Two-stage cultivation of a Nannochloropsis mutant for biodiesel feedstock

Abstract

Optimization of mass microalgae cultures is required for the efficient production of biodiesel feedstock in terms of total fatty acid (TFA) content and a conducive fatty acid profile. A mutant strain of Nannochloropsis sp. (MT-I5), as modified via random mutagenesis and flow cytometric cell sorting, was investigated in both a single- and two-stage cultivation using 250 L laboratory raceway ponds. Culture was based on photoautotrophic biomass production (stage 1) followed by a switch to photomixotrophic growth induced by adding sodium acetate (2 mM) (stage 2). The biomass yield of the mutant in two-stage cultivation was maintained at a level similar to that of the one-stage photoautotrophic culture, but TFA content was increased by 2.3-fold. The fatty acid profile of MT-I5 also had an increased level of desirable saturated fatty acids (SFA) for use as a biodiesel feedstock, i.e. from 43 to 48 % of TFA, as well as a decreased level of less desirable polyunsaturated fatty acids (PUFA), i.e. from 22 to 7 % of TFA. The two-stage cultivation process is of interest for the mass culture of microalgae for biofuel feedstocks, as biomass productivity can be maximized during the first stage of culture until N-starvation is achieved, followed by the enhanced synthesis of SFA in the second stage of culture by adding sodium acetate as a fixed-carbon source.