Amino acid profile and protein bioaccessibility of two Galdieria sulphuraria strains cultivated autotrophically and mixotrophically in pilot-scale photobioreactors

Galdieria sulphuraria is considered one of the most promising microalgae for food applications. In this study, we compared two strains of G. sulphuraria cultivated autotrophically and mixotrophically over 35 days in pilot-scale photobioreactors under nonsterile conditions. The low pH (<1.9) used for cultivation successfully prevented microbial contamination. The two strains had similar autotrophic and mixotrophic biomass productivities, the latter being 2.3 times higher than autotrophic productivity. Comparing the two strains, G. sulphuraria SAG 108.79 and ACUF 064 had 51% and 64% (w/w) protein and 4% and 9% (w/w) C-phycocyanin content, respectively. Interestingly, G. sulphuraria SAG 108.79 showed a protein bioaccessibility of 62%, in line with other microalgal species, whereas G. sulphuraria ACUF 064 had a protein bioaccessibility of only 14%. No differences in the amino acid profile were found between the two strains or between trophic modes. Stable and well-balanced protein profiles are encouraging results for future applications of this species. The main focus of this study was the production of single-cell proteins using two strains of the polyextremophilic microalgae Galdieria sulphuraria. The acidic cultivation condition was sufficient to guarantee axenic production in not sterile conditions, even in the presence of glucose. Both strains were rich in proteins with a similar amino acid profile rich in all of the essential amino acids. Interestingly, there was a 4.4-fold difference in protein bioaccessibility between the two strains. Simple production of axenic microalgal biomass rich in protein is an encouraging result for future food applications of this species.