enhance the efficiency of bioethanol conversion through the microalgal biomass
(Gallego et al. 2015).
After biorefining of all available sugar or cellulosic-based materials in algal cells,
protein by-products can be accumulated; however, there are no strategies to convert
these by-products into biofuels. Recent research showed that biobutanol can be
produced from cumulative protein after biorefining, which includes proteinogenic
amino acid (Lan and Liao 2013). The key intermediates for the alcohol formation
from protein in algal cell are the respective α-keto acids which could derive either
catabolically from exogenous amino acids (Ehrlich pathway) or could be the result
of amino acid in anabolic biosynthetic pathways (Hazelwood et al. 2008; Kondo
et al. 2012). The isoleucine biosynthesis pathway generates α-keto acid that
α-ketovalerate can be converted into butanol (Shen and Liao 2008). Using this
strategy, only two nonnative steps are needed to produce biofuels by shunting
intermediates from amino acid biosynthesis pathways to alcohol production.
3.3.3 Biohydrogen Production from Microalgae
Most algal hydrogen production processes use anaerobic metabolism systems by
controlling oxygenic photosynthesis under sulfur deprivation in culture media (Florin et al. 2001; Melis et al. 2000), which leads to an inactivation of photosystem II
(PSII) and thus initiates H 2 evolution by hydrogenase (Melis et al. 2000). Increasing
hydrogenase tolerance to oxygen (O 2 ) is one of the challenges toward commercial
feasibility of microalgal hydrogen production. Sulfur deprivation can interrupt
photosynthetic production of oxygen and thus the cease of PSII activity; however,
sulfur control requires specific synthetic feedstocks and is not practical to control in
real-world applications.
Another approach is to exchange anion-binding site in PSII because most chloride
preferentially combines to S oxidizing transitions (S 2 to S 3 and S 3 to S 0 ) for oxygen
producing in oxygen-evolving complex (OEC) at PSII; however, oxygen evolution
can be inhibited by exchanging chloride to acetate or other anions (e.g., NO 3
À and
Br
À ) (Wincencjusz et al. 1997). Acetate has an advantage of capability of ceasing
oxygen evolution at PSII, thus not requiring additional sulfur or chloride deprivation
in the substrate. More research is required to produce hydrogen using acetate as a
practical oxygen regulator for photosynthetic microalgal hydrogen production in
wastewater streams.
A recent eukaryotic microalgal hydrogen study reported an oxygen-tolerant
[FeFe] hydrogenase in C. vulgaris during photolysis, showing the expression and
the specific activity of hydrogenase under atmospheric oxygen levels (Hwang et al.
2014). Further research on oxygen-tolerant [FeFe] hydrogenase could identify the
gene of the hydrogenase, which could provide an opportunity to improve the
artificial oxygen-tolerant hydrogenase technology.
292
J.-H. Hwang et al.
(Gallego et al. 2015).
After biorefining of all available sugar or cellulosic-based materials in algal cells,
protein by-products can be accumulated; however, there are no strategies to convert
these by-products into biofuels. Recent research showed that biobutanol can be
produced from cumulative protein after biorefining, which includes proteinogenic
amino acid (Lan and Liao 2013). The key intermediates for the alcohol formation
from protein in algal cell are the respective α-keto acids which could derive either
catabolically from exogenous amino acids (Ehrlich pathway) or could be the result
of amino acid in anabolic biosynthetic pathways (Hazelwood et al. 2008; Kondo
et al. 2012). The isoleucine biosynthesis pathway generates α-keto acid that
α-ketovalerate can be converted into butanol (Shen and Liao 2008). Using this
strategy, only two nonnative steps are needed to produce biofuels by shunting
intermediates from amino acid biosynthesis pathways to alcohol production.
3.3.3 Biohydrogen Production from Microalgae
Most algal hydrogen production processes use anaerobic metabolism systems by
controlling oxygenic photosynthesis under sulfur deprivation in culture media (Florin et al. 2001; Melis et al. 2000), which leads to an inactivation of photosystem II
(PSII) and thus initiates H 2 evolution by hydrogenase (Melis et al. 2000). Increasing
hydrogenase tolerance to oxygen (O 2 ) is one of the challenges toward commercial
feasibility of microalgal hydrogen production. Sulfur deprivation can interrupt
photosynthetic production of oxygen and thus the cease of PSII activity; however,
sulfur control requires specific synthetic feedstocks and is not practical to control in
real-world applications.
Another approach is to exchange anion-binding site in PSII because most chloride
preferentially combines to S oxidizing transitions (S 2 to S 3 and S 3 to S 0 ) for oxygen
producing in oxygen-evolving complex (OEC) at PSII; however, oxygen evolution
can be inhibited by exchanging chloride to acetate or other anions (e.g., NO 3
À and
Br
À ) (Wincencjusz et al. 1997). Acetate has an advantage of capability of ceasing
oxygen evolution at PSII, thus not requiring additional sulfur or chloride deprivation
in the substrate. More research is required to produce hydrogen using acetate as a
practical oxygen regulator for photosynthetic microalgal hydrogen production in
wastewater streams.
A recent eukaryotic microalgal hydrogen study reported an oxygen-tolerant
[FeFe] hydrogenase in C. vulgaris during photolysis, showing the expression and
the specific activity of hydrogenase under atmospheric oxygen levels (Hwang et al.
2014). Further research on oxygen-tolerant [FeFe] hydrogenase could identify the
gene of the hydrogenase, which could provide an opportunity to improve the
artificial oxygen-tolerant hydrogenase technology.
292
J.-H. Hwang et al.