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5.5 Microbial Community Dynamics
Interactions between microbial communities are of high importance in microbial
mixed culture systems. In a system studying a change in feedstock, it was found
both the microbial communities of the fermentation step and polyhydroxyalkanoates accumulation steps in a three-stage system underwent significant changes when
the feedstock changed from molasses to cheese whey, with higher hydroxyvalerate
precursors during feeding with molasses. Despite this, both acidogenic and polyhydroxyalkanoates production performance were similar after pseudo-steady-state
acclimation demonstrating functional redundancy in the system (Carvalho
et al. 2018).
Experiments conducted by Albuquerque et al. revealed that bacterial strains have
preferences to the substrates they consume. For instance, Azoarcus consumed acetate, Thauera consumed butyrate, while Paracoccus consumed a larger range of
substrates (Albuquerque et al. 2012). In various experiments, it was revealed that
Thauera is linked to the production of hydroxyvalerate monomers (Carvalho et al.
2014), while Paracoccus is able to accumulate polyhydroxybutyrate, polyhydroxyvalerate, and polyhydroxybutyrate-co-valerate when fed with methanol, pentanol, and a mixture of methanol and pentanol, respectively (Yamane et al. 1996;
Maehara et al. 2001). Wang et al. studied the effect of different organic loading
rates, sludge retention time, and dissolved oxygen on the startup of polyhydroxyalkanoates producing sequencing batch reactor systems operated under feast-famine
regime (Wang et al. 2017). One reactor was operated with a high organic loading
rate, high dissolved oxygen (3.47 mg/L), and sludge retention time of 4 days, while
the second was initially operated under a low dissolved oxygen (0.87 mg/L) and
organic loading rate with an 8 d sludge retention time. The organic loading rate was
gradually increased to that of the first reactor and then the sludge retention time
reduced to 4 days also. It was found that the initial and final microbial communities
were very similar in composition despite the differing culture conditions with time
and dissolved oxygen concentrations and were dominant in Plasticicumulans,
Zoogloea, Paracoccus, and Flavobacterium, suggesting dissolved oxygen did not
largely impact the microbial community. However, at the midpoint of the experiment, where organic loading rate was similar but the sludge retention time was still
varied, the microbial community in the second sequencing batch reactor was dominated by Amaricoccus and Azoarcus, polyhydroxyalkanoates producers that are
known to require a longer sludge retention time (Wang et al. 2017).
5.6 Process Design
A variety of processes exist that can produce polyhydroxyalkanoates by mixed and
pure cultures. The selection of the process will be dependent on the nature of the
feedstock (or wastewater), expected microbial community competition, and
S. Sali and H. R. Mackey
5.5 Microbial Community Dynamics
Interactions between microbial communities are of high importance in microbial
mixed culture systems. In a system studying a change in feedstock, it was found
both the microbial communities of the fermentation step and polyhydroxyalkanoates accumulation steps in a three-stage system underwent significant changes when
the feedstock changed from molasses to cheese whey, with higher hydroxyvalerate
precursors during feeding with molasses. Despite this, both acidogenic and polyhydroxyalkanoates production performance were similar after pseudo-steady-state
acclimation demonstrating functional redundancy in the system (Carvalho
et al. 2018).
Experiments conducted by Albuquerque et al. revealed that bacterial strains have
preferences to the substrates they consume. For instance, Azoarcus consumed acetate, Thauera consumed butyrate, while Paracoccus consumed a larger range of
substrates (Albuquerque et al. 2012). In various experiments, it was revealed that
Thauera is linked to the production of hydroxyvalerate monomers (Carvalho et al.
2014), while Paracoccus is able to accumulate polyhydroxybutyrate, polyhydroxyvalerate, and polyhydroxybutyrate-co-valerate when fed with methanol, pentanol, and a mixture of methanol and pentanol, respectively (Yamane et al. 1996;
Maehara et al. 2001). Wang et al. studied the effect of different organic loading
rates, sludge retention time, and dissolved oxygen on the startup of polyhydroxyalkanoates producing sequencing batch reactor systems operated under feast-famine
regime (Wang et al. 2017). One reactor was operated with a high organic loading
rate, high dissolved oxygen (3.47 mg/L), and sludge retention time of 4 days, while
the second was initially operated under a low dissolved oxygen (0.87 mg/L) and
organic loading rate with an 8 d sludge retention time. The organic loading rate was
gradually increased to that of the first reactor and then the sludge retention time
reduced to 4 days also. It was found that the initial and final microbial communities
were very similar in composition despite the differing culture conditions with time
and dissolved oxygen concentrations and were dominant in Plasticicumulans,
Zoogloea, Paracoccus, and Flavobacterium, suggesting dissolved oxygen did not
largely impact the microbial community. However, at the midpoint of the experiment, where organic loading rate was similar but the sludge retention time was still
varied, the microbial community in the second sequencing batch reactor was dominated by Amaricoccus and Azoarcus, polyhydroxyalkanoates producers that are
known to require a longer sludge retention time (Wang et al. 2017).
5.6 Process Design
A variety of processes exist that can produce polyhydroxyalkanoates by mixed and
pure cultures. The selection of the process will be dependent on the nature of the
feedstock (or wastewater), expected microbial community competition, and
S. Sali and H. R. Mackey