Page 63 - Advances in Eco-Fuels for a Sustainable Environment

40 Advances in Eco-Fuels for a Sustainable Environment

study of engineered microorganisms with the ability to produce fuels from feedstock
they could not use previously. Processes that previously required multiple feedstock
pretreatment steps are being consolidated into single-step microbial processes using
metabolically engineered species.
Compared to traditional random mutagenesis for strain improvement, metabolic
engineering is rapid, and very powerful: entire new pathways can be introduced into
microorganisms to enable production of biofuels and precursors of interest, or, alter-
natively, parts of specific biochemical pathways may be engineered to optimize
biofuel production. Diffuse commercialization of biofuels as true substitutes for their
fossil counterpart will require microbial cell factories with substantially superior
performance to the engineered strains currently available. Genetically engineered
Clostridia species already showed high productivity of butanol from the ABE fermen-
tation process.
Biobutanol production from starch-based microalgae feedstock could be a simpler
process than that of bioethanol production because Clostridia species are sac-
charolytic. The steps for starch liquefaction by amylase and saccharification of dex-
trins by glucoamylase required by bioethanol fermentation with Saccharomyces are
no longer needed for biobutanol production. Efremenko et al. [48] examined the effi-
ciency of ABE fermentation from various microalgal biomasses by fermentation with
poly(vinyl alcohol)-immobilized C. acetobutylicum cells. The highest biobutanol
yields were obtained when using thermolysis pretreated Arthrospira platensis and
Nannochloropsis sp. Ellis et al. [49] investigated the feasibility of using microalgae
biomass cultivated with wastewater as a feedstock for ABE fermentation with Clos-
tridium saccharoperbutylacetonicum N1-4. They found that proper pretreatment and
enzymatic hydrolysis significantly improved the ABE yield while adding 1% glucose
also led to a 1.6-fold increase in total ABE production. In addition to the conversion of
starch components in microalgae for biobutanol production, the cellulose content in
microalgae can also be converted to biobutanol after appropriate hydrolysis processes.
Converting microalgal biomass that contains both starch and cellulose into biobutanol
via fermentation with Clostridia species is likely to be growing as a trend in biobutanol
production.

2.5 Biofuels ecological footprint

Notwithstanding their advantages, some concerns with biofuels are still stirring
debate on their widespread use. Biofuels (especially first- and second-generation)
run into land-use issues (fuel versus food in first-generation, and fuel versus forest
in both), and how growing feedstock on nonagricultural soil could actually lead to
worsening of the global carbon balance. There is, in fact, not enough land currently
in use to meet all the world fuel needs with biofuels. This means that forested areas
would need to be cleared for feedstock cultivation. If the land used to grow feedstock
is cleared of native vegetation, ecological damage is perpetrated in three ways: the
first is the destruction of local habitat (animal dwellings, microcosms, reducing the