Biohydrogen

Hydrogen that is produced biologically

Biohydrogen is H2 that is produced biologically. Interest is high in this technology because H2 is a clean fuel and can be readily produced from certain kinds of biomass, including biological waste. Furthermore some photosynthetic microorganisms are capable to produce H2 directly from water splitting using light as energy source. Besides the promising possibilities of biological hydrogen production, many challenges characterize this technology. First challenges include those intrinsic to H2, such as storage and transportation of an explosive noncondensible gas. Additionally, hydrogen producing organisms are poisoned by O2 and yields of H2 are often low.

The biological hydrogen production with algae is a method of photobiological water splitting which is done in a closed photobioreactor based on the production of hydrogen as a solar fuel by algae. Algae produce hydrogen under certain conditions. In 2000 it was discovered that if C. reinhardtii algae are deprived of sulfur they will switch from the production of oxygen, as in normal photosynthesis, to the production of hydrogen. Green algae express [FeFe] hydrogenases, being some of them considered the most efficient hydrogenases with turnover rates superior to 104 s−1. This remarkable catalytic efficiency is nonetheless shadowed by its extreme sensitivity to oxygen, being irreversibly inactivated by O2 Fe Fe hydrogenases (enzymes) combine them into hydrogen gas. In Chlamydomonas reinhardtii Photosystem II produces in direct conversion of sunlight 80% of the electrons that end up in the hydrogen gas. In 2020 scientists reported the development of algal cell based micro emulsion for multicellular spheroid microbial reactors capable of producing hydrogen alongside either oxygen or CO2 via photosynthesis in daylight under air. Enclosing the microreactors with synergistic bacteria was shown to increase levels of hydrogen production via reduction of O2 concentrations.

The chlorophyll (Chl) antenna size in green algae is minimized, or truncated, to maximize photobiological solar conversion efficiency and H2 production. It has been shown that Light harvesting complex photosystem II light harvesting protein LHCBM9 promotes efficient light energy dissipation. The truncated Chl antenna size minimizes absorption and wasteful dissipation of sunlight by individual cells, resulting in better light utilization efficiency and greater photosynthetic efficiency when the green alga are grown as a mass culture in bioreactors.

With current reports for algae based biohydrogen, it would take about 25,000 square kilometre algal farming to produce biohydrogen equivalent to the energy provided by gasoline in the US alone. This area represents approximately 10% of the area devoted to growing soya in the US.

Restriction of photosynthetic hydrogen production by accumulation of a proton gradient. Competitive inhibition of photosynthetic hydrogen production by carbon dioxide. Requirement for bicarbonate binding at photosystem II (PSII) for efficient photosynthetic activity. Competitive drainage of electrons by oxygen in algal hydrogen production. Economics must reach competitive price to other sources of energy and the economics are dependent on several parameters. A major technical obstacle is the efficiency in converting solar energy into chemical energy stored in molecular hydrogen. Attempts are in progress to solve these problems via bioengineering.

In 1933, Marjory Stephenson and her student Stickland reported that cell suspensions catalysed the reduction of methylene blue with H2. Six years later, Hans Gaffron observed that the green photosynthetic alga Chlamydomonas reinhardtii, would sometimes produce hydrogen. In the late 1990s Anastasios Melis discovered that deprivation of sulfur induces the alga to switch from the production of oxygen (normal photosynthesis) to the production of hydrogen. He found that the enzyme responsible for this reaction is hydrogenase, but that the hydrogenase lost this function in the presence of oxygen. Melis also discovered that depleting the amount of sulfur available to the algae interrupted their internal oxygen flow, allowing the hydrogenase an environment in which it can react, causing the algae to produce hydrogen. Chlamydomonas moewusii is also a promising strain for the production of hydrogen.