Page 46 - Refining Biomass Residues for Sustainable Energy and Bioproducts

26 Refining Biomass Residues for Sustainable Energy and Bioproducts

faster growth using simple carbon source derived from corn, sugarcane, and cellulosic
biomass (Meng et al., 2009; Liang et al., 2012; Gao et al., 2014). Various species of
yeast, such as Rhodosporidium, Rhodotorula, Lipomyces,and Cryptococcus, store intra-
cellular lipids in the form of TAGs up to 70% of their cell dry weight (CDW) (Li et al.,
2007). Oleaginous bacterial strains, such as Rhodococcus opacus (Alvarez and
Steinbu ¨chel, 2002), Arthrobacter sp. (Meng et al., 2009), and Acinetobacter calcoaceti-
cus (Alvarez et al., 1997b), are able to store the components of fatty acids up to 87% of
their CDW and generate large biomass in short span of time due to high growth rates.
Bacteria are able to synthesize and accumulate fatty acids and its derivatives, which
also act as precursor molecules in the synthesis of their own cell envelopes (Moazami
et al., 2011). Similar to plants, bacteria synthesize fatty acids and their derivatives with
the help of acetyl-CoA using ATP (adenosine triphospate) as an energy source and
NADPH as a source of reducing equivalents. Bacterial synthesis of fatty acids is cata-
lyzed by acetyl-CoA carboxylase in ATP-dependent manner through the biosynthesis
of malonyl-CoA from acetyl-CoA and bicarbonate. Multisubunit fatty acid synthase
synthesizes fatty acyl ACPs (acyl carrier proteins) by using malonyl-CoA, and further
this fatty acyl moiety is finally transferred to glycerol derivatives (or similar alcohols
derivatives) with the help of enzyme glycerol-3-phosphate acyl transferase and forms
fatty acids (Lu et al., 2008; Rottig and Steinbu ¨chel, 2013).
Production of alkanes directly from fatty acid derivatives is reported previously
(Schirmer et al., 2010) by the decarbonylation of fatty aldehyde (Fig. 2.3). The bio-
synthesis pathway involves the conversion of fatty acid metabolic intermediates
into hydrocarbons (alkanes and alkenes) by collective efforts of an aldehyde decar-
bonylase and an acyl ACP. The biosynthesis of microbial hydrocarbons consider-
ably depends on growth media condition adopted by the microbes to regulate their
physiology (Bharti et al., 2014a,b). The mechanism for the biosynthesis of hydro-
carbon is diverse and varies from one microbe to another. Similarly the contents of
hydrocarbon are different among diverse sets of microbes and plants. The biosyn-
thesis and production of mixture of hydrocarbon ranging from C 13 to C 17 is previ-
ously described in photoautotrophic bacteria (Kumar et al., 2017b). The production
of an ideal fuel is highly dependent upon the molecular weight and range of the
involved hydrocarbons. The genus Clostridium that belongs to Gram-positive bacte-
ria shows the synthesis and storage of hydrocarbon ranging from C 11 to C 35 with
the majority of middle-chain n-alkanes (C 18 C 27 ) and higher range n-alkanes
(C 25 C 35 )(Ladygina et al., 2006). Vibrio furnissii bacterium belongs to the cate-
gory of halotolerance microbes, which synthesizes and produces intra- and extracel-
lular hydrocarbons ranging between C 15 and C 24 having similar properties to
kerosene and light oil (Park et al., 2005).

2.3 Fatty acid production using different carbon sources

The large-scale bacterial production of fatty acids and their derivatives using
sucrose or starch-derived sugars is not viable commercially and ethically. Diverse