Algae can be a source of third-generation gaseous transport biofuel (either biomethane or biohydrogen). An optimal system is suggested which combines dark fermentation, anaerobic digestion, and carbon dioxide biofi xation. Biogas upgrading through removal of carbon dioxide is required prior to utilization as a transport biofuel. Traditional upgrading systems are energy intensive and
expensive. Carbon dioxide biofi xation by microalgae, with a bicarbonate/carbonate cycle, offers energy and cost savings over traditional upgrading systems. The microalgae, which remove carbon
dioxide, can be used as substrate for biogas, forming an energy effi cient closed loop system. The proposed system has signifi cant advantages in reducing parasitic energy demand in upgrading biogas, enhancing the energy yield three-fold, and minimizing carbon dioxide emission