Algal optics

Nearly a decade ago, it was discovered that the spherical cell body of the alga Chlamydomonas reinhardtii can act as a lens to concentrate incoming light onto the cell's membrane-bound photoreceptor and thereby affect phototaxis. Since many nearly transparent cells in marine environments have complex, often nonaxisymmetric shapes, this observation raises fundamental, yet little-explored questions in biological optics about light refraction by the bodies of microorganisms. There are two distinct contexts for such questions: the absorption problem for incoming light, typified by photosynthetic activity taking place in the chloroplasts of green algae, and the emission problem for outgoing light, where the paradigm is bioluminescence emitted from scintillons within dinoflagellates. Here we examine both of these aspects of "algal optics" in the special case where the absorption or emission is localized in structures that are small relative to the overall organism size, taking into account both refraction and reflections at the cell-water boundary. Analytical and numerical results are developed for the distribution of light intensities inside and outside the body, and we establish certain duality relationships that connect the incoming and outgoing problems. For strongly nonspherical shapes, we find lensing effects that may have implications for photosynthetic activity and for the angular distribution of light emitted during bioluminescent flashes.