Microalgal Metabolism and their Utilisation 45
the pigment ratios and decline in pigment content is consistent with decreases in the size and number of
PS I and PS II reaction centers protecting the photosynthetic apparatus from excess energy during growth
under sustained saturating irradiances.
Relatively rapid fluctuations in light as would be expected, for example, by the passage of a cloud
during the day, expose the cells to rapid changes between high and low irradiances. Microalgae have
evolved a number of photoprotective mechanisms such as the PS II and PS I electron cycles, statetransitions, fast repair of the D1 protein of the PS II reaction centre, and the scavenging of reactive
oxygen species (Falkowski and Raven 2007; Lavaud 2007; Minagawa 2011) to cope with rapid changes
in irradiance. Amongst these mechanisms the xanthophyll cycle and the dependent thermal dissipation of
the excess light energy (non-photochemical quenching—NPQ) plays a central role (Brunet and Lavaud
2010). NPQ (allows the thermal dissipation of the excess of energy and is one of the faster photoprotective
processes activated by algal cells). NPQ decreases the flow of excitation energy to PS II reaction centers
and helps to minimize the production of harmful oxygen radicals in the PS II antenna. NPQ has three
components: qE which is a fast quenching component regulated by the build-up of a transthylakoid ΔpH
and the operation of the xanthophyll cycle, qI which is an intermediate quenching component due to
state transition, and qI which is a slowly relaxing quenching component due to photoinhibition (Rascher
and Nedbal 2006). qE involves mostly the xanthophyll cycle activity (Brunet and Lavaud 2010), which
corresponds to the reversible de-epoxidation of violaxanthin into zeaxanthin through antheraxanthin
for green algae. In most chlorophyll c–containing microalgae, the photoprotective cycle involves the
conversion between diadinoxanthin and diatoxanthin. Diatoms are one of the groups with the greatest
NPQ and xanthophyll cycle activity (Dimier et al. 2007; Goss and Jakob 2010) and this may be the
mechanism by which they can better cope with variable irradiances (Wagner et al. 2006).
Photosynthesis and photorespiration are also influenced by temperature, with higher temperatures
increasing oxygenase and PSII activities (Belay and Fogg 1978; Verity 1981; Morris and Kronkamp 2003).
Different algae species have different temperature optima and therefore their response to high irradiance
will vary with temperature. For example, Fig. 1 shows the effect of temperature at different irradiances
on the effect of increasing [O 2 ] for Isochrysis galbana. The gross photosynthetic rate decreased with
increasing {O 2 ] above 50% sat air for all conditions except for the 23ºC treatment at 1200 μmol photons .m
–2
.s
–1
which was inhibited at concentrations above 80% sat air . The rate of decrease in gross photosynthetic rate
(O 2 inhibition rate) was affected by both irradiance and temperature. The gross photosynthetic rate at
1200 μmol photons .m
–2
.s
–1
was highest at 23ºC whereas at 2500 μmol photons .m
–2
.s
–1
the highest temperature
specific maximum gross photosynthetic rate was observed at 26ºC.
Respiration
Algae cultures can lose a significant part of their biomass at night due to respiration (Grobbelaar and
Soeder 1985) and it is therefore important to minimise these respiratory losses in order to maximise
productivity. Dark respiration is cell size dependent (Banse 1976) and is strongly influenced by the
growth irradiance and temperature (Raven 1981; Torzillo et al. 1991; Ogbonna and Tanaka 1996) with
suboptimal temperatures and high irradiances resulting in higher night-time respiration. Dark respiration
is also influenced by the nitrogen source (Vanlerberghe et al. 1992; Post 1993). More on dark respiration
can be found in Raven and Beardall (2016).
Inorganic carbon utilisation
Algae, being aquatic organisms, face a different inorganic carbon environment than land plants. In water,
inorganic carbon exists in three forms (CO 2 , HCO 3
–
and CO 3
2–
), and the inorganic carbon system is
the main buffering system in water with the relative proportion of these three inorganic carbon species
dependent on pH, salinity, and temperature. For example, less than 1% of dissolved inorganic carbon
(DIC) is present as CO 2 at normal seawater pH (8.1–8.2) and more than 90% is in the form of HCO 3
–
.
Furthermore, the rate of diffusion of CO 2 in water is 10
4
times slower than that in the atmosphere, and the
rate of conversion of HCO 3
–
to CO 2 is very slow at alkaline pH. These factors cause a limitation in the
the pigment ratios and decline in pigment content is consistent with decreases in the size and number of
PS I and PS II reaction centers protecting the photosynthetic apparatus from excess energy during growth
under sustained saturating irradiances.
Relatively rapid fluctuations in light as would be expected, for example, by the passage of a cloud
during the day, expose the cells to rapid changes between high and low irradiances. Microalgae have
evolved a number of photoprotective mechanisms such as the PS II and PS I electron cycles, statetransitions, fast repair of the D1 protein of the PS II reaction centre, and the scavenging of reactive
oxygen species (Falkowski and Raven 2007; Lavaud 2007; Minagawa 2011) to cope with rapid changes
in irradiance. Amongst these mechanisms the xanthophyll cycle and the dependent thermal dissipation of
the excess light energy (non-photochemical quenching—NPQ) plays a central role (Brunet and Lavaud
2010). NPQ (allows the thermal dissipation of the excess of energy and is one of the faster photoprotective
processes activated by algal cells). NPQ decreases the flow of excitation energy to PS II reaction centers
and helps to minimize the production of harmful oxygen radicals in the PS II antenna. NPQ has three
components: qE which is a fast quenching component regulated by the build-up of a transthylakoid ΔpH
and the operation of the xanthophyll cycle, qI which is an intermediate quenching component due to
state transition, and qI which is a slowly relaxing quenching component due to photoinhibition (Rascher
and Nedbal 2006). qE involves mostly the xanthophyll cycle activity (Brunet and Lavaud 2010), which
corresponds to the reversible de-epoxidation of violaxanthin into zeaxanthin through antheraxanthin
for green algae. In most chlorophyll c–containing microalgae, the photoprotective cycle involves the
conversion between diadinoxanthin and diatoxanthin. Diatoms are one of the groups with the greatest
NPQ and xanthophyll cycle activity (Dimier et al. 2007; Goss and Jakob 2010) and this may be the
mechanism by which they can better cope with variable irradiances (Wagner et al. 2006).
Photosynthesis and photorespiration are also influenced by temperature, with higher temperatures
increasing oxygenase and PSII activities (Belay and Fogg 1978; Verity 1981; Morris and Kronkamp 2003).
Different algae species have different temperature optima and therefore their response to high irradiance
will vary with temperature. For example, Fig. 1 shows the effect of temperature at different irradiances
on the effect of increasing [O 2 ] for Isochrysis galbana. The gross photosynthetic rate decreased with
increasing {O 2 ] above 50% sat air for all conditions except for the 23ºC treatment at 1200 μmol photons .m
–2
.s
–1
which was inhibited at concentrations above 80% sat air . The rate of decrease in gross photosynthetic rate
(O 2 inhibition rate) was affected by both irradiance and temperature. The gross photosynthetic rate at
1200 μmol photons .m
–2
.s
–1
was highest at 23ºC whereas at 2500 μmol photons .m
–2
.s
–1
the highest temperature
specific maximum gross photosynthetic rate was observed at 26ºC.
Respiration
Algae cultures can lose a significant part of their biomass at night due to respiration (Grobbelaar and
Soeder 1985) and it is therefore important to minimise these respiratory losses in order to maximise
productivity. Dark respiration is cell size dependent (Banse 1976) and is strongly influenced by the
growth irradiance and temperature (Raven 1981; Torzillo et al. 1991; Ogbonna and Tanaka 1996) with
suboptimal temperatures and high irradiances resulting in higher night-time respiration. Dark respiration
is also influenced by the nitrogen source (Vanlerberghe et al. 1992; Post 1993). More on dark respiration
can be found in Raven and Beardall (2016).
Inorganic carbon utilisation
Algae, being aquatic organisms, face a different inorganic carbon environment than land plants. In water,
inorganic carbon exists in three forms (CO 2 , HCO 3
–
and CO 3
2–
), and the inorganic carbon system is
the main buffering system in water with the relative proportion of these three inorganic carbon species
dependent on pH, salinity, and temperature. For example, less than 1% of dissolved inorganic carbon
(DIC) is present as CO 2 at normal seawater pH (8.1–8.2) and more than 90% is in the form of HCO 3
–
.
Furthermore, the rate of diffusion of CO 2 in water is 10
4
times slower than that in the atmosphere, and the
rate of conversion of HCO 3
–
to CO 2 is very slow at alkaline pH. These factors cause a limitation in the