It is well known that biogenic guanine crystals are widely used by various animals to manipulate light. 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.
Crystalline inclusions composed of guanine or related purines were recently identified also in different phylogenetically unrelated photosynthetic microalgae. In the case of dinoflagellate Amphidinium carterae they were shown to serve as a long-term, high capacity store of nitrogen.
However, nitrogen storage does not exclude other roles, and light manipulation was already speculated for photosynthetic eukaryotes.