278 Marine Macro- and Microalgae: An Overview
A first approach to increase the light conversion efficiency would be to extend the range of radiation
wavelengths that can be captured by the photosynthetic machinery. Due to different algae having slightly
shifted ranges of PAR, mixed cultures of different species may cover the solar spectrum better than
axenic ones. Alternatively, algae that are susceptible of genetic transformation may be manipulated for
extended absorption – and this may be achieved in several ways. The discovery and characterization of
the cyanobacterial chlorophylls d (Larkum and Kühl 2005) and f (Chen et al. 2010), which absorb light
beyond the visible limit into the near infrared (Li et al. 2012), prompted suggestions for introducing
such pigments in selected algae, thereby expanding the spectrum of PAR (Chen and Blankenship 2011).
Cyanobacteria using almost exclusively chlorophyll d are known to carry out oxygenic photosynthesis
with an energy conversion efficiency that is comparable to, or higher than, other species typically relying
on chlorophyll a (Mielke et al. 2011). This is possible because chlorophyll d substitutes chlorophyll a
also at the photosynthetic reaction center, thus shifting its absorption peak to longer wavelengths and
letting the excitation energy collected by the antennae at the far red to run downhill to the reaction center.
Chlorophyll d differs from the more common chlorophyll a in that the C3 vinyl group at ring A has been
oxidized to a formyl group. Oxidation of chlorophyll a is carried out by molecular oxygen (Schliep et
al. 2010) in a reaction that is apparently catalyzed by a P450 oxygenase. Chlorophyll f contains also
a formyl group substitution at the C2 position of the same ring, but details of its biosynthesis are still
unknown. In any case, the intended goal would be to transform algae with genes coding for the enzymes
needed to synthesize these red-shifted chlorophylls. This would extend the span of light harvested by the
antenna complexes, and would potentially increase the photosynthetic efficiency—although this is yet to
be experimentally demonstrated.
A different strategy has been followed by introducing fluorescent proteins. These are variants of
the well known green fluorescent protein (GFP) from the jellyfish Aequorea victoria, which acquires
fluorescence through a spontaneous condensation of three vicinal amino acid residues into a high
quantum yield fluorophore upon folding (reviewed by Stepanenko et al. 2011). Therefore, fluorescence
can be introduced directly by transformation with a gene encoding GFP (or any of its mutants with shifted
excitation and/or emission wavelengths). The aim is to express proteins that can harvest sunlight at the
near-ultraviolet region (the so-called UV-A) and re-emit it in the visible range to be subsequently captured
by the antenna complexes (and its energy funnelled into the reaction center). This approach, protected
by a US patent (Gressel et al. 2010), may be currently under research. Nevertheless, the introduction of
fluorescent dyes in solution (with the same aim of converting unusable radiation into photosynthetically
profitable wavelengths) has been shown to increase growth and lipid accumulation in microalgal cultures
(Seo et al. 2015). In addition, the expression of light-harvesting phycobilisomes of cyanobacteria in
algae has been proposed also to improve absorption at the “green” window of transmission of natural
photosynthetic pigments (Stephenson et al. 2011).
Under relatively high light intensity, the photosynthetic centers of exposed cells may become
saturated and a significant part of the absorbed energy may be lost to heat or fluorescence. Even worse,
intense illumination in bioreactors may result in photosynthetic center damage and photoinhibition in
cells of the most exposed layer (Simionato et al. 2013). This is unfortunate because other cells located
deeper inside the culture (i.e., receiving less intense light) may not be saturated and could potentially
channel this energy in a productive fashion. This problem may be alleviated in shallow or constantly
mixed liquid cultures, but there is always a certain degree of unavoidable self-shading – especially in the
case of dense cultures as intended for high biomass production. In order to reduce this effect, different
algal strains have been equipped with defective antenna complexes, which capture less light and thus
allow photons to penetrate deeper in the culture. This has been achieved in several species by different
strategies (reviewed in Stephens et al. 2010; Simionato et al. 2013), including targeted interferenceRNA repression and several forms of mutagenesis to disrupt light harvesting components. Under such
conditions, which distribute radiation more evenly among cells of dense cultures, a significant increase
in photosynthetic efficiency and productivity has been reported for C. reinhardtii grown in mass culture
(Polle et al. 2003; Mussgnug et al. 2007).
A first approach to increase the light conversion efficiency would be to extend the range of radiation
wavelengths that can be captured by the photosynthetic machinery. Due to different algae having slightly
shifted ranges of PAR, mixed cultures of different species may cover the solar spectrum better than
axenic ones. Alternatively, algae that are susceptible of genetic transformation may be manipulated for
extended absorption – and this may be achieved in several ways. The discovery and characterization of
the cyanobacterial chlorophylls d (Larkum and Kühl 2005) and f (Chen et al. 2010), which absorb light
beyond the visible limit into the near infrared (Li et al. 2012), prompted suggestions for introducing
such pigments in selected algae, thereby expanding the spectrum of PAR (Chen and Blankenship 2011).
Cyanobacteria using almost exclusively chlorophyll d are known to carry out oxygenic photosynthesis
with an energy conversion efficiency that is comparable to, or higher than, other species typically relying
on chlorophyll a (Mielke et al. 2011). This is possible because chlorophyll d substitutes chlorophyll a
also at the photosynthetic reaction center, thus shifting its absorption peak to longer wavelengths and
letting the excitation energy collected by the antennae at the far red to run downhill to the reaction center.
Chlorophyll d differs from the more common chlorophyll a in that the C3 vinyl group at ring A has been
oxidized to a formyl group. Oxidation of chlorophyll a is carried out by molecular oxygen (Schliep et
al. 2010) in a reaction that is apparently catalyzed by a P450 oxygenase. Chlorophyll f contains also
a formyl group substitution at the C2 position of the same ring, but details of its biosynthesis are still
unknown. In any case, the intended goal would be to transform algae with genes coding for the enzymes
needed to synthesize these red-shifted chlorophylls. This would extend the span of light harvested by the
antenna complexes, and would potentially increase the photosynthetic efficiency—although this is yet to
be experimentally demonstrated.
A different strategy has been followed by introducing fluorescent proteins. These are variants of
the well known green fluorescent protein (GFP) from the jellyfish Aequorea victoria, which acquires
fluorescence through a spontaneous condensation of three vicinal amino acid residues into a high
quantum yield fluorophore upon folding (reviewed by Stepanenko et al. 2011). Therefore, fluorescence
can be introduced directly by transformation with a gene encoding GFP (or any of its mutants with shifted
excitation and/or emission wavelengths). The aim is to express proteins that can harvest sunlight at the
near-ultraviolet region (the so-called UV-A) and re-emit it in the visible range to be subsequently captured
by the antenna complexes (and its energy funnelled into the reaction center). This approach, protected
by a US patent (Gressel et al. 2010), may be currently under research. Nevertheless, the introduction of
fluorescent dyes in solution (with the same aim of converting unusable radiation into photosynthetically
profitable wavelengths) has been shown to increase growth and lipid accumulation in microalgal cultures
(Seo et al. 2015). In addition, the expression of light-harvesting phycobilisomes of cyanobacteria in
algae has been proposed also to improve absorption at the “green” window of transmission of natural
photosynthetic pigments (Stephenson et al. 2011).
Under relatively high light intensity, the photosynthetic centers of exposed cells may become
saturated and a significant part of the absorbed energy may be lost to heat or fluorescence. Even worse,
intense illumination in bioreactors may result in photosynthetic center damage and photoinhibition in
cells of the most exposed layer (Simionato et al. 2013). This is unfortunate because other cells located
deeper inside the culture (i.e., receiving less intense light) may not be saturated and could potentially
channel this energy in a productive fashion. This problem may be alleviated in shallow or constantly
mixed liquid cultures, but there is always a certain degree of unavoidable self-shading – especially in the
case of dense cultures as intended for high biomass production. In order to reduce this effect, different
algal strains have been equipped with defective antenna complexes, which capture less light and thus
allow photons to penetrate deeper in the culture. This has been achieved in several species by different
strategies (reviewed in Stephens et al. 2010; Simionato et al. 2013), including targeted interferenceRNA repression and several forms of mutagenesis to disrupt light harvesting components. Under such
conditions, which distribute radiation more evenly among cells of dense cultures, a significant increase
in photosynthetic efficiency and productivity has been reported for C. reinhardtii grown in mass culture
(Polle et al. 2003; Mussgnug et al. 2007).