Abstract
Current mixotrophic culture systems for Dunaliella salina have technical limitations to achieve high growth and productivity. The purpose of this study was to optimize the mixotrophic conditions imposed by glycerol, light, and salinity that lead to the highest biomass and β-carotene yields in D. salina. The combination of 12.5 mM glycerol, 3.0 M salinity, and 50 μmol photons m−2 s−1 light intensity enabled significant assimilation of glycerol by D. salina and consequently enhanced growth (2.1 × 106 cell mL−1) and β-carotene accumulation (4.43 pg cell−1). The saline and light shock induced the assimilation of glycerol by this microalga. At last stage of growth, the increase in light intensity (300 μmol photons m−2 s−1) caused the β-carotene to reach values higher than 30 pg cell−1 and tripled the β-carotene values obtained from photoautotrophic cultures using the same light intensity. Increasing the salt concentration from 1.5 to 3.0 M NaCl (non-isosmotic salinity) produced higher growth and microalgal β-carotene than the isosmotic salinity 3.0 M NaCl. The mixotrophic strategy developed in this work is evidenced in the metabolic capability of D. salina to use both photosynthesis and organic carbon, viz., glycerol that leads to higher biomass and β-carotene productivity than that of an either phototrophic or heterotrophic process alone. The findings provide insights into the key role of exogenous glycerol with a strategic combination of salinity and light, which evidenced unknown roles of this polyol other than that in osmoregulation, mainly on the growth, pigment accumulation, and carotenogenesis of D. salina.
Highlights
Microalgae are one of the primary aquatic resources with the potential for exploitation and cultivation to obtain economically valuable compounds
This paper presents the individual and combined effect of glycerol level, salinity, and light intensity on the nutritional and metabolic capacities of D. salina to maximize the cellular growth and carotenoids content under mixotrophy
Due to the heterogeneity of both the salinities and glycerol levels, and the cell growth reported from previous studies [14,20,28], the effect of glycerol concentration on the culture of D. salina under better growth isosmotic conditions (1.5 M NaCl) [29] was evaluated
Summary
Microalgae are one of the primary aquatic resources with the potential for exploitation and cultivation to obtain economically valuable compounds. The biomass and β-carotene of this microalga are used extensively in the food, feed, pharmaceutical, biomedical, and nutraceutical and cosmeceutical industries [3,4]. This halophilic microorganism is a promising source of other carotenoids (zeaxanthin, lutein, and α-carotene), vitamins, lipids, and proteins [3,5,6]. Mixotrophic systems are attractive due to their low light requirements and large availability of carbon (inorganic and organic), contributing to improved growth rate, biomass, and extensive synthesis of metabolites. Organic carbon nutrition and mixotrophy linked to metabolic pathways represent new scenarios in expanding knowledge and diversification of the metabolism and physiology of microalgae of environmental and economic importance
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