Effects of nitrogen delivery on chitin nanofiber production during batch cultivation of the diatom Cyclotella sp. in a bubble column photobioreactor

Abstract

The photosynthetic diatom Cyclotella sp. extrudes chitin nanofibers following cell division. This diatom requires silicon for cell wall biosynthesis and division, as well as nitrogen for biosynthesis of intracellular material and extracellular chitin, an N-acetyl glucosamine biopolymer. The initial nitrogen/silicon molar ratio was the critical parameter for assessing the limits of nitrogen delivery on cell number and chitin production during batch cultivation of Cyclotella in a bubble column photobioreactor under silicon-limited growth conditions, using nitrate as the nitrogen source. The peak rate of volumetric chitin production increased linearly, from 3.0 to 46 mg chitin L⁻¹ day⁻¹, with increasing N/Si ratio over the range studied (0.82 to 8.6 mol N mol⁻¹ Si). However, the cell number yield and the chitin yield per cell increased asymptotically with increasing N/Si ratio, achieving a final cell number yield of 5.3 × 10⁹ ± 2.6 × 10⁸ cells mol⁻¹ Si and chitin yield of 28.7 ± 1.2 mg chitin per 10⁹ cells (1.0 S.E.). An N/Si ratio of at least 4.0 mol N mol⁻¹ Si achieved 90% of the asymptotic chitin yield. This study has shown that scalable cultivation systems for maximizing chitin nanofiber production will require delivery of both silicon and optimal nitrogen under silicon-limiting growth conditions to promote cell division and subsequent chitin formation.