Page 55 - Refining Biomass Residues for Sustainable Energy and Bioproducts

32 Refining Biomass Residues for Sustainable Energy and Bioproducts

(polyunsaturated fatty acid) having important application in medical or food industries.
Moreover, few other unfamiliar oleaginous Gram-negative bacterial strains have been
reported: a bacterial strain belonging to the genus Nitratireductor (α-proteobacterium),
which could be potentially applied in wastewater treatment, uses short-chain organic
acids as carbon source. It further produces a complex mixture of fatty acids having squa-
lene, TAG, and light oils such as methyl ester of 2-butenoic acid up to 70% of its CDW
(Okamura et al., 2016). In addition, a chemolithotrophic CO 2 concentrating Serratia sp.
ISTD04 produced approximately 67% fatty acid of its CDW and 466 mg extracellular
lipids/L of bacterial culture (Bharti et al., 2014a,b). Nevertheless, there are various reports
of fatty acids producing oleaginous Pseudomonas and Bacillus strains, but that need
some further validation as these bacterial species are well known for the accumulation of
PHAs instead of neutral lipids (De Andre `s et al., 1991; Patnayak and Sree, 2005; Morya
et al., 2018). The potency of these strains might not have been properly investigated at
the taxonomic level. Likewise, the synthesis and production of neutral lipids by
Pseudomonas aeruginosa 44T1 strain is not established in an independent investigation
(Alvarez and Steinbu ¨chel, 2002). The cyanobacterial group also appears distinctively, as
these microbes have not been reported to accumulate intracellular TAG. The published
genome of cyanobacteria did not have WS/DGAT gene. In spite of this, the photosyn-
thetic membrane of cyanobacteria acts as a large reservoir for the accumulation of mem-
brane lipids and diacylglycerols, which can further harness biodiesel production by
transesterification trailedbymethanoland catalysts(Modiri et al., 2015; Liu et al., 2011).
Some cyanobacterial species such as Cyanobacterium aponinum or Synechococcus sp.
are able to produce lipids more than 40% of their CDW using CO 2 as asolecarbon
source (Table 2.2), but owing to the complexity of these produced lipids, hydrolysis is
required to extract fatty acids. Biodiesel production requires bulk amount of lipids as raw
material similar to this context; robust bacteria strains belonging to the Actinomycetes
group, such as Rhodococcus or Streptomyces sp., which efficiently utilizes cheap carbon
source for the production of large quantity of TAG, prove useful. Although WE produc-
ing Gram-negative bacterial strains generally do not attain higher lipid contents and need
more expensive and simple carbon sources, still a number of WEs with appropriate
chemical compositions can be synthesized for further use as high-value products, such as
cosmetics. For enhancing the production of lipids, the potential bacterial strains from nat-
ural environments need to be isolated and screened. Genetic engineering becomes a
potential tool and can be applied to optimize the selection of carbon sources and produc-
tion of fatty acids by these strains. In addition, Escherichia coli, which is a nonoleaginous
bacteria, can be modified into a potential strain by applying genetic engineering to
enhance the production of FFA, TAG, or fatty acid ethyl ester (FAEE) (Ro ¨ttig and
Steinbu ¨chel, 2016).

2.3.3 Production of free fatty acid and triacylglycerol by
genetically modified bacteria
Synthesis and production of FFA by naturally isolated bacterial strains is rare with
few exceptional studies (mentioned earlier). Overproduction and intracellular accu-
mulation of FFAs is generally not obtained in natural bacterial strains, due to the
cytotoxic nature of FFA as compared to TAG or WE (Lennen and Pfleger, 2012).