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the latter group of organisms is triggered through a feast-famine regime in a fedbatch fermenter. The first group includes bacteria such as Cupriavidus necator and
Pseudomonas, while the second group includes Azohydromonas lata and Escherichia
coli (Muralidharan et al. 2013).
Feeding regime affects the polyhydroxyalkanoates yield as well as the monomer
composition. Under transient feeding, where specific amount of carbon substrate is
supplied periodically, the organism experiences periods of feast, where there is
external substrate availability; and famine, where there is no available external substrate. When the biomass is subjected to successive periods of feast and famine, it
experiences an unbalanced growth. This approach is considered the most promising
due to the high polyhydroxyalkanoates accumulation as it promotes the conversion
of carbon substrate to polyhydroxyalkanoates over other intracellular materials
(Salehizadeh and van Loosdrecht 2004). Additionally, the bacteria that contain
polyhydroxyalkanoates showed improved tolerance against transient environmental
changes (Tan et al. 2014). The biomass is forced to adapt in order to survive and
grow in a famine environment. This naturally selects for polyhydroxyalkanoateaccumulating organisms that can convert substrate to polyhydroxyalkanoates during the feast period and maintain an overall higher average growth rate over the
feast and famine periods through growth on stored polyhydroxyalkanoates
(Valentino et al. 2016). Typically, the feast-famine cycle is started out with long
famine duration to create a strong selection pressure for polyhydroxyalkanoates
accumulating organisms and is then decreased to increase the overall rate of substrate addition and polyhydroxyalkanoates production prior to harvest.
The feast-famine regime usually produces suitable conditions for the storage of
volatile fatty acids as polyhydroxyalkanoates since they are rapidly converted by the
bacteria (Basset et al. 2016), with polyhydroxyalkanoates accumulation using
microbial mixed cultures and volatile fatty acids as substrate producing polyhydroxyalkanoates contents of up to 90% cell dry weight (Korkakaki et al. 2016;
Johnson et al. 2009a).
In an experiment comparing the effect of feeding mode, the semicontinuous
pulse feeding produced a 64.5% polyhydroxyalkanoates content from an activated
sludge from wastewater treatment and food waste plant with fermented volatile fatty
acids as substrate, while the one-time pulse-feeding mode was able to produce only
51.5% polyhydroxyalkanoates content (Chen et al. 2013). Moreover, the semicontinuous feeding was able to produce more diverse monomers compared to the feastfamine mode (Albuquerque et al. 2011). These experiments illustrated that a more
frequent pulse-feeding mode with a short cycle will result in more diverse polyhydroxyalkanoates and a higher yield. In an experiment conducted by Marang et al., it
was revealed that the feeding regime impacts significantly the accumulation of
polyhydroxyalkanoates. They tested a continuous feed, which resulted in the accumulation of 53% cell dry weight of polyhydroxyalkanoates, while a true feast feed
where 50% of the substrate was supplied at the beginning led to a higher accumulation of 85% cell dry weight (Marang et al. 2018). Moreover, organic loading rate
and sludge retention also play an important role when comparing the impact of
feeding mode. The improvement of feast-famine over continuous feeding is much
S. Sali and H. R. Mackey
the latter group of organisms is triggered through a feast-famine regime in a fedbatch fermenter. The first group includes bacteria such as Cupriavidus necator and
Pseudomonas, while the second group includes Azohydromonas lata and Escherichia
coli (Muralidharan et al. 2013).
Feeding regime affects the polyhydroxyalkanoates yield as well as the monomer
composition. Under transient feeding, where specific amount of carbon substrate is
supplied periodically, the organism experiences periods of feast, where there is
external substrate availability; and famine, where there is no available external substrate. When the biomass is subjected to successive periods of feast and famine, it
experiences an unbalanced growth. This approach is considered the most promising
due to the high polyhydroxyalkanoates accumulation as it promotes the conversion
of carbon substrate to polyhydroxyalkanoates over other intracellular materials
(Salehizadeh and van Loosdrecht 2004). Additionally, the bacteria that contain
polyhydroxyalkanoates showed improved tolerance against transient environmental
changes (Tan et al. 2014). The biomass is forced to adapt in order to survive and
grow in a famine environment. This naturally selects for polyhydroxyalkanoateaccumulating organisms that can convert substrate to polyhydroxyalkanoates during the feast period and maintain an overall higher average growth rate over the
feast and famine periods through growth on stored polyhydroxyalkanoates
(Valentino et al. 2016). Typically, the feast-famine cycle is started out with long
famine duration to create a strong selection pressure for polyhydroxyalkanoates
accumulating organisms and is then decreased to increase the overall rate of substrate addition and polyhydroxyalkanoates production prior to harvest.
The feast-famine regime usually produces suitable conditions for the storage of
volatile fatty acids as polyhydroxyalkanoates since they are rapidly converted by the
bacteria (Basset et al. 2016), with polyhydroxyalkanoates accumulation using
microbial mixed cultures and volatile fatty acids as substrate producing polyhydroxyalkanoates contents of up to 90% cell dry weight (Korkakaki et al. 2016;
Johnson et al. 2009a).
In an experiment comparing the effect of feeding mode, the semicontinuous
pulse feeding produced a 64.5% polyhydroxyalkanoates content from an activated
sludge from wastewater treatment and food waste plant with fermented volatile fatty
acids as substrate, while the one-time pulse-feeding mode was able to produce only
51.5% polyhydroxyalkanoates content (Chen et al. 2013). Moreover, the semicontinuous feeding was able to produce more diverse monomers compared to the feastfamine mode (Albuquerque et al. 2011). These experiments illustrated that a more
frequent pulse-feeding mode with a short cycle will result in more diverse polyhydroxyalkanoates and a higher yield. In an experiment conducted by Marang et al., it
was revealed that the feeding regime impacts significantly the accumulation of
polyhydroxyalkanoates. They tested a continuous feed, which resulted in the accumulation of 53% cell dry weight of polyhydroxyalkanoates, while a true feast feed
where 50% of the substrate was supplied at the beginning led to a higher accumulation of 85% cell dry weight (Marang et al. 2018). Moreover, organic loading rate
and sludge retention also play an important role when comparing the impact of
feeding mode. The improvement of feast-famine over continuous feeding is much
S. Sali and H. R. Mackey