Conversion of SPORL pretreated Douglas fir forest residues into microbial lipids with oleaginous yeasts

Authors: Dien, Bruce; ZHU, J.Y - Us Forest Service (FS); Slininger, Patricia - Pat; Kurtzman, Cletus; Moser, Bryan; O Bryan, Patricia; GLEISNER, ROLAND - Us Forest Service (FS); Cotta, Michael

Submitted to: Royal Society of Chemistry Advances
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2016
Publication Date: 2/10/2016
Citation: Dien, B.S., Zhu, J.Y., Slininger, P.J., Kurtzman, C.P., Moser, B.R., O'Bryan, P.J., Gleisner, R., Cotta, M.A. 2016. Conversion of SPORL pretreated Douglas fir forest residues into microbial lipids with oleaginous yeasts. RSC Advances. 6(25):20695-20705. doi: 10.1039/c5ra24430g.

Interpretive Summary: Some microorganisms are capable of storing sugars in the form of lipids when starved for a macro nutrient (i.e. nitrogen or phosphate) that is required for continued growth. Oleaginous yeasts in particular accumulate at least 20% of their cell weight in lipids and some have even been observed to achieve 70% lipid contents. In this study, Douglas fir residue is evaluated as a source for production for single cell oils using the yeast Lipomyces tetrasporus. Douglas-fir is the most important commercial wood in the United States by production. The woody residue was pretreated using the SPORL (Sulfite Pretreatment to Overcome Recalcitrance of Lignocellulose) process and the freed fibers converted to sugars using commercial cellulases. The final sugar concentration was > 70 g/l at moderate enzyme loading. The sugars were converted to lipids using a batch and two multistage processes. In the most promising scheme, the yeast were grown on rich medium and transferred to undiluted SPORL hydrolysate, in the absence of a nitrogen source, for “fattening”. The end result was the production of 13.4 g/l of lipids in 3 days. Furthermore the fatty composition appeared to be compatible for conversion to biodiesel. This paper will be of interest to those employed in the pulp and paper industry and interested in production of advanced biofuels and chemicals.

Technical Abstract: Douglas fir is the dominant commercial tree grown in the United States. In this study Douglas fir residue was converted to single cell oils using oleaginous yeasts. Monosaccharides were extracted from the woody biomass by pretreating with sulfite and dilute sulfuric acid (SPORL process) and hydrolyzing using commercial cellulases. A new SPORL process that uses pH profiling was compared to the traditional method. Either process yielded 77 g/l concentration of sugars. The SPORL generated sugars were evaluated for conversion to SCO using yeasts Lipomyces tetrasporus and Yarrowia lipolytica in batch cultures containing SPORL sugars diluted to 60% v/v supplemented with nitrogen at an appropriate C:N ratio of 75:1. An extended lag phase was observed for both yeasts, which was eliminated by including SPORL sugars diluted to 40% v/v in the seed cultures to acclimate them. The maximum lipid concentrations were 3.56 – 5.02 g/l. This corresponded to yields of 0.07 – 011 g lipid per g beginning sugars and productivities of 0.99 – 1.42 g/l/d. Lipid concentrations for L. tetrasporus were further amplified by using two schemes incorporating multiple batch cultures. In the first, the yeast was grown in 40% v/v SPORL sugars and the entire contents of this fermentation transferred to undiluted SPORL sugars not supplemented with nitrogen. The result was the production of 13.4 g/l lipids within 3 days. This corresponds to a yield of 0.174 g/g and a productivity of 4.47 g/l/d. The second approach was to thrice transfer the yeast cells in 60% v/v SPORL sugars supplemented with limited nitrogen to promote further lipid formation. The end result was 18.1 g/l of lipids with a process yield and productivity of 0.104 g/g and 1.29 g/l/d, respectively. This is the first report that the authors are aware of demonstrating the feasibility of converting woody biomass to SCO.