Microbial biodiesel production from lignocellulosic biomass: New insights and future challenges

In many countries, biodiesel production is obstructed because of a high production cost accounting for raw materials, the large acreage needed for the cultivation of oil-yielding vegetable crops, and competition between food and feed. Therefore, biodiesel production requires new approaches for which microbial oils offer a potential solution. Among several microorganisms available, oleaginous microorganisms (yeast and fungi) accumulate more than 20–70% oils inside their cells when grown in specific environmental conditions. Moreover, microbial oils or single cell oils (SCOs) offer numerous advantages over vegetable oils or animal fats such as similar fatty acid profile, short life cycles of the microbes, relatively lower environmental impact, reduced labor demand, and convenient scalability. Microbial lipids production using lignocellulosic biomass (LCB), which are naturally available in abundance, as a renewable raw material for producing second-generation biodiesel, has become a fundamental approach for tackling the challenges we face of higher energy costs, protection of the environment, and rapid depletion of crude oil reserves. This review compares and examines the extent to which different microbes can accumulate a productive level of lipids using lignocellulosic biomass as substrates, pretreatment strategies used for converting LCB into SCOs, and future challenges in using LCB for biodiesel production.