Although systems study of microalgae on carbohydrate metabolisms is currently
in its infant stage, omics studies on microalgae have made significant progress.
Such a strategy will open a door for efficient carbohydrate metabolic regulation and
genetic engineering of microalgae for biofuels’ production.
3 Technologies of Microalgal Carbohydrates
to Bioethanol
The main technological routes for bioethanol production by microalgal biomass
involve hydrolysis–yeast fermentation, the use of metabolic pathways in dark
conditions, and “photofermentation.”
The hydrolysis of biomass is the most used method for the use of microalgal
carbohydrates. Hydrolysis–fermentation of microalgal biomass is based on the
production of microalgae biomass succeeded by pretreatment steps, involving
breakdown of the cell structure and hydrolysis of the biomass, and frequently by the
addition of enzymes. The treated biomass is then fermented with yeasts or bacteria
to obtain ethanol. The main drawbacks of this route are the multistep processes
required, which demands more energy, and the use of enzymes and yeasts, which
accounts for a considerable proportion of the costs. On the contrary, the hydrolysis/
fermentation process converts biomass at the highest rate, because of the
well-known high efficiency of enzymes and yeasts in converting biomass into
products.
Markou et al. (2013) studied the potential of bioethanol production using
carbohydrate-enriched biomass of the cyanobacteria A. platensis. For the saccharification of the carbohydrate-enriched biomass, four acids (H 2 SO 4 , HNO 3 , HCl, and
H 3 PO 4 ) were investigated. The hydrolysates then were used as substrate for ethanol
fermentation by a salt stress-adapted Saccharomyces cerevisiae strain. According to
the authors, the highest bioethanol yield was observed at acid concentration of
0.5 N. At this concentration, fermentation of hydrolysates with HCl as catalyst had
the lowest bioethanol yield (13.41% gram of ethanol per gram dry biomass), while
hydrolysates with H 2 SO 4 and HNO 3 as catalysts had bioethanol yield of 16.27 and
16.32%, respectively. Chlorella biomass was hydrolyzed in the presence of 2% HCl
and 2.5% MgCl 2 , a sugar concentration of nearly 12%, and a sugar recovery of
about 83% was obtained. Fermentation experiments demonstrated that glucose in
the Chlorella biomass hydrolysates was converted into ethanol by S. cerevisiae
with a yield of 0.47 g/g, which is 91% of the theoretical yield (Zhou et al. 2011).
Rizza et al. (2017) researched Desmodesmus sp. strain for production of
biomass fermentable. Hydrolyzed preparations were brought to pH 5.5–6.0 with
Mg(OH) 2 crystals and used directly or after concentration by freeze-drying for
ethanol fermentation. A detailed time-course analysis of the increase in biomass and
accumulation of total carbohydrates and proteins indicated that Desmodesmus
sp. strain FG grew robustly, its reaching ratios of carbohydrates to protein over 2.
238
R. G. Bastos
in its infant stage, omics studies on microalgae have made significant progress.
Such a strategy will open a door for efficient carbohydrate metabolic regulation and
genetic engineering of microalgae for biofuels’ production.
3 Technologies of Microalgal Carbohydrates
to Bioethanol
The main technological routes for bioethanol production by microalgal biomass
involve hydrolysis–yeast fermentation, the use of metabolic pathways in dark
conditions, and “photofermentation.”
The hydrolysis of biomass is the most used method for the use of microalgal
carbohydrates. Hydrolysis–fermentation of microalgal biomass is based on the
production of microalgae biomass succeeded by pretreatment steps, involving
breakdown of the cell structure and hydrolysis of the biomass, and frequently by the
addition of enzymes. The treated biomass is then fermented with yeasts or bacteria
to obtain ethanol. The main drawbacks of this route are the multistep processes
required, which demands more energy, and the use of enzymes and yeasts, which
accounts for a considerable proportion of the costs. On the contrary, the hydrolysis/
fermentation process converts biomass at the highest rate, because of the
well-known high efficiency of enzymes and yeasts in converting biomass into
products.
Markou et al. (2013) studied the potential of bioethanol production using
carbohydrate-enriched biomass of the cyanobacteria A. platensis. For the saccharification of the carbohydrate-enriched biomass, four acids (H 2 SO 4 , HNO 3 , HCl, and
H 3 PO 4 ) were investigated. The hydrolysates then were used as substrate for ethanol
fermentation by a salt stress-adapted Saccharomyces cerevisiae strain. According to
the authors, the highest bioethanol yield was observed at acid concentration of
0.5 N. At this concentration, fermentation of hydrolysates with HCl as catalyst had
the lowest bioethanol yield (13.41% gram of ethanol per gram dry biomass), while
hydrolysates with H 2 SO 4 and HNO 3 as catalysts had bioethanol yield of 16.27 and
16.32%, respectively. Chlorella biomass was hydrolyzed in the presence of 2% HCl
and 2.5% MgCl 2 , a sugar concentration of nearly 12%, and a sugar recovery of
about 83% was obtained. Fermentation experiments demonstrated that glucose in
the Chlorella biomass hydrolysates was converted into ethanol by S. cerevisiae
with a yield of 0.47 g/g, which is 91% of the theoretical yield (Zhou et al. 2011).
Rizza et al. (2017) researched Desmodesmus sp. strain for production of
biomass fermentable. Hydrolyzed preparations were brought to pH 5.5–6.0 with
Mg(OH) 2 crystals and used directly or after concentration by freeze-drying for
ethanol fermentation. A detailed time-course analysis of the increase in biomass and
accumulation of total carbohydrates and proteins indicated that Desmodesmus
sp. strain FG grew robustly, its reaching ratios of carbohydrates to protein over 2.
238
R. G. Bastos