. "I think what they found here is a new modification of chlorophyll that shows the flexibility of photosynthetic organisms to use whatever light is available."
Robert Blankenship, a photosynthesis expert at Washington University in St. Louis, agrees that the discovery is significant. "I think this is a very important new development and is the first new type of chlorophyll discovered in an oxygenic organism in sixty years," he wrote via e-mail.
Other researchers are more cautious about the findings. John Clark Lagarias, a molecular biologist at the University of California, Davis, points out that earlier research suggests some oxygen-producing cyanobacteria can harvest energy from near-infrared light using chlorophyll d—one of the four known varieties of chlorophyll, which also include chlorophylls a, b and c. But the new paper still interests Lagarias: "It's an exciting potential discovery, and if it's true it provides a second example of a red-shifted-chlorophyll-containing organism," he says. "We don't know for sure that it's used for photosynthesis, but we know it's absorbing light and it's likely to be involved in photosynthetic apparatus somehow. It could be a bona fide new form of chlorophyll that exists in something living."
In July 2008, Min's colleagues collected samples of stromatolites—structures formed from layers of cyanobacteria, calcium carbonate and sediments— and microbial mats from Hamelin Pool in Shark Bay, Western Australia, which is known to contain some of the most diverse and oldest stromatolites in the world. Cyanobacteria and other microorganisms build stromatolites in shallow water as they grow, gradually trapping and binding sediments into the small rock-like towers and mounds. Chen ground up the samples in a mortar and pestle and cultured the microorganisms in petri dishes under continuous illumination by near-infrared LEDs. Eventually, only microorganisms like cyanobacteria capable of photosynthesis using near-infrared light survived in the cultures.