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

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 9.10(6) 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 mu mol P/10 (9) 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 1.1.10(6) 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.