Considering the sugar concentration of the open pond-grown S.
obliquus biomass, a maximum H2 yield of 2.74 mol/mol of glucose
equivalents was achieved. This value corresponds to 67.5% of the
maximum theoretical yield from glucose (4 mol/mol) and is
comparable to 2.9 mol/mol obtained in a previous work where S.
obliquus acid hydrolysate was used for bioH2 production by C.
butyricum [15]. The production of organic acids stabilised after
36 h (Fig. 3). The main organic acids produced were acetate and
butyrate, achieving a maximum of 26.0 and 28.8 mM after 36 h
of incubation, respectively. Traces of formate were also produced
during the first 12 h of incubation. The production of acetic and
butyric acid by clostridia is associated with high-H2 production
and the fact that the maximum concentrations were determined within the first 36 h of incubation indicates higher metabolic rates
at the beginning of the fermentation [53]. In opposition to the
major diversity of fermentation metabolites produced by mixed
cultures, the fermentative H2 production by pure cultures enables
the simultaneous production of specific metabolites with industrial interest. Butyric acid, for example, possesses a high potential
to be used in multiple applications, either as biobutanol precursor
or for the production of polyhydroxbutyrate bioplastics [54], or
simply as additive in the pharmaceutical or food industry [55]. In
this case, the incorporation of additional value by the exploitation
of fermentation co-products has the potential to improve the
economic feasibility of bioH2 production.