Environmental Engineering Reference
In-Depth Information
host for incorporating the PET vector, “producing more than 1000 mM ethanol from hemicellulose
hydrolysate sugars” (Lu and Mosier 2008).
In addition to having improved microorganisms, incorporating these enzymes (namely cellu-
lases) directly into plants gives them a distinct advantage over microorganisms, namely, a lower
energy input than microbial production (Sticklen 2008). These hydrolytic enzymes are mainly
microbial in nature and require codon alterations to become suitable for plant expression. Also,
for the correct expression of these enzymes, proper protein folding is necessary. This problem can
be fixed if the enzymes accrue in subcellular components rather than in the cytosol. Extraction
from the plants will enable researchers to apply these enzymes as “part of the plant total soluble
protein (TSP)” (Sticklen 2008) to biomass for the conversion into sugars. This process is relatively
simple and can become incorporated into traditional cellulosic ethanol production. More research is
needed to increase the levels of production and the biological activity of the heterologous enzymes
(Sticklen 2008). An additional area of research is incorporating heat-induced enzymes into the
plant to help increase its biomass conversion efficiency. One such enzyme is the
Acidothermus
celluloyticus
cellulase E1 (Yuan et al. 2008).
Lignin plays an important role in structural support, so the crop does not lodge, making
harvesting difficult (Tew and Cobill 2008). Also, lignin is physically important in protecting against
pathogens and helps water transport through the xylem (Torney et al. 2007). Downregulating the
biosynthesis of lignin is an important part of reducing the amount of lignin present in plant cell
walls, thereby reducing the cost of pretreatment (Sticklen 2008; Yuan et al. 2008). Cinnamyl alcohol
dehydrogenase (CAD) downregulation in poplar resulted in improved lignin solubility in an alka-
line medium, leading to more efficient delignification (Abramson et al. 2010; Harfouche et al. 2011).
Having a cell wall more uniform in nature could also help in lowering the amount of pretreatment
needed. Another method is to redirect carbon flux from lignin biosynthesis to overall biomass built
up, helping to increase sugar release during enzymatic hydrolysis. This was seen in aspen when
4-coumarate CoA ligase (4CL) was downregulated .Although more research is needed in bioenergy
crops and the downregulation of lignin biosynthesis, this is an important alternative in reducing
pretreatment costs (Sticklen 2008).
Use of genetically modified (GM) feedstocks for the production of bioenergy has been com-
plicated by public acceptance of these crops. The public (environmental organizations, consumer
advocacy groups, and scientific community) scrutinizes these crops because of their concerns over
“health and environmental safety and socioeconomic considerations” (Chapotin and Wolt 2007).
Because biofuels are considered a greener technology and are intended to replace petroleum usage,
this technology could be held to a higher environmental standard. Concerns regarding the use of
genetic engineering have the potential to slow down the adoption of GM crops. General public
perception of GM crops has been affected based on incidents of agricultural biotechnology crops
such as the StarLink
™
corn, co-mingling of pharmaceutical plants and food crops, unapproved GM
rice, and
Bt
corn effects on monarch butterflies.
3.4
BIoenerGy croPs
3.4.1 c
orn
Corn is an important cereal grain and a staple food. Corn is more productive as a biofuel crop than
wheat because of its more efficient C
4
photosynthetic pathway compared with the C
3
pathway of
wheat. C
4
plants are generally more efficient users of water and nitrogen compared with C
3
plants
(Karp and Shield 2008). Corn is adapted to various soils and environmental conditions. Because
corn is a widely available starchy feedstock, it is the principal source of ethanol production in the
United States. Corn is in high demand as a food and feed, therefore criticism for its use as a biofuel
feedstock is mounting (Fargione et al. 2008; Stoeglehner and Narodoslawsky 2009; Ajanovic 2010).
One way to help reduce competition between food and fuel is the use of corn stover, the remaining
