Microalgal Hydrogen Production: Achievements and Perspectives
Photosynthesis and Hydrogen from Photosynthetic Microorganisms
Published:19 Mar 2018
P. C. Hallenbeck, C. Z. Lazaro, and E. Sagir, in Microalgal Hydrogen Production: Achievements and Perspectives, ed. M. Seibert and G. Torzillo, The Royal Society of Chemistry, 2018, pp. 1-30.
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Although rarely thought of this way, in fact humankind is almost completely dependent on photosynthesis for both food and fuel. In addition to the organic carbon, newly fixed through photosynthesis, that is used as food, we also rely on photosynthesis when we use fossil fuels, which represent ancient photosynthetic products that have been stored and transformed over millions of years. More recently, biofuels, such as bioethanol and biodiesel, are now being made directly on a large scale, using recent photosynthesis. Hydrogen is an almost ideal alternative fuel, which can be produced biologically in a number of ways. Here, various processes that rely on photosynthesis as the transformative energy source behind hydrogen production are briefly introduced and discussed. These include photofermentation by purple non-sulfur photosynthetic bacteria using captured solar energy to drive H2 production from organic compounds and systems that are capable of the direct conversion of captured solar energy to hydrogen. In the most attractive system, biophotolysis, the solar energy captured by photosystem II (PSII) and photosystem I (PSI) is used to reduce ferredoxin. Subsequently, the reduced ferredoxin reduces a hydrogen-evolving enzyme. Hydrogen can also be produced by indirect biophotolysis, where photosynthetic carbon fixation occurs in the first, illuminated stage, and the stored carbohydrate is then used in a second, anaerobic, hydrogen-producing stage. Such a process thus separates in both time and space, oxygen-producing photosynthesis from the oxygen-sensitive proton reduction reaction. Here, we briefly review how these processes take place in cyanobacteria and green algae.