Abstract
It is commonly known that biogenic guanine crystals are widely used by various animals to manipulate light [1]. Because of extremely high refraction index and plate-like arrangement, multiple arrays of guanine crystals can act as diffuse scatterers, broad- and narrowband reflectors, tunable photonic crystals, and image-forming mirrors [1].
Crystalline inclusions composed of guanine or related purines were recently identified also in different phylogenetically unrelated photosynthetic microalgae [2]. In the case of dinoflagellate Amphidinium carterae they were shown to serve as a long-term, high capacity store of nitrogen [2].
However, nitrogen storage does not exclude other roles, and light manipulation was already speculated for photosynthetic eukaryotes [3]. Using confocal Raman microscopy for visualization of crystalline guanine within intact cells, we show that location of guanine crystals in A. carterae depends on the intensity of illumination.
When the cells are cultivated under continuous supply of inorganic nitrogen but exposed to low light intensity (ca 5 µmol(photons) m-2 s-1), their adaptation consists of increasing amount of plastids and synthesis of guanine crystals situated behind the plastids, closer to the center of the cell. Acting as photonic mirrors or diffusers for redirecting untrapped photons back to plastids, guanine crystals might increase efficiency of photosynthesis.
In the case of high light intensity (ca 200 µmol(photons) m-2 s-1), extensive layers of guanine crystals are organized between the cell wall and plastids, shielding plastids from the excessive illumination.