Ca2+ enhances algal photolysis hydrogen production by improving the direct and indirect pathways

In this study, Ca2+ was applied to enhance the photolysis H2 production of Chlamydomonas reinhardtii grown in pure culture or co-cultivated with bacterium. With addition of Ca2+, the chlorophyll and protein contents in algal cells were maintained at high levels and the reactive oxygen species (ROS) were scavenged to tolerable levels by activities of catalase (CAT) and superoxide dismutase (SOD). High chlorophyll and low ROS are benefit to protect the photosystem II (PS II) activity that responsible for the direct photolysis H2 production. Moreover, addition of Ca2+ was able to promote the acetic acid assimilation as well as the starch synthesis. The starch degradation, in return, contributed to the H2 production via the indirect photolysis pathway. Based on the improvements of direct and indirect photolysis pathways, the H2 production of algae grown in co-cultivation adding with 5 mM Ca2+ reached a maximum value of 306 mL L−1, which was 2.4 times higher than the production without Ca2+ addition. This study gives an efficient and easy strategy to improve the algal photolysis H2 production, and gets an insight into the mechanisms of the improvement.