54 Marine Macro- and Microalgae: An Overview
density cultures are required. However, not all cells in the culture receive the same amount of light in
high cell density cultures because of mutual shading (Tamiya 1957). Light passing through the culture
is absorbed by the cells so that it rapidly attenuates and cells deeper in the culture receive less or no
light. The higher the cell density, the shorter the depth that the light penetrates in the culture. Oswald
(1988) empirically found that the maximum depth that light penetrated in a pond of green algae was
6000/C c , where C c is the cell density in mg.L
–1
. The average amount of light, sometimes also called
the average irradiance, received by the cells in a well-mixed culture is a function of the cell density
(Myers and Graham 1958) and there is an optimum cell density (OCD) at which photosynthetic efficiency
and productivity are at a maximum (Richmond and Vonshak 1978; Hu and Richmond 1994; Hu et al.
1998). The optimum average irradiance is equivalent to the saturating irradiance (E s ) as measured in a
photosynthesis/irradiance curve using dilute algae suspensions. The OCD and light utilization efficiency,
and therefore also productivity, can potentially be improved by altering the photosynthetic antenna size in
the algae (Benemann 1989; Polle et al. 2002). This has been shown in the laboratory for several species
of algae (Nakajima and Ueda 1997; Melis et al. 1999; Nakajima and Ueda 1999, 2000) but whether such
algae can be practically grown in large-scale cultures is not yet known.
In order to accommodate the seasonal differences in irradiance, the average irradiance received by
the algae cells can be altered either by changing the cell density by varying the harvesting frequency and/
or the proportion of the culture harvested, or by changing the culture depth (in the case of pond cultures)
(Vonshak 1997).
However, the irradiance outdoors also changes over the day as does the culture temperature and the
oxygen concentration in the water. In the morning, outdoor cultures are often too cool to use light efficiently
(Vonshak et al. 2001), and by noon on sunny days, algae cultures are generally photoinhibited, even in
very dense cultures (Vonshak and Guy 1992; Hu et al. 1996; Torzillo et al. 1996; Sukenik et al. 2009). The
high oxygen concentration leads to photorespiration and the production of active oxygen species, such as
the superoxide radical (O 2
.
), the hydroxyl radical (
.
OH), hydrogen peroxide (H 2 O 2 ), and singlet oxygen
(
1
Δg O 2 ), which can cause photoinhibition of photosynthesis (Belay and Fogg 1978; Demmig-Adams and
Adams 1992; Singh et al. 1995; Cadenus 2005). Not only does photoinhibition reduce photosynthesis,
but the repair of the photodamage requires energy and thus has a metabolic cost to the alga (Raven 2011),
thus reducing productivity. Supraoptimal temperatures also increase photorespiration and the degree of
photodamage (Kromkamp et al. 2009) and this can lead to increased dark respiration and biomass loss at
night (Raven 1981; Torzillo et al. 1991; Raven 2011). Temperature and irradiance also interact and affect
the chemical composition of the algae (Payer et al. 1980; Carvalho et al. 2009).
Since different algae species and strains have different temperature optima, their response to high
irradiance will vary with temperature (Fig. 1). It is therefore important to select a strain which is best able
to utilize high light at the temperature range which will be encountered at the site of culture (Borowitzka
2013b) and maintain the culture cell density near the OCD. This means that the cell density needs to
be varied between summer and winter if operating at a constant pond depth. Alternatively, the average
irradiance received by the algae can also be adjusted by varying the pond depth while maintaining the
cell density constant. It is potentially possible to use different strains with different temperature optima
in summer and winter (Belay 1997).
High O 2 concentrations in open ponds and photobioreactors arising from algal photosynthesis are a
significant factor in limiting productivity (Ogawa et al. 1980; Kliphuis et al. 2011) and selection of algae
strains whose Rubisco is less sensitive to oxygen and/or strains which have a more efficient CCM and
thus an increased level of CO 2 at the site of Rubisco are highly desirable (Borowitzka 2013b).
Optimisation of growth and the cell content of specific products requires a good understanding of
algal physiology (Borowitzka 2016). Furthermore, scaling-up to the very large volumes required for
commercial production is not easy and is a major challenge for the transition from the laboratory to an
industrial process (Borowitzka and Vonshak 2017).
density cultures are required. However, not all cells in the culture receive the same amount of light in
high cell density cultures because of mutual shading (Tamiya 1957). Light passing through the culture
is absorbed by the cells so that it rapidly attenuates and cells deeper in the culture receive less or no
light. The higher the cell density, the shorter the depth that the light penetrates in the culture. Oswald
(1988) empirically found that the maximum depth that light penetrated in a pond of green algae was
6000/C c , where C c is the cell density in mg.L
–1
. The average amount of light, sometimes also called
the average irradiance, received by the cells in a well-mixed culture is a function of the cell density
(Myers and Graham 1958) and there is an optimum cell density (OCD) at which photosynthetic efficiency
and productivity are at a maximum (Richmond and Vonshak 1978; Hu and Richmond 1994; Hu et al.
1998). The optimum average irradiance is equivalent to the saturating irradiance (E s ) as measured in a
photosynthesis/irradiance curve using dilute algae suspensions. The OCD and light utilization efficiency,
and therefore also productivity, can potentially be improved by altering the photosynthetic antenna size in
the algae (Benemann 1989; Polle et al. 2002). This has been shown in the laboratory for several species
of algae (Nakajima and Ueda 1997; Melis et al. 1999; Nakajima and Ueda 1999, 2000) but whether such
algae can be practically grown in large-scale cultures is not yet known.
In order to accommodate the seasonal differences in irradiance, the average irradiance received by
the algae cells can be altered either by changing the cell density by varying the harvesting frequency and/
or the proportion of the culture harvested, or by changing the culture depth (in the case of pond cultures)
(Vonshak 1997).
However, the irradiance outdoors also changes over the day as does the culture temperature and the
oxygen concentration in the water. In the morning, outdoor cultures are often too cool to use light efficiently
(Vonshak et al. 2001), and by noon on sunny days, algae cultures are generally photoinhibited, even in
very dense cultures (Vonshak and Guy 1992; Hu et al. 1996; Torzillo et al. 1996; Sukenik et al. 2009). The
high oxygen concentration leads to photorespiration and the production of active oxygen species, such as
the superoxide radical (O 2
.
), the hydroxyl radical (
.
OH), hydrogen peroxide (H 2 O 2 ), and singlet oxygen
(
1
Δg O 2 ), which can cause photoinhibition of photosynthesis (Belay and Fogg 1978; Demmig-Adams and
Adams 1992; Singh et al. 1995; Cadenus 2005). Not only does photoinhibition reduce photosynthesis,
but the repair of the photodamage requires energy and thus has a metabolic cost to the alga (Raven 2011),
thus reducing productivity. Supraoptimal temperatures also increase photorespiration and the degree of
photodamage (Kromkamp et al. 2009) and this can lead to increased dark respiration and biomass loss at
night (Raven 1981; Torzillo et al. 1991; Raven 2011). Temperature and irradiance also interact and affect
the chemical composition of the algae (Payer et al. 1980; Carvalho et al. 2009).
Since different algae species and strains have different temperature optima, their response to high
irradiance will vary with temperature (Fig. 1). It is therefore important to select a strain which is best able
to utilize high light at the temperature range which will be encountered at the site of culture (Borowitzka
2013b) and maintain the culture cell density near the OCD. This means that the cell density needs to
be varied between summer and winter if operating at a constant pond depth. Alternatively, the average
irradiance received by the algae can also be adjusted by varying the pond depth while maintaining the
cell density constant. It is potentially possible to use different strains with different temperature optima
in summer and winter (Belay 1997).
High O 2 concentrations in open ponds and photobioreactors arising from algal photosynthesis are a
significant factor in limiting productivity (Ogawa et al. 1980; Kliphuis et al. 2011) and selection of algae
strains whose Rubisco is less sensitive to oxygen and/or strains which have a more efficient CCM and
thus an increased level of CO 2 at the site of Rubisco are highly desirable (Borowitzka 2013b).
Optimisation of growth and the cell content of specific products requires a good understanding of
algal physiology (Borowitzka 2016). Furthermore, scaling-up to the very large volumes required for
commercial production is not easy and is a major challenge for the transition from the laboratory to an
industrial process (Borowitzka and Vonshak 2017).