Production of α-tocopherol by sequential heterotrophic-photoautotrophic cultivation of Euglena gracilis

Photoautotrophic cultivation of Euglena gracilis results in cells with high a-tocopherol content but the final cell concentration is usually very low due to the difficulty of supplying light efficiently to the photobioreactor. On the other hand, Euglena grows heterotrophically to high cell concentrations, using various organic carbon sources, but the alpha-tocopherol contents of heterotrophically grown cells are usually very low. Sequential heterotrophic/photoautotrophic cultivation, by which cells are grown heterotrophically to high cell concentrations and then transferred to photoautotrophic culture for accumulation of alpha-tocopherol was therefore investigated for efficient alpha-tocopherol production. In batch culture, using glucose as the organic carbon source, the cellular alpha-tocopherol content increased from 120 mu g g(-1) at the end of heterotrophic phase to more than 400 mu g g(-1) at the end of the photoautotrophic phase. By using ethanol as the organic carbon source during the heterotrophic phase, adding corn steep liquor as a nitrogen source and optimizing light supply during the photoautotrophic phase, the alpha-tocopherol content of the cells at the end of the photoautotrophic phase increased to 1700 mu g g(-1). A system consisting of a mini-jar fermenter (for the heterotrophic phase) and an internally illuminated photobioreactor (for the photoautotrophic phase) was then constructed for continuous sequential heterotrophic/photoautotrophic cultivation. The cells were continuously cultivated heterotrophically in the mini-jar fermenter and the effluent was continuously passed through the photobioreactor for cr-tocopherol accumulation. In this way, it was possible to produce 7 gl(-1) cells containing about 1100 mu g alpha-tocopherol per g-cell continuously for more than 420 h. The continuous process resulted in alpha-tocopherol productivity of 100 mu g l(-1) h(-1) which is about 9.5 and 4.6 times higher than those obtained in batch photoautotrophic culture and batch heterotrophic cultures, respectively. (C) 1999 Elsevier Science B.V. All rights reserved.