115
low-cost mixed cultures and high productivity recombinant microbial strains could
help decrease the costs further (Visakh 2014).
There are many factors involved in the accumulation of polyhydroxyalkanoates
that could be modified and improved to increase the production. Earlier research
focused on the feeding of the system and alternating aerobic and anaerobic environments (Kourmentza et al. 2017). Recently, new strategies are under development to
improve and combine polyhydroxyalkanoate-producing strains with advanced fermentation designs that could lower the costs of polyhydroxyalkanoates production
and simultaneously reduce the waste in the environment. Another strategy would be
to develop functional strains with technology that have the ability to control the
structures of polyhydroxyalkanoates monomers developed (Wang et al. 2014). In
fact, many variables influence the production of polyhydroxyalkanoates including
the aeration, feeding process, electron donor/acceptor availability, carbon and
energy sources, temperature and pH of the environment, and nutrient availability
(Kourmentza et al. 2017; Montiel-Jarillo et al. 2017). However, the easiest approach
to reduce the costs considerably is to employ cheap, renewable carbon substrates
readily available in industrial wastewaters, particularly agroindustrial, with the use
of low-maintenance mixed cultures for combined polyhydroxyalkanoates production and wastewater treatment.
5.2.2 Biological Aspects of Production
Many different types of bacteria produce polyhydroxyalkanoates as part of their
usual metabolism, including archaea and Gram-positive and Gram-negative bacteria (Chee et al. 2010). Polyhydroxyalkanoates support the survival of bacteria in
case of nutrient-scarce or fluctuating organic conditions, being accumulated under
these conditions within the cell for later use as a carbon and energy source (Fradinho
et al. 2016).
There are three main pathways for the production of polyhydroxyalkanoates by
microorganisms. The first pathway depends on the carbon substrate used and relies
on three enzymes: ketothiolase (PhaA), acetoacetyl-CoA reductase (PhaB), and
PHA synthase (PhaC) (Możejko-Ciesielska and Kiewisz 2016). This pathway usually results in scl-polyhydroxyalkanoates due to the involvement of short-chain
volatile fatty acids such as acetic acids, propionic acid, butyric acid, and valeric acid
and is summarized in Fig. 5.3 (Serafim et al. 2008). The biosynthesis of polyhydroxyalkanoates, or polyhydroxybutyrate specifically, from sugars begins by taking
up acetate as a substrate and converting it to acetyl-CoA. Then two molecules of
acetyl-CoA are condensed into acetoacetyl-CoA, which is then reduced to
3-hydroxybutyl-CoA and finally polymerized into polyhydroxybutyrate
(Salehizadeh and van Loosdrecht 2004; Bugnicourt et al. 2014). Propionate can
result in three different types of polyhydroxyalkanoates depending on the precursor.
The polymer poly-3-hydroxy-2-methylvalerate is a result of two molecules of
propionyl- CoA. Either polyhydroxyvalerate or poly-3-hydroxy-2-methylbutyrate
5 Integration of Polyhydroxyalkanoates Production with Industrial Wastewater…
low-cost mixed cultures and high productivity recombinant microbial strains could
help decrease the costs further (Visakh 2014).
There are many factors involved in the accumulation of polyhydroxyalkanoates
that could be modified and improved to increase the production. Earlier research
focused on the feeding of the system and alternating aerobic and anaerobic environments (Kourmentza et al. 2017). Recently, new strategies are under development to
improve and combine polyhydroxyalkanoate-producing strains with advanced fermentation designs that could lower the costs of polyhydroxyalkanoates production
and simultaneously reduce the waste in the environment. Another strategy would be
to develop functional strains with technology that have the ability to control the
structures of polyhydroxyalkanoates monomers developed (Wang et al. 2014). In
fact, many variables influence the production of polyhydroxyalkanoates including
the aeration, feeding process, electron donor/acceptor availability, carbon and
energy sources, temperature and pH of the environment, and nutrient availability
(Kourmentza et al. 2017; Montiel-Jarillo et al. 2017). However, the easiest approach
to reduce the costs considerably is to employ cheap, renewable carbon substrates
readily available in industrial wastewaters, particularly agroindustrial, with the use
of low-maintenance mixed cultures for combined polyhydroxyalkanoates production and wastewater treatment.
5.2.2 Biological Aspects of Production
Many different types of bacteria produce polyhydroxyalkanoates as part of their
usual metabolism, including archaea and Gram-positive and Gram-negative bacteria (Chee et al. 2010). Polyhydroxyalkanoates support the survival of bacteria in
case of nutrient-scarce or fluctuating organic conditions, being accumulated under
these conditions within the cell for later use as a carbon and energy source (Fradinho
et al. 2016).
There are three main pathways for the production of polyhydroxyalkanoates by
microorganisms. The first pathway depends on the carbon substrate used and relies
on three enzymes: ketothiolase (PhaA), acetoacetyl-CoA reductase (PhaB), and
PHA synthase (PhaC) (Możejko-Ciesielska and Kiewisz 2016). This pathway usually results in scl-polyhydroxyalkanoates due to the involvement of short-chain
volatile fatty acids such as acetic acids, propionic acid, butyric acid, and valeric acid
and is summarized in Fig. 5.3 (Serafim et al. 2008). The biosynthesis of polyhydroxyalkanoates, or polyhydroxybutyrate specifically, from sugars begins by taking
up acetate as a substrate and converting it to acetyl-CoA. Then two molecules of
acetyl-CoA are condensed into acetoacetyl-CoA, which is then reduced to
3-hydroxybutyl-CoA and finally polymerized into polyhydroxybutyrate
(Salehizadeh and van Loosdrecht 2004; Bugnicourt et al. 2014). Propionate can
result in three different types of polyhydroxyalkanoates depending on the precursor.
The polymer poly-3-hydroxy-2-methylvalerate is a result of two molecules of
propionyl- CoA. Either polyhydroxyvalerate or poly-3-hydroxy-2-methylbutyrate
5 Integration of Polyhydroxyalkanoates Production with Industrial Wastewater…