Utilization of cane molasses towards cost-saving astaxanthin production by a Chlorella zofingiensis mutant

Abstract

The aim of the present study was to survey the growth and astaxanthin production of E17, an astaxanthin-rich mutant of Chlorella zofingiensis, through feeding the low-cost carbon source cane molasses. In heterotrophic batch cultivation, E17 fed with pretreated molasses achieved biomass (1.79 g L⁻¹ day⁻¹) and astaxanthin (1.99 mg L⁻¹ day⁻¹) productivities comparable to those with glucose, which were about 2- and 2.8-fold of those fed with untreated molasses, respectively. Molasses-induced astaxanthin accumulation may be attributed to the elicited expression of carotenogenic genes, in particular the genes specifically responsible for the ketolation and hydroxylation of β-carotene to form astaxanthin. A two-stage fed-batch strategy was employed to grow E17 and induce astaxathin accumulation, resulting in 45.6 g L⁻¹ biomass and 56.1 mg L⁻¹ astaxanthin, the highest volumetric astaxanthin yield ever reported for this alga. In addition, the astaxanthin production by E17 was tested with a semi-continuous culture method, where the directly diluted raw molasses (giving 5 g L⁻¹ sugar) was used as the carbon source. Little growth inhibition of E17 was observed in the semi-continuous culture with a biomass productivity of 1.33 g L⁻¹ day⁻¹ and an astaxanthin productivity of 0.83 mg L⁻¹ day⁻¹. The mixotrophic semi-continuous cultures enhanced the biomass and astaxanthin productivities by 29.3 % and 42.2 %, respectively. This study highlights the potential of using the industrially cheap cane molasses towards large-scale cost-saving production of the high-value ketocarotenoid astaxanthin.