Simultaneous Saccharification and Fermentation of Cellulose by Microalgae Schizochytrium Limacinum for Lipid Production

Simultaneous Saccharification and Fermentation of Cellulose by Microalgae Schizochytrium limacinum for Lipid Production

Yubin Zheng, Xiaochen Yu, Jijiao Zeng, Zhanyou Chi, Shulin Chen

Department of Biological Systems Engineering,

Washington State University, Pullman, WA 99164

Abstract

Due to the limitation of the agricultural oil resources, the accumulation of lipids from microalgae has received tremendous attention recently. Heterotrophic microalgae culture to produce biodiesel showed the advantages of fast growth, high production rate, convenience for harvesting and scaling up. However, the fatal limiting factor for biodiesel production is the availability and cost of feedstock in the heterotrophic process. The cellulosic material is the potential candidate because of its sustainability, abundance and low value. In this study, we investigated the feasibility for lipid production via simultaneous saccharification and fermentation (SSF) of Avicel, an insoluble crystalline cellulose polymer, by microalgae Schizochytrium limacinum which was reported to produce more than 50% lipid as well as high-value by-product docosahexaenoic acid (DHA). The effects of enzyme loading were studied firstly. The lipid production rate was detected on 3% (w/w) Avicel with enzyme loading from 5 to 60 FPU/g cellulose (the ratio of cellobiase to cellulase was fixed at 5:1). The optimal enzyme loading was 30 FPU/g cellulose and the lipid was 46% of the theoretical value based on pure glucose as the substrate. The lipid profile analysis showed that the enzyme loading had significant effects on fatty acid compositions, especially the DHA, and lower enzyme loading led to higher DHA content. Then, the ratio of cellobiase to cellulase (0-5:1) with fixed enzyme loading of 30 FPU/g cellulose was evaluated for lipid production. Finally, the SSF of Avicel by S. limacinum was conducted in fermentor under the optimal enzyme loading and ratio of cellobiase to cellulase. The kinetics of glucose, cellobiose, total lipid and DHA were investigated. These results demonstrated that the microalgae S. limacinum had the capability to use cellulose for lipid production. This is the first description of Avicel cellulose for biodiesel production by microalgae. The finding is quite significant because it provides the potential to produce biodiesel by heterotrophically culturing microalgae on lignocellulosic biomass, such as wheat straw, switch grass, corn stover, etc, and a cost reduction for algal oil production can be expected by using this type of feedstock in the future.