147
PHYA, PHYB, CRY1, phototropin 1 (PHO1), and phototropin 2-like have been
identified (Kong et al. 2014; Olsen et al. 2016). Phytochromes C and UVR8 transcripts are, however, absent in Z. marina (Olsen et al. 2016).
Although existing studies (Greco et al. 2013; Dattolo et al. 2014; Kong et al.
2014; Olsen et al. 2016) suggest that seagrass physiological responses may be regulated by photoreceptors. In aquatic condition, light quality is altered with depth. As
red and far-red wavelengths are largely attenuated by the water column (Ragni and
Ribera d’Alcala 2004), the possible functions of phytochromes in such condition
remain unclear and how they are involved in light signaling may be different from
the terrestrial systems. Further research should investigate how light is sensed by
seagrasses at different light environments and the signaling pathways associated
with light-dependent essential processes such as photosynthesis as it might reveal
one of the key adaptations of this group of plants to the sea.
7.3.2 Mechanistic Understanding of Photosynthetic Carbon
Fixation
It is generally taken for granted that seagrass possesses the same photosynthetic
pathway as terrestrial higher plants. Nevertheless, the key photosynthetic processes
of seagrasses are still not completely understood. Recent genome assemblies of the
two seagrasses Z. marina and Z. muelleri have pointed out that seagrass adaptation
to the aquatic environment has shaped their physiological features so that they in
many ways differ significantly from those of the terrestrial higher plants. While
chlorophyll fluorescence technique allows extensive investigation of the lightdependent reactions of photosynthesis, little is known about the mode of photosynthetic carbon fixation of seagrasses. In general, seagrasses are regarded as C3 plants
(discussed in the previous section); however, certain seagrass species exhibit C4
characteristics, while others were identified as C3–C4 intermediates (Benedict and
Scott 1976; Andrews and Abel 1979; Beer and Waisel 1979; Waghmode and Joshi
1983; Bowes and Salvucci 1989). It is difficult to compare these results since different measurements and parameters have been used such as a photosynthetic quantum
efficiency, δ
13
C values, first stable photosynthetic products and the activities of photosynthetic enzymes. Although there are indications that C4 photosynthesis occurs
in certain seagrass species, and that it might be inducible under certain environmental conditions, no further investigation has been conducted on this aspect. On the
contrary, much more information has been accumulated in the terrestrial plant systems regarding mode of carbon assimilation. Differential gene expressions between
C3 and C4 species have been revealed, whereas the responsive genes in the facultative C4 plant and the key genes attributing to C3–C4 intermediate characteristics in
certain plant species have been identified (Rao et al. 2008; Bräutigam et al. 2011;
Külahoglu et al. 2014; Schulze et al. 2016).
7 Photobiology of Seagrasses: A Systems Biology Perspective
PHYA, PHYB, CRY1, phototropin 1 (PHO1), and phototropin 2-like have been
identified (Kong et al. 2014; Olsen et al. 2016). Phytochromes C and UVR8 transcripts are, however, absent in Z. marina (Olsen et al. 2016).
Although existing studies (Greco et al. 2013; Dattolo et al. 2014; Kong et al.
2014; Olsen et al. 2016) suggest that seagrass physiological responses may be regulated by photoreceptors. In aquatic condition, light quality is altered with depth. As
red and far-red wavelengths are largely attenuated by the water column (Ragni and
Ribera d’Alcala 2004), the possible functions of phytochromes in such condition
remain unclear and how they are involved in light signaling may be different from
the terrestrial systems. Further research should investigate how light is sensed by
seagrasses at different light environments and the signaling pathways associated
with light-dependent essential processes such as photosynthesis as it might reveal
one of the key adaptations of this group of plants to the sea.
7.3.2 Mechanistic Understanding of Photosynthetic Carbon
Fixation
It is generally taken for granted that seagrass possesses the same photosynthetic
pathway as terrestrial higher plants. Nevertheless, the key photosynthetic processes
of seagrasses are still not completely understood. Recent genome assemblies of the
two seagrasses Z. marina and Z. muelleri have pointed out that seagrass adaptation
to the aquatic environment has shaped their physiological features so that they in
many ways differ significantly from those of the terrestrial higher plants. While
chlorophyll fluorescence technique allows extensive investigation of the lightdependent reactions of photosynthesis, little is known about the mode of photosynthetic carbon fixation of seagrasses. In general, seagrasses are regarded as C3 plants
(discussed in the previous section); however, certain seagrass species exhibit C4
characteristics, while others were identified as C3–C4 intermediates (Benedict and
Scott 1976; Andrews and Abel 1979; Beer and Waisel 1979; Waghmode and Joshi
1983; Bowes and Salvucci 1989). It is difficult to compare these results since different measurements and parameters have been used such as a photosynthetic quantum
efficiency, δ
13
C values, first stable photosynthetic products and the activities of photosynthetic enzymes. Although there are indications that C4 photosynthesis occurs
in certain seagrass species, and that it might be inducible under certain environmental conditions, no further investigation has been conducted on this aspect. On the
contrary, much more information has been accumulated in the terrestrial plant systems regarding mode of carbon assimilation. Differential gene expressions between
C3 and C4 species have been revealed, whereas the responsive genes in the facultative C4 plant and the key genes attributing to C3–C4 intermediate characteristics in
certain plant species have been identified (Rao et al. 2008; Bräutigam et al. 2011;
Külahoglu et al. 2014; Schulze et al. 2016).
7 Photobiology of Seagrasses: A Systems Biology Perspective