Improvement of hydrogen production by reduction of the photosynthetic oxygen in microalgae cultures of Chlamydomonas gloeopara and Scenedesmus obliquus

One of the most significant challenges in hydrogen (H-2) production by microalgae is the incompatibility between oxygenic photosynthesis and anaerobic hydrogen production due to the high sensitivity of hydrogenase to oxygen (O-2). One strategy for mitigating this problem is to decrease the concentration of photosynthetic O-2 as soon as it is produced. This work proposes that if the photosynthetic O-2 in the culture medium is depleted will be possible to produce hydrogen. To this end, we studied the effects of 4 chemical reducing agents (cysteine, sodium sulfite, ascorbic acid, and hydroquinone) on the growth and consumption of photosynthetic O-2 in cultures of Chlamydomonas gloeopara and Scenedesmus obliquus. All reducing agents were tested at 3 concentrations in triplicate. Then, anaerobic hydrogen production was improved through photosynthetic oxygen consumption using a chemical reducing agent. Of the compounds, only cysteine did not have inhibitory effects against algal growth (even stimulating it) and maintained low O-2 concentrations for 192 h. In descending order of activity, hydroquinone, ascorbic acid, and sodium sulfite inhibited the growth of both algal species at the concentrations that were tested. Thus, cysteine was used to induce the anaerobic H-2 production in algal cultures with a sulfur-containing TAP medium. Hydrogen production was between 3 and 5 times greater when photosynthetic O-2 was eliminated with cysteine than when the sulfur-deprived protocol was applied. Also, algal cultures with cysteine experienced similar growth and carbohydrate metabolism as those in TAP medium, indicating that direct and indirect photolysis supplies electrons to hydrogenase for H-2 production. Based on our results, oxygenic photosynthesis and hydrogen production were reconciled for over 40 h by capturing the produced O-2 by a reducing agent. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.