Bioethanol production from marine biomass alginate by metabolically engineered bacteria

Bioethanol production from algae is a promising approach that resolves problems associated with biofuel production from land biomass, such as bioethanol-food conflicts and the indirect land use change. However, it presents several technical difficulties because existing ethanologenic microbes can neither degrade alginate, a major component of brown algae, nor assimilate alginate degradation products. We developed an integrated bacterial system for converting alginate to ethanol using a metabolically modified, alginate-assimilating, pit-forming bacterium, Sphingomonas sp. A1 (strain A1). Overexpression of Zymomonas mobilis pdc and adhB was achieved using a strong constitutive expression promoter newly identified in strain A1 and by inserting multiple gene copies using the methylation sensitivity of XbaI. Metabolome analysis revealed by-product accumulation, and its synthesis pathway was blocked by gene disruption. The ethanologenic recombinant strain A1 accumulated 13.0 g L-1 ethanol in 3 d using alginate as the sole carbon source.