Page 185 - Lignocellulosic Biomass to Liquid Biofuels

150 Lignocellulosic Biomass to Liquid Biofuels

The pathway of triglyceride synthesis in microalgae consists of three
steps [150]:
1. Formation of acetyl-CoA in the cytoplasm—Acetyl-CoA is an initiator and
its formation takes place in the chloroplast, where it is formed as an
intermediate, glyceraldehyde phosphate (GAP), that is transferred to
the cytoplasm and subsequently consumed. After the export of GAP
from chloroplast to cytoplasm, the carbon source is directed to the
sugar synthesis (that represent the main storage products in the cyto-
plasm of plant cells) or oxidation through the glycolytic pathway to
pyruvate. Therefore a part of the exogenous glucose is directly con-
verted into starch and the rest is oxidized via glycolysis.
2. Elongation and desaturation of carbon chains of fatty acids—Elongation
depends mainly on the reaction of two enzyme systems that include
acetyl-CoA and requires the presence of malonyl-CoA.
3. Biosynthesis of triglycerides—Generally, L-α-phosphoglycerol and acetyl-
coA are two major primers.
So far, few works have been reported about the direct effects of ligno-
cellulosic hydrolysates on microalgae cultures (Table 4.5). Lignocellulosic
hydrolysates contain organic carbon, such as fermentable sugars, volatile
organic acids, and inhibitors, which should be considered to evaluate
microalgae cultivation growth.

4.7 Fermentation process
The fermentation processes carried out for lipid production need a high
C/N ratio, where nitrogen is not limiting. The residual carbon-to-
nitrogen ratio increases during the cultivation time, and the control of this
parameter is essential to prevent citric acid production instead of lipid
accumulation. Continuous reactors are almost always operated under
steady-state conditions, and the C/N ratio and the concentrations of C
and N can be kept constant at a given dilution rate. The optimization of
the process, therefore, involves determining the optimal dilution rate to
obtain an optimal intermediate C/N ratio.
In fed-batch culture, nitrogen and carbon flows have to be accurately
monitored to control the specific growth rate and residual C/N ratio
[157].
Aeration is another important factor, affecting yeast cell growth as well
as total lipid level. The amount of dissolved oxygen in liquid cultures can
also influence the fatty acids composition of cell lipids. In an aerated