Page 80 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Microbial-derived natural bioproducts for a sustainable environment 55

(gas chromatography mass spectrometry), FTIR (Fourier-transform infrared spec-
troscopy), and NMR (nuclear magnetic resonance) analysis.
There are basically two types of PHA on the basis of the chain length—(1) SCL-
PHA (3 5 carbon short chain length), for example, poly(hydroxybutyrate) (PHB)
and (2) MCL-PHA (6 14 carbon medium chain length), for example, poly-b-
hydroxyvalerate (3) LCL-PHA (long chain length with 15 or more carbon atoms)
(Maheshwari et al., 2018). Copolymers, such as poly(3-hydroxybutyrate-co-3-
hydroxyvalerate), also exist. Each type has its own specific properties, such as
durability, flexibility, and brittleness, which helps in deciding the usage sector and
promotes commercialization, a major research application. Efficient microbes accu-
mulate nearly 70% 90% of their biomass in the form of PHA as a storage material
(Madison and Huisman, 1999). A whole range of functions is served by PHA
including membrane consistency, ion balance and transport, nitrogen fixation, sig-
naling, etc. (Anderson and Dawes, 1990; Reddy et al., 2003). Commercially, it is
used in the regular plastic commodities, such as cosmetic bottles, utensils, and
product handles. It is a significant drug delivery and also helps in drug preparation
such as Truspot (antiglaucoma drug) owing to its depolymerization products.
Starting from its rudimentary actions till being a bioplastic, PHA presents an excel-
lent picture as a compound.
Table 3.1 describes the various possibilities of hydroxybutyrate (HB) and its
polymers along with describing their structure. Different possibilities include copo-
lymers with HA [P(3HB-3HV) copolymers] and PHB polymer blends [P(3HB) with
polyvinylalcohol]. Each variation comes with different changes in structure as
described in the following:

3.2.1.2 Factors governing polyhydroxyalkanoate production

Growth conditions are critical governing factors to decide the metabolism of PHA.
The following paragraph discusses the parameters controlling the fate of PHA.

1. Concentration of nutrients: According to Macrae and Wilkinson’s observation, there is a
direct proportionality between carbon to nitrogen (C:N) ratio and PHA accumulation. It
was realized that nutrient limitations (nitrogen, oxygen, phosphate, etc.) in microbes lead
to growth imbalance which restricts the processes, such as cell division, but the carbon
source is sufficient enough to synthesize storage materials, such as PHA. This helped us
in deducing its physiological role; however, there are also examples of PHA production
under no nutrient deficiency conditions.
2. Substrate and other parameters: Different rich carbon sources are available to promote
the production of PHA while with the advancing issues, waste products are favored as a
better substrate. Wastes, such as contaminated water, sludge, and landfill-enriched micro-
bial culture, are used. The microbial growth is influenced under different conditions (tem-
perature, salt concentration, C:N ratio, etc.) leading to different amount of production
which is higher in case of a stressed environment causing cell protection.
3. Metabolic growth of microbes: It is observed that microbes usually synthesize such biopo-
lymers during their late exponential or initial stationary growth period. However, fluctuat-
ing results are obtained owing to various influential parameters of growth rate.