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In a new study, Yale researcher Alison Sweeney found that giant clams in the Western Pacific may be the most efficient solar energy system on the planet.

Solar panel and biorefinery designers could learn from iridescent giant clams living near tropical coral reefs, according to a new Yale-led study. This is because giant clams have precise geometries — dynamic, vertical columns of photosynthetic receptors covered by a thin, light-scattering layer — that may make them the most efficient solar energy systems on Earth.

“It’s counter-intuitive to a lot of people, because clams operate in intense sunlight, but actually they’re really dark on the inside,” said Alison Sweeney, associate professor of physics and of ecology and evolutionary biology in Yale’s Faculty of Arts and Sciences. “The truth is that clams are more efficient at solar energy conversion than any existing solar panel technology.”

In the new study, published in the journal PRX: Energy, a research team led by Sweeney presents an analytical model for determining the maximum efficiency of photosynthetic systems based on the geometry, movement, and light-scattering characteristics of giant clams. It is the latest in a series of research studies from Sweeney’s lab that highlight biological mechanisms from the natural world that could inspire new sustainable materials and designs.

The researchers looked specifically at the impressive solar energy potential of iridescence giant clams in Palau's shallow waters in the Western Pacific. The clams are photosymbiotic, with vertical cylinders of single-celled algae growing on their surface. The algae absorb sunlight after it has been scattered by a layer of cells called iridocytes.

Both the geometry of the algae and the light scattering by iridocytes are important. The algae’s arrangement in vertical columns — parallel to incoming light — enables them to absorb sunlight efficiently as it has been filtered and scattered by iridocytes. The light then wraps uniformly around each vertical algae cylinder.

Based on this geometry, Sweeney and her colleagues developed a model to calculate quantum efficiency — converting photons into electrons. They also factored in fluctuations in sunlight during a typical day in tropics with sunrise, midday sun intensity, and sunset. Initially, quantum efficiency was 42%.

Then researchers added how giant clams stretch themselves reacting to changes in sunlight. “Clams like to move and groove throughout the day,” Sweeney said. “This stretching moves the vertical columns farther apart, effectively making them shorter and wider.” With this adjustment, quantum efficiency jumped to 67%. By comparison, Sweeney noted that green leaf systems' quantum efficiency is about 14% in tropical environments.

An intriguing comparison would be northern spruce forests; boreal spruce forests surrounded by fluctuating fog layers share similar geometries with giant clams but on a larger scale with nearly identical quantum efficiency.

“One lesson from this is how important it is to consider biodiversity writ large,” Sweeney said. “My colleagues and I continue to brainstorm about where else on Earth this level of solar efficiency might happen. It is also important to recognize we can only study biodiversity where it is maintained.” She added: “We owe a major debt to Palauans who put vital cultural value on their clams and reefs working to keep them pristine.”

Such examples may offer inspiration for more efficient sustainable energy technology solutions.

“One could envision a new generation of solar panels that grow algae or inexpensive plastic solar panels made out of stretchy material,” Sweeney suggested.

The study’s first author is Amanda Holt, an associate research scientist in Sweeney’s lab. Co-author Lincoln Rehm is Palauan-American; former graduate student at Drexel University; researcher at Palau International Coral Reef Center; now at National Oceanography Atmospheric Administration.

The research was funded by Packard Foundation fellowship and National Science Foundation.