Engineering the xylose metabolism in Schizochytrium sp. to improve the utilization of lignocellulose

Background: Schizochytrium sp. is a heterotrophic, oil-producing microorganism that can efficiently produce lipids. However, the industrial production of bulk chemicals using Schizochytrium sp. is still not economically viable due to high-cost culture medium. Replacing glucose with cheap and renewable lignocellulose is a highly promising approach to reduce production costs, but Schizochytrium sp. cannot efficiently metabolize xylose, a major component of lignocellulosic biomass. Results: : In order to improve the utilization of lignocellulose by Schizochytrium sp., we cloned and functionally characterized the genes encoding enzymes involved in the xylose metabolism. The results showed that the endogenous xylose reductase and xylulose kinase genes possess corresponding functional activities. Additionally, attempts were made to use genetic engineering techniques to introduce exogenous xylitol dehydrogenase/xylose isomerase to construct a strain of Schizochytrium sp. that can efficiently utilize xylose. The results showed that the engineered strain 308-XI with an exogenous xylose isomerase could utilize xylose, consuming 8.2 g/L xylose over 60 h of cultivation. Xylose consumption was further elevated to 11.1 g/L when heterologous xylose isomerase and xylulose kinase were overexpressed simultaneously. Furthermore, culturing 308-XI-XK(S) in a medium with lignocellulosic hydrolysate as the main carbon source resulted in a 22.4 g/L of dry cell weight and 5.3 g/L of total lipid titer, respectively representing 42.7 and 30.4% increases compared to the wild type. Conclusion: This study shows that engineering of Schizochytrium sp. to efficiently utilize xylose is conducive to improve its utilization of lignocellulose, which can reduce the costs of industrial lipid production.