Galactose, mannose and xylose were identified as soluble
sugars that were present at the beginning of the fermentation
and were depleted within the first 12 h of incubation (Fig. 3).
Glucose was rapidly converted up to 24 h of incubation. H2 evolution progressed rapidly in the first 24 h and then stabilised,
displaying a minor volumetric production between 48 and 96 h
(Fig. 3). The maximum H2 production was 116.3 mL/g vs after
96 h, in accordance to values previously obtained by the authors
[21]. 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
Galactose, mannose and xylose were identified as solublesugars that were present at the beginning of the fermentationand were depleted within the first 12 h of incubation (Fig. 3).Glucose was rapidly converted up to 24 h of incubation. H2 evolution progressed rapidly in the first 24 h and then stabilised,displaying a minor volumetric production between 48 and 96 h(Fig. 3). The maximum H2 production was 116.3 mL/g vs after96 h, in accordance to values previously obtained by the authors[21]. In terms of H2 productivity, the fermentation can be dividedinto three different stages. The maximum H2 production rate was153.9 mL/L h within the first 24 h of incubation. In a second stagefrom 24 to 48 h this rate decreased to 25.0 mL/L h, and it decreasedfurther 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 onlyafter biomass pretreatment with dilute acid hydrolysis [48]. Thedifference between both assays lies in the sugars made availablefor 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 arereadily 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 useof corrosive and hazardous acids whose recycling and disposal ofneutralising salts are associated with major process disadvantagesand environmental harmfulness [50]. Clostridia are consideredsuperior bioprocessing strains due to their capacity to easilydegrade and ferment complex sugars with high bioconversionyields, therefore considered excellent biological platforms for consolidated bioprocessing (CBP), that excludes any previous hydrolysis of the biomass
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