carbohydrates, nucleic acids, vitamins and lipids (Richmond 2004). Microalgae
have inorganic carbon assimilation processes: diffusion (5.0 < pH < 7.0) and active
transport (pH > 7.0) (Gonçalves et al. 2017). In order to achieve high autotrophic
production rates, CO 2 and bicarbonates (HCO 3
− ) supply is the most important
(Richmond 2004). For certain species of microalgae that grow in mixotrophic
conditions, organic compounds (e.g. sugars, acids and alcohols) can be used as
carbon source.
Nitrogen has also an important role, since it is a basic element for the formation
of proteins, nucleic acids, vitamins and photosynthetic pigments (Richmond 2004).
The assimilation mechanism of nitrate and ammonium (NH 4
+
) by microalgae is
active transport (Gonçalves et al. 2017). Nitrogen is mainly provided in the form of
nitrate (NO 3
− ), but sometimes ammonium (NH 4
+ ) and urea can also be used
(Richmond 2004). Silva et al. (2015) evaluated the preferred source of nitrogen
(NO 3
− and NH 4
+
) for two species of microalgae (Chlorella vulgaris and
Pseudokirchneriella subcapitata). The authors concluded that the ammonium was
preferred source of nitrogen for microalgae C. vulgaris, since its assimilation by the
microalgae involves lower energy consumption (Jia et al. 2016). When the
microalgae are limited by nitrogen a discoloration of the cells usually occurs (reduction of chlorophylls and carotenoids increase) and a build-up of organic compounds such as polysaccharides and some oils (Becker 1994). Goiris et al. (2015)
studied the impact of nutrient limitation in the production of antioxidants in three
species of microalgae (Phaeodactylum tricornutum, Tetraselmis suecica and C.
vulgaris). The content of chlorophyll a in biomass was significantly lower when the
microalgae were limited by nitrogen.
Phosphorus is essential nutrient for growth and for many cellular metabolic
activities, such as energy transfer, synthesis of nucleic acids, deoxyribonucleic acid
(DNA), among others (Richmond 2004). Similarly to nitrogen, phosphorus is also
assimilated by the microalgae through active transport (Gonçalves et al. 2017). This
chemical element is preferentially added in the form of orthophosphate (PO 4
3− ),
and its absorption is energy dependent (Richmond 2004). The supply of phosphorus
also influences the composition of the produced biomass (Borowitzka 1988). The
content of lipids and carbohydrates is especially affected by internal and external
phosphorus supply. The N:P ratio in the culture medium is also important, as it
influences not only the productivity, but also the dominant species in culture
(Richmond 2004). In 1934, Alfred C. Redfield estimated the N:P ratio of 16:1
(known as Redfield ratio) through the elemental composition of microalgal cells.
However, several studies have tested different ratios (Martin et al. 1987; Minster
and Boulahdid 1987; Shaffer et al. 1999; Takahashi et al. 1985). Silva et al. (2015)
evaluated the effect of N:P ratio on the growth of microalgae C. vulgaris and
P. subcapitata. The N:P ratios of 8:1, 16:1 and 24:1 were evaluated. The N:P ratio
of 8:1 was the one that more favoured the growth of microalgae C. vulgaris.
In addition to C, N and P, other nutrients are also important for cell growth, such
as the sulphur (S), potassium (K), sodium (Na), iron (Fe), magnesium (Mg) and
calcium (Ca) (Richmond 2004). In addition to these, other trace elements
38
A. P. de Carvalho Lopes et al.
have inorganic carbon assimilation processes: diffusion (5.0 < pH < 7.0) and active
transport (pH > 7.0) (Gonçalves et al. 2017). In order to achieve high autotrophic
production rates, CO 2 and bicarbonates (HCO 3
− ) supply is the most important
(Richmond 2004). For certain species of microalgae that grow in mixotrophic
conditions, organic compounds (e.g. sugars, acids and alcohols) can be used as
carbon source.
Nitrogen has also an important role, since it is a basic element for the formation
of proteins, nucleic acids, vitamins and photosynthetic pigments (Richmond 2004).
The assimilation mechanism of nitrate and ammonium (NH 4
+
) by microalgae is
active transport (Gonçalves et al. 2017). Nitrogen is mainly provided in the form of
nitrate (NO 3
− ), but sometimes ammonium (NH 4
+ ) and urea can also be used
(Richmond 2004). Silva et al. (2015) evaluated the preferred source of nitrogen
(NO 3
− and NH 4
+
) for two species of microalgae (Chlorella vulgaris and
Pseudokirchneriella subcapitata). The authors concluded that the ammonium was
preferred source of nitrogen for microalgae C. vulgaris, since its assimilation by the
microalgae involves lower energy consumption (Jia et al. 2016). When the
microalgae are limited by nitrogen a discoloration of the cells usually occurs (reduction of chlorophylls and carotenoids increase) and a build-up of organic compounds such as polysaccharides and some oils (Becker 1994). Goiris et al. (2015)
studied the impact of nutrient limitation in the production of antioxidants in three
species of microalgae (Phaeodactylum tricornutum, Tetraselmis suecica and C.
vulgaris). The content of chlorophyll a in biomass was significantly lower when the
microalgae were limited by nitrogen.
Phosphorus is essential nutrient for growth and for many cellular metabolic
activities, such as energy transfer, synthesis of nucleic acids, deoxyribonucleic acid
(DNA), among others (Richmond 2004). Similarly to nitrogen, phosphorus is also
assimilated by the microalgae through active transport (Gonçalves et al. 2017). This
chemical element is preferentially added in the form of orthophosphate (PO 4
3− ),
and its absorption is energy dependent (Richmond 2004). The supply of phosphorus
also influences the composition of the produced biomass (Borowitzka 1988). The
content of lipids and carbohydrates is especially affected by internal and external
phosphorus supply. The N:P ratio in the culture medium is also important, as it
influences not only the productivity, but also the dominant species in culture
(Richmond 2004). In 1934, Alfred C. Redfield estimated the N:P ratio of 16:1
(known as Redfield ratio) through the elemental composition of microalgal cells.
However, several studies have tested different ratios (Martin et al. 1987; Minster
and Boulahdid 1987; Shaffer et al. 1999; Takahashi et al. 1985). Silva et al. (2015)
evaluated the effect of N:P ratio on the growth of microalgae C. vulgaris and
P. subcapitata. The N:P ratios of 8:1, 16:1 and 24:1 were evaluated. The N:P ratio
of 8:1 was the one that more favoured the growth of microalgae C. vulgaris.
In addition to C, N and P, other nutrients are also important for cell growth, such
as the sulphur (S), potassium (K), sodium (Na), iron (Fe), magnesium (Mg) and
calcium (Ca) (Richmond 2004). In addition to these, other trace elements
38
A. P. de Carvalho Lopes et al.