Environmental Engineering Reference
In-Depth Information
PHAs are benign materials from an environmental perspective for several reasons. First,
substrates necessary for bacterial growth of PHA-producing bacteria are available from
renewable resources, and second, PHAs are highly biodegradable, not only by their bacterial
producers, but also by numerous other aquatic and terrestrial microorganisms. Chowdhury
first isolated poly(3HB)-degrading bacteria in 1963 [19], and additional studies on the
isolation and characterization of PHA-degrading microorganisms began to appear in the
1990s [18]. Since that time, concern about the environmental impacts of plastic wastes has led
to expanded investigations into PHA-degrading microorganisms. At this point, a number of
extracellular PHA-degrading enzymes from various microorganisms have been purified and
characterized, and PHA-derived products appear to decompose readily in both composting
and activated sludge systems [20, 21].
3. State of the Science
3.1. Synthesis
Poly (3HB) is the most common biological polyester and is produced by numerous
microorganisms in nature [22]. From this basic starting point, however, three primary
approaches have been investigated to produce novel polymers with a wide range of
properties.
3.3.1. Feedstock manipulation
. Because the physical and thermal properties of PHA
polymers and copolymers can be regulated by varying their molecular structure and
copolymer compositions, the simplest metabolic engineering strategy is to provide specific
carbon sources to the microbes that bias the monomer production in favor of desired
compounds. Polymer composition can be further controlled by varying the feed ratio of
various substrates.
For example, a random copolymer of (R)-3HB and (R)-3 -hydroxyvalerate, poly(3HB-co-
3HV), has been produced in Ralstonia eutropha by feeding pentanoic acid and butyric acid as
the carbon sources [5]. The poly(3HB) homopolymer was produced from butyric acid, while
a poly(3HB-co-3HV) copolymer was produced from pentanoic acid. By varying the ratio of
pentanoic acid to butyric acid in the feed, variable composition copolymers were produced.
Similarly, using 3-hydroxypropionic acid as the substrate, R. eutropha produced a random
copolyester of (R)-3HB and 3-hydroxypropionate, poly(3HB-co-3HP) [23]. The same
copolymer can be produced by Alcaligenes latus [24]. Using olive oil as a substrate,
Aeromonas caviae produced a random copolymer of (R)-3HB and (R)-3-hydroxyhexanoate,
poly(3HB-co3HHx) [25]. Feeding 4-hydroxybutyric acid, 1,4-butanediol, or butyrolactone as
the carbon source produces a random copolyester of (R)-3HB and 4-hydroxybutyrate,
poly(3HB-co-4HB) when R. eutropha, [5] A. latus, [26] or Comamonas acidovorans [27] are
utilized. Recently, a number of unusual sulfur-containing polymers have been generated by
feeding alkylthionates to R. eutropha, and external substrate manipulation has even been used
to generate block copolymers by intermittent addition of one substrate. In the latter example,
R. eutropha was fed pulses of valerate-a precursor substrate of 3 -hydroxyvalerate-to
medium containing an excess of fructose-a precursor of PHB-to generate the block
copolymer [12].
PHAs with functional groups in the side chains can also be produced when functionalized
organic substrates are employed, allowing the engineering of specific physical properties and
