However, this production does not seems to be completely correlated with H2 production,
as the accumulated organic acids in the case of LE37 are much
higher than those of C. butyricum, without the correspondent
increase in H2 production. (Fig. 2, Table 1). This disparity likely
resides in the higher complexity of the metabolic routes present
in the mixed culture, where a multiplicity of metabolically active
hydrogenotrophs and hydrogenic microorganisms may coexist.
For example, the acetogenic Wood–Ljungdahl pathway corresponds to a H2 consumption route present in some clostridial strains, where acetate is formed directly by reduction of CO2,
preferentially with H2 as reducing agent [43]. In principle, microbial consortia provide useful combinations of metabolic pathways,
important to a more efficient bioconversion of complex substrates.
However, these mixed cultures must be well controlled since bacterial successions with negative impact on H2 production may
occur [44]. Unexpected process changes, such as in the substrate
composition, may also generate difficulties in controlling the
microbial populations and reproducing the maximum H2 yields.