9.5.1 Biodiesel Production with Microalgae
A. Environmental Stress Response and Lipid Accumulation
Most microalgae are autotrophs that grow by using CO 2 as a carbon source, performing photosynthesis when exposed to light in the presence of inorganic nutrients
(compounds containing nitrogen, phosphorus, potassium, etc.). Reports also indicate
that some of them are capable of growth under mixotrophic or heterotrophic conditions using organic nutrients, although these properties are not yet fully understood.
Some microalgae are reported to accumulate lipids in the form of oil droplets as
their cells are subjected to stress from large changes in their optimal growth
environment. They accumulate oil especially quickly when deprived of nitrogen,
which makes them useful when selecting strains that accumulate lipids. Other
microalgae accumulate large amounts of lipids on a continuous basis even under
ordinary culturing conditions, although these species tend to have relatively slow
growth rates.
Other potential sources of environmental stress besides nitrogen deprivation
include physical factors such as light quantity, temperature, and CO 2 concentration;
chemical factors owing to changes in pH, nutrients, and toxins; and biological
factors related to growth, symbiosis, and bacteria.
Microalgae in the natural world are understood to accumulate lipids as a
response to these environmental stresses, although the physiological relationship
between lipid accumulation and the molecular response mechanism toward environment stresses has not been identified.
Like the lipids contained in the higher plant seeds used for food or as raw
material for biodiesel, the lipids accumulated by known microalgae consist mainly
of triacylglycerols. Triacylglycerols are neutral lipids with a three fatty acid ester
linked to a single molecule of glycerol. As shown in Fig. 9.10, the length of the
fatty acid carbon chain synthesized in the cell and degree of unsaturation varies by
species and growth environment. Since increased efficiency of triacylglycerol
production may reduce biofuel production costs, control of the fatty acid carbon
number can result in fuel quality management and high value-added (Hu et al.
2008; Mata et al. 2010; Chen et al. 2011; Bulgariu 2015).
B. Microalgae Characteristics
Microalgae inhabit a broad range of environments. They can be understood as
inhabiting any place in which sunlight can be exploited, including polar and snowy
regions, hot springs, freshwater and seawater, soil, plant surfaces, building surfaces,
and deserts. Symbiotic algae are also present in the cells and bodies of animals such
as corals and jellyfish; some species have been found in caves, where they have
adapted to environments with very weak light. Not all of these have led to the
discovery of suitable strains for algae-based biofuel. This raises the question of
what characteristics a strain should have to make it appropriate for algae-based
biofuel production. At least three points must be considered:
332
9 Marine Bioenergy Production
A. Environmental Stress Response and Lipid Accumulation
Most microalgae are autotrophs that grow by using CO 2 as a carbon source, performing photosynthesis when exposed to light in the presence of inorganic nutrients
(compounds containing nitrogen, phosphorus, potassium, etc.). Reports also indicate
that some of them are capable of growth under mixotrophic or heterotrophic conditions using organic nutrients, although these properties are not yet fully understood.
Some microalgae are reported to accumulate lipids in the form of oil droplets as
their cells are subjected to stress from large changes in their optimal growth
environment. They accumulate oil especially quickly when deprived of nitrogen,
which makes them useful when selecting strains that accumulate lipids. Other
microalgae accumulate large amounts of lipids on a continuous basis even under
ordinary culturing conditions, although these species tend to have relatively slow
growth rates.
Other potential sources of environmental stress besides nitrogen deprivation
include physical factors such as light quantity, temperature, and CO 2 concentration;
chemical factors owing to changes in pH, nutrients, and toxins; and biological
factors related to growth, symbiosis, and bacteria.
Microalgae in the natural world are understood to accumulate lipids as a
response to these environmental stresses, although the physiological relationship
between lipid accumulation and the molecular response mechanism toward environment stresses has not been identified.
Like the lipids contained in the higher plant seeds used for food or as raw
material for biodiesel, the lipids accumulated by known microalgae consist mainly
of triacylglycerols. Triacylglycerols are neutral lipids with a three fatty acid ester
linked to a single molecule of glycerol. As shown in Fig. 9.10, the length of the
fatty acid carbon chain synthesized in the cell and degree of unsaturation varies by
species and growth environment. Since increased efficiency of triacylglycerol
production may reduce biofuel production costs, control of the fatty acid carbon
number can result in fuel quality management and high value-added (Hu et al.
2008; Mata et al. 2010; Chen et al. 2011; Bulgariu 2015).
B. Microalgae Characteristics
Microalgae inhabit a broad range of environments. They can be understood as
inhabiting any place in which sunlight can be exploited, including polar and snowy
regions, hot springs, freshwater and seawater, soil, plant surfaces, building surfaces,
and deserts. Symbiotic algae are also present in the cells and bodies of animals such
as corals and jellyfish; some species have been found in caves, where they have
adapted to environments with very weak light. Not all of these have led to the
discovery of suitable strains for algae-based biofuel. This raises the question of
what characteristics a strain should have to make it appropriate for algae-based
biofuel production. At least three points must be considered:
332
9 Marine Bioenergy Production