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The most efficient solar energy farm might not look like what you’d imagine. There are no acres of flat desert, no industrial-sized mirrors or refrigerator-sized batteries. Rather, in the crystalline waters of Palau, the giant clam Tridacna boasts a peculiar trait—near sixty-seven percent optimal photosynthesis. As strange as it sounds, these irregular creatures may possess the anatomy to conduct the most efficient form of photosynthesis currently known.
Clams are members of the bivalve family—organisms that are bilaterally symmetrical with an “in” siphon and an “out” siphon. As they take in and expel water, clams capture vital foods such as microscopic plankton. However, the clear waters of Palau are not the best areas for filter-feeding these small critters. Imagine trying to bob for apples in the Great Lakes…not effective!
To sustain itself, the giant clam has evolved a unique anatomical structure: a stunningly iridescent mantle filled with special photosynthetic algae. The pairing is evolutionarily elegant; as the algae provide the clam with sugars from photosynthesis, the clam upholds its end of the bargain by supplying the algae with nitrogen and other nutrients. What makes the clam’s structure interesting isn’t just this symbiotic relationship, but its efficiency. Food crops grown under high solar energy, similar to the tropical environment of the clam, only convert around three percent of the energy in sunlight into usable sugars. Then, what is the secret behind the giant clam’s solar efficiency?
“As it turns out, the algae inside the clam tissue are organized in a very specific geometry, kind of like they’re in these cylindrical pillars. Like spaghetti pillars growing straight toward the sun,” said Amanda Holt, a Yale physicist and co-author of the study published in PRX: Energy.
During Holt’s postdoc at UCSB, she came to appreciate the interdisciplinary collaboration used to investigate interesting optical properties of marine animals and their potential applications in future technologies. It was there that Holt met Alison Sweeney, a Yale professor of ecology and evolutionary biology and physics. They began working on clam biophysics together and eventually moved to Yale, where Holt currently works as an associate research assistant in Sweeney’s lab. Her lab focuses on the interaction of light with biological materials, like clam tissue, and how these materials naturally evolve.
Along with third co-author Lincoln Rehm, who is Palauan-American, the researchers created a model to calculate the clam’s quantum efficiency, or its ability to convert photons into electrons. The clam contains layers of spherical cells called iridocytes, which scatter sunlight outward into a cone-like shape. This conical beam of diffused light then reaches the columns of algae, which are nearly perfectly aligned to receive the light. Everything about this system works in tandem to maximize the output. Being exposed to such strong sunlight can be damaging to molecules, which is why diffusing the light is essential to preserve the integrity of the proteins involved.
What they found was that the tissues had a forty-two percent quantum efficiency, a number significantly greater than the fourteen percent efficiency boasted by terrestrial photosynthetic plants in the same region. But the team took their model one step further, accounting for the clams’ daily movement; as the sun changes position throughout the day, the clam readjusts and spreads out its fleshy mantle to account for this fluctuation. The new calculation came out to around sixty-seven percent.
Holt said these findings hold major applications across numerous industries. Algae compounds are incorporated into various products, including cosmetics, foods, and, most importantly, fuel. By creating tiny devices mimicking the pillar-like geometry of the giant clam’s algae, growers can maximize algal production for commercial use.
It’s worth noting that several species of Tridacna clam are listed as protected species due to habitat threats and climate change. Valued highly for their ivory-like shells, these centuries-old organisms have also become victims of poaching. Because of their conservation status, Holt said that researchers would purchase giant clams from local farmers and perform tissue biopsies in the lab, rather than collect specimens from the wild.
With these bountiful applications, it’s no surprise that unsuspecting consumers may soon reap the benefits of the remarkable giant clam, whether for algal-based cosmetics, food additives, or even the energy powering our homes.