The physiology and biotechnology of dark fermentative biohydrogen production.

A CO2-neutral energy production alternative compared to conventional fossil fuel utilization is biohydrogen (H2) production. Three basic mechanisms for microbial H2 production exist: photosynthetic H2 production, photo-fermentative H2 production, and dark fermentative H2 production (DFHP). Despite surmounting reports in literature on the characterization and optimization of DFHP systems, H2 production has not yet reached an industrial scale. Here, DFHP characteristics of pure culture of microorganisms from more than one century were reviewed and analysed. Analysing pure culture DFHP has the advantage that the physiology and the biotechnological potential of a specific organism can be exploited with the aim to optimize and establish a straightforward H2 production bioprocess. Essential to this effort is the analysis of reported values across phylogenetically distinct groups of microorganisms. Therefore, an extensive review and subsequent in-depth meta-data analysis of DFHP from pure cultures was performed with the goals of providing: a comprehensive overview to their physiology, reviewing closed batch, batch, and continuous culture DFHP from an energy production perspective, and to integrate physiology and biotechnology through comprehensive meta-data analyses, statistics, and modelling. We revealed that a comparison of H2 productivity and H2 yield (Y(H2/S)) could unambiguously be performed on a carbon molar level. Clear dependencies between Y(H2/S) and the metabolic pathways of specific phylogenetic DFHP groups were found. With respect to specific H2 productivity and Y(H2/S) the superior phylogenetic group for DFHP was Thermococcaceae. Moreover, a distinct correlation between high Y(H2/S) and high H2 productivity was identified. The best substrate for H2 production was found to be formate. Statistical analysis and modelling provided the input parameter sets that could be used to optimize of H2 production of Clostridiaceae and Enterobacteriaceae. With respect to the overall goal to improve H2 production beyond reported values, we suggest to utilize Thermococcaceae, and to integrate these organisms into a H2 production set-up encompassing a cell retention system that would allow the accumulation of a high biomass density. Then both, high H2 production and Y(H2/S) might be achieved at the same time. Such an integrated system could finally render DFHP a biotechnologically useful process.