Phosphate addition strategies for enhancing the co-production of lipid and chitin nanofibers during fed-batch cultivation of the diatom Cyclotella sp.

Abstract

Diatom microalgae require dissolved silicon for cell division. This study assessed three phosphate addition modes for controlling the co-production of lipids and extracellular chitin nanofibers by the centric diatom Cyclotella sp. under silicon-limited cell division during two-stage, fed-batch photobioreactor cultivation. In Stage I, diatom cells were grown on an initial charge of dissolved silicon (Si), nitrate (N), and phosphate (P) to initialize the silicon-starved state. In Stage II, cells were continuously fed 0.20 mmol L−1 day−1 Si with 1.9 mol N/mol Si for 7–9 days to provide 9·106 cells mL−1 by Si-delivery limited cell division. The culture was fed phosphate by three modes (A, B, C). In mode A, phosphate was added at the beginning of Stage I (0.01 to 0.04 mM) with no additional phosphate added to Stage II. In mode B, phosphate was also added as a single pulse to the beginning of Stage II, whereas in mode C, a concentrated feed phosphate solution was pumped to the culture during Stage II to provide 0.006–0.04 mmol P L−1 culture day−1. Cells took up all phosphate delivered in each mode. In mode A, a controlled P-limited state was superimposed on silicon-delivery limited cell division, with minimum requirement for cell division of 9.4 ± 0.7 μmol P/109 cells. Cell number and chitin production rate scaled linearly to phosphate delivery, but lipid yields exceeding 60 wt% were not sustained beyond one cell division in Stage II. In mode C, cell division and co-production of lipid and chitin were sustained, with productivities of 1.1·106 cells mL−1 day−1, 31 ± 2 mg lipid L−1 day−1 and 17 ± 2 mg chitin L−1 day−1 in Stage II, with similar results obtained by mode B. Overall, these phosphate delivery strategies identified minimum phosphate requirements, and controlled the rate and selectivity of lipid and chitin nanofiber production.