Hao Fang

　　Northwest A&F University, Yangling, Shaanxi, China,

　　Corn stover and rice straw are the representative agricultural residues in the North and South China, respectively, which are always treated improperly such as combusted directly, thereby causing serious environmental problem. Issues about how to utilize them in a sustainable and environmental-friendly way, therefore, are becoming more and more important as the concerns on energy security and environment protection increase in China.

　　On-site enzyme production is another important way to reduce the costs of enzymes in the biorefinery based on lignocelluloses because it has several cost savingadvantages. Furthermore, use of lignocellulosic biomass as substrate to induce cellulase production has an increased enzymatic hydrolysis specificity for the substrate itself than others. These advantages make on-site enzyme production very promising as a solution of enzyme autarky in lignocellulose-based bioprocesses.

　　Single cell oil (SCO) from microorganisms is considered as an alternative oil source because of the high productivity and low land requirement. Amongst different oleaginous microbes, oleaginous fungi have attracted increasing attention these years due to multiple advantages: (1) accumulate high lipid content and produce some value-added fatty acids; (2) show good lipid profiles for making high quality biodiesel; (3) use a variety of carbon sources for lipid production; (4) produce lipids with low capital cost and energy expenditure; and (5) tend to form pellets, improving fermentation and downstream processing.

　　Mortierella isabellina, one of oleaginous fungi, is able to accumulate considerable amount of lipids, up to 80% of dry cell biomass. M. isabellina could be cultured on various substrates, including monomeric sugars, glycerol, lignocellulosic biomass hydrolyzate, etc. Moreover, M. isabellina has good tolerance to inhibitors ubiquitously existing in lignocellulosic hydrolyzates. These features make M. isabellina an ideal candidate for lipid production from low cost lignocellulosic materials.

　　We established on-site cellulase production using a mixed culture system composed of Trichoderma reesei and Aspergillus niger to supply cellulase to the bioprocess from corn stover and rice straw to SCO. We found that the three-stage enzymatic hydrolysis could greatly improve the efficiency of the bioprocess from high solid loading corn stover and rice straw to SCO by M. isabellina . Our work testified the application value of three-stage enzymatic hydrolysis in lignocellulose-based bioprocesses. The bioprocess with three-stage enzymatic hydorlysis used the lowest enzyme input 20 FPIU cellulase/g glucan and the shortest time 222 h, but produced 44.94 g dry cell biomass and 25.77 g lipid from 327.63 g dry steam exploded corn stover. It had the highest lipid content 57.34%, and its productivities and yields were much higher than the processes with one stage enzymatic hydrolysis.

　　Moreover, the SCO produced by M. isabellina could be transformed by self-metathesis to long chain α,ω-dicarboxylic acids, the high-value chemical in industry. The picture is the paradigm we established.