Combined “de novo” and “ex novo” lipid fermentation in a mix-medium of corncob acid hydrolysate and soybean oil by Trichosporon dermatis

BACKGROUND Microbial oil is one important bio-product for its important function in energy, chemical, and food industry. Finding suitable substrates is one key issue for its industrial application. Both hydrophilic and hydrophobic substrates can be utilized by oleaginous microorganisms with two different bio-pathways ("de novo" lipid fermentation and "ex novo" lipid fermentation). To date, most of the research on lipid fermentation has focused mainly on only one fermentation pathway and little work was carried out on both "de novo" and "ex novo" lipid fermentation simultaneously; thus, the advantages of both lipid fermentation cannot be fulfilled comprehensively. RESULTS In this study, corncob acid hydrolysate with soybean oil was used as a mix-medium for combined "de novo" and "ex novo" lipid fermentation by oleaginous yeast Trichosporon dermatis. Both hydrophilic and hydrophobic substrates (sugars and soybean oil) in the medium can be utilized simultaneously and efficiently by T. dermatis. Different fermentation modes were compared and the batch mode was the most suitable for the combined fermentation. The influence of soybean oil concentration, inoculum size, and initial pH on the lipid fermentation was evaluated and 20 g/L soybean oil, 5% inoculum size, and initial pH 6.0 were suitable for this bioprocess. By this technology, the lipid composition of extracellular hydrophobic substrate (soybean oil) can be modified. Although adding emulsifier showed little beneficial effect on lipid production, it can modify the intracellular lipid composition of T. dermatis. CONCLUSIONS The present study proves the potential and possibility of combined "de novo" and "ex novo" lipid fermentation. This technology can use hydrophilic and hydrophobic sustainable bio-resources to generate lipid feedstock for the production of biodiesel or other lipid-based chemical compounds and to treat some special wastes such as oil-containing wastewater.