Revisiting biocrystallization: purine crystalline inclusions are widespread in eukaryotes

Abstract

Despite the widespread occurrence of intracellular crystalline inclusions in unicellular eukaryotes, scant attention has been paid to their composition, functions, and evolutionary origins. Using Raman microscopy, we examined >200 species from all major eukaryotic supergroups. We detected cellular crystalline inclusions in 77% species out of which 80% is composed of purines, such as anhydrous guanine (62%), guanine monohydrate (2%), uric acid (12%) and xanthine (4%). Our findings shifts the paradigm assuming predominance of calcite and oxalates. Purine crystals emerge in microorganisms in all habitats, e.g., in freshwater algae, endosymbionts of reef-building corals, deadly parasites, anaerobes in termite guts, or slime molds. Hence, purine biocrystallization is a general and ancestral eukaryotic process likely present in the last eukaryotic common ancestor (LECA) and here we propose two proteins omnipresent in eukaryotes that are likely in charge of their metabolism: hypoxanthine-guanine phosphoribosyl transferase and equilibrative nucleoside transporter. Purine crystalline inclusions are multifunctional structures representing high-capacity and rapid-turnover reserves of nitrogen and optically active elements, e.g., used in light sensing. Thus, we anticipate our work to be a starting point for further studies spanning from cell biology to global ecology, with potential applications in biotechnologies, bio-optics, or in human medicine.

Guanine crystal formation by bacteria

“…In the case of eukaryotic microorganisms, although purine crystals have been known for many years, their composition has been investigated in detail in the last years [ 3 ]. In a very recent study that investigated crystalline inclusions in diverse unicellular eukaryotes, almost all guanine crystals contained anhydrous guanine with just a few examples containing the monohydrate form [ 33 ]. It is possible that detailed analysis of other bacteria could lead in the future to the discovery of other kinds of organic crystals, as already observed in eukaryotes [ 33 ].…”

Massive Accumulation of Strontium and Barium in Diplonemid Protists

“…For the in situ determination of the chemical composition of intracellular structures, a confocal Raman microscope (alpha300 RSA; WITec, Germany) was used as previously described ( 56 – 60 ). To immobilize the fast-moving flagellates on the quartz slide, 5 μL of the cell pellet was mixed with 5 μL of 1% (wt/vol) solution of low-melting-point agarose (catalog number 6351.5; Carl Roth, Germany), immediately spread as a single-cell layer between a quartz slide and coverslip, and sealed with CoverGrip sealant (Biotium, USA).…”

Guanine crystals discovered in bacteria

“…In the case of eukaryotic microorganisms, although purine crystals have been known for many years, their composition has been investigated in detail in the last years 3 . In a very recent study that investigated crystalline inclusions in diverse unicellular eukaryotes, almost all guanine crystals contained anhydrous guanine with just a few examples containing the monohydrate form 31 . Guanine crystals are associated with melanin in many organisms 2,32,33 .…”

“…Their occurrence in other organisms, such as unicellular eukaryotes, was studied many years ago 9,36 , and thought to be limited to a few cases. Recent renewed interest in guanine crystals and the application of new technologies has expanded this knowledge, and a very recent study showed them to be widespread among eukaryotic microorganisms 31 . Our work has demonstrated their presence in prokaryotes, extending the range of guanine crystal producing organisms to a new Domain of life.…”