Xylose, glucose and acetate as feedstock for three microalgal species cultivated in heterotrophy

Microalgae are known as good producers of high-added value bioproducts useful in many applications such as pharmaceuticals, nutrition or biofuel production. In contrast to phototrophy, heterotrophy emerges as a promising strategy for algal biomass production due to high cell densities, controlled conditions and reduced space requirement. Hemicellulose is the second most abundant material in land plants. Its hydrolysis liberates xylose, glucose, acetate. Our study focuses on three microalgal species, Galdieria sulphuraria, Euglena gracilis, and Auxenochlorella protothecoides, cultivated under heterotrophy with the above-mentioned carbon sources, supplemented alone or in combination. Growth parameters and biomass analysis revealed distinctive characteristics. G. sulphuraria, despite a modest fatty acid content (5–15 % w/w), displayed potential for hemicellulose valorization, demonstrating high biomass yield using xylose as a sole carbon source (approx. 0.5 gDW gxylose−1) and high saturated fatty acid (SFA) content (45–63 %). In our cultivation conditions, E. gracilis only assimilated acetate with low fatty acid content (approx. 6 % w/w), but high SFA content (60–77 %) and high paramylon content (47 % w/w), convertible to wax-esters under anaerobiosis. A. protothecoides exhibited biomass yields of 0.42–0.54 gDW gsubstrate−1 depending on the carbon substrate supplied but maintained constant fatty acid content (16–18 % w/w) in the presence of all substrates except xylose. Surprisingly, despite an inability to grow with xylose alone, sugar depletion analysis indicated decreasing xylose concentration when other carbon sources were present in the cultivation medium for this alga. This comparative study discusses the strengths and weaknesses of each strain, providing insights into their potential when grown on hemicellulose carbon sources.