From CO2 to proteins and oils: innovative approach for valorizing acetate-rich gas fermentation stream using microalgae through a two-step process

Microalgae represent an innovative source of proteins and fatty acids for food and feed, and their production's economic and environmental sustainability can be improved using novel carbon sources. Acetate is a source of organic carbon that can be produced through innovative processes in a more sustainable way compared to sugars. Among the various biotechnological synthesis processes, gas fermentation enables the production of an acetate-rich stream, where biological contamination is easier to manage compared to other streams or wastewaters. This study presents an innovative two-step process valorizing acetate produced through Thermoanaerobacter kivui gas fermentation as carbon source for Chlorella sorokiniana heterotrophic cultivation. The real acetate-rich stream (13.3 g L−1) was tested at different dilutions, achieving biomass productivities up to 1.6 g L−1day−1. The biomass obtained was characterized for protein, lipid and carbohydrate content. The heterotrophic biomass was characterized by high protein content (39%) and balanced essential amino acid profile, with high nutritional value suitable for human and fish nutrition. The lipid profile also confirmed excellent nutritional characteristics, contained 16% of valuable omega-3 fatty acids and ω6:ω3 ≈ 1. Additionally, the nutritional potential of T. kivui biomass was also evaluated, elucidating its amino acids profile and highlighting a nitrogen to protein conversion factor of 2.65. This innovative process, combining gas fermentation with microalgae cultivation, offers a promising approach to enhancing proteins and lipid production for food and feed applications, compared to traditional photoautotrophic methods.