Power Plants
That we've embraced green as the color of energy efficiency is a bit of a paradox. Granted, green is everywhere outdoors, courtesy of the pigment chlorophyll, a remarkable technology that, by harnessing solar energy, has enabled photosynthetic life to thrive for 3 billion years. Chlorophyll excels at absorbing and utilizing light in the blue and red portions of the electromagnetic spectrum -- but the rest, including green light, is largely ignored, reflected, squandered. Green, in nature, is the color of extravagant waste.
"The light that chlorophyll doesn't like is the color we see," notes Robert Blankenship, a photosynthesis expert at Washington University in St. Louis. That may seem like a glaring loss of potential, but chlorophyll makes up for it in other ways. Blankenship has been working with a team of Australian biologists who recently discovered a type of chlorophyll that can gather energy from infrared light, which lies below human vision and, it was thought, photosynthetic capacity. As humankind strives to emulate the zero-cost, low-impact way in which plants harness sunlight -- a business that clearly still holds surprises -- the subvisible turns out, fortunately, to be illuminating.
In the photosynthesis factory, chlorophyll has two jobs. First, it absorbs the sun's energy, which it then converts into chemical energy for the plant to use and store. Several types of chlorophyll are known, each varying slightly in molecular structure and function. The most common, chlorophyll-a, is found in all plants and cyanobacteria (single-celled blue-green algae) and does the core work of converting photons into food. Most plants also use chlorophyll-a to absorb, or "harvest", sunlight. Some, though, harvest light with chlorophyll-b, which absorbs light at slightly shorter wavelengths (though still largely in the red and blue parts of the spectrum), while others use chlorophyll-c. The infrared spectrum (wavelengths longer than 700 nanometers) was thought to lack sufficient energy to drive photosynthesis -- until 1996, when biologists confirmed the existence of chlorophyll-d, which has a peak absorption of about 710 nanometers. This pigment turns up in marine cyanobacteria, including some that live on the underside of other photosynethetic structures, gleaning the thin light that filters down.
Min Chen, a biologist at the University of Sydney, was hunting for chlorophyll-d when she stumbled into an altogether new variety. Out in the tidal pools of Western Australia are odd, ancient structures called stromatolites; they look like misshapen rocks but, when cut open, reveal themselves as multilayered colonies of cyanobacteria. The most deeply buried (and thus darkest) layers, Chen figured, might harbor microbial employers of chlorophyll-d. She was right -- and "as a bonus," she says, she also found evidence of chlorophyll-f, a pigment capable of absorbing light of 725 nanometers.
The discovery is "extremely exciting," says Michael Strano, a chemical engineer at the Massachusetts Institute of Technology. Older photovoltaic cells, which employ silicon substrates, and newer technologies that enlist light-absorbing dyes, can already harvest some light from the infrared spectrum. Still, the solar industry is constantly scouring nature for "good absorbers," Strano says, and the biochemistry of the infrared chlorophylls will surely interest chemical engineers, either as a model or as a material to be incorporated into future technologies.
Yet even as we model machines after plants, we tweak plants to serve us as machines do. It doesn't take much for a cell to convert chlorophyll-a to chlorophyll-d. Recently Blankenship and Chen identified the gene responsible; they are now working to insert it into another photosynthetic organism to extend its -- and ultimately our -- light-gathering ability.
The world of genetically modified plants is already so weird: some rice contains daffodil genes to boost its vitamin A content; potatoes carry fish genes to better tolerate the cold. Why not engineer plants to harvest infrared light? One can imagine plants that are photosynthetically tiered, like stromatolites: the upper leaves harvest sunlight with chlorophyll-a, while the lower ones employ chlorophyll-d or -f to catch "waste" light. Crops could be grown in denser stands, to increase yield; biofuels could be made more efficient. "If one could insert it into plants and make it functional," Blankenship says, "you could gain access to about 10 percent additional energy from the solar spectrum."
The biochemistry of infrared sheds deeper light as well. That living cells can so easily convert chlorophyll-a to chlorophyll-d, and probably to -f too, suggests that the latter pigments evolved sometime after the former, as life adapted to occupy the murky niches. Human industry is at a similar juncture: to sustain ourselves, we must begin to look beyond what we merely see and make better use of darkness. If the single cells of the world can swing it, surely we can too.