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energy released from glycolysis is used to incorporate intracellular fermentation
products in the form of polyhydroxybutyrate. Both polyphosphate-accumulating
organisms and glycogen-accumulating organisms thrive in a cyclic environment
where the substrate is periodically present when a preferred electron acceptor is
absent (Salehizadeh and van Loosdrecht 2004). However, their polyhydroxyalkanoates production is typically limited to only 20–30% cell dry weight (Serafim et al.
2008), though values up to 60% have been reported (Albuquerque et al. 2007).
Archaea, which are typically extremophiles, are another group of microorganisms with the ability to produce polyhydroxyalkanoates. Known archaeal polyhydroxyalkanoates producers are all halophiles, requiring high salinity environments
to support their growth (Saharan et al. 2014). The advantage of producing polyhydroxyalkanoates with halophiles is the reduction and elimination of contamination
from unwanted species under open or mixed culture cultivation (Kourmentza et al.
2017). The first case of archaea-producing polyhydroxyalkanoates was recorded in
the Dead Sea in 1970. Since then, many experiments were conducted where archaea
produced up to 65% polyhydroxyalkanoates of cell dry weight under nutrientlimiting conditions (Saharan et al. 2014). Currently, the best archaea for polyhydroxyalkanoates production is Haloferax mediterranei, frequently found in the
coast of Spain (Kourmentza et al. 2017). Studies conducted on this type of bacteria
revealed that low-cost feedstocks result in a high polyhydroxyalkanoates content. In
an experiment, pretreated vinasse was used as the carbon source and produced
66–70% of polyhydroxyalkanoates (Bhattacharyya et al. 2014).
Pure cultures have historically been used to produce polyhydroxyalkanoates, but
their use is still limited by the high costs of maintaining cultures through sterilization in special equipment (Kourmentza et al. 2017). In contrast, microbial mixed
cultures are less costly to maintain, particularly with feedstocks already rich in
microorganisms (Kourmentza et al. 2017; Valentino et al. 2016). Microbial mixed
cultures reduce fermentation equipment expenses and typically require less process
control (Serafim et al. 2008). Additionally, microbial mixed cultures can be manipulated in order to produce specific polyhydroxyalkanoate polymers by varying the
carbon source composition as well as the feeding cycle (Gumel et al. 2013). These
factors make mixed cultures an excellent candidate to couple with wastewater treatment where the feedstocks are effectively free or even better can be offset against
costs typically associated with treatment.
Activated sludge is a generic term for flocculated microbial mixed cultures frequently used in wastewater treatment. Dominant polyhydroxyalkanoate-producing
bacteria found in wastewaters include Bacillus, Pseudomonas, Alcaligenes,
Chromobacterium (Koller et al. 2011), Plasticicumulans (Jiang et al. 2011b, c),
Azoarcus, Paracoccus, and Thauera (Albuquerque et al. 2012). These cultures have
the ability to store polyhydroxyalkanoates as carbon and energy storage material.
They can accumulate polyhydroxyalkanoates under conditions of irregular nutrient
availability due to an intermittent feeding regime and presence of an electron acceptor (Serafim et al. 2008). The microorganisms present are able to adapt to rapidly
changing conditions of nutrient availability and continuous changes in substrate
(Salehizadeh and van Loosdrecht 2004). Beccari et al. reported that with a microbial
S. Sali and H. R. Mackey
energy released from glycolysis is used to incorporate intracellular fermentation
products in the form of polyhydroxybutyrate. Both polyphosphate-accumulating
organisms and glycogen-accumulating organisms thrive in a cyclic environment
where the substrate is periodically present when a preferred electron acceptor is
absent (Salehizadeh and van Loosdrecht 2004). However, their polyhydroxyalkanoates production is typically limited to only 20–30% cell dry weight (Serafim et al.
2008), though values up to 60% have been reported (Albuquerque et al. 2007).
Archaea, which are typically extremophiles, are another group of microorganisms with the ability to produce polyhydroxyalkanoates. Known archaeal polyhydroxyalkanoates producers are all halophiles, requiring high salinity environments
to support their growth (Saharan et al. 2014). The advantage of producing polyhydroxyalkanoates with halophiles is the reduction and elimination of contamination
from unwanted species under open or mixed culture cultivation (Kourmentza et al.
2017). The first case of archaea-producing polyhydroxyalkanoates was recorded in
the Dead Sea in 1970. Since then, many experiments were conducted where archaea
produced up to 65% polyhydroxyalkanoates of cell dry weight under nutrientlimiting conditions (Saharan et al. 2014). Currently, the best archaea for polyhydroxyalkanoates production is Haloferax mediterranei, frequently found in the
coast of Spain (Kourmentza et al. 2017). Studies conducted on this type of bacteria
revealed that low-cost feedstocks result in a high polyhydroxyalkanoates content. In
an experiment, pretreated vinasse was used as the carbon source and produced
66–70% of polyhydroxyalkanoates (Bhattacharyya et al. 2014).
Pure cultures have historically been used to produce polyhydroxyalkanoates, but
their use is still limited by the high costs of maintaining cultures through sterilization in special equipment (Kourmentza et al. 2017). In contrast, microbial mixed
cultures are less costly to maintain, particularly with feedstocks already rich in
microorganisms (Kourmentza et al. 2017; Valentino et al. 2016). Microbial mixed
cultures reduce fermentation equipment expenses and typically require less process
control (Serafim et al. 2008). Additionally, microbial mixed cultures can be manipulated in order to produce specific polyhydroxyalkanoate polymers by varying the
carbon source composition as well as the feeding cycle (Gumel et al. 2013). These
factors make mixed cultures an excellent candidate to couple with wastewater treatment where the feedstocks are effectively free or even better can be offset against
costs typically associated with treatment.
Activated sludge is a generic term for flocculated microbial mixed cultures frequently used in wastewater treatment. Dominant polyhydroxyalkanoate-producing
bacteria found in wastewaters include Bacillus, Pseudomonas, Alcaligenes,
Chromobacterium (Koller et al. 2011), Plasticicumulans (Jiang et al. 2011b, c),
Azoarcus, Paracoccus, and Thauera (Albuquerque et al. 2012). These cultures have
the ability to store polyhydroxyalkanoates as carbon and energy storage material.
They can accumulate polyhydroxyalkanoates under conditions of irregular nutrient
availability due to an intermittent feeding regime and presence of an electron acceptor (Serafim et al. 2008). The microorganisms present are able to adapt to rapidly
changing conditions of nutrient availability and continuous changes in substrate
(Salehizadeh and van Loosdrecht 2004). Beccari et al. reported that with a microbial
S. Sali and H. R. Mackey