Heterotrophic single-cell protein production in Galdieria sulphuraria: amino acid profile and protein bioaccessibility

Abstract

Protein production with single-cell heterotrophic microalgae may result in at least threefold lower green gas emissions, water consumption and land usage per Kg of protein produced compared to soybeans1,2. When produced heterotrophically, some microalgal species may completely lose their pigmentation, simplifying the integration of algae biomass into food products. Galdieria sulphuraria is one of the most promising microalgae for protein production. When cultivated in the presence of light it can contain up to 66% w/w amino acids with a high in vitro bioaccessibility (69%) 3 and an ideal profile for human consumption. When cultivated heterotrophically the protein content of G. sulphuraria has been reported to drop down to 30% w/w 4. In this work, we cultivated heterotrophically two G. sulphuraria strains, known to lose their pigmentation in the absence of light, and two strains known to maintain their pigmentation. Protein content and bioaccessibility were followed during four growth phases: exponential growth (EX), early stationary phase (ES), 24 (S24) and 48 (S48) hours after substrate depletion. The four strains had an exponential growth rate between 0.90 and 1.65 day-1. After substrate depletion, the growth terminated and dry weight decreased 12-17% from ES to S48. Substrate depletion strongly affected protein content and bioaccessibility. In all the strains nitrogen content began to increase after substrate depletion, reaching 8-12% w/w of the biomass within 24h (S24). The analysis of amino acid content confirmed the increase in proteins after substrate depletion and the high protein (50-60% w/w). Our results indicate that also heterotrophically grown G. sulphuraria has a high protein quality fulfilling FAO dietary requirements for adults, and that it is particularly rich in sulphurated amino acids (2-2.5% w/w). Interestingly, protein bioaccessibility had an opposite trend decreasing from a maximum of 80% during EX to a minimum of 20% in S48 with substantial differences among the strains. Our results highlight the large variation in protein content and bioavailability among G. sulphuraria strains during different growth phases. Selecting the right strain and harvesting at the right time are keys to efficient single-cell protein production.