Biohydrogen

[1] Interest is high in this technology because H2 is a clean fuel and can be readily produced from certain kinds of biomass,[2] including biological waste.

[3] Furthermore some photosynthetic microorganisms are capable to produce H2 directly from water splitting using light as energy source.

[7] Enzymes within this widely diverse family are commonly sub-classified into three different types based on the active site metal content: [FeFe]-hydrogenases (iron-iron), [NiFe]-hydrogenases (nickel-iron) hydrogenases, and [Fe]-hydrogenases (iron-only).

Notable examples are members of the genera Clostridium, Desulfovibrio, Ralstonia or the pathogen Helicobacter, being most of them strict-anaerobes or facultative microorganisms.

In Chlamydomonas reinhardtii Photosystem II produces in direct conversion of sunlight 80% of the electrons that end up in the hydrogen gas.

[20] 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.

[21][18] The chlorophyll (Chl) antenna size in green algae is minimized, or truncated, to maximize photobiological solar conversion efficiency and H2 production.

Under nitrogen-limited conditions some cells can specialize and form heterocysts, which ensures an anaerobic intracellular space to ease N2 fixation by the nitrogenase enzyme expressed also inside.

Under nitrogen-fixation conditions, the nitrogenase enzyme accepts electrons and consume ATP to break the triple dinitrogen bond and reduce it to ammonia.

Nevertheless, since the production of H2 is an important loss of energy for the cells, most of nitrogen fixing cyanobacteria also feature at least one uptake hydrogenase.

[26] Uptake hydrogenases exhibit a catalytic bias towards oxygen oxidation, thus can assimilate the produced H2 as a way to recover part of the energy invested during the nitrogen fixation process.

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.