Investigation of the Physiology of Hydrogen Production in the Green Alga Chlamydomonas reinhardtii Using Spectral-Selective Photosystem I Light

Abstract

With increasing global populations and demand for energy, greater strain is placed on the limited supply of fossil derived fuels, which in turn drives the need for development of alternative energy sources. The discovery of biophotolysis in Chlamydomonas reinhardtii and the development of a spectral-selective photosystem I activating/photosystem II deactivating light (PSI-light) method provides a promising platform for commercial hydrogen production systems. The PSI-light method allows electrons to pass through the photosynthetic electron transport chain while reducing radiation available for photosynthetic oxygen evolution that inactivates hydrogenase. Exploring the physiology of photohydrogen production using the PSI-light method can provide insight on how to optimize conditions for maximum hydrogen production. Through the use of photosynthetic mutant strains of C. reinhardtii, it was possible to suppress photosynthetic oxygen evolution further than using photosystem I light alone to extend photohydrogen production longevity and total yield. A preliminary investigation of an iterating light treatment revealed that longevity and yield could be increased further by providing a period of darkness to allow cells to consume evolved oxygen and resynthesize hydrogenase. Work with these mutants provided understanding that a balance of radiation was required to provide electrons to hydrogenase while limiting oxygen evolution, and that when no light was provided, fermentation of stored starch was the major contributor of electrons to hydrogen production. To determine the role of starch during hydrogen production, wild type cells were exposed to different media and light treatments and monitored for starch consumption and hydrogen production. The results indicated that starch was required for hydrogen production in the dark, but for photohydrogen production, starch likely played a minor role in contributing electrons to hydrogenase. The experiments also showed the importance of acetate in the medium during the hydrogen production phase to allow any significant photohydrogen production. The role of acetate was further investigated as a growth medium constituent that stimulates metabolic activity while reducing photosynthetic oxygen evolution when added to cells grown auto- or mixotrophically. By exposing cells to CO₂ during growth, photohydrogen production was significantly increased over cells grown only in the presence of acetate.