Characterization of plant carbon substrate utilization by Auxenochlorella protothecoides

Algae hold great potential as a source of renewable fuel due to their ability to produce refinery-compatible diesel and jet fuel precursors. Significant effort has been made to maximize productivity under photoautotrophic growth conditions; however, little progress has been made to discover and understand reduced carbon assimilation pathways or enzymatic degradation of complex carbon substrates in algae. We purport that utilization of plant-based carbon substrates in addition to photosynthesis (mixotrophic growth) for biochemical assimilation into biomass, biofuels, and bioproducts, can increase cultivation productivity and improve the economic viability of algal-derived biofuels. Herein we report that a freshwater production strain of microalgae, Auxenochlorella protothecoides UTEX 25, is capable of directly degrading and utilizing non-food plant substrates, such as switchgrass, for cell growth. Glycome profiling of plant substrates before and after addition to A. protothecoides cultures demonstrates the utilization of xyloglucans. Genomic, proteomic and transcriptomic analyses revealed the identity of many enzymes that are hypothesized to be involved in complex carbohydrate degradation, including several family 5 and 9 glycosyl hydrolases. This work paves the way for future designer engineering of plant‑carbon utilization to further improve productivity of algal production strains.