Soluble germanium addition to silicon-starved cultures of the diatom Cyclotella sp. limits β-chitin nanofiber formation

Abstract

Diatoms are unicellular algae that make nanostructured, porous cell walls called frustules through uptake and biomineralization of soluble silicon (Si) to silica during the cell division process. After cell division, the diatom Cyclotella sp. also synthesizes and extrudes nanofibers composed of poly N-acetyl glucosamine (β-chitin) through conical pore structures called fultoportulae lining the rim of the frustule valve. The nanofibers are 50–60 nm in width and 40–80 μm in length. After cultivation of Cyclotella to the Si-starved state, nominally 80 fibers per cell (approximately 2 fibers per fultoportula) were produced following the final cell division. However, the co-addition of soluble Si and germanium (Ge) to Si-starved cultures Cyclotella sp. limited diatom growth to one cell division cycle and induced aberrations in the nanostructure of daughter frustule, including fusion of pore arrays and closure fortuportulae openings. This process led to a twofold reduction in the number of β-chitin nanofibers extruded per cell, although fiber length was not affected. These observations were consistent with a process where the parent frustule continued to form chitin nanofibers, whereas the aberrant daughter frustule did not. This study also demonstrated how diatom biomineralization processes can be harnessed to create novel biogenic nanostructured materials consisting of both Si-Ge oxide nanocomposites and pendant biopolymer nanofibers.