Effect of Light Intensity and Wavelength on Biomass Growth and Protein and Amino Acid Composition of Dunaliella salina

Abstract

Dunaliella salina is a halotolerant, photoautotrophic marine microalga and one of the richest sources of natural carotenoids but also shows potential as a novel food source with high protein quality. This study sought to optimise the production of biomass, protein and amino acids from D. salina, alongside carotenoids using a two-stage cultivation approach based on the use of light of different intensities and quality, i.e., white, red and blue LED light. In stage 1, four white LED light intensities were tested. In stage 2, the same four light intensities from either blue or red LEDs were applied once exponential growth ceased and cells reached the stationary phase under white LED light in stage 1. Remarkably, both biomass concentration and biomass productivity showed a 1.3–1.7-fold increase in stage 2, without medium replenishment, while protein concentration and protein productivity showed an ~1.1-fold increase. The amino acid content and amino acid index remained unchanged from stage 1 to stage 2, and minimum difference was found across different light intensities. Overall, D. salina delivered so-called high protein quality, with an essential amino acid index (EAAI) of 0.99, and red light, which has previously been shown to increase carotenoid production, boosted further biomass production over and above white light, at all light intensities tested.

A Comparison of β-Carotene, Phytoene and Amino Acids Production in Dunaliella salina DF 15 (CCAP 19/41) and Dunaliella salina CCAP 19/30 Using Different Light Wavelengths

“…On the other hand, cultivation under white, red or blue light wavelengths, which were previously reported to alter β-carotene isomeric composition in D. salina DF15 when nitrogen was abundant [5,14], did not seem to change the EAA profiles [11]. This effect might be strain-dependent, because carotenoid profiles are differently affected depending on the nature of the D. salina strain used [14], rather than an effect of nitrogen abundance failing to discriminate differences between white, red or blue light on the EAA profile [11]. The potential of D. salina to co-produce high-quality protein and carotenoids has previously been demonstrated, where two-phase cultivation using nitrogen limitation and high light intensity was adopted [9].…”

“…In several studies, cultivation with either nitrogen, phosphorus or sulphur limitations each modified the amino acid profile and changed the EAAI in D. salina [9,12,13]. On the other hand, cultivation under white, red or blue light wavelengths, which were previously reported to alter β-carotene isomeric composition in D. salina DF15 when nitrogen was abundant [5,14], did not seem to change the EAA profiles [11]. This effect might be strain-dependent, because carotenoid profiles are differently affected depending on the nature of the D. salina strain used [14], rather than an effect of nitrogen abundance failing to discriminate differences between white, red or blue light on the EAA profile [11].…”

“…Protein quality, measured in terms of EAA content and the EAAI, may improve as cells redirect their metabolism to increase the degradation of certain amino acids and repress the biosynthesis pathways of others; glutamate levels may decline along with the cellular amino acid pool, but stress-related amino acids such as proline, lysine and tyrosine may increase [22][23][24][25][26][27]. Both D. salina CCAP 19/18 and D. salina DF15 have been shown to support a high protein quality measured in terms of EAAI and EAA content under N-limiting conditions [9,11]. Protein quality can also depend on light quality: in Nannochloropsis gaditana under highenergy blue light, carbohydrate concentrations decreased compared to under red light, and the stress-related amino acid proline increased [28].…”

“…In recent years, the potential of D. salina to be used as a sustainable protein source has bloomed, even after the separation of carotenoids [7,8]. However, it has been reported that D. salina protein quantity and quality can be affected by various factors, including nutrient levels, light intensities and light wavelengths, among others [9][10][11]. As a result, the essential amino acid (EAA) content and essential amino acid index (EAAI) have also been reported with different values, indicating variable protein qualities [9][10][11].…”

“…However, it has been reported that D. salina protein quantity and quality can be affected by various factors, including nutrient levels, light intensities and light wavelengths, among others [9][10][11]. As a result, the essential amino acid (EAA) content and essential amino acid index (EAAI) have also been reported with different values, indicating variable protein qualities [9][10][11]. In several studies, cultivation with either nitrogen, phosphorus or sulphur limitations each modified the amino acid profile and changed the EAAI in D. salina [9,12,13].…”