In the quest for developing solar panels and manufacturing biofuel, scientists have come a long way in a relatively short amount of time. However, giant clams may be able to teach them a thing or two, as they have already perfected the process through millions of years of evolution. A new study has revealed that the iridescent colors help giant clams create a greenhouse for algae, which live symbiotically with the clams and provide them with food. The research was led by Alison Sweeney of the University of Pennsylvania and the paper was published in the Journal of the Royal Society Interface.
"Many mollusks, like squid, octopuses, snails and cuttlefish have iridescent structures, but almost all use them for camouflage or for signaling to mates," Sweeney said in a press release. "We knew giant clams weren't doing either of those things, so we wanted to know what they were using them for."
Giant clams, and thus the algae embedded within its tissue, can be found in tropical regions where sunlight is abundant. Though it is counterintuitive, excessive exposure to sunlight actually reduces the algae's ability to photosynthesize to create food. The team hypothesized that the iridocytes, which give giant clams their sheen, aided the algae in some way.
The answer, it turned out, was the geometrical arrangement of the algal cells and iridocytes. The algae is stacked into pillars, which is not the best way to receive sunlight for photosynthesis. However, there is a layer of iridocytes on top, aiding in the process. These structures act like a prism, scattering the sunlight as it passes through. Red and blue light, which is the most beneficial during photosynthesis, is directed downward and spread widely. This lets the light reach deep into the stacked algae and allow for efficient photosynthesis, while preventing the cells from becoming over inundated with sunlight. This allowed the clam to act as a functional greenhouse for the algae.
"We see that, at any vertical position within the clam tissue, the light comes in at just about the highest rate at which these algae can make use of photons most efficiently," explained Sweeney. "The entire system is scaled so the algae absorb light exactly at the rate where they are happiest."
To test if the blue and red wavelengths were really that pervasive throughout the clam's flesh, the team needed to detect the light. A flat-tipped, light-sensitive probe that had been stuck straight down was not able to pick up any light dispersed through the iridocytes. However, small fiber optic cables with rounded ends that had been woven into the tissue were able to pick up on the scattered light.
"The clam has to make every square inch count when it comes to efficiency," Sweeney concluded. "Likewise, all of our alternatives are very expensive when it comes to surface area, so it makes sense to try to solve that problem the way evolution has."
Understanding how iridocytes maximize potential for photosynthesis could allow scientists to design solar panels that are also able to filter the most productive wavelengths of light, boosting potential. This could also be used in bioreactors that grow algae for biofuel; if light can be coaxed into penetrating the algae deeply rather than needing to be stirred and mixed to rotate exposure to sunlight, the process could be greatly streamlined.
