Establishing Oleaginous Microalgae Research Models for Consolidated Bioprocessing of Solar Energy - Biotechnology in China: Biofuels and Bioenergy - page 71

Fig. 1 Consolidated Bioprocessing of Solar Energy (CBP-SE)

energy crisis, CO 2 emission and environmental concerns [ 3 ]. The current pilot-scale
algal oil production systems typically consist of identification and preservation of
microalgae feedstock, large-scale cultivation to produce algal biomass, and biomass
processing that typically includes harvesting, oil extraction and transesterification to
produce biodiesel. However, there are presently a number of major technological
bottlenecks, including low oil content of wild microalgae (usually less than 25%),
low productivity (usually less than 15 g/m 2 /day on average), difficulty in oil
extraction (high energy consumption when dehydrating biomass over 99%), low
efficiency in converting biomass energy to biodiesel (large amounts of biomass
residue are wasted), low quality of mature liquid fuel produced (of high oxygen
content or containing large amounts of long carbon-chain unsaturated fatty acids) [ 5 ].
Most of these challenges can be traced to the microalgal feedstock. It was
estimated that if the algal biomass productivity increased from 20 to 60 g/m 2 /day
and lipid content from 25 to 60%, algal fuel cost per gallon would fall from nearly
$10 to $2, a price level competitive with fossil fuels [ 5 ]. However, at present,
autotrophic growth rates of microalgae are generally less than 15 g/m 2 /day and
neutral lipid content less than 25%. In addition to oil productivity, stress tolerance
of the microalgal feedstock is of particular importance, which is reflected as the
ability to establish and maintain dominant culture at large-scale, typically outdoor
cultivation under abiotic and biotic environmental perturbation such as seasonal
and diurnal climate variation, water source fluctuation, and invasion by competi-
tors and predators. Furthermore, chemical parameters of the cellular products
synthesized by the microalgal feedstock, such as length of the carbon chain,
number of double bonds and content of unsaturated fatty acids, directly influence
the quality of biofuel products. For example, fatty acids with more than four
double bonds are vulnerable to oxidation and therefore represent an challenge for
storage [ 5 ]. The viability of algal biofuels industry is therefore dependent on the
availability of algal feedstock with much higher biomass productivity and lipid