320 Marine Macro- and Microalgae: An Overview
It has been proposed (e.g., Benemann 1989) that smaller antennae in microalgae would result
in higher light utilization efficiency by eliminating the need to dissipate absorbed light energy thus
eliminating waste. It is also generally expected that cells with smaller antenna would allow more
light into the culture thus possibly increasing the carrying capacity of such a system. Some, but not
all, published studies have supported the concept that reduction in antenna size can lead to improved
photosynthetic efficiency. Nakajima and Ueda (1997, 1999) showed that productivity of Synechocystis
PCC6714 and Chlorella pyrenoidosa strains with smaller antenna sizes each increased of 30 to 50%
in dense cell cultures. Antenna size was controlled by light intensity for Chlorella and by the use of
a phycocyanin-deficient mutant for Synechocystis. These researchers observed similar properties for a
Chlamydomonas mutant strain lacking an antenna complex protein (Nakajima and Ueda 2000; Nakajima
et al. 2001). Polle et al. (2003) screened 129 low chlorophyll fluorescence mutants and isolated one stable
mutant from an insertional knockout library in Chlamydomonas that had a functional chlorophyll antenna
size for PSI and PSII of 50% and 65% relative to wild type, respectively. This mutant required a higher
light intensity for the saturation of photosynthesis (Melis 2009; Mitra and Melis 2010). It also exhibited
a greater photosynthetic efficiency and higher cell density in greenhouse grown cultures (Polle et al.
2003). Beckmann et al. (2009) engineered a small antenna strain of Chlamydomonas by permanently
expressing an LHC repressor; this mutation led to a 17% decrease in antenna size, resulting in a 50%
increase in photosynthetic efficiency in saturating light. They also observed significant increases in
biomass productivity compared to wild type cells in laboratory photobioreactors. Note, however, that
certain attempts to increase photosynthetic efficiency by antenna reduction have been unsuccessful. For
example, Huesemann et al. (2009) examined chemically- and UV-induced mutants of Cyclotella that
had smaller antenna sizes; although these strains required a higher light intensity to reach saturation, no
improvements in biomass productivities were observed in either semi-continuous laboratory cultures
or outdoor ponds. This may be due to the decrease in antenna size occurring out of balance with other
aspects of the acclimation response, leading to aberrant photosynthetic control; an appropriate antenna
composition must occur at each reaction center in order to ensure that photosynthetic electron transport
(PET) is poised to adequately match Calvin cycle activity. During photosynthetic acclimation, most algae
alter antenna size and composition along with Calvin cycle activity and PET to ensure that the saturation
point for photosynthesis is reached in a balanced manner. Therefore, obtaining decreased antenna strains,
with higher light saturation points, that retain all aspects of the high light acclimation response, should
ensure that the light and dark reactions saturate in balance.
Although some success has been achieved some hurdles remain to understand the actual changes in
the cell’s photosystem and their significance:
• How stable are these transformed strains?
• Are we decreasing the amount of chl-a in the cell and the number of reaction centers in parallel? Or
are we decreasing the size of the antenna complex per reaction center?
• How is cellular respiration changing?
• How competitive are these small antenna strains in an outdoor competitive environment?
We believe that answering these questions will help realize large gains in microalgal photosynthetic
efficiency which will result in concomitant increases in economic and environmental efficiency.
Redirect photosynthate towards desired products
In photosynthetic algae, “carbon partitioning” refers to the proportion of photosynthetically fixed carbon
that is directed into the various biochemical components of a cell (i.e., carbohydrates, proteins, and
lipids). In general, carbon partitioning is determined by the relative activities of the enzymatic pathways
that are responsible for biosynthesis of the major cell components. This is particularly important at the
branch points of metabolism (Fig. 10).
The flux of carbon through these pathways is controlled both at the local enzyme level and by
means of a global regulatory network that responds to the sensed environment. Global regulatory systems
utilize specialized regulatory proteins and cascades that usually perceive the environment through the
It has been proposed (e.g., Benemann 1989) that smaller antennae in microalgae would result
in higher light utilization efficiency by eliminating the need to dissipate absorbed light energy thus
eliminating waste. It is also generally expected that cells with smaller antenna would allow more
light into the culture thus possibly increasing the carrying capacity of such a system. Some, but not
all, published studies have supported the concept that reduction in antenna size can lead to improved
photosynthetic efficiency. Nakajima and Ueda (1997, 1999) showed that productivity of Synechocystis
PCC6714 and Chlorella pyrenoidosa strains with smaller antenna sizes each increased of 30 to 50%
in dense cell cultures. Antenna size was controlled by light intensity for Chlorella and by the use of
a phycocyanin-deficient mutant for Synechocystis. These researchers observed similar properties for a
Chlamydomonas mutant strain lacking an antenna complex protein (Nakajima and Ueda 2000; Nakajima
et al. 2001). Polle et al. (2003) screened 129 low chlorophyll fluorescence mutants and isolated one stable
mutant from an insertional knockout library in Chlamydomonas that had a functional chlorophyll antenna
size for PSI and PSII of 50% and 65% relative to wild type, respectively. This mutant required a higher
light intensity for the saturation of photosynthesis (Melis 2009; Mitra and Melis 2010). It also exhibited
a greater photosynthetic efficiency and higher cell density in greenhouse grown cultures (Polle et al.
2003). Beckmann et al. (2009) engineered a small antenna strain of Chlamydomonas by permanently
expressing an LHC repressor; this mutation led to a 17% decrease in antenna size, resulting in a 50%
increase in photosynthetic efficiency in saturating light. They also observed significant increases in
biomass productivity compared to wild type cells in laboratory photobioreactors. Note, however, that
certain attempts to increase photosynthetic efficiency by antenna reduction have been unsuccessful. For
example, Huesemann et al. (2009) examined chemically- and UV-induced mutants of Cyclotella that
had smaller antenna sizes; although these strains required a higher light intensity to reach saturation, no
improvements in biomass productivities were observed in either semi-continuous laboratory cultures
or outdoor ponds. This may be due to the decrease in antenna size occurring out of balance with other
aspects of the acclimation response, leading to aberrant photosynthetic control; an appropriate antenna
composition must occur at each reaction center in order to ensure that photosynthetic electron transport
(PET) is poised to adequately match Calvin cycle activity. During photosynthetic acclimation, most algae
alter antenna size and composition along with Calvin cycle activity and PET to ensure that the saturation
point for photosynthesis is reached in a balanced manner. Therefore, obtaining decreased antenna strains,
with higher light saturation points, that retain all aspects of the high light acclimation response, should
ensure that the light and dark reactions saturate in balance.
Although some success has been achieved some hurdles remain to understand the actual changes in
the cell’s photosystem and their significance:
• How stable are these transformed strains?
• Are we decreasing the amount of chl-a in the cell and the number of reaction centers in parallel? Or
are we decreasing the size of the antenna complex per reaction center?
• How is cellular respiration changing?
• How competitive are these small antenna strains in an outdoor competitive environment?
We believe that answering these questions will help realize large gains in microalgal photosynthetic
efficiency which will result in concomitant increases in economic and environmental efficiency.
Redirect photosynthate towards desired products
In photosynthetic algae, “carbon partitioning” refers to the proportion of photosynthetically fixed carbon
that is directed into the various biochemical components of a cell (i.e., carbohydrates, proteins, and
lipids). In general, carbon partitioning is determined by the relative activities of the enzymatic pathways
that are responsible for biosynthesis of the major cell components. This is particularly important at the
branch points of metabolism (Fig. 10).
The flux of carbon through these pathways is controlled both at the local enzyme level and by
means of a global regulatory network that responds to the sensed environment. Global regulatory systems
utilize specialized regulatory proteins and cascades that usually perceive the environment through the