To enhance the economics of microalgae-based biofuels, utilization of every ingredient of the raw biomass is important (Georgianna and Mayfield, 2012; Sheehan et al., 1998). Whereas the majority of fuels derived from microalgae have been focused on storage oils, the extracted oil accounts for only 37.9% of the energy and 27.4% of the initial fixed carbon (Lardon et al.,

2009) . The remaining carbon is stored in the leftover oil cakes composed of abundant proteins and carbohydrates. Hence, recycling these nutrient elements may help increase biomass margins of microalgae-based fuels (Lardon et al., 2009). Recycling algal waste by anaerobic digestion has been proposed to support the microalgae production process (Ras et al., 2011; Zamalloa et al., 2012).

Several innovative metabolic engineering strategies have been proposed recently to reduce the energy debt and increase the margins of microalgae-based fuels. One of the approaches is to establish an integrated system that takes advantage of the amenable genetic modification capability of the Escherichia coli (E. coli) system. Although microalgae can grow photosynthet­ically to accumulate biomass for biodiesel purposes, the leftover paste can be utilized for alcohol-fuel production by feeding it into an engineered bacterial system. Huo et al. accom­plished this by genetically engineering an E. coli strain that is capable of converting the back­bone and side chains of amino acids in pretreated biomass into two-, four — and five-carbon alcohol fuels, ammonia, and other chemicals (Huo et al., 2011). In a small-scale experiment, the authors successfully converted hydrolyzed microalgal protein biomass into alcohol fuels. This demonstration supports the potential of using microalgal biomass as a feedstock for protein-based biorefinaries.