Biology of Fuel Production
The model system for understanding hydrogen production in algae is Chlamydomonas reinhardtii. This area will describe the current understanding of the sulfur delpeted hydrogen production mechanism. It will also discuss strategies for improving the efficiency of hydrogen production.
Photosynthetic hydrogen production
There are two types of hydrogen production, fermentative and photosynthetic. These terms describe from where the electrons delivered to the hydrogenase are obtained. In photosynthetic hydrogen production, electrons are obtained from the oxidation of water. Fermentative hydrogen production is produced from the oxidation of organic molecules.
Mechanism of Hydrogen production in C. reinhardtii
C. reinhardtii was long known to produce H2 gas when exposed to light after a period of anaerobic incubation in the dark. This H2 production only occured for a period of seconds to a few minutes. Aerobic respiration consumed all the available oxygen, causing anaerobic conditions in the culture. These anaerobic conditions induced the expression of the Fe hydrogenase and other genes. Upon exposure to light, P680 in photosystem II reduced H2O to produce O2, H+, and elections. The electrons were passed down the electron transport chain (ETC), through photosystem I, and to ferrodoxin. From ferrodoxin, the electrons could be passed to ferrodoxin NADP reductase, or to the hydrogenase to combine with H+ to form H2. Upon reduction of H2O by PSII, the medium became again aerobic, and the oxygen sensitive hydrogenase was inhibited, ceasing H2 production.
It has been found that deficiency of Sulfate in the growth medium has the effect of inhibiting methionine and cysteine production. This in turn inhibits the production of new protein. The D1 component of PSII has a particularly high turnover, and sulfate deficiency results to reduced H2O oxidation by PSII as a result of insufficient D1. In sealed containers deprived of sulfate, there comes a point where the consumption of O2 by aerobic respiration exceeds the production of O2 by oxidation of water and the medium becomes anaerobic. At this point, the hydrogenase genes are activated and begin to siphon electrons from ferrrodoxin on the ETC. PSII aparently continues to oxidize water, albeit at a reduced rates in order to provide these electrons. Additionally, some electons appear to be derived from the catabolism of starches. As much as 60 hours of H2production is obtained before rates slow and sulfate stress conditions must be relieved.
Strategies for Improving Hydrogen Production
There are many aspects of the algal hydrogen production mechanism which could be modified for more efficient hydrogen production.
- Sulfate Permease