Microalgal Metabolism and their Utilisation 49
Algae can also use urea as an N source. Algae either have a urease which catalyses the conversion of
urea to ammonium generating CO 2 in the process:
CO(NH 2 ) 2 + H 2 O → CO 2 + 2NH 3
or urea amidolyase which is a two enzyme, ATP requiring, system:
a urea carboxylase:
Urea + ATP + HCO 3
–
→ Allophanate + ADP + P i
and allophanate hydrolase: Allophanate + 3H 2 O + H
+
→ 2NH 4
+
+ 2HCO 3
–
Urea amidolyase is only found in green algae, and all other algae have urease (Leftley and Syrett
1973; Syrett 1988).
Further details on Nitrogen and microalgae can be found in (Raven and Giodano 2016).
Phosphorous
Phosphorous is essential for many cellular processes such as energy transfer, nucleic acid synthesis, etc.
(Dyhrman 2016). Orthophosphate (PO 4
2–
) is the preferred form for microalgae although many microalgae
have inducible extracellular phosphatases allowing the utilization of organic P sources (Burczyk and
Loos 1995). Many algae also accumulate excess P intracellularly mainly in the form of polyphosphates
bodies when an excess of P is available (Eixler et al. 2006). High concentrations of phosphate may inhibit
algal growth and there is great variation between species in their tolerance to high phosphate.
Silicon
Silicon is generally sufficiently abundant in natural waters so as not to be limiting to microalgae; however,
diatoms present a special case with a high Si requirement because of their silica valves and for most, Si
must be added to the growth medium. Insufficient Si can lead to cessation of DNA synthesis and cell
division (Darley and Volcani 1969; Sullivan and Volcani 1981) and, in some species such as the diatom
Skeletonema costatum, to cell death.
Metabolism and the production of some compounds of commercial interest
Microalgae are currently used for the commercial production of carotenoids, specifically β-carotene and
astaxanthin.
Dunaliella salina and β-carotene
The halophilic green alga, Dunaliella salina, accumulates up to 14% of dry weight as β-carotene in oil
droplets located in the chloroplast (Borowitzka and Borowitzka 1988a). The β-carotene in the globules is
mainly in the form of two stereoisomers: all-trans and 9-cis, with the rest a few other mono-cis and di-cis
stereoisomers of the β-carotene and no xanthophylls. Both the amount of the accumulated β-carotene and
the 9-cis to all-trans ratio depend on irradiance and on the algal division time, which is determined by
the growth conditions (Ben-Amotz et al. 1988). The synthesis of this β-carotene is mainly influenced by
salinity and irradiance. Salinity mainly regulates the maximum content of β-carotene possible, whereas
irradiance regulates the rate of β-carotene formation (Semenenko and Abdullayev 1980; Ben-Amotz and
Avron 1983; Borowitzka et al. 1990). Upon an upward shift in salinity D. salina rapidly accumulates
β-carotene, but if the salinity is reduced the β-carotene is only very slowly metabolised (Borowitzka et
al. 1985; Borowitzka et al. 1990). Nitrate limitation will also increase carotenoid accumulation (Mil’ko
1963; Ben-Amotz and Avron 1983) whereas P-limitation has little effect (Ben-Amotz et al. 1982). The
source of N used also affects carotenogenesis with ammonia inhibiting β-carotene synthesis (Borowitzka
and Borowitzka 1988b). In general, carotenogenesis is greatest under sub-optimal growth conditions
when the specific growth rate is low (Borowitzka et al. 1984); that is, β-carotene is a typical secondary
metabolite. The accumulation of β-carotene requires the concomitant synthesis of fatty acids (especially
Algae can also use urea as an N source. Algae either have a urease which catalyses the conversion of
urea to ammonium generating CO 2 in the process:
CO(NH 2 ) 2 + H 2 O → CO 2 + 2NH 3
or urea amidolyase which is a two enzyme, ATP requiring, system:
a urea carboxylase:
Urea + ATP + HCO 3
–
→ Allophanate + ADP + P i
and allophanate hydrolase: Allophanate + 3H 2 O + H
+
→ 2NH 4
+
+ 2HCO 3
–
Urea amidolyase is only found in green algae, and all other algae have urease (Leftley and Syrett
1973; Syrett 1988).
Further details on Nitrogen and microalgae can be found in (Raven and Giodano 2016).
Phosphorous
Phosphorous is essential for many cellular processes such as energy transfer, nucleic acid synthesis, etc.
(Dyhrman 2016). Orthophosphate (PO 4
2–
) is the preferred form for microalgae although many microalgae
have inducible extracellular phosphatases allowing the utilization of organic P sources (Burczyk and
Loos 1995). Many algae also accumulate excess P intracellularly mainly in the form of polyphosphates
bodies when an excess of P is available (Eixler et al. 2006). High concentrations of phosphate may inhibit
algal growth and there is great variation between species in their tolerance to high phosphate.
Silicon
Silicon is generally sufficiently abundant in natural waters so as not to be limiting to microalgae; however,
diatoms present a special case with a high Si requirement because of their silica valves and for most, Si
must be added to the growth medium. Insufficient Si can lead to cessation of DNA synthesis and cell
division (Darley and Volcani 1969; Sullivan and Volcani 1981) and, in some species such as the diatom
Skeletonema costatum, to cell death.
Metabolism and the production of some compounds of commercial interest
Microalgae are currently used for the commercial production of carotenoids, specifically β-carotene and
astaxanthin.
Dunaliella salina and β-carotene
The halophilic green alga, Dunaliella salina, accumulates up to 14% of dry weight as β-carotene in oil
droplets located in the chloroplast (Borowitzka and Borowitzka 1988a). The β-carotene in the globules is
mainly in the form of two stereoisomers: all-trans and 9-cis, with the rest a few other mono-cis and di-cis
stereoisomers of the β-carotene and no xanthophylls. Both the amount of the accumulated β-carotene and
the 9-cis to all-trans ratio depend on irradiance and on the algal division time, which is determined by
the growth conditions (Ben-Amotz et al. 1988). The synthesis of this β-carotene is mainly influenced by
salinity and irradiance. Salinity mainly regulates the maximum content of β-carotene possible, whereas
irradiance regulates the rate of β-carotene formation (Semenenko and Abdullayev 1980; Ben-Amotz and
Avron 1983; Borowitzka et al. 1990). Upon an upward shift in salinity D. salina rapidly accumulates
β-carotene, but if the salinity is reduced the β-carotene is only very slowly metabolised (Borowitzka et
al. 1985; Borowitzka et al. 1990). Nitrate limitation will also increase carotenoid accumulation (Mil’ko
1963; Ben-Amotz and Avron 1983) whereas P-limitation has little effect (Ben-Amotz et al. 1982). The
source of N used also affects carotenogenesis with ammonia inhibiting β-carotene synthesis (Borowitzka
and Borowitzka 1988b). In general, carotenogenesis is greatest under sub-optimal growth conditions
when the specific growth rate is low (Borowitzka et al. 1984); that is, β-carotene is a typical secondary
metabolite. The accumulation of β-carotene requires the concomitant synthesis of fatty acids (especially