metabolic pathways that allow dissipation of electrons in response to the redox state
of the consumed carbon and nitrogen sources [96]. In the particular case of using
PPB for treating wastewaters or valorizing fermented agro-wastes, the uptake of
reduced carbon sources (e.g., organic acids), present in the waste stream, requires
electron dissipation which is possible through PHA accumulation, CO 2 fixation via
CBB cycle or through N 2 fixation/H 2 production via nitrogenase enzyme [96, 97].
Since wastewaters and fermented streams usually contain NH 4
+
, and this compound
is a product of nitrogenase activity, nitrogenase synthesis is repressed and its
activity is inhibited [97]. Thus, N 2 fixation/H 2 production is no longer a viable
pathway for electron dissipation. Between CO 2 fixation and PHA accumulation, the
first is an ATP demanding process, while the later, does not require ATP and cells
can directly reoxidize reduced agents like NADH, by for example, reducing
acetoacetyl-CoA molecules to 3-hydroxybutyryl-CoA, a precursor of PHB [82].
Both pathways can be used in parallel by the cells to dissipate electrons, but the
more or less utilization of each one is dependent on the amount of ATP available,
and on the type and redox state of the organic acids present in the feedstock. Cells
are more likely to use the PHA synthesis pathway when they are fed with molecules
that are easily converted to this polymer, like acetate and propionate [98], but will
also resort to CO 2 fixation to dispose excess electrons if the organic acid is more
reduced than the cell material (e.g., butyrate and valerate) [97]. These particularities
of PHA accumulation in PPB are very important when designing strategies to
promote PHA accumulation and choosing feedstocks, especially in mixed microbial
cultures systems. The dual function of PHA that is stored as carbon and reducing
power enables the implementation of selection strategies that specifically target
each function, as detailed in Sect. 5.1.
5.1 Phototrophic Mixed Culture PHA Production:
From Laboratory to Demo PPBPonds
As in other phototrophic processes, the main purpose of approaching mixed culture
PHA production with phototrophic organisms is (i) to eliminate the high costs of
intensive aeration observed in aerobic systems, aiming a more cost-effective PHA
production and (ii) to promote a high carbon conversion efficiency to biomass and
PHA, minimizing substrate losses as CO 2 . While numerous studies on the PHA
production capability of PPB as single strains have been reported (reviewed in [26,
95]), to the best of our knowledge, there are only a few studies on PHA production
with phototrophic mixed cultures (PMC).
Studies with PMC started by targeting PPB capability of storing PHA as carbon
reserves. By following the feast and famine (FF) strategy typically applied to aerobic
mixed cultures, [99] proposed the combination of PPB with microalgae, in a consortia
where microalgae would be the oxygen providers. Using a synthetic acetate feed and
under continuous illumination (halogen lamp), sequential FF cycles led to the
enrichment of a PMC (Fig. 5) where PPB stored PHB during the feast phase and
consumed it during the famine phase, using the oxygen produced by microalgae [99].
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of the consumed carbon and nitrogen sources [96]. In the particular case of using
PPB for treating wastewaters or valorizing fermented agro-wastes, the uptake of
reduced carbon sources (e.g., organic acids), present in the waste stream, requires
electron dissipation which is possible through PHA accumulation, CO 2 fixation via
CBB cycle or through N 2 fixation/H 2 production via nitrogenase enzyme [96, 97].
Since wastewaters and fermented streams usually contain NH 4
+
, and this compound
is a product of nitrogenase activity, nitrogenase synthesis is repressed and its
activity is inhibited [97]. Thus, N 2 fixation/H 2 production is no longer a viable
pathway for electron dissipation. Between CO 2 fixation and PHA accumulation, the
first is an ATP demanding process, while the later, does not require ATP and cells
can directly reoxidize reduced agents like NADH, by for example, reducing
acetoacetyl-CoA molecules to 3-hydroxybutyryl-CoA, a precursor of PHB [82].
Both pathways can be used in parallel by the cells to dissipate electrons, but the
more or less utilization of each one is dependent on the amount of ATP available,
and on the type and redox state of the organic acids present in the feedstock. Cells
are more likely to use the PHA synthesis pathway when they are fed with molecules
that are easily converted to this polymer, like acetate and propionate [98], but will
also resort to CO 2 fixation to dispose excess electrons if the organic acid is more
reduced than the cell material (e.g., butyrate and valerate) [97]. These particularities
of PHA accumulation in PPB are very important when designing strategies to
promote PHA accumulation and choosing feedstocks, especially in mixed microbial
cultures systems. The dual function of PHA that is stored as carbon and reducing
power enables the implementation of selection strategies that specifically target
each function, as detailed in Sect. 5.1.
5.1 Phototrophic Mixed Culture PHA Production:
From Laboratory to Demo PPBPonds
As in other phototrophic processes, the main purpose of approaching mixed culture
PHA production with phototrophic organisms is (i) to eliminate the high costs of
intensive aeration observed in aerobic systems, aiming a more cost-effective PHA
production and (ii) to promote a high carbon conversion efficiency to biomass and
PHA, minimizing substrate losses as CO 2 . While numerous studies on the PHA
production capability of PPB as single strains have been reported (reviewed in [26,
95]), to the best of our knowledge, there are only a few studies on PHA production
with phototrophic mixed cultures (PMC).
Studies with PMC started by targeting PPB capability of storing PHA as carbon
reserves. By following the feast and famine (FF) strategy typically applied to aerobic
mixed cultures, [99] proposed the combination of PPB with microalgae, in a consortia
where microalgae would be the oxygen providers. Using a synthetic acetate feed and
under continuous illumination (halogen lamp), sequential FF cycles led to the
enrichment of a PMC (Fig. 5) where PPB stored PHB during the feast phase and
consumed it during the famine phase, using the oxygen produced by microalgae [99].
430
J. C. Fradinho et al.