production. They concluded that these fuels are better than petro-based fuels and
have less environmental implications. Nagarajan et al. (2020) have reviewed the
different kinds of pretreatment methods for algal hydrogen generation. They have
suggested that carbohydrates present in algae are effective carbon source for dark
fermentation. Further, they stated that mechanical methods for hydrogen generation
have high extraction efficiency but are energy-intensive; while chemical methods are
less energy intensive, they generate chemical compounds which inhibit fermentation
(Nagarajan et al. 2020). Fonseca et al. (2020) optimized the pretreatment process for
Kappaphycus alvarezii biomass which was used by the Clostridium beijerinckii
Br21 to produce hydrogen and hence suggested that macroalgae biomass can be used
as feedstock for hydrogen production. Chang et al. (2020) studied the effect of
microwave power on hydrogen production using microalgae which was pyrolyzed.
With the increase in microwave power, the yield increased twice. The potential of
using macroalgae Ulva sp. was reported by Margareta et al. (2020). In that study,
macroalgal biomass was used as feedstock for enhancing hydrogen production
through dark fermentation. Biohydrogen generation rate of 812 mL/L/h was seen.
The green macroalgal biomass Ulva sp. was subjected to mild acid-thermal combined pretreatment for the effective release of fermentable sugars for biohydrogen
production. The impact of pH and BESA addition on gas production by mixed
microalgae biomass was investigated by Kumar et al. (2016). Dadak et al. (2016) had
done the eco-exergy analysis for gas production. Yin and Wang (2019) suggested
pretreatment for bioenergy recovery from algae. They studied various pretreatment
methods and concluded that all the pretreatment methods could enhance the hydrogen production, while the combined pretreatment showed significant enhancement.
Hydrogen yield of 17.5 mL/g TS added was seen in heat-base pretreatment and heatacid pretreatment with the highest total energy conversion efficiency of 35.4% (Yin
and Wang 2019). Kosourov et al. (2011) immobilized a tla1 mutant (CC-4169) in
thin alginate films and cultured it under sulfate-limited and low-intensity light
conditions. They found that hydrogen production rates were significantly reduced.
Radha and Murugesan (2017) increased the biohydrogen production by using
different pretreatment processes in the marine macroalgae Padina tetrastromatica.
7.6.3 Substrate and Salt Concentration
Kim et al. (2006) observed the consequences of substrate concentration on production of hydrogen. Although some algae have adapted to tolerate a very high salt
concentration, for example, the halophilic Dunaliella salina (Oren et al. 2008), most
algae thrive in either freshwater, brackish water, or marine environments. When
salinities increase above optimum for growth, algae may suffer from hyperosmotic
stress, leading to impaired electron transfer between antenna pigments, and in PSII
and PSI reaction centers, again leading to photoinhibition and oxidative stress.
Among several different adaptive responses to salinity, stress is the production of
osmolytes or production of secondary carotenoids (Hadi et al. 2008). In few cases,
7 Sustainable Production of Hydrogen by Algae: Current Status and Future. . .
199
have less environmental implications. Nagarajan et al. (2020) have reviewed the
different kinds of pretreatment methods for algal hydrogen generation. They have
suggested that carbohydrates present in algae are effective carbon source for dark
fermentation. Further, they stated that mechanical methods for hydrogen generation
have high extraction efficiency but are energy-intensive; while chemical methods are
less energy intensive, they generate chemical compounds which inhibit fermentation
(Nagarajan et al. 2020). Fonseca et al. (2020) optimized the pretreatment process for
Kappaphycus alvarezii biomass which was used by the Clostridium beijerinckii
Br21 to produce hydrogen and hence suggested that macroalgae biomass can be used
as feedstock for hydrogen production. Chang et al. (2020) studied the effect of
microwave power on hydrogen production using microalgae which was pyrolyzed.
With the increase in microwave power, the yield increased twice. The potential of
using macroalgae Ulva sp. was reported by Margareta et al. (2020). In that study,
macroalgal biomass was used as feedstock for enhancing hydrogen production
through dark fermentation. Biohydrogen generation rate of 812 mL/L/h was seen.
The green macroalgal biomass Ulva sp. was subjected to mild acid-thermal combined pretreatment for the effective release of fermentable sugars for biohydrogen
production. The impact of pH and BESA addition on gas production by mixed
microalgae biomass was investigated by Kumar et al. (2016). Dadak et al. (2016) had
done the eco-exergy analysis for gas production. Yin and Wang (2019) suggested
pretreatment for bioenergy recovery from algae. They studied various pretreatment
methods and concluded that all the pretreatment methods could enhance the hydrogen production, while the combined pretreatment showed significant enhancement.
Hydrogen yield of 17.5 mL/g TS added was seen in heat-base pretreatment and heatacid pretreatment with the highest total energy conversion efficiency of 35.4% (Yin
and Wang 2019). Kosourov et al. (2011) immobilized a tla1 mutant (CC-4169) in
thin alginate films and cultured it under sulfate-limited and low-intensity light
conditions. They found that hydrogen production rates were significantly reduced.
Radha and Murugesan (2017) increased the biohydrogen production by using
different pretreatment processes in the marine macroalgae Padina tetrastromatica.
7.6.3 Substrate and Salt Concentration
Kim et al. (2006) observed the consequences of substrate concentration on production of hydrogen. Although some algae have adapted to tolerate a very high salt
concentration, for example, the halophilic Dunaliella salina (Oren et al. 2008), most
algae thrive in either freshwater, brackish water, or marine environments. When
salinities increase above optimum for growth, algae may suffer from hyperosmotic
stress, leading to impaired electron transfer between antenna pigments, and in PSII
and PSI reaction centers, again leading to photoinhibition and oxidative stress.
Among several different adaptive responses to salinity, stress is the production of
osmolytes or production of secondary carotenoids (Hadi et al. 2008). In few cases,
7 Sustainable Production of Hydrogen by Algae: Current Status and Future. . .
199