4.2 Biofuels
Cyanobacteria are the unique group of photosynthetic bio-agents that can grow at a
fast rate due to their simple cell structure and minimum requirement of nutrients
accompanied by the capacity to produce bio-energy including bio-hydrogen (Mona
et al., 2011) and bioethanol (Rawat et al. 2011). Cyanobacteria have been used to
produce hydrogen gas that constitutes an alternative future energy source to the
limited fossil fuel resources (Dutta et al. 2005). Cyanobacteria produce hydrogen
either as a by-product of nitrogen fixation, when nitrogenase-containing heterocystous cyanobacteria are grown under nitrogen-limiting conditions, or by the reversible
activity of hydrogenase enzymes. Many genera of cyanobacteria such as Anabaena,
Oscillatoria, Cyanothece, Synechococcus, Nostoc, Microcystis, Calothrix,
Gloeobacter, Aphanocapsa, Chroococcidiopsis, and Microcoleus produce hydrogen
gas under various culture conditions.
The production of ethanol via biological route has received widespread attention in
recent years. Traditionally, bioethanol is produced in a two-step route to first collect
plant-derived biomass, and subsequent conversion of the biomass to fuels by microbial
fermentation is employed (Stephanopoulos 2007). This two-step bioethanol production is inefficient in comparison than photosynthetic microbes who directly convert
carbon dioxide to fuels. Although some cyanobacterial strains naturally produce low
quality ethanol but, it could be enhance the production efficiency of ethanol production by genetic engineering. An attempt to introduce the pyruvate decarboxylase (pdc)
and alcohol dehydrogenase II (adh) genes from Zymomonas mobilis into the chromosome of Synechocystis sp. PCC 6803 was reported (Deng and Coleman 1999). Further
engineering of Synechocystis sp. by overexpressing endogenous alcohol dehydrogenase and disrupting polyhydroxyalkanoate biosynthetic pathway increased ethanol
production up to 5500 mg/L (Gao et al. 2012).
4.3 Bio-fertilizer
Cyanobacteria can fix atmospheric N 2 and are good source of bio-fertilizer. They can
fix atmospheric nitrogen in the form of free-living or in the symbiotic associations
with partners such as water fern Azolla, cycads, Gunnera, etc. Some cyanobacteria
have specialized cells known as heterocyst which are considered site of nitrogen
fixation by nitrogenase enzyme. Nitrogenase enzyme is a complex in nature and
catalyzes the conversion of the molecular N 2 into reduced form like ammonia (Singh
et al. 2011). The fixed nitrogen may be released in the form of ammonia, polypeptides, and free amino acids, either by secretion or by microbial degradation after
the cell death. Nitrogen-fixing ability is not limited by heterocyst-containing
cyanobacteria but also by several non-heterocystous unicellular and filamentous
genera which can also fix nitrogen. Cyanobacteria as bio-fertilizer is advantageous
over the chemical fertilizer as it is cheap and eco-friendly (Issa et al. 2014).
404
S. Mona et al.
Cyanobacteria are the unique group of photosynthetic bio-agents that can grow at a
fast rate due to their simple cell structure and minimum requirement of nutrients
accompanied by the capacity to produce bio-energy including bio-hydrogen (Mona
et al., 2011) and bioethanol (Rawat et al. 2011). Cyanobacteria have been used to
produce hydrogen gas that constitutes an alternative future energy source to the
limited fossil fuel resources (Dutta et al. 2005). Cyanobacteria produce hydrogen
either as a by-product of nitrogen fixation, when nitrogenase-containing heterocystous cyanobacteria are grown under nitrogen-limiting conditions, or by the reversible
activity of hydrogenase enzymes. Many genera of cyanobacteria such as Anabaena,
Oscillatoria, Cyanothece, Synechococcus, Nostoc, Microcystis, Calothrix,
Gloeobacter, Aphanocapsa, Chroococcidiopsis, and Microcoleus produce hydrogen
gas under various culture conditions.
The production of ethanol via biological route has received widespread attention in
recent years. Traditionally, bioethanol is produced in a two-step route to first collect
plant-derived biomass, and subsequent conversion of the biomass to fuels by microbial
fermentation is employed (Stephanopoulos 2007). This two-step bioethanol production is inefficient in comparison than photosynthetic microbes who directly convert
carbon dioxide to fuels. Although some cyanobacterial strains naturally produce low
quality ethanol but, it could be enhance the production efficiency of ethanol production by genetic engineering. An attempt to introduce the pyruvate decarboxylase (pdc)
and alcohol dehydrogenase II (adh) genes from Zymomonas mobilis into the chromosome of Synechocystis sp. PCC 6803 was reported (Deng and Coleman 1999). Further
engineering of Synechocystis sp. by overexpressing endogenous alcohol dehydrogenase and disrupting polyhydroxyalkanoate biosynthetic pathway increased ethanol
production up to 5500 mg/L (Gao et al. 2012).
4.3 Bio-fertilizer
Cyanobacteria can fix atmospheric N 2 and are good source of bio-fertilizer. They can
fix atmospheric nitrogen in the form of free-living or in the symbiotic associations
with partners such as water fern Azolla, cycads, Gunnera, etc. Some cyanobacteria
have specialized cells known as heterocyst which are considered site of nitrogen
fixation by nitrogenase enzyme. Nitrogenase enzyme is a complex in nature and
catalyzes the conversion of the molecular N 2 into reduced form like ammonia (Singh
et al. 2011). The fixed nitrogen may be released in the form of ammonia, polypeptides, and free amino acids, either by secretion or by microbial degradation after
the cell death. Nitrogen-fixing ability is not limited by heterocyst-containing
cyanobacteria but also by several non-heterocystous unicellular and filamentous
genera which can also fix nitrogen. Cyanobacteria as bio-fertilizer is advantageous
over the chemical fertilizer as it is cheap and eco-friendly (Issa et al. 2014).
404
S. Mona et al.