Co-production of high density biomass and high-value compounds via two-stage cultivation of Chlorella vulgaris using light intensity and a combination of salt stressors

Abstract

In this study, the effects of combined stress factors involving light intensity and salinity (NaCl, MgCl2, CaCl2, and their combinations) on the two-stage cultivation of Chlorella vulgaris for simultaneous production of biomass and high-value products, were investigated. The two-stage strategies comprised a 25-day vegetative stage in BG11 medium, followed by a 15-day combined stress stage. During salt stress conditions, the addition of 15 g L−1 CaCl2 or 7.5 g L−1 MgCl2 / 7.5 g L−1 CaCl2 mixture with 140 µmol m−2 s−1 light intensity significantly promoted the growth of C. vulgaris achieving maximum biomass productivity of 50.50 ± 0.50 and 50.25 ± 3.25 mg L−1 d−1, respectively. Cultivation of C. vulgaris in a medium containing 7.5 g L−1 NaCl/ 7.5 g L−1 CaCl2 had remarkably increased the lipid content (31.15 ± 1.18%) and lipid productivity (14.55 ± 1.48 mg L−1 d−1). The saturated fatty acids (SFAs) at 39.52–59.29%, monounsaturated fatty acids (MUFAs) at 27.16–35.47%, and polyunsaturated fatty acids (PUFAs) at 7.18–29.97%, were obtained with palmitic (C16:0), oleic (C18:1), stearic (C18:0), and linolenic (C18:3) acids as predominant fatty acids. Cultures supplemented with 5 g L−1 NaCl / 5 g L−1 MgCl2 / 5 g L−1 CaCl2 and high light intensity exposure attained consistently high carbohydrate content (52.71 ± 2.50%). The combination of 7.5 g L−1 NaCl / 7.5 g L−1 MgCl2 also resulted in a marked increase in the protein content (35.32 ± 2.20%) and total carotenoids (0.31 ± 0.03 μg mL−1) as compared to the Controls. The highest antioxidant activity (86.16%) was achieved with a 7.5 g L−1 NaCl / 7.5 g L−1 CaCl2 combination in the growth stage. The antioxidant activities were attributed to the presence of phenolics, flavonoids, and tannins due to the stressed conditions. One of the key benefits of using a combined stress strategy in this study is that if one factor has a low impact on enhancing target metabolites, other factors can compensate.