Revisiting biocrystallization: Purine biocrystals are widespread in eukaryotes
Each of us produce biocrystals – in our bones and teeth. Some of them could be malignant causing urolithiases in our kidneys, bile stones or atheromatous plaques. Different types of crystals are present in insects and fish, having brilliant optical properties enabling beautiful color changes of chameleons and iridescent fish scales, they also form mirror-like layers in the back of the eyes of nocturnal insects and deep-sea fish to improve their vision under low light.
Interestingly, unicellular microorganisms – protists and algae – represent 70 % of estimated diversity of living organisms on earth and many of them also possess biocrystals that have been observed since the very beginning of microscopy, being mentioned even by pioneers of biology – Charles Darwin and Ernst Haeckel and repeatedly admired by laymen (see Fig. 1, http://y2u.be/HbwV-EzP67Q). But since then still (!), the chemical nature of intracellular crystals remained unclear with traditionally repeated myth about calcites or oxalates. According to our systematical revision, this is not the case (https://y2u.be/UtygkzDmz8U).
Fortunately, nowadays we have a powerful method of Raman microscopy – enabling measurement of vibrational spectra reflecting the chemical composition in vivo and in situ. Our initial serendipitous discovery of guanine crystals in microalgae led me to a bold guess that it might not be limited to algae, as it is not connected to chloroplasts, plus algae emerged many times independently, so they are as closely related to each other as, for example, we are related to amoebae. Since the study tested more than 200 species from model and biotechnologically exploited species, medically important parasites, and environmental samples from a broad variety of habitats – ranging from toxic algal bloom freshwater species to the anaerobes of termite guts and slime molds, researchers eventually found that my bold guess was correct – purine crystals are universally present in microscopic eukaryotes. So they proposed a paradigm shift in the concept of eukaryotic biocrystallization originally assuming the presence of previously found calcite or oxalates in microorganisms. They have also found other minor types of crystalline inclusions: calcium oxalate, strontianite, calcite, baryte, celestite, lipophilic crystals of sterols, fatty acids and carotenoids.
Moreover, purine crystals might have occurred in the last eukaryotic common ancestor and be the oldest form of biocrystallization. Hence, they outlined an evolutionary scenario for purine crystal formation inside cells due to highly conserved purine transporters. Purines are surprisingly versatile molecules: building blocks of DNA and RNA, sources of chemical energy (ATP, GTP) and messengers in cellular signaling (cAMP). Caffeine is another type of purine that is our favorite and luckily more soluble than those forming biocrystals. They found a new function of purines in the cells. They act as high-capacity nitrogen storage that is reusable once cells are deprived for the nitrogen sources (https://www.pnas.org/doi/10.1073/pnas.2005460117). A single cell can store up to 150 pg guanine per cell that can cover up to 3 consecutive cell divisions solely supplied from internal sources of nitrogen after inoculation to nitrogen depleted medium. Thus, they may be a great competitive advantage in the environment with fluctuating supplies of nitrogen. They are key to the understanding of the global nitrogen cycle, which is, in turn, tightly interconnected with the carbon cycle and global climate change. The extra nitrogen load might be further used in green biofertilizers that currently raise in popularity. However, purine crystals in algal-based food supplements may potentially cause hyperuricemia and its complications, such as gouty arthritis or nephropathy, if consumed regularly.
Simultaneously, purine crystals exhibit exquisite optical properties enabling cells to modulate light – that might be mostly prominent in photosynthetic algae – either increasing the light intensity in respect to the plastids, or shielding them from excessive light irradiation that may be deleterious to photosystems (https://www.sciencedirect.com/science/article/abs/pii/S1047847719300759). The high reflectivity of crystalline purine inclusions (e.g., photonic mirrors) may be exploited in the field of optics and cosmetic beauty products due to the magnetic tunable reflectivity of guanine biocrystals.
Final statistics: so far, there have been published a million research articles on microbial eukaryotes, half a million on purine metabolism and tens of thousands dedicated to biomineralization, nitrogen cycle, and microbial biotechnologies according to PubMed. All that without the basic awareness that crystalline purines are the most common crystalline inclusion of microscopic eukaryotes. ,,Wedare to believe that their findings have similar inspiring potential as the discovery of the biological role of inorganic polyphosphates by the Nobel Prize winner Arthur Kornberg eighty years ago, who initiated extensive research of that mysterious substance, considered to be just a molecular fossil, and thus contributed to the discovery of a multitude of their biological functions,” says Jana Pilátová from the Department of Experimental Plant Biology, Faculty of Science at Charles University.