Bacterial production and secretion of water-insoluble fuel compounds from cellulose without the supplementation of cellulases.

Abstract

Achieving economic biofuel production from cellulosic biomass will require significant cost reductions. Enzymatic degradation of cellulosic biomass and distillation of water-soluble fuel compounds substantially increase the cost of biofuel production. Consolidated bioprocessing is a strategy to circumvent expensive biofuel production steps. Clostridium thermocellum is a promising bacterium for consolidated bioprocessing because it does not require the supplementation of lignocellulose-degrading enzymes. To produce water-insoluble fuel compounds, C. thermocellum was engineered to express a fatty acyl-acyl carrier protein reductase and an aldehyde-deformylating oxygenase from Synechococcus elongatus PCC 7942. Expression of the aldehyde-deformylating oxygenase gene was clearly detected, whereas only slight expression of the fatty acyl-acyl carrier protein reductase gene was detected. Cells expressing the fatty acyl-acyl carrier protein reductase and the aldehyde-deformylating oxygenase accumulated fatty aldehydes (higher alcohol precursors). After cultivation with cellulose, the higher alcohols, decanol and dodecanol, were detected in the organic solvent phase of the culture broth, indicating that the strain secreted the higher alcohols. These results suggest that the engineered C. thermocellum strain, expressing fatty acyl-acyl carrier protein reductase and aldehyde-deformylating oxygenase genes, directly produces and secretes higher alcohols from cellulose without the supplementation of cellulases. The higher alcohols can be collected by phase separation.