Page 502 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Marine biomass toward biofuel production 459

The “pdc and adh” genes from Z. mobilis are inserted into B. subtilis which elimi-
nated butanediol synthesis and inserting E. coli udhA transhydrogenase in “alsS”
locus. This recombinant B. subtilis strain possesses the ability to produce ethanol as
a sole product (Romero et al., 2007).

20.3.5 Yeast

Yeast, a single-celled eukaryotic microorganism, is currently the leading microor-
ganism that is highly employed for fuel production (Hahn-H¨ agerdal et al., 2006).
Yeast is generally grown slightly in acidic condition and used for the production of
food. At present, the application of yeast in biofuel production by fermentation has
the greatest demand due to their economic sustainability and environment-friendly
nature. The sugars present in biomass can be converted into ethanol by breaking
down of sugars by amylase enzyme and then allow it to ferment to produce ethanol.
Most commonly, coculturing of yeast and recombinant yeast were reported to be
very effective in utilizing both xylose and glucose to increase the bioethanol yield
(Fu et al., 2009). But, the problem in coculturing is inhibition of one yeast by the
other. Saccharomyces cerevisiae has the special compatibility to grow along with
other yeast strains. It is well known for bioethanol production in spite of toxic che-
micals present in biomass. S. cerevisiae is grown in optimal condition to give the
maximum yield. Very few yeast strains, such as, Pachysolen tannophilus, Candida
shehatae, and Pichia stipitis, were reported to have the ability to uptake both C 5
and C 6 sugars (Agbogbo and Coward-Kelly, 2008). When the biomass is subjected
to pretreatment by biological or chemical method, it is reported to improve ethanol
yield. Enzymes play an important role in converting biomass into biofuels and
many other value-added by-products which is economically and environmental sus-
tainable. In addition, it has various applications in food, feed, and textile industries.
S. cerevisiae is grown along with other microbes to increase the production yield
and to reduce the cost and time. Xylose reductase gene from Scheffersomyces
stipites has been introduced into S. cerevisiae to increase the yield of ethanol
production (Runquist et al., 2010).

20.4 Challenges and future perspective

Producing biofuels from marine resource is more challenging because of the
biomass complexity and the problems during harvesting and transporting to the
centralized refinery section. It also includes the pretreatment methods, microbial
fermentation, and microbial contamination during the process. Reducing the cost of
biofuel is also a major challenge faced by the biorefinery (Balan, 2014). If this cost
issue is addressed, biofuels can be considered as "fuel of future" for transportation
in the next few years. New companies, new competencies, and job opportunities are
expected to emerge in upcoming years for the biorefineries. It also requires further
innovation in abovementioned aspects (De Jong and Jungmeier, 2015).