| Photobiological production of hydrogen requires a combination of optimized reductant production by photosynthesis coupled to high reductant-utilization by the algal hydrogenases. The latter, although known to have the highest catalytic rates of all known hydrogenases, are easily inactivated by oxygen gas, a by-product of photosynthesis. Moreover, algal hydrogenase are not expressed nor assembled in the presence of oxygen. The regulation of algal hydrogenase gene transcription is complex, and seems to involve at least two types of sensors, respectively, for oxygen and redox potential. Similarly, the assembly of the catalytic center is dependent on oxygen levels. Finally, photosynthetic electron transport rates are down-regulated under anaerobic conditions that favor hydrogenase activity (such as sulfur-deprivation). In order to circumvent these issues, artificial systems that utilize photochemical charge separation devices coupled to [FeFe]-hydrogenases are being investigated. I will describe current work at NREL addressing each of these issues.
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