Meeting Abstract
21.4 Friday, Jan. 4 Photonic enhancement of symbiotic photosynthesis in giant clams HOLT, A.L.*; GAGNON, Y.; VAHIDINIA, S.; MORSE, D.E.; SWEENEY, A.M.; UCSB; Duke University; NASA-Ames; UCSB; Univ. of Pennsylvania holt@lifesci.ucsb.edu
Giant Tridacnid clams, like reef-building corals, harbor and exchange nutrients with the photosynthetic dinoflagellate Symbiodinium. These clams are also notable for their bright color patterns, caused by clam cells called iridocytes. These cells are photonically unique, having a structure that is a superposition of sub-wavelength scale layers with a super-wavelength scale sphere, suggesting that they may serve an undiscovered optical function. Here we show that these iridocytes function to reshape and propagate incident sunlight deep into the clam tissue, in spite of the presence of the strongly absorbing algae, producing a remarkably deep photic zone inside the animal. This expanded photic zone likely increases the photosynthetic capacity of the clams, contributing to the animals’ storied size. In field experiments in Palau, we used custom-built fiber optic microprobes to measure scalar irradiance with ~10 µm spatial resolution inside the tissue of several species of Tridacna. Non-iridescent individuals occur occasionally in wild populations of the species Tridacna crocea; these animals allowed us to compare the optical performance of an iridocyte-bearing clam to one without iridocytes. Clams with iridocytes had five-fold more photons available for photosynthesis within their tissues than did clams without iridocytes. This relative enhancement of photosynthesis increases with clam tissue depth. To explain the optical performance of iridocyte-bearing clams, we developed a model of radiative transfer in clam tissue using the discrete dipole approximation to estimate the scattering of a single iridocyte, and Monte Carlo simulation of photon transport. Our models demonstrate that the unique scattering function of the iridocytes is likely responsible for the optical performance of the real clam.
