Environmental Engineering Reference
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Figure 11.12. Effect of growth regime on lipid content. Dunaliella tertiolecta under (a) phototrophic and
(b) heterotrophic growth. Data from Tang et al. (2011). Nannochloropsis oculata under
conditions of (c) nitrogen sufficiency and (d) nitrogen deficiency. Data from Su et al. (2011).
dramatic, but similar compositional trends were reported for Chlorella protothecoides (Santos
et al. , 2011). The ideal oil would be in the C10-C14 range, which is closer to conventional jet, and
would require less energy for refining. With this in mind, the shift toward lower carbon number is
beneficial for the manufacture of jet-grade fuel, although the hydrocarbon chains are still longer
than desirable. Since the C16 components are fully saturated, they will be more difficult to iso-
merize (section 11.5.3) to bring down the freeze temperature to acceptable levels. Consequently,
this oil may still require energy-intensive hydrocracking to bring it to jet-range quality.
When under stress such as conditions of nutrient deficiency, algae do not waste energy on cell
division. Cells become larger, and lipid production increases, resulting in a net accumulation of
lipids. The Aquatic Species Program concluded that growing algae in a nutrient-deficient mode
was counterproductive since the gain in lipid production was more than offset by the decrease in
growth rate (Sheehan et al. , 1998). On the other hand, it could reduce production cost directly
since nutrients do not have to be supplied during the deprivation phase, and indirectly by reducing
the amount of biomass that must be processed. For example, one study found that after four
days of nitrogen deprivation in the lab, Dunaliella tertiolecta cells accumulated five times more
lipids than the control cells (Chen et al. , 2011). Similarly, Nannochloropsis oculata increased
its lipid content from 35.0
2.2% after four days of nitrogen deprivation (Su
et al. , 2011). The lipid composition also exhibited a shift toward saturated compounds at C18, as
shown in Figures 11.11(c) and (d). Many other studies have examined the utility of mixotrophic
growth, initially setting growth conditions for high biomass production, followed by several days
of nutrient deficiency to increase the lipid content; see, e.g., Ben-Amotz (1995), Levine et al .
(2010), and Xiong et al. (2010).
Another interesting concept is the growth of multi-species algal communities in open ponds.
Recent work studied the impact of species diversity using cultures of one to four algal strains with
six different species compositions that were drawn from a collection of 22 standard algal strains.
The algae represented all major algal classes, including chlorophytes, cyanobacteria, cryptomon-
ades, crysophytes and diatoms. In addition, eight samples of naturally occurring phytoplankton
±
1.2% to 44.5
±
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