Bio-Inspired Crystalline Core-Shell Guanine Spherulites.

Spherical particles with diameters within the wavelength of visible light, known as spherulites, manipulate light uniquely due to their spatial organization and their structural birefringence. Most of the known crystalline spherulites are branched, and composed of metals, alloys, and semi-crystalline polymers. Recently, a different spherulite architecture was discovered in the vision systems of decapod crustaceans - core-shell spherulites composed of highly birefringent (Δn ∼ 30%) organic single-crystal platelets, with exceptional optical properties. These metastructures, which efficiently scatter light even in dense aqueous environments, have no synthetic equivalence and serve as a natural proof-of-concept as well as synthetic inspiration for thin scattering media. Here, we present the synthesis of core-shell spherulites composed of guanine crystal platelets (Δn ∼ 25%) in a two-step emulsification process in which we use a water/oil/water emulsion and induced pH changes to promote interfacial crystallization. Carboxylic acids neutralize the dissolved guanine salts to form spherulites composed of single, radially stacked, ß-guanine platelets, which are oriented tangentially to the spherulite surface. Using Mie theory calculations and forward scattering measurements from single spherulites, we find that due to the single-crystal properties and orientation, the synthetic spherulites possess a high tangential refractive index, similarly to biogenic particles. This article is protected by copyright. All rights reserved.

Intra- to extracellular crystallization of calcite in the freshwater green algae Phacotus lenticularis.

Phacotus lenticularis is a freshwater unicellular green alga that forms lens-shaped calcitic shells around the cell. We documented P. lenticularis biomineralization pathways in live daughter cells while still within the reproductive complex, using scanning confocal microscopy and after vitrification using cryo-scanning electron microscopy (cryo-SEM). We show that some or all of the calcium ions required for mineral formation enter the cell through endocytosis, as inferred from the uptake of calcein fluorescent dye. Ions first concentrate inside intracellular vesicles to form small crystals that were detected by birefringence, reflectance, and cryo-SEM of cells in near-native, hydrated state. The crystals later exit the cell and build up the lens-shaped shell. The small crystals first cover the outer lorica surface and later fuse to form a thin continuous shell. This is most likely followed by a second shell maturation phase in which the shell undergoes thickening and crystal reorganization. Crystal assembly within the confined protected volume of the reproduction complex allows controlled shell formation outside the daughter cell. Only two other unicellular marine calcifiers, coccolithophores and miliolid foraminifera, are known to perform intracellular crystal formation. STATEMENT OF SIGNIFICANCE: Calcium carbonate (CaCO3) deposition in aquatic environments is a major component of the global carbon cycle, which determines the CO2 content of the atmosphere. In freshwater ecosystems, the green alga Phacotus lenticularis is considered the main contributor of autochthonous calcite precipitation and the only algal species known to form its shell through a controlled process. The chemical and ecological effects of P. lenticularis are intensively investigated, but our understanding of its shell formation is limited. We used advanced confocal laser scanning microscopy and cryo-scanning electron microscopy (cryo-SEM) to provide new insights into mineral formation and trafficking in the calcifying P. lenticularis cells.