Page 43 - Refining Biomass Residues for Sustainable Energy and Bioproducts

24 Refining Biomass Residues for Sustainable Energy and Bioproducts

Owing to the recent development in the field of genomics and metabolomics
engineering, the biosynthetic pathways of microbes, such as bacteria, yeast, and
microalgae, can be effortlessly altered (Hollinshead et al., 2014; Xu et al., 2014;
Pfleger et al., 2015). In the recent few decades, biofuels having short carbon chain
such as bioethanol and butanol were successfully produced by engineered yeast
cells. However, due to lower energy efficiency and vapor pressure along with its
corrosive nature, the utilization of short carbon chain fuels is not widespread (Yan
and Liao, 2009). Thus research endeavors are progressively running toward fatty
acid derived advanced fuels, which are more suitable and compatible for industrial
applications. Fatty acid is the precursor material for an improved biofuel production
industry, because of its wide application in the production of different chain length
hydrocarbons (alkanes, alkenes), bioethanol, and fatty acids derived biodiesel
(Steen et al., 2010; Lennen and Pfleger, 2013). Therefore by employing a joint
strategy of natural and altered synthetic (fatty acid biosynthesis) pathways in bacte-
ria, the production of valuable chemicals and fuels is expected to be enhanced.
This chapter comprehensively illustrates the bacterial production of lipids and
TAG for the production of biodiesel by utilizing diverse range of carbon source and
its future prospects, opportunities, and challenges along with brief discussion of cat-
alytic transesterification process.

2.2 Fatty acid and hydrocarbon production

The application of the components of fatty acids at industrial level is expected to
increase as it has diverse potential and used as a substitute of fossil resources
(Ro ¨ttig and Steinbu ¨chel, 2016). At commercial and industrial level, incredible prog-
ress has been made so far regarding the use of oils and lipids as sustainable and
unconventional raw material to produce a multitude of elementary oleochemicals
for the synthesis of organic compounds, enzymes, or biotechnological products
(Biermann et al., 2011). The synthesis of lipids, its composition, and content varies
from one microbe to another, which depends upon the biochemical pathways fol-
lowed by the microbes. These synthesis pathways can be modified by changing the
culture conditions, such as temperature, pH, duration, carbon sources, and nutrient
availability (Tripathi et al., 2015). The nitrogen-deprived condition and excess car-
bon source in the growth media typically trigger the accumulation of lipids and
fatty acids in oleaginous microorganisms, but growth is repressed directing the car-
bon into TAG’s synthesis (Kumar et al., 2016d; Meng et al., 2009). Fig. 2.2 repre-
sents the bacterial synthesis of fatty acid and its derivatives. Microalgae, such as
Nannochloropsis sp., Chlorella sp., and Scenedesmus sp., are potential candidates
for microbial production of biofuel as they are able to accumulate high lipids intra-
cellularly and high growth rate as compared to other energy crops (Chisti, 2007;
Ratledge and Wynn, 2002). Even though photoautotrophic microalgae show higher
growth rate than energy crops, their growth rate is much slower than various hetero-
trophic oleaginous microbes. As compared to autotrophic microorganisms, hetero-
trophs such as yeasts and fungi are promising oleaginous microorganisms, which show