Hydrogen biproduction by photosynthetic microorganisms

Many micro-organisms have the ability to produce hydrogen. For several species of fermentative bacteria, hydrogen production enables energy to be extracted from organic compounds, in the absence of respiratory substrates such as oxygen. In this way, protons in solution in the intracellular medium serve as electron acceptors for the reoxidation of cofactors needed to metabolize organic compounds. In some photosynthetic micro-organisms, these fermentation pathways are also present; moreover, solar energy can stimulate hydrogen production, either from organic matter in the case of anoxygenic photosynthetic bacteria (known as "photofermentation"), or from water in the case of certain cyanobacteria and microalgae (water biophotolysis).

These various microbial production methods have potential applications, but their implementation comes up against different types of biological and technological hurdles, the nature of which depends on the characteristics of the micro-organisms in question. Hydrogen synthesis coupled with oxygenic photosynthesis offers the prospect of clean, renewable production methods, using water and solar energy as the main resources. But the sensitivity of hydrogen-producing enzymes to oxygen is a major limitation to the development of processes based on this principle. Diverse engineering strategies, at the enzymatic, metabolic and process levels, need to be developed to ensure sustainable modes of H ₂ production by these processes. Hydrogen production from photosynthetic microorganisms is implemented in devices known as "photobioreactors", in which production conditions are sought to be optimized. In the case of microalgae, the hydrogen production process, due to the succession of oxygenic and anoxic phases, can be carried out in two different photobioreactors or in a single one, either using strict control of nutrients and light received, or by fixing the microalgae on supports so as to easily change the culture medium.