A gigantic, shiny shell hides the most efficient solar panel ever discovered
Inside the giant rainbow-colored shell is an algae farm that could inspire highly efficient bioreactors.
Close-up underwater photograph of a giant shell detailing its internal patterns and vibrant colors in a tropical coral reef in Palau.
Erin Donalson/Getty Images
The shallow tropical coral reefs off the coast of Palau are home to this genus of mollusks that at first glance appear to be unremarkable but gigantic. Giant clamBut peer inside the rippling innards between its 4-foot-long shell and you’ll find glistening blue flesh that, according to a new study, houses the most efficient solar panels scientists have yet found.
“What really stuck out to me was the fact that no one could explain why clams are iridescent,” says Alison Sweeney, a biophysicist at Yale University and co-author of the new study.
In previous work investigating the incredible brilliance, Sweeney and her colleagues found that despite their striking iridescent shine, the animal’s fleshy mantle reflects only about 5 percent of the bright sunlight that hits it.
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The rest of the incoming light is absorbed, much of it sent to photosynthetic algae that the clam grows inside its body as a food source. Absorbing about 95 percent of the incoming light provides a very strong basis for photosynthesis. By comparison, terrestrial forests such as the Amazon reflect much more light, making photosynthesis less efficient to begin with. Sweeney and his colleagues also found that specialized cells called iridocytes that cover the surface of the mantle are made up of neatly stacked, transparent, protein-rich platelets that diffuse light while pushing it deeper into the clam.
In a new study published in PRX EnergySweeney and her team studied the arrangement of the clams’ symbiotic algae, which settle in tiny modified tubes that extend from their digestive systems. Unlike the random distribution of photosynthetic machinery in leaves, the clams’ algae form remarkably orderly patterns, arranged in thin columns that extend from the iris cells to the flesh. “The clams essentially plant the algae as if they were farmland,” Sweeney says. (The algae can also be transferred between clams as fecal pellets.)
Sweeney’s team modeled the system and calculated that the theoretical efficiency of the first stage of photosynthesis, in which chlorophyll absorbs one photon, is 43%. That’s more than double the efficiency of most current solar panels and three times that of tropical leaves, Sweeney says. But previous measurements of the photosynthetic efficiency of these clams in the wild put the efficiency even higher, at over 60%. In the new study, the researchers resolved this discrepancy by taking into account the clams’ behavioral quirks. There is evidence that the clams may inflate and contract their mantle sections throughout the day. This may allow the clams to further optimize their exposure to sunlight, the scientists determined, and the modeled efficiency now reaches 67%.
Curiosity led the researchers to study satellite photos of old spruce forests, looking for other examples of photosynthetic systems that reflect very little light. Sweeney says these images reminded her strongly of the micrographs of clam tissue. “If you don’t know the scale of the image you’re looking at, you immediately intuitively feel there’s a striking similarity,” she says. Just as the clam’s iris cells scatter light toward the algae, the clouds and fog in these forests scatter light onto each individual tree, with each tree acting as a mountain of algae.
Sweeney hopes that this work will be useful in designing algae-stocked bioreactors — just one example of how evolution’s creative approach to a single problem can provide inspiration for tackling technological challenges.
“Fundamental research into biological systems provides new ideas and new strategies that can be applied in unexpected areas,” says Gabriella Schrau-Cohen, a physical chemist at Massachusetts Institute of Technology who was not involved in the study. “Given the scale of the energy crisis, we need all kinds of strategies.”
That meant exploring the natural world far from home, says Sweeney, who grew up in the Midwest. “My sense of photosynthesis comes from deciduous forests and cornfields, and it turns out those forests and cornfields are really bad at it,” she says. “And this humble bivalve mollusk was just the perfect place to look for clever solutions.”
