In terms of H2 productivity, the fermentation can be divided
into three different stages. The maximum H2 production rate was
153.9 mL/L h within the first 24 h of incubation. In a second stage
from 24 to 48 h this rate decreased to 25.0 mL/L h, and it decreased
further to 4.3 mL/L h in the interval of 48 up to 96 h. Liu and coworkers attained a higher H2 productivity from C. vulgaris but only
after biomass pretreatment with dilute acid hydrolysis [48]. The
difference between both assays lies in the sugars made available
for the fermentation with the acid pretreatment. Advantageously,
the polymeric sugars, both part of the cellular wall and those present as storage compounds, are hydrolysed and the sugars are
readily made available for fermentation [49]. However, the introduction of acid hydrolysis of the microalgal biomass involves a series of additional energetically costly stages [15], as well as the use
of corrosive and hazardous acids whose recycling and disposal of
neutralising salts are associated with major process disadvantages
and environmental harmfulness [50]. Clostridia are considered
superior bioprocessing strains due to their capacity to easily
degrade and ferment complex sugars with high bioconversion
yields, therefore considered excellent biological platforms for consolidated bioprocessing (CBP), that excludes any previous hydrolysis of the biomass