Treatment With A Moderate Concentration Of Na₂S Improves Photobiological H₂Production In Chlamydomonas Reinhardtii

Increased awareness of energy crisis and environmental pollution has led toextensive efforts to develop H2photoproduction in microalgae. Chlamydomonasreinhardtii, a unicellular green alga, has been recognized as an ideal system forsustainable hydrogen photoproduction under anaerobic conditions because of itsclearly genetic background, easy culture, rapid growth, easy manipulation and highactivity of H2ase. However, the extreme sensitivity of hydrogenase (H2ase) to O2greatly limits the development of H2photoproduction in microalgae. To breakthroughthis limitation, many methods were developed by decreasing O2content andincreasing hydrogenase activity, such as NaHSO3treatment and sulfur deprivation.Although these methods can greatly increase the hydrogen photoproduction in C.reinhardtii, the accumulated amount of hydrogen cannot meet still the requirement ofindustrial application. Therefore, it is necessary to seek new approaches to improvehydrogen photoproduction in C. reinhardtii.The purpose of this study was to search new methods to improve hydrogenphotoproduction in C. reinhardtii. We found that treatment with Na2S cansignificantly improve the yield of H2production in C. reinhardtii by screening thesechemical regents. Under steady state conditions, the accumulated H2levels in cellstreated with1.5mM of Na2S were about10times higher than that in untreated cells. We also found that treatment with Na2S reduced the dissolved O2content in thecultures, activated H2ase activity, thereby enhancing the yield of H2photoproductionin C. reinhardtii.Compared to the NaHSO3treatment and sulfur deprivation strategies, the H2yield caused by Na2S treatment was lowest, but the impairment of photosystem IIcaused by the treatment was the best. This suggests that we can further optimize theH2yield in C. reinhardtii via different strategies, and were listed as follows:1) The yield of H2can be enhanced approximately30times by adding1.5mMNa2S repeat every12hours for3times.2) The yield of H2can be enhanced approximately15times by adjusting theinitial pH to8.0;3) The yield of H2can be enhanced approximately15times upon exposure ofcells to500μEm-2s-1;4) The yield of H2can be enhanced approximately20times via adjusting theinitial pH to8.0and illuminating using500μEm-2s-1.On the basis of these results, we conclude that Na2S treatment is a novel strategyto improve H2photoproduction in the green alga C. reinhardtii and its yield can befurther improved via different strategies. Our study diversifies the strategy ofhydrogen photoproduction and provides a theoretical possibility for industrialapplication of H2energy in microalgae.