Page 49 - Refining Biomass Residues for Sustainable Energy and Bioproducts

28 Refining Biomass Residues for Sustainable Energy and Bioproducts

groups of fatty acids and their derivatives were produced efficiently using substrates
such as alkanoic acids and alkanes. Nevertheless, their high toxicity and lower misci-
bility, along with the shooting market prices, make these processes noneconomical
and challenging (Ro ¨ttig and Steinbu ¨chel, 2016). The major challenge in the produc-
tion of lipids-derived fuels from microbes is carbon source as it contributes up to
85% of the overall production cost, making the production process expensive (Kumar
and Thakur, 2018). Therefore the most desirable step to achieve a cost-effective pro-
duction requires the use of inexpensive carbon or nitrogen sources from municipal,
agricultural, or industrial waste and excess available materials, such as hydrolyzed
plant biomass, molasses, crude glycerol from biodiesel industry, whey from cheese
industry, and sludge from wastewater treatment plant. (Rude and Schirmer, 2009;
Ro ¨ttig et al., 2010; Sun et al., 2007; Kumar et al., 2018a,c). Lignocellulosic biomass
obtained from agriculture, industry, and forest is the largest and economically viable
source of sugars with roughly 4.15 billion tons of agricultural waste production annu-
ally (Ro ¨ttig and Steinbu ¨chel, 2016). The utilization of lignocellulosic biomass for the
production of lipids not only makes the process cost effective, but it also reduces the
environmental burden. Nevertheless, it is very difficult to extract fermentable sugar
from lignocellulosic biomass because of its complex and stable structure, and it is
also an energy- and time-consuming process (Kumar et al., 2016b, 2018c). Hence, a
better strategy would be to employ bacterial strains that may break the hemicellulosic
component of lignocellulosic biomass and proficiently use the released hexoses and
pentoses, such as L-arabinose, D-xylose, D-mannose, D-galactose, and D-glucose, to
produce “second-generation biofuels” (Himmel et al., 2007; Kim and Yun, 2006;
Stephanopoulos, 2007). Other emerging tactics to produce cost-effective microbial
lipids are recently reported from municipal sludge or carbon dioxide (CO 2 ), forming
the “third-generation biofuels” (Kumar et al., 2016a; Kumar and Thakur, 2018;
Bharti et al., 2014a,b). During wastewater treatment, specialized anaerobic microor-
ganisms accumulate lipid mixtures composed of TAG, wax esters (WE), or polyhy-
droxyalkanoate (PHA) (Kumar et al., 2018a). Oleaginous photosynthetic microbes
can proficiently utilize solar energy and fix CO 2 in the form of lipids. Microalgae
accumulate TAG under environmental hassle conditions, while some group of cyano-
bacteria store substantial amounts of fatty acids in their thylakoid membrane as dia-
cylglycerol, which might be genetically modified to enhance the production of free
fatty acids (FFA) (Liu et al., 2011).

2.3.1 Production of lipids and triacylglycerol from Gram-positive
bacteria
The Gram-positive bacteria belonging to the order Actinomycetales, such as
Arthrobacter, Dietzia, Gordonia, Nocardia, Rhodococcus,or Streptomyces sp.
(Table 2.1), have excellent capability to store substantial quantities of TAG as intra-
cellular storage material (Ro ¨ttig and Steinbu ¨chel, 2016). The bacterial strain R. opa-
cus PD630 is well known for its remarkable TAG accumulation ability that is up to
80% of its CDW. It is realized as a potential strain for industrial TAG production,
because of its fast growth, utilization of diverse range of carbon sources, and