Lipid production and characterization in waste water
grown algae
The fatty acid composition of raw material greatly influences biodiesel characteristics
such as cetane number, cold-flow properties, viscosity and oxidative stability.
Especially in cold climates, the fuel’s cold-flow properties should be noted. Saturated
and mono-unsaturated fatty acids with 14–18 carbon atoms are most suitable
for biodiesel production. Saturated fatty acids with long carbon chain have
unsatisfactory cold-flow properties, but inversely cetane number increases with
increasing chain length and decreasing saturation (Stansell et al. 2012). Fatty
acids with several double bonds have poor oxidative stability and the proportion of
these fatty acids should be low (Stansell et al. 2012).
In our research E. gracilis and Selenastrum sp. had a higher capability to produce
lipids suitable for biodiesel production than C. pyrenoidosa (Table 4). All
species demonstrated the highest lipid content when grown in organic press water
(diluted 10%). The total fatty acid content in E. gracilis was 14.5% of DW, whereas
in Selenastrum and C. pyrenoidosa the corresponding values were only 7.1% and
only 4.1%, respectively. A substantial proportion of fatty acids in Selenastrum sp.
in different waste water cultivations were saturated or monounsaturated with a
carbon chain length 16–18. The most abundant fatty acid in all Selenastrum cultivations
was oleic acid (18:1n9c) (23.6–36.8% of total FA:s). The amount of palmitic
acid (16:0) was also high (19.4–22.1%). C. pyrenoidosa had a high proportion of
palmitic acid (20.9–32.2%), but the amount of oleic acid was low (2.6–8.0%). Instead,
C. pyrenoidosa had a significant amount of rumenic acid (18:2n6c) (16.5–
23.5%). In press water, in E. gracilis over half of the total fatty acids were myristic
acid (14:0), but in other waste waters the proportion of this fatty acid in E. gracilis
was highly variable (5.6–35.5%). The proportion of alphalinolenic acid (18:3n3)
which is the precursor for other polyunsaturated fatty acids (PUFAs), was generally
higher in Selenastrum and C. pyrenoidosa strains than in E. gracilis (9.9–33.5,
16.9–36.5 and 3.3–20.3) but E. gracilis had a distinct profile, with relatively high
amounts of polyunsaturated fatty acids with more than three double bonds.
According to our results Selenastrum sp. had a more suitable fatty acid profile
for biodiesel than E. gracilis or C. pyrenoidosa, since it had a higher proportion of
monounsaturated fatty acids and lower proportion of polyunsaturated fatty acids
(Table 4). The unsaturated fatty acids with several double bonds present in
3. Algal cultivation for lipid production in waste water
21
E. gracilis would not be optimal for biodiesel production, but are excellent in functional
food products, as also recognized by the Japanese company Euglena Co.
Ltd. (http://www.euglena.jp/english/.) Synthesis of polyunsaturated fatty acids with
more than 18 atoms of carbon is limited in humans and thus these must be obtained
from food.
Nitrogen limitation is known to enhance lipid production in algae. Since most of
the waste waters assessed here contained abundant nitrogen (Table 2), high
concentrations of lipid in the biomass were not expected. However, even though
the nitrogen content of fish farm water was low (Table 2), lipid accumulation in fish
farm water was not higher than in other waste waters. Other stress factors may
have contributed to differences in lipid production obtained in different conditions,
but the complexity of waste waters hampered identification of specific factors. In
general, the fatty acid profiles were more dependent on the species than on the
culture medium, but growth conditions had some effect on the relative amounts of
fatty acids in the different species.