Giant, Sparkly Clams Hide the Most Efficient Solar Panels Ever Found
Inside giant, iridescent clams are algal farms that could inspire highly efficient bioreactors
Detail of design and vibrant colors inside giant clam taken close up underwater on a tropical reef in Palau.
Erin Donalson/Getty Images
In the shallow tropical reefs off Palau lie what at first glimpse look like unremarkable, albeit huge, clams in the genus Tridacna. But a peek at the undulous innards tucked between their four-foot-long shells reveals sparkly blue flesh—that hosts what new research shows to be the most efficient solar panels that scientists have ever found.
“The fact that nobody could explain why a clam was iridescent really just stuck with me,” says Alison Sweeney, a Yale University biophysicist and co-author of the new research.
In previous work to investigate the startling sparkle, Sweeney and her colleagues determined that despite impressive iridescence, the animals’ fleshy mantles reflect as little as about 5 percent of the bright sunlight hitting them.
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The rest of the incoming light is absorbed, and much of it is channeled to photosynthetic algae that the clams cultivate within their body as a food source. And absorbing around 95 percent of incoming light is a remarkably strong basis for photosynthesis; in comparison, terrestrial forests such as the Amazon reflect far more light, reducing their photosynthetic efficiency from the outset. Sweeney and her colleagues also determined that specialized cells called iridocytes, which line the mantle’s surface, contain neatly aligned stacks of transparent, protein-rich platelets that diffuse light while pushing it deeper inside the clam.
In new research published in PRX Energy, Sweeney and her team studied the arrangement of the clams’ symbiotic algae, which settle in tiny modified tubes extending up from the digestive system. Whereas a leaf’s photosynthetic machinery is distributed randomly, the clam’s algae form a strikingly orderly pattern, arranged in thin columns that stretch from the iridocyte down into the flesh. “The clam basically plants them as if it were an agricultural field,” Sweeney says. (The algae also travel between clams in pellets of poop.)
Sweeney’s team modeled this system and calculated that its theoretical efficiency at the first step of photosynthesis, during which chlorophyll absorbs a single photon, is 43 percent—more than twice the efficiency of most current solar panels and three times that of a tropical leaf, Sweeney says. Yet previous measurements of these clams’ photosynthetic efficiency in the wild had put it even higher, at more than 60 percent. In the new study, the researchers solved this discrepancy by factoring in a quirk of clam behavior: there is evidence that clams might inflate and deflate their mantle throughout the day. This could let the clams further optimize their sunlight exposure, the scientists determined—now clocking in at a modeled 67 percent efficiency.
Curious, the researchers then looked for examples of other photosynthetic systems that reflect little light and found themselves studying satellite photos of old spruce forests. Sweeney says that these images reminded her powerfully of microscopic views of clam tissue. “There’s an immediate, visceral, striking resemblance if you don’t know the scale of the image you’re looking at,” she says. Like the clams’ iridocytes, which scatter light inward toward algae, these forests’ clouds and fog scatter light down to individual trees, each of which acts like a stack of algae.
Sweeney hopes this work can inform the design of algae-stocked bioreactors, in just one example of how evolution’s creative approaches to a single problem can offer inspiration for tackling technological challenges.
“Fundamental studies of biological systems give us new ideas and new strategies that can be applicable in unexpected spaces,” says Gabriela Schlau-Cohen, a physical chemist at the Massachusetts Institute of Technology, who was not involved in the new research. “Given the scale of the energy crisis, we need all the strategies we can get.”
And that means exploring the natural world far from home, says Sweeney, who grew up in the U.S. Midwest. “My sense of what photosynthesis is comes from deciduous forests and cornfields—and it turns out they’re really bad at it,” she says. “This lowly bivalve was really the right place to look for smart solutions.”
