Re-routing photosynthetic energy for continuous hydrogen production in vivo

Background: Hydrogen is considered a promising energy vector that can be produced from sustainable resources such as sunlight and water. In green algae, such as Chlamydomonas reinhardtii, photoproduction of hydrogen is catalyzed by the enzyme [FeFe]-hydrogenase (HydA). Although highly efficient, this process is transitory and thought to serve as a release valve for excess reducing power. Up to date, prolonged production of hydrogen was achieved by the deprivation of either nutrients or light, thus, hindering the full potential of photosynthetic hydrogen production. Previously we showed that the enzyme superoxide dismutase (SOD) can enhance HydA activity in vitro, specifically when tied together to a fusion protein. Results: In this work, we explored the in vivo hydrogen production phenotype of HydA-SOD fusion. We found a sustained hydrogen production, which is dependent on linear electron flow, although other pathways feed it as well. In addition, other characteristics such as slower growth and oxygen production were also observed in Hyd-SOD-expressing algae. Conclusions: The Hyd-SOD fusion manages to outcompete the Calvin-Benson cycle, allowing sustained hydrogen production for up to 14 days in non-limiting conditions.