Minimum nitrogen cell quota for maximal growth rate in cycloturbidostat cultures of Picochlorum oklahomense

Abstract

Replicate cycloturbidostats cultures of Picochlorum oklahomense were manipulated to quantify the minimum nitrogen cell quota supporting maximum growth rate and to study the effects of nitrogen limitation on cycloturbidostat cultures growing at near-μmax under steady-state, balanced growth conditions. Results show that cyclical diurnal neutral lipid accumulation and ultradian growth are not mutually exclusive, and that cycloturbidostat cultures may be maintained near or at the threshold of N-limitation to control cell composition and modulate biomass and lipid productivity. Using this novel approach, we have identified the threshold of N limitation in P. oklahomense and investigated the effects of N limitation on replicate cycloturbidostat cultures. Mildly N-limited cultures grew rapidly (87 % μmax) and maintained near optimal physiology, while also accumulating neutral lipids during the photoperiod. Flow cytometry revealed differences in temporal patterns between treatments, demonstrating the importance of the cell-based approach in manipulating microalgal cultures. This study highlights the utility of turbidostats in experimental phycology and defines a scantly explored concept in the study of N limitation, the threshold of N cell quota necessary to support μmax below which cells are forced to re-allocate resources and slow growth, denoted QN, T.