Continuous production of selenomethionine-enriched Chlorella sorokiniana biomass in a photobioreactor

This article describes the enrichment of the fresh-water green microalga Chlorella sorokiniana in selenomethionine (SeMet). The microalga was cultivated in a 2.2 L glass-vessel photobioreactor, in a culture medium supplemented with selenate (SeO42−) concentrations ranging from 5 to 50 mg L−1. Although selenate exposure lowered culture viability, C. sorokiniana grew well at all tested selenate concentrations, however cultures supplemented with 50 mg L−1 selenate did not remain stable at steady state. A suitable selenate concentration in fresh culture medium for continuous operation was determined, which allowed stable long-term cultivation at steady state and maximal SeMet productivity. In order to do that, the effect of dilution rate on biomass productivity, viability and SeMet content of C. sorokiniana at several selenate concentrations were determined in the photobioreactor. A maximal SeMet productivity of 21 μg L−1 day−1 was obtained with 40 mg L−1 selenate in the culture medium. Then a continuous cultivation process at several dilution rates was performed at 40 mg L−1 selenate obtaining a maximum of 246 μg L−1 day−1 SeMet at a low dilution rate of 0.49 day−1, calculated on total daily effluent volume. This paper describes for the first time an efficient long-term continuous cultivation of C. sorokiniana for the production of biomass enriched in the high value amino acid SeMet, at laboratory scale.