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International audience; Strictly anaerobic bacteria of the Clostridium genus have attracted great interest as potential cell factories for molecular hydrogen production purposes. In addition to being a useful approach to this process, dark fermentation has the advantage of using the degradation of cheap agricultural residues and industrial wastes for molecular hydrogen production. However, many improvements are still required before large-scale hydrogen production from clostridial metabolism is possible. Here we review the literature on the basic biological processes involved in clostridial hydrogen production, and present the main advances obtained so far in order to enhance the hydrogen productivity, as well as suggesting some possible future prospects.

The electrochemistry of membrane-bound [NiFe] hydrogenase I ([NiFe]-hase I) from the hyperthermophilic bacterium Aquifex aeolicus was investigated at gold and graphite electrodes. Direct and mediated H(2) oxidation were proved to be efficient in a temperature range of 25-70 degrees C, describing a potential window for H(2) oxidation similar to that of O(2)-tolerant hydrogenases. Search for enhancement of current densities and enzyme stability was achieved by the use of carbon nanotube coatings. We report high catalytic currents for H(2) oxidation up to 1 mA cm(-2), 10 times higher than at the bare electrode. Interestingly, high stability of the direct catalytic process was observed when encapsulating A. aeolicus [NiFe]-hase I into a carboxylic functionalized single walled carbon nanotube network. This suggests a peculiar interaction between the enzyme and the electrode material. The parameters that governed the orientation of the enzyme before electron transfer were thus investigated using self-assembled-monolayer gold electrodes. No control of the orientation by the charge or the hydrophobicity of the interface was demonstrated. This behavior was explained on the basis of a structural comparison between A. aeolicus [NiFe]-hase I and Desulfovibrio fructosovorans [NiFe] hydrogenase, which revealed the absence of acidic residues and an additional loop in the environment of the [4Fe-4S] distal cluster in A. aeolicus [NiFe]-hase I. Finally, the effect of inhibitors on the direct oxidation of H(2) by A. aeolicus [NiFe]-hase I encapsulated in a single walled carbon nanotube network was investigated. No inhibition by CO and tolerance toward O(2) were observed. Discussion of the reasons for such tolerance was undertaken on the basis of structural comparison with hydrogenases from aerobic bacteria.