Several studies have shown that limiting the amount of nitrogen in the culture
medium is one of the main factors that leads to the accumulation of carbohydrates
by microalgae (Dragone et al. 2011). According to Behrenset et al. (1989),
microalgae in a nitrogen-deprived culture medium direct the flow of carbon to the
synthesis of carbohydrates in detriment of the production of proteins. Thus, effort to
increase yields of biofuels produced by microalgae is underway, including the
optimization of light technologies to modify the carbon uptake pathways, aimed at a
higher accumulation of biomass or specific compounds such as carbohydrates and
lipids or, more recently, the use of genetic engineering for producing bioethanol,
biohydrogen, and other special fermentation products (de Farias Silva and Bertucco
2016). Photosynthetic organisms are favorable for the production of biofuels,
mainly because of their low cost of cultivation, but biofuel yields obtained under
normal conditions are not satisfactory. In addition to the production of biodiesel,
microalgae and cyanobacteria serve as attractive feedstock for the production of
bioethanol, although the scientific and technological knowledge on this context is
still scarce. On the contrary, studies have documented that the contents of oil and
carbohydrates in microalgae cells can be increased under stress conditions, resulting, for instance, in a decrease of the protein content under nitrogen depletion
(Ho et al. 2013; Wang et al. 2013). This approach could be applied to cultivate
microalgae biomass richer in carbohydrates, thereby leveraging their use for the
production of bioethanol, which is currently the most widely used biofuel in the
world.
However, under or non-optimized growth conditions, some microalgae strains
have been receiving special attention because they present the potential of industrial
application for the production of bioethanol of the third generation. Hirano et al.
(1997) found two with high starch: Chlamydomonas reinhardtii (UTEX 2247) with
45% starch (dry basis) and C. vulgaris (IAM C-534) with 37% starch. The
microalgae yields were, respectively, 11 and 32 g dry mass/(m
2 day). Dragone
et al. (2011) produced biomass of C. vulgaris with up to 41% starch (dry basis)
under low nitrogen culture conditions. According to Doucha and Lívanský (2009),
a mutant strain for the production of starch from Chlorella sp. can accumulate 70%
starch (dry basis) under conditions of suppression of protein production.
Technologies for the first (sugar or starch feedstock) and second generations
(lignocellulosic feedstock) of bioethanol basically involve two stages: the conversion of sunlight into chemical energy (such as carbohydrates and lipids) and the
conversion of chemical energy into biofuel. These two stages are related to each
other and result in increased production costs. As an improvement of this process,
the use of a single-stage system that is capable of capturing sunlight directly and
converting it into biofuel (bioethanol) would avoid one step, thereby reducing the
cost of production and increasing the sustainability of the bioethanol production
process. Three possible routes involving the use of microalgae and cyanobacteria
biomass for bioethanol production are discussed in the literature, accordingly
summarized in Fig. 1 (de Farias Silva and Bertucco 2016). The first one is the
traditional process in which the biomass undergoes pretreatment steps, enzymatic
hydrolysis, and yeast fermentation. The second route is the use of metabolic
232
R. G. Bastos
medium is one of the main factors that leads to the accumulation of carbohydrates
by microalgae (Dragone et al. 2011). According to Behrenset et al. (1989),
microalgae in a nitrogen-deprived culture medium direct the flow of carbon to the
synthesis of carbohydrates in detriment of the production of proteins. Thus, effort to
increase yields of biofuels produced by microalgae is underway, including the
optimization of light technologies to modify the carbon uptake pathways, aimed at a
higher accumulation of biomass or specific compounds such as carbohydrates and
lipids or, more recently, the use of genetic engineering for producing bioethanol,
biohydrogen, and other special fermentation products (de Farias Silva and Bertucco
2016). Photosynthetic organisms are favorable for the production of biofuels,
mainly because of their low cost of cultivation, but biofuel yields obtained under
normal conditions are not satisfactory. In addition to the production of biodiesel,
microalgae and cyanobacteria serve as attractive feedstock for the production of
bioethanol, although the scientific and technological knowledge on this context is
still scarce. On the contrary, studies have documented that the contents of oil and
carbohydrates in microalgae cells can be increased under stress conditions, resulting, for instance, in a decrease of the protein content under nitrogen depletion
(Ho et al. 2013; Wang et al. 2013). This approach could be applied to cultivate
microalgae biomass richer in carbohydrates, thereby leveraging their use for the
production of bioethanol, which is currently the most widely used biofuel in the
world.
However, under or non-optimized growth conditions, some microalgae strains
have been receiving special attention because they present the potential of industrial
application for the production of bioethanol of the third generation. Hirano et al.
(1997) found two with high starch: Chlamydomonas reinhardtii (UTEX 2247) with
45% starch (dry basis) and C. vulgaris (IAM C-534) with 37% starch. The
microalgae yields were, respectively, 11 and 32 g dry mass/(m
2 day). Dragone
et al. (2011) produced biomass of C. vulgaris with up to 41% starch (dry basis)
under low nitrogen culture conditions. According to Doucha and Lívanský (2009),
a mutant strain for the production of starch from Chlorella sp. can accumulate 70%
starch (dry basis) under conditions of suppression of protein production.
Technologies for the first (sugar or starch feedstock) and second generations
(lignocellulosic feedstock) of bioethanol basically involve two stages: the conversion of sunlight into chemical energy (such as carbohydrates and lipids) and the
conversion of chemical energy into biofuel. These two stages are related to each
other and result in increased production costs. As an improvement of this process,
the use of a single-stage system that is capable of capturing sunlight directly and
converting it into biofuel (bioethanol) would avoid one step, thereby reducing the
cost of production and increasing the sustainability of the bioethanol production
process. Three possible routes involving the use of microalgae and cyanobacteria
biomass for bioethanol production are discussed in the literature, accordingly
summarized in Fig. 1 (de Farias Silva and Bertucco 2016). The first one is the
traditional process in which the biomass undergoes pretreatment steps, enzymatic
hydrolysis, and yeast fermentation. The second route is the use of metabolic
232
R. G. Bastos