48 Marine Macro- and Microalgae: An Overview
Nutrients
Algae require about 30 different elements for growth (Kaplan et al. 1986). In this section, however, I will
only consider those which are known to influence growth and photosynthesis most. Ever since Redfield
(Redfield 1934, 1958) proposed the C:N:P ratio of 106:16:1 (known as the Redfield Ratio) as representing
the average atomic ratio of phytoplankton there has been discussion on how representative it is. It is clear
that there are significant variations from this ratio depending on growth conditions, between taxa and with
cell size (Falkowski 2000; Geider and Roche 2002; Ho et al. 2005), but ‘healthy’ phytoplankton cells
appear to have a composition close to this ratio. Quigg et al. (2003) extended the Redfield ratio to include
trace elements with the average composition of phytoplankton in this study found to be C 124 N 16 P 1 S 1.3 K 1.7
Mg 0.56 Ca 0.5 Fe 0.0075 Zn 0.0008 Cu 0.00038 Cd 0.00021 Co 0.00019 . This ratio serves as a good working approximation of the
nutrient requirements of microalgae and serves as a start for medium design and optimisation.
Nitrogen
As it is a key component of amino acids and proteins, nitrogen is critical for the production of biomass.
The nitrogen content of algal biomass ranges from about 1% to more than 10%, and although it varies
between taxa, the available nitrogen in the medium is a major determinant. Algae can utilize nitrate (NO 3
–
),
ammonia (NH 4
+
), urea, and a range of organic N sources. Although ammonium is generally the N-source
most rapidly utilized by microalgae, probably because of a lower energy demand for its metabolism, high
concentrations of ammonium are detrimental, if not lethal, to most algal species because of the speciation
of ammonia (NH 3 )/ammonium (NH 4
+
) in water which is temperature and pH dependent:
NH 3 (aq) + H 2 O ↔ NH 4
+
(aq) + OH
–
(aq) pK a (25ºC) = 9.25
Ammonia is toxic to algae (whereas ammonium is a valuable N-source) but one which may be
toxic at high concentrations (Abeliovich and Azov 1976). The mechanisms of this toxicity is not well
understood (Britto and Kronzucker 2002), but there is some variation in the sensitivity of different algae
to ammonia. Alkaline pH and high temperatures lead to the formation of ammonia rather than ammonium
(see equation above). High rates of algal photosynthesis lead to alkalinisation of the medium and can result
in high concentrations of ammonia; however uptake of ammonium by algae can lead to acidification of
the medium and either of these can lead to algal death (Borowitzka and Borowitzka 1988b). For example,
Levine (2011) reported toxic levels of NH 4
+
at 100 mg L
–1
for a culture of Neochloris oleoabundans. Other
species can acclimate to high levels of nutrients and can more efficiently utilize ammonium as a nitrogen
source (typically Chlorella and Scenedesmus sp. in wastewater treatment), but the exact limitations are
very specific to individual algal strains and culture conditions. No significant effect on the growth of
Chlorella sorokiniana was observed at 400 mg NH4 .L
–1
while Spirulina platensis was nearly completely
inhibited at 200 mg NH4 .L
–1
(Ogbonna et al. 2000). Gonzalez et al. (2008) reported successful culture of
a Chlorella sp. at concentrations as high as 373 mg.L
–1
but Przytocka-Jusiak et al. (1984) reported that
C. vulgaris cell growth was reduced by 50% at 330 mg.L
–1
ammonium, while 700 mg.L
–1
completely
inhibited growth at pH 8–9. Scenedesmus will grow at 150 mg NH4 .L
–1
but not at 300 mg NH4 .L
–1
(De Godos
et al. 2010). However, the effects of high ammonia/ammonium concentrations in large-scale cultures is
little understood as ammonium-N salts are not normally used as the N-source in large-scale algal cultures
and laboratory studies have paid little attention to the speciation of ammonia/ammonium and therefore
the data in the studies cited above must be interpreted with caution.
Control of pH is important when using ammonia. Gonzalez et al. (2008) was also able to demonstrate
that a much higher NH 4
+
load (~ 1100 mg L
–1
) could be tolerated by an enclosed Chlorella culture when
the pH was maintained at pH 7 where most of this N will be in the form of NH 4
+
. Tam and Wong (1996)
found very similar results with Chlorella able to maintain a specific growth rate of 0.2 d
–1
at ammonia
concentrations of 1000 mg.L
–1
when pH was maintained at neutral.
Uptake of nitrate is by an H
+
/nitrate co-transport and is stimulated by light and enhanced by CO 2
(Grant and Turner 1969; Eisele and Ullrich 1977; Tischner 2000). Although the uptake of nitrate leads to
alkalinisation of the medium, high nitrate concentrations are not toxic to algae.
Nutrients
Algae require about 30 different elements for growth (Kaplan et al. 1986). In this section, however, I will
only consider those which are known to influence growth and photosynthesis most. Ever since Redfield
(Redfield 1934, 1958) proposed the C:N:P ratio of 106:16:1 (known as the Redfield Ratio) as representing
the average atomic ratio of phytoplankton there has been discussion on how representative it is. It is clear
that there are significant variations from this ratio depending on growth conditions, between taxa and with
cell size (Falkowski 2000; Geider and Roche 2002; Ho et al. 2005), but ‘healthy’ phytoplankton cells
appear to have a composition close to this ratio. Quigg et al. (2003) extended the Redfield ratio to include
trace elements with the average composition of phytoplankton in this study found to be C 124 N 16 P 1 S 1.3 K 1.7
Mg 0.56 Ca 0.5 Fe 0.0075 Zn 0.0008 Cu 0.00038 Cd 0.00021 Co 0.00019 . This ratio serves as a good working approximation of the
nutrient requirements of microalgae and serves as a start for medium design and optimisation.
Nitrogen
As it is a key component of amino acids and proteins, nitrogen is critical for the production of biomass.
The nitrogen content of algal biomass ranges from about 1% to more than 10%, and although it varies
between taxa, the available nitrogen in the medium is a major determinant. Algae can utilize nitrate (NO 3
–
),
ammonia (NH 4
+
), urea, and a range of organic N sources. Although ammonium is generally the N-source
most rapidly utilized by microalgae, probably because of a lower energy demand for its metabolism, high
concentrations of ammonium are detrimental, if not lethal, to most algal species because of the speciation
of ammonia (NH 3 )/ammonium (NH 4
+
) in water which is temperature and pH dependent:
NH 3 (aq) + H 2 O ↔ NH 4
+
(aq) + OH
–
(aq) pK a (25ºC) = 9.25
Ammonia is toxic to algae (whereas ammonium is a valuable N-source) but one which may be
toxic at high concentrations (Abeliovich and Azov 1976). The mechanisms of this toxicity is not well
understood (Britto and Kronzucker 2002), but there is some variation in the sensitivity of different algae
to ammonia. Alkaline pH and high temperatures lead to the formation of ammonia rather than ammonium
(see equation above). High rates of algal photosynthesis lead to alkalinisation of the medium and can result
in high concentrations of ammonia; however uptake of ammonium by algae can lead to acidification of
the medium and either of these can lead to algal death (Borowitzka and Borowitzka 1988b). For example,
Levine (2011) reported toxic levels of NH 4
+
at 100 mg L
–1
for a culture of Neochloris oleoabundans. Other
species can acclimate to high levels of nutrients and can more efficiently utilize ammonium as a nitrogen
source (typically Chlorella and Scenedesmus sp. in wastewater treatment), but the exact limitations are
very specific to individual algal strains and culture conditions. No significant effect on the growth of
Chlorella sorokiniana was observed at 400 mg NH4 .L
–1
while Spirulina platensis was nearly completely
inhibited at 200 mg NH4 .L
–1
(Ogbonna et al. 2000). Gonzalez et al. (2008) reported successful culture of
a Chlorella sp. at concentrations as high as 373 mg.L
–1
but Przytocka-Jusiak et al. (1984) reported that
C. vulgaris cell growth was reduced by 50% at 330 mg.L
–1
ammonium, while 700 mg.L
–1
completely
inhibited growth at pH 8–9. Scenedesmus will grow at 150 mg NH4 .L
–1
but not at 300 mg NH4 .L
–1
(De Godos
et al. 2010). However, the effects of high ammonia/ammonium concentrations in large-scale cultures is
little understood as ammonium-N salts are not normally used as the N-source in large-scale algal cultures
and laboratory studies have paid little attention to the speciation of ammonia/ammonium and therefore
the data in the studies cited above must be interpreted with caution.
Control of pH is important when using ammonia. Gonzalez et al. (2008) was also able to demonstrate
that a much higher NH 4
+
load (~ 1100 mg L
–1
) could be tolerated by an enclosed Chlorella culture when
the pH was maintained at pH 7 where most of this N will be in the form of NH 4
+
. Tam and Wong (1996)
found very similar results with Chlorella able to maintain a specific growth rate of 0.2 d
–1
at ammonia
concentrations of 1000 mg.L
–1
when pH was maintained at neutral.
Uptake of nitrate is by an H
+
/nitrate co-transport and is stimulated by light and enhanced by CO 2
(Grant and Turner 1969; Eisele and Ullrich 1977; Tischner 2000). Although the uptake of nitrate leads to
alkalinisation of the medium, high nitrate concentrations are not toxic to algae.