Maximizing the hydrogen photoproduction yields in Chlamydomonas reinhardtii cultures: The effect of the H2 partial pressure

Photoproduction of H2 gas has been examined in sulfur/phosphorus-deprived Chalmydomonas reinhardtii cultures, placed in photobioreactors (PhBRs) with different gas phase to liquid phase ratios (Vg.p./Vl.p.). The results demonstrate that an increase in the ratio stimulates H2 photoproduction activity in both algal suspension cultures and in algae entrapped in thin alginate films. In suspension cultures, a 4× increase (from ∼0.5 to ∼2) in Vg.p./Vl.p results in a 2× increase (from 10.8 to 23.1 mmol l−1 or 264–565 ml l−1) in the total yield of H2 gas. Remarkably, 565 ml of H2 gas per liter of the suspension culture is the highest yield ever reported for a wild-type strain in a time period of less than 190 h. In immobilized algae, where diffusion of H2 from the medium to the PhBR gas phase is not affected by mixing, the maximum rate and yield of H2 photoproduction occur in PhBRs with Vg.p./Vl.p above 7 or in a PhBR with smaller headspace, if the H2 is effectively removed from the medium by continuous flushing of the headspace with argon. These experiments in combination with studies of the direct inhibitory effect of high H2 concentrations in the PhBR headspace on H2 photoproduction activity in algal cultures clearly show that H2 photoproduction in algae depends significantly on the partial pressure of H2 (not O2 as previously thought) in the PhBR gas phase.