FF conditions, and (3) PHA accumulation in a batch reactor using sludge from the
selected culture fed with the fermented effluent [88].
By applying this process to cultures fed with fermented agro-industrial wastes,
high PHA accumulation values in the order of 75% using sugar cane molasses [89]
and paper mill wastewater [90] have been achieved. Moreover, the accumulated
PHAs were co-polymers composed of hydroxybutyrate (HB) and hydroxyvalerate
(HV) monomers which confer better thermoplastic properties do the polymer [82].
Currently, many other fermented agro-industrial wastes have been successfully
tested as potential feedstock for PHA production, like waste activated sludge [86],
cheese whey [87], and waste mixtures as reviewed by [85]. While all of these
studies were conducted with aerobic organisms, de fact is that a wider diversity of
bacterial species can produce and accumulate PHA, including phototrophic bacteria, such as cyanobacteria and purple bacteria.
Cyanobacteria
PHA production in cyanobacteria has drawn great attention in recent years due to
their ability to accumulate PHB under photoautothrophic conditions with just some
requirements of simple inorganic nutrients (e.g., phosphate and nitrogen) and
micronutrients [91]. PHB production is usually stimulated by applying a deficiency
or limitation of nutrients, and thus far, several PHA accumulating cyanobacteria
species have been identified (e.g., Anabaena spp., Spirulina spp., Synechococcus
spp., Synechocystis spp., Nostoc spp., and Aulosira spp.) [91, 92]. Typically, their
PHA content under autotrophy is low, around 2–20% of cell dry weight (cdw) [91,
92]. Higher PHA contents up to 70–80% cdw have been reported, but they were
only achieved by multistage cultivation processes under heterotrophic conditions
with additives supplementation such as glucose, acetate, and valerate [92]. Currently, metabolic engineering is being used to improve cyanobacteria PHA accumulation capacity, monomer diversity, and more importantly, productivity [91, 92]
since the highest volumetric value reported thus far from CO 2 is in the order of
260 mg PHB L
−1 d
−1 obtained with a genetically modified Synechhocytis spp.[93].
However, this is still a recent field of research and the CO 2 fixation capacity of
cyanobacteria is an attractive characteristic with great potential for CO 2 recovery in
the form of PHA during off-gas streams treatment.
Purple phototrophic bacteria
If cyanobacteria are suited for photoautotrophic PHA production, then purple
bacteria are the choice for phototrophic PHA production from organic rich wastes.
As mentioned in previous sections, PPB are highly versatile organisms which
includes their capability to synthetize PHA, preferably from VFAs like acetate and
propionate. High PHA accumulation capacities up to 80% cdw have been reported
on VFAs, both for sulfur bacteria (e.g., Thiosystis spp., Ectothiorhodospira spp.,
Chromatium spp.) and non-sulfur purple bacteria (e.g., Rhodobacter spp.,
Rhodopseudomonas spp.) [94, 95]. It is important to stress that in PPB, PHA
granules are internally stored not only as a carbon reserves but also, very importantly, as a sink for reduced power. Since PPB thrive in illuminated anaerobic
environments, the cells’ redox homeostasis is maintained through activation of
New Phototrophic Factories for Resource Recovery
429
selected culture fed with the fermented effluent [88].
By applying this process to cultures fed with fermented agro-industrial wastes,
high PHA accumulation values in the order of 75% using sugar cane molasses [89]
and paper mill wastewater [90] have been achieved. Moreover, the accumulated
PHAs were co-polymers composed of hydroxybutyrate (HB) and hydroxyvalerate
(HV) monomers which confer better thermoplastic properties do the polymer [82].
Currently, many other fermented agro-industrial wastes have been successfully
tested as potential feedstock for PHA production, like waste activated sludge [86],
cheese whey [87], and waste mixtures as reviewed by [85]. While all of these
studies were conducted with aerobic organisms, de fact is that a wider diversity of
bacterial species can produce and accumulate PHA, including phototrophic bacteria, such as cyanobacteria and purple bacteria.
Cyanobacteria
PHA production in cyanobacteria has drawn great attention in recent years due to
their ability to accumulate PHB under photoautothrophic conditions with just some
requirements of simple inorganic nutrients (e.g., phosphate and nitrogen) and
micronutrients [91]. PHB production is usually stimulated by applying a deficiency
or limitation of nutrients, and thus far, several PHA accumulating cyanobacteria
species have been identified (e.g., Anabaena spp., Spirulina spp., Synechococcus
spp., Synechocystis spp., Nostoc spp., and Aulosira spp.) [91, 92]. Typically, their
PHA content under autotrophy is low, around 2–20% of cell dry weight (cdw) [91,
92]. Higher PHA contents up to 70–80% cdw have been reported, but they were
only achieved by multistage cultivation processes under heterotrophic conditions
with additives supplementation such as glucose, acetate, and valerate [92]. Currently, metabolic engineering is being used to improve cyanobacteria PHA accumulation capacity, monomer diversity, and more importantly, productivity [91, 92]
since the highest volumetric value reported thus far from CO 2 is in the order of
260 mg PHB L
−1 d
−1 obtained with a genetically modified Synechhocytis spp.[93].
However, this is still a recent field of research and the CO 2 fixation capacity of
cyanobacteria is an attractive characteristic with great potential for CO 2 recovery in
the form of PHA during off-gas streams treatment.
Purple phototrophic bacteria
If cyanobacteria are suited for photoautotrophic PHA production, then purple
bacteria are the choice for phototrophic PHA production from organic rich wastes.
As mentioned in previous sections, PPB are highly versatile organisms which
includes their capability to synthetize PHA, preferably from VFAs like acetate and
propionate. High PHA accumulation capacities up to 80% cdw have been reported
on VFAs, both for sulfur bacteria (e.g., Thiosystis spp., Ectothiorhodospira spp.,
Chromatium spp.) and non-sulfur purple bacteria (e.g., Rhodobacter spp.,
Rhodopseudomonas spp.) [94, 95]. It is important to stress that in PPB, PHA
granules are internally stored not only as a carbon reserves but also, very importantly, as a sink for reduced power. Since PPB thrive in illuminated anaerobic
environments, the cells’ redox homeostasis is maintained through activation of
New Phototrophic Factories for Resource Recovery
429