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the same composition of light-harvesting pigments for photosynthesis as other
higher plants, i.e., chlorophyll a, chlorophyll b, and a variety of xanthophylls and
carotenoids (Casazza and Mazzella 2002). However, the siphonaxanthin-like pigment has been reported in the seagrasses Posidonia oceanica and Halophila stipulacea, and its concentration increases with depth (Casazza and Mazzella 2002).
This group of pigment is specifically found in the light-harvesting complex of green
alga and has been proposed to play a role in harvesting light in the green region
(510–550 nm) and transfer absorbed energy to chlorophylls (Wang et al. 2013).
From such finding, it is suggested that seagrass might have undergone lightharvesting adaptation to the underwater light environment.
Recent research provides further evidence of photosynthetic adaptation associated with light-harvesting and the photosynthetic machinery of seagrass. Wissler
et al. (2011) identified candidate genes associated with the molecular evolution of
seagrass. This work revealed that gene class encoding photosynthetic proteins
have been significantly enriched in Zostera marina and Posidonia oceanica,
whereas a number of genes encoding photosynthetic antenna proteins and involved
in photosynthetic electron transport such as light-harvesting complex 5 (LHCB5),
ferredoxin 3 (ATFD3), PSI subunit L (PSAL), PSII subunit O-2, oxygen-evolving
binding (PSBO2), PSII subunit R (PSBR), and chlorophyll-binding protein
(LHCA3) have been identified as positively selected genes, indicating that many
constituents of the photosynthetic pathway have acquired sequence changes upon
adaptation to the sea (Wissler et al. 2011). Olsen et al. (2016) examined the genome
of the seagrass Zostera marina and showed that PSI and PSII of Z. marina remain
similar to those of other terrestrial and aquatic plants, but chlorophyll a/b-binding
proteins of light-harvesting complex II subtype Lhcb of Z. marina become more
diversified. There are experimental indications from the studies in higher plants
and algae that these PSII-associated chlorophyll a/b-binding proteins are the main
targets of regulation under changing light condition (Kouřil et al. 2013).
7.1.2 Carbon Reactions
Similar to many other aquatic plants, seagrass lacks stomata and dissolved CO 2 is
absorbed directly into the epidermal cells. Correspondingly, the sequenced genome
of the seagrass Zostera marina has revealed a complete loss of the genes associated
with stomatal differentiation (Olsen et al. 2016). Seagrasses are able to utilize
HCO 3
− in addition to CO 2 as an exogenous carbon source. Such ability gives seagrasses competitive advantage in a marine environment where HCO 3
− is the dominant form of dissolved inorganic carbon and diffusion rate of CO 2 is slow. The thick
stagnant layer adjacent to the leaf surface or boundary layer plays a part in impeding
the CO 2 acquisition of seagrass from the water body as CO 2 can only be transferred
via molecular diffusion in such layer (Beer 1989). The most common carbonconcentrating system of seagrasses is associated with extracellular carbonic anhydrase. It has been proposed that this enzyme catalyzes the formation of CO 2 from
7 Photobiology of Seagrasses: A Systems Biology Perspective
the same composition of light-harvesting pigments for photosynthesis as other
higher plants, i.e., chlorophyll a, chlorophyll b, and a variety of xanthophylls and
carotenoids (Casazza and Mazzella 2002). However, the siphonaxanthin-like pigment has been reported in the seagrasses Posidonia oceanica and Halophila stipulacea, and its concentration increases with depth (Casazza and Mazzella 2002).
This group of pigment is specifically found in the light-harvesting complex of green
alga and has been proposed to play a role in harvesting light in the green region
(510–550 nm) and transfer absorbed energy to chlorophylls (Wang et al. 2013).
From such finding, it is suggested that seagrass might have undergone lightharvesting adaptation to the underwater light environment.
Recent research provides further evidence of photosynthetic adaptation associated with light-harvesting and the photosynthetic machinery of seagrass. Wissler
et al. (2011) identified candidate genes associated with the molecular evolution of
seagrass. This work revealed that gene class encoding photosynthetic proteins
have been significantly enriched in Zostera marina and Posidonia oceanica,
whereas a number of genes encoding photosynthetic antenna proteins and involved
in photosynthetic electron transport such as light-harvesting complex 5 (LHCB5),
ferredoxin 3 (ATFD3), PSI subunit L (PSAL), PSII subunit O-2, oxygen-evolving
binding (PSBO2), PSII subunit R (PSBR), and chlorophyll-binding protein
(LHCA3) have been identified as positively selected genes, indicating that many
constituents of the photosynthetic pathway have acquired sequence changes upon
adaptation to the sea (Wissler et al. 2011). Olsen et al. (2016) examined the genome
of the seagrass Zostera marina and showed that PSI and PSII of Z. marina remain
similar to those of other terrestrial and aquatic plants, but chlorophyll a/b-binding
proteins of light-harvesting complex II subtype Lhcb of Z. marina become more
diversified. There are experimental indications from the studies in higher plants
and algae that these PSII-associated chlorophyll a/b-binding proteins are the main
targets of regulation under changing light condition (Kouřil et al. 2013).
7.1.2 Carbon Reactions
Similar to many other aquatic plants, seagrass lacks stomata and dissolved CO 2 is
absorbed directly into the epidermal cells. Correspondingly, the sequenced genome
of the seagrass Zostera marina has revealed a complete loss of the genes associated
with stomatal differentiation (Olsen et al. 2016). Seagrasses are able to utilize
HCO 3
− in addition to CO 2 as an exogenous carbon source. Such ability gives seagrasses competitive advantage in a marine environment where HCO 3
− is the dominant form of dissolved inorganic carbon and diffusion rate of CO 2 is slow. The thick
stagnant layer adjacent to the leaf surface or boundary layer plays a part in impeding
the CO 2 acquisition of seagrass from the water body as CO 2 can only be transferred
via molecular diffusion in such layer (Beer 1989). The most common carbonconcentrating system of seagrasses is associated with extracellular carbonic anhydrase. It has been proposed that this enzyme catalyzes the formation of CO 2 from
7 Photobiology of Seagrasses: A Systems Biology Perspective