Paradigm shift in eukaryotic biocrystallization

Despite the widespread occurrence of crystalline inclusions in unicellular eukaryotes, scant attention has been paid to their composition, functions, and evolutionary origins, assuming just their inorganic contents. The advent of Raman microscopy, still scarcely used for biological samples, allowed chemical characterization of cellular inclusions in vivo. Using this method, herein we provide a substantial revision of the cellular crystalline inclusions across the broad diversity of eukaryotes examining all major supergroups. Surprisingly, here we show that 80 % of these crystalline inclusions contain purines, mostly anhydrous guanine (62 %), guanine monohydrate (2 %), uric acid (12 %) and xanthine (4 %). Hence, our findings indicate that purine biocrystallization is a very general and an ancestral eukaryotic process operating by an as-yet-unknown mechanism. Purine crystalline inclusions are high-capacity and rapid-turnover reserves of nitrogen of a great metabolic importance, as well as optically active elements, e.g., present in the light sensing eyespots of flagellates, possessing even more hypothetical functions. Thus, we anticipate our work to be a starting point for more in-depth studies of this phenomenon on the detailed level spanning from cell biology to global ecology, with further potential applications in biotechnologies, bio-optics or in human medicine. ### Competing Interest Statement The authors have declared no competing interest.