Photoautotrophic Production of Docosahexaenoic Acid- and Eicosapentaenoic Acid-Enriched Biomass by Co-Culturing Golden-Brown and Green Microalgae

Marine microalgae offer a sustainable alternative source for the human diet’s essential omega-3-fatty acids, including docosahexaenoic acid (DHA, C22:6) and eicosapentaenoic acid (EPA, C20:5). However, none of them can produce DHA and EPA in a nutritionally balanced ratio of 1:1. As shown recently, the phototrophic co-cultivation of the golden-brown microalgae Tisochrysis lutea (DHA producer) with the green microalgae Microchloropsis salina (EPA producer) can provide microalgae biomass with a balanced DHA-to-EPA ratio with increased productivity compared to monocultures. This study evaluates whether other golden-brown (Isochrysis galbana) and green microalgae (Nannochloropsis oceanica, Microchloropsis gaditana) can enable the phototrophic batch production of omega-3 fatty acids in a nutritionally balanced ratio in co-culture. All co-cultivations applying a physically dynamic climate simulation of a repeated sunny summer day in Australia in LED-illuminated flat-plate gas lift photobioreactors resulted in increased biomass concentrations compared to their respective monocultures, achieving balanced DHA-to-EPA ratios of almost 1:1. Using urea instead of nitrate as a nitrogen source increased the EPA content by up to 80% in all co-cultures. Light spectra measurements on the light-adverted side of the photobioreactor showed that increased biomass concentrations in co-cultures could have been related to enhanced light use due to the utilization of different wavelengths of the two microalgae strains, especially with the use of green light (500–580 nm) primarily by golden-brown microalgae (I. galbana) and orange light (600–620 nm) predominantly used by green microalgae (N. oceanica). Phototrophic co-cultivation processes thus promise higher areal biomass yields if microalgae are combined with complimentary light-harvesting features.