Metabolite types and ratios vary greatly by microorganism and fermentation
substrate. For example, ethanol is produced along with short-chain fatty acids from
glucose (a product of hydrolysis of the fibrin contain in algae fiber) under anaerobic
conditions.
In contrast, ethanol production is unlikely to be achieved under anaerobic
conditions with alginic acid, a major component of brown algae. This is due to the
fact that the alginic acid monomer b-D-mannuronic acid and its C-5 epimer a-Lgluronic acid are acidic sugars lacking the reduction capability needed for ethanol
production. As per the following reaction formula, the chief product from alginic
acid under anaerobic conditions is acetic acid:
C 6 H 8 O 6 ! 2CH 3 COOH þ 2CO 2
In contrast, mannitol (C 6 H 14 O 6 ), which is another main component of brown
algae, may provide an excellent substrate for ethanol production. Because the
enzymes normally used in ethanol production do not degrade mannitol, other
bacteria besides those enzymes must be used to degrade mannitol unless genetic
manipulation is applied. Seaweed proteins, which are also present in large quantities in brown algae in addition to sugars, lack the necessary reduction to produce
ethanol, and are typically converted to short-chain fatty acids by anaerobic
microorganisms. The process of brown algae hydrolysis and organic acid generation by anaerobic microorganisms thus results in short-chain fatty acids as a major
metabolite, without much alcohol production.
Short-chain fatty acid such as propionic acid and butyric acid that are created
during the organic acid formation process as broken down by the
hydrogen-producing acetic acid bacteria into acetic acid, hydrogen, and carbon
dioxide. The organic matter that methane-producing bacteria are capable of using is
very limited; acetic acid and hydrogen serve as direct substrates for methane formation in artificial methane fermentation. Depending on the methane-producing
bacteria, this can be divided into production of hydrogen and carbon dioxide and
production of methane and carbon dioxide from alcohol and short-chain fatty acids.
The growth substrate for methane-producing bacteria is also supplied by different
microorganisms.
In the case of hydrogen- and acetic acid-forming bacteria, hydrogendecomposition methane bacteria typically break down the hydrogen, which is
removed to sustain growth. As this indicates, a large number of microorganisms are
involved in the methane fermentation process. It is chiefly the methane production
and hydrolysis processes, however, that serve to limit the rate.
9.3.4 Yield of Methane Fermentation of Algae
Algae contain sugars that are easily hydrolyzed and have lower lignin content than
land-based plants. For example, while corn has lignin content of 15.1%w/w relative
9.3 Methane Production from Algae
309
substrate. For example, ethanol is produced along with short-chain fatty acids from
glucose (a product of hydrolysis of the fibrin contain in algae fiber) under anaerobic
conditions.
In contrast, ethanol production is unlikely to be achieved under anaerobic
conditions with alginic acid, a major component of brown algae. This is due to the
fact that the alginic acid monomer b-D-mannuronic acid and its C-5 epimer a-Lgluronic acid are acidic sugars lacking the reduction capability needed for ethanol
production. As per the following reaction formula, the chief product from alginic
acid under anaerobic conditions is acetic acid:
C 6 H 8 O 6 ! 2CH 3 COOH þ 2CO 2
In contrast, mannitol (C 6 H 14 O 6 ), which is another main component of brown
algae, may provide an excellent substrate for ethanol production. Because the
enzymes normally used in ethanol production do not degrade mannitol, other
bacteria besides those enzymes must be used to degrade mannitol unless genetic
manipulation is applied. Seaweed proteins, which are also present in large quantities in brown algae in addition to sugars, lack the necessary reduction to produce
ethanol, and are typically converted to short-chain fatty acids by anaerobic
microorganisms. The process of brown algae hydrolysis and organic acid generation by anaerobic microorganisms thus results in short-chain fatty acids as a major
metabolite, without much alcohol production.
Short-chain fatty acid such as propionic acid and butyric acid that are created
during the organic acid formation process as broken down by the
hydrogen-producing acetic acid bacteria into acetic acid, hydrogen, and carbon
dioxide. The organic matter that methane-producing bacteria are capable of using is
very limited; acetic acid and hydrogen serve as direct substrates for methane formation in artificial methane fermentation. Depending on the methane-producing
bacteria, this can be divided into production of hydrogen and carbon dioxide and
production of methane and carbon dioxide from alcohol and short-chain fatty acids.
The growth substrate for methane-producing bacteria is also supplied by different
microorganisms.
In the case of hydrogen- and acetic acid-forming bacteria, hydrogendecomposition methane bacteria typically break down the hydrogen, which is
removed to sustain growth. As this indicates, a large number of microorganisms are
involved in the methane fermentation process. It is chiefly the methane production
and hydrolysis processes, however, that serve to limit the rate.
9.3.4 Yield of Methane Fermentation of Algae
Algae contain sugars that are easily hydrolyzed and have lower lignin content than
land-based plants. For example, while corn has lignin content of 15.1%w/w relative
9.3 Methane Production from Algae
309