(1) relatively simple cell walls with no lignin therefore requires milder pretreatment,
(2) high ability of CO 2 fixation, (3) higher productivity, (4) no need of arable land
for mass cultivation and (5) can grow in brackish, saline and wastewaters thus
reducing the freshwater footprint (Monlau et al. 2014; Sambusiti et al. 2015; Xia
et al. 2015). During their growth, microalgae can synthesize and accumulate lipid,
carbohydrate and protein (Monlau et al. 2014). The percentage of different components of microalgae varies according to algal species, environmental and cultivation conditions (Sambusiti et al. 2015).The first and most important task during
the utilization of microalgal biomass as feedstock for dark fermentation is the
selection of appropriate microalgal species having high biomass productivity and
carbohydrate content (Wang et al. 2017). Microalgae store the polysaccharides
either in the form of starch or glycogen.
Indeed, in microalgae the carbohydrates are also found entrapped within the cell
wall mainly in the form of cellulose, hemicellulose, pectin and sulphated
polysaccharide (Chen et al. 2013). Some microalgae can accumulate carbohydrate
higher than 50% of their dry weight (Markou et al. 2012b). The microalgal storage
and the cell wall polysaccharides upon efficient pretreatment can be released in the
form of simple sugars (glucose) and contribute as a potential feedstock for dark
fermentation. Appropriate cultivation condition and nutritional strategy can maximize the carbohydrate content and its productivity by altering the metabolic pathway of microalgae (Markou et al. 2012b). For instance, threefold higher
carbohydrate content (39.19%) was obtained in Scenedesmus sp. CCNM 1077
under mixotrophic condition (glucose-supplemented medium) (Pancha et al. 2015).
Ho et al. (2012) reported an increase in the carbohydrate content of S. obliquus
CNW-N from 38.25 to 51.8%, when it was cultivated under high light intensity
with nitrogen deficient condition. Moreover, Vitova et al. (2015) suggested sulphur
deprivation as the most effective method of maximizing the carbohydrate content
and productivity. Microalgae are also rich in macro- and microelements which are
required for the growth of H 2 -producing bacteria (HPB) (Sambusiti et al. 2015).
The H 2 yield in dark fermentation mainly depends upon the monosaccharide
content of the microalgal biomass because fermentation of lipid and protein by the
HPB is thermodynamically unfavourable (Xia et al. 2015). Despite the lower
potentiality of proteins for biohydrogen production, they are essential for balancing
the C/N ratio of algal feedstock (Sambusiti et al. 2015). In dark fermentation,
optimal C/N ratio of the substrate is an important factor for the growth and biological activity of HPB (Lay et al. 2013). The high protein content of the algal
biomass results in low C/N ratio, which decreases the rate of H 2 production and
limits the use of algal biomass as sole substrate. Excessive protein content leads to
release and accumulation of nitrogen in the form of ammonium ion. High concentration of ammonium ion decreases the pH of the fermentation media, which
may inhibit the growth of HPB or activity of enzymes participating in fermentative
H 2 production. The C/N ratio of the algae can be increased by applying selected
growth conditions (Montingelli et al. 2015). Moreover, an appropriate C/N ratio can
be achieved by the addition of carbon-rich biomass with the microalgal biomass
having high protein content. Xia et al. (2014) observed an increase in hydrogen
10 Biofuels from Microalgae: Biohydrogen
213
(2) high ability of CO 2 fixation, (3) higher productivity, (4) no need of arable land
for mass cultivation and (5) can grow in brackish, saline and wastewaters thus
reducing the freshwater footprint (Monlau et al. 2014; Sambusiti et al. 2015; Xia
et al. 2015). During their growth, microalgae can synthesize and accumulate lipid,
carbohydrate and protein (Monlau et al. 2014). The percentage of different components of microalgae varies according to algal species, environmental and cultivation conditions (Sambusiti et al. 2015).The first and most important task during
the utilization of microalgal biomass as feedstock for dark fermentation is the
selection of appropriate microalgal species having high biomass productivity and
carbohydrate content (Wang et al. 2017). Microalgae store the polysaccharides
either in the form of starch or glycogen.
Indeed, in microalgae the carbohydrates are also found entrapped within the cell
wall mainly in the form of cellulose, hemicellulose, pectin and sulphated
polysaccharide (Chen et al. 2013). Some microalgae can accumulate carbohydrate
higher than 50% of their dry weight (Markou et al. 2012b). The microalgal storage
and the cell wall polysaccharides upon efficient pretreatment can be released in the
form of simple sugars (glucose) and contribute as a potential feedstock for dark
fermentation. Appropriate cultivation condition and nutritional strategy can maximize the carbohydrate content and its productivity by altering the metabolic pathway of microalgae (Markou et al. 2012b). For instance, threefold higher
carbohydrate content (39.19%) was obtained in Scenedesmus sp. CCNM 1077
under mixotrophic condition (glucose-supplemented medium) (Pancha et al. 2015).
Ho et al. (2012) reported an increase in the carbohydrate content of S. obliquus
CNW-N from 38.25 to 51.8%, when it was cultivated under high light intensity
with nitrogen deficient condition. Moreover, Vitova et al. (2015) suggested sulphur
deprivation as the most effective method of maximizing the carbohydrate content
and productivity. Microalgae are also rich in macro- and microelements which are
required for the growth of H 2 -producing bacteria (HPB) (Sambusiti et al. 2015).
The H 2 yield in dark fermentation mainly depends upon the monosaccharide
content of the microalgal biomass because fermentation of lipid and protein by the
HPB is thermodynamically unfavourable (Xia et al. 2015). Despite the lower
potentiality of proteins for biohydrogen production, they are essential for balancing
the C/N ratio of algal feedstock (Sambusiti et al. 2015). In dark fermentation,
optimal C/N ratio of the substrate is an important factor for the growth and biological activity of HPB (Lay et al. 2013). The high protein content of the algal
biomass results in low C/N ratio, which decreases the rate of H 2 production and
limits the use of algal biomass as sole substrate. Excessive protein content leads to
release and accumulation of nitrogen in the form of ammonium ion. High concentration of ammonium ion decreases the pH of the fermentation media, which
may inhibit the growth of HPB or activity of enzymes participating in fermentative
H 2 production. The C/N ratio of the algae can be increased by applying selected
growth conditions (Montingelli et al. 2015). Moreover, an appropriate C/N ratio can
be achieved by the addition of carbon-rich biomass with the microalgal biomass
having high protein content. Xia et al. (2014) observed an increase in hydrogen
10 Biofuels from Microalgae: Biohydrogen
213