HighlightsThe effects of the carbon/nitrogen ratios of 40, 90, 140, and 190 on hydrogen production are evaluated by varying the nitrogen source in an upflow fixed-bed anaerobic reactor.An optimal C/N ratio of 137 to produce 3.5 mol H2 mol−1 sucrose is estimated by a mathematical approximation.Continuous decreases in the specific organic loading rate as a function of time seemed to be responsible for the instability of the system.A microbial biology analysis identified hydrogen-producing and -consuming microorganisms from natural inoculum.

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To provide new insight into the dark fermentation process, a multi-lateral study was performed to study the microbiology of 20 different lab-scale bioreactors operated in four different countries (Brazil, Chile, Mexico, and Uruguay). Samples (29) were collected from bioreactors with different configurations, operation conditions, and performances. The microbial communities were analyzed using 16S rRNA genes 454 pyrosequencing. The results showed notably uneven communities with a high predominance of a particular genus. The phylum Firmicutes predominated in most of the samples, but the phyla Thermotogae or Proteobacteria dominated in a few samples. Genera from three physiological groups were detected: high-yield hydrogen producers (Clostridium, Kosmotoga, Enterobacter), fermenters with low-hydrogen yield (mostly from Veillonelaceae), and competitors (Lactobacillus). Inocula, reactor configurations, and substrates influence the microbial communities. This is the first joint effort that evaluates hydrogen-producing reactors and operational conditions from different countries and contributes to understand the dark fermentation process.

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