culture is higher than that of acid hydrolysis. Carbohydrates in microalgae biomass
are mainly cellulose and starch. Cellulose molecules are glucose polymers linked
together by b-1,4 glucosidic bonds, as opposed to the a-1,4 and a-1,6 glucosidic
bonds for starch. In the enzymatic pretreatment of algae, b-glucosidase/cellulase
hydrolyzed b-1,4 glucosidic bonds of algal cellulose, whereas a-amylase liquefied
algal starch to oligosaccharides through the hydrolysis of the a-1,4 glucosidic
linkages, and then amyloglucosidase hydrolyzed a-1,4 and a-1,6 glucosidic bonds
of oligosaccharides into glucose. Therefore, it is desirable to use three enzymes in
the enzymatic pretreatment of microalgae, thus improving the hydrolysis yields
even further.
Another process known as “dark fermentation” refers to the conversion of
organic substrates into biohydrogen (de Farias Silva and Bertucco 2016).
Fermentative and hydrolytic microorganisms hydrolyze complex organic polymers
into monomers, which are subsequently converted into a mixture of organic acids of
low molecular weight and alcohols, mainly acetic acid and ethanol. Various
microalgae and cyanobacteria that are capable of expelling ethanol through the cell
wall by means of intracellular process in the absence of light include C. reinhardtii,
Chlamydomonas moewusii, C. vulgaris, Oscillatoria limnetica, Oscillatoria limosa,
Gleocapsa alpícola, Cyanothece sp., Chlorococcum littorale, and Spirulina sp. and
Synechococcus sp. However, dark fermentation is disadvantageous in terms of
hydrogen productivity, because approximately 80–90% of the initial chemical
oxygen demand (COD) remains in the form of acids and alcohols after the process.
Even under optimal operating conditions, typical yields vary only between 1 and
2 mol H 2 per mol of glucose. The production of ethanol is favored by the accumulation of carbohydrates in the microalgae cells through photosynthesis, and then,
the microalgae are forced to synthesize ethanol through fermentative metabolism
directly from their carbohydrate and lipid reserves when switching the growth to
dark conditions. However, it can be concluded that dark fermentation of microalgae
is not an efficient process for the production of bioethanol.
“Photofermentation” is a process of growing interest principally after the
announcement of the installation of industrial plants where modified cyanobacteria
are used to produce bioethanol directly. The “photofermentative” route (simply,
photanol) is a natural mechanism of converting sunlight into products of fermentation through a highly efficient metabolic pathway. Photanol is not only limited to
ethanol production, but it is also used for a large number of naturally occurring
products resulting from glycolysis-based fermentation (Rai and Singh 2016). Thus,
several cyanobacteria species can be genetically modified by introducing specific
fermentation cassettes through molecular engineering procedures, and then tested as
a fermentative organism. Synechococcus sp. is a unicellular cyanobacterium living
in freshwater that has been relatively well characterized. It is capable of tolerating
insertion of foreign DNA to be transformed and replicated using shuttle vectors
between Escherichia coli and cyanobacteria, or insertion of foreign DNA into the
chromosome through homologous recombination at selected active sites.
Synechocystis sp. PCC 6803 was the first photosynthetic organism that had its
genome sequenced and one of the best characterized cyanobacteria.
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R. G. Bastos
are mainly cellulose and starch. Cellulose molecules are glucose polymers linked
together by b-1,4 glucosidic bonds, as opposed to the a-1,4 and a-1,6 glucosidic
bonds for starch. In the enzymatic pretreatment of algae, b-glucosidase/cellulase
hydrolyzed b-1,4 glucosidic bonds of algal cellulose, whereas a-amylase liquefied
algal starch to oligosaccharides through the hydrolysis of the a-1,4 glucosidic
linkages, and then amyloglucosidase hydrolyzed a-1,4 and a-1,6 glucosidic bonds
of oligosaccharides into glucose. Therefore, it is desirable to use three enzymes in
the enzymatic pretreatment of microalgae, thus improving the hydrolysis yields
even further.
Another process known as “dark fermentation” refers to the conversion of
organic substrates into biohydrogen (de Farias Silva and Bertucco 2016).
Fermentative and hydrolytic microorganisms hydrolyze complex organic polymers
into monomers, which are subsequently converted into a mixture of organic acids of
low molecular weight and alcohols, mainly acetic acid and ethanol. Various
microalgae and cyanobacteria that are capable of expelling ethanol through the cell
wall by means of intracellular process in the absence of light include C. reinhardtii,
Chlamydomonas moewusii, C. vulgaris, Oscillatoria limnetica, Oscillatoria limosa,
Gleocapsa alpícola, Cyanothece sp., Chlorococcum littorale, and Spirulina sp. and
Synechococcus sp. However, dark fermentation is disadvantageous in terms of
hydrogen productivity, because approximately 80–90% of the initial chemical
oxygen demand (COD) remains in the form of acids and alcohols after the process.
Even under optimal operating conditions, typical yields vary only between 1 and
2 mol H 2 per mol of glucose. The production of ethanol is favored by the accumulation of carbohydrates in the microalgae cells through photosynthesis, and then,
the microalgae are forced to synthesize ethanol through fermentative metabolism
directly from their carbohydrate and lipid reserves when switching the growth to
dark conditions. However, it can be concluded that dark fermentation of microalgae
is not an efficient process for the production of bioethanol.
“Photofermentation” is a process of growing interest principally after the
announcement of the installation of industrial plants where modified cyanobacteria
are used to produce bioethanol directly. The “photofermentative” route (simply,
photanol) is a natural mechanism of converting sunlight into products of fermentation through a highly efficient metabolic pathway. Photanol is not only limited to
ethanol production, but it is also used for a large number of naturally occurring
products resulting from glycolysis-based fermentation (Rai and Singh 2016). Thus,
several cyanobacteria species can be genetically modified by introducing specific
fermentation cassettes through molecular engineering procedures, and then tested as
a fermentative organism. Synechococcus sp. is a unicellular cyanobacterium living
in freshwater that has been relatively well characterized. It is capable of tolerating
insertion of foreign DNA to be transformed and replicated using shuttle vectors
between Escherichia coli and cyanobacteria, or insertion of foreign DNA into the
chromosome through homologous recombination at selected active sites.
Synechocystis sp. PCC 6803 was the first photosynthetic organism that had its
genome sequenced and one of the best characterized cyanobacteria.
240
R. G. Bastos