Screening for heterotrophy in microalgae of various taxonomic positions and potential of mixotrophy for production of high-value compounds

We tested 37 photoautotrophic microalgal strains of various taxonomic positions for their heterotrophic growth on glucose. We showed that facultative heterotrophy was quite common among certain groups of green algae (Chlorellales or Sphaeropleales) and of Xanthophyceae, whereas it was completely absent from Eustigmatophyceae. For the first time, we found heterotrophy in some members of green algae (Botryosphaerella sudetica, Bracteacoccus sp., Dictyosphaerium spp., Lemmermannia sp., Parachlorella kessleri). Of the tested facultative heterotrophs, three strains, Scenedesmus sp., Dictyosphaerium chlorelloides, and Tribonema aequale, were selected to investigate, in detail, their mixotrophic growth and production of specific metabolites, carotenoids, exopolysaccharides, and eicosapentaenoic acid, respectively. Among them only T. aequale showed significantly higher production of the target compound, eicosapentaenoic acid, under the mixotrophic cultivation mode than photoautotrophically. Together with the ease of biomass harvest, Tribonema microalgae are therefore proposed as a possible source of high-quality -3 polyunsaturated fatty acids.