Today, about half of the global production of oxygen is carried out by algal photosynthesis in the oceans. Guanine crystals are used by certain animals, including vertebrates, to produce structural colors or to enhance vision, because of their distinctive reflective properties.

The crystalline inclusions in a single-celled photosynthesizing marine dinoflagellate species, Calciodinellum operosum aff., are identified spectroscopically as blocky crystals of anhydrous guanine in the β-polymorph. The deposits of anhydrous guanine crystals are closely associated with the chloroplast network.

Cryo-SEM micrograph of guanine crystals in C. operosum aff. (left) and 3D reconstruction of a cryo-FIB-SEM dataset (right) showing the distribution of guanine crystals (blue) in correlation with the chloroplast network (red)

One possible function of the guanine crystals might be scattering light that was missed in the first passage, back into the chloroplasts and thereby influence photosynthetic performance. This is consistent with the crystal locations within the cell, their shapes and their sizes. As the dinoflagellates are extremely abundant in the oceans and are a major group of photosynthesizing marine organisms, this observation may have broad significance.