Environmental Engineering Reference
In-Depth Information
2006). On the other hand, other species of algae such as
Dunaliella tertiolecta
,
Tetraselmis
sp.,
Nannochloris atomus
,
Biddulphia aurita
, and
Synedra ulna
have a reduced lipid content when
nutrient limited (Shifrin and Chisholm 1981; Siron et al. 1989; Reitan et al. 1994). In the green
alga
Chlorella vulgaris
, iron supplementation has also been shown to increase the neutral lipid
content (Liu et al. 2008). The addition of CO
2
or bicarbonate to the cultures may also increase the
lipid content of some (e.g., Muradyan et al. 2004; Chiu et al. 2009; Guihéneuf et al. 2009; Widjaja
et al. 2009) but not all species of algae (Raghavan et al. 2008). However, because algal biomass
productivity is generally stimulated by CO
2
addition, the lipid productivity may still be enhanced
even if the lipid content per cell is not. A more detailed treatment of the effects of nutrition and
environmental factors on algal lipid content and the fatty acid profile may be found in Borowitzka
(1988), Hu et al. (2008), and Harwood and Guschina (2009).
26.6 hydrocarBon Producers
The hydrocarbon content of most microalgae is quite low (Borowitzka 1988), with the marked
exception of the colonial green alga
Botryococcus braunii
, which is found in freshwater, brackish
lakes, and other water bodies in temperate and tropical zones and which has a very high content
of hydrocarbons and ether lipids (Metzger and Largeau 2005). There are three chemical “races” of
Botryococcus
: (1) the A-race that produces essentially n-alkadiene and triene hydrocarbons, odd-
carbon-numbered from C
23
to C
33
; (2) the B-race, which produces C
30
-C
37
triterpenoid hydrocar-
bons, the botryococcenes, and C
34
methylated squalenes; and (3) the L-race, which produces a
single tetraterpenoid hydrocarbon, lycopadiene. Hydrocarbon contents range from 0.4 to 61% of
dry weight for A-race strains, approximately 9-40% for B-race strains, and 0.1-8% for L-race
strains (Metzger and Largeau 2005). In the Berkeley strain of
B. braunii
, approximately 7% of the
botryococcenes, mainly C
30
and C
32
botryococcenes, are located in the cells, whereas the external
colonial matrix contains more than 99% of the C
33
and C
34
compounds as well as lower-chain-length
botryococcenes (Wolf et al. 1985).
B. braunii
also contains “normal” lipids and sterols (Metzger
and Largeau 1999).
Hydrocarbon productivity is greatest during the exponential growth phase and does not occur in
nitrogen- and phosphorus-deficient media (Largeau et al. 1980; Casadevall et al. 1985; Dayanandra
et al. 2007). The hydrocarbon productivity can be enhanced by bubbling the culture with CO
2
-
enriched air (Casadevall et al. 1985; Ranga Rao et al. 2007). A day/night cycle rather than continuous
light also seems to favor hydrocarbon production (Dayanandra et al. 2007). Trials of growing this
alga outdoors in tubular photobioreactors up to a volume of 200 L have been carried out (Gudin and
Chaumont 1983).
Botryococcus
also grows well on secondarily treated piggery wastewater (An et
al. 2003). Recovery of the hydrocarbons can be by solvent extraction (Metzger and Largeau 1999)
or by extraction using supercritical CO
2
(Mendes et al. 1994). Frenz et al. (1989a, 1989b) used a
novel process in which they extracted the hydrocarbons by a short contact of the wet biomass with
a nontoxic solvent such as hexane without reducing cell viability. This process recovered up to 70%
of the total hydrocarbons.
The other group of microalgae that produces significant amounts of hydrocarbons and long-chain
methyl and ethyl ketones (alkenones) and may have a high lipids content is the coccolithophorids
(Fernandez et al. 1994; Bell and Pond 1996). At least one species,
Pleurochrysis carterae
, grows
very well in outdoor raceway ponds (Moheimani and Borowitzka 2006, 2007) and is therefore also
of interest as a potential source of renewable fuel.
26.7 larGe-scale ProductIon oF mIcroalGae
Commercial-scale algae culture has been carried out all over the world for the last 30+ years.
The two largest commercial algae production plants are at Hutt Lagoon, Western Australia and
Whyalla, South Australia, growing the halophilic algae
Dunaliella salina
for the production of
