136
HCO 3
− at the boundary layer. The newly formed CO 2 then diffuses into the cell
(Moroney et al. 1985; Mercado et al. 1997, 1998; Larsson and Axelsson 1999; Beer
and Rehnberg 1997; Invers et al. 1999; Borum et al. 2016; Ow et al. 2016). An additional system proposed in seagrasses involves the formation of CO 2 from HCO 3
− in
the diffusion boundary layer by lowering pH and shifting DIC equilibrium via protons extrusion (Hellblom et al. 2001; Uku et al. 2005). The efficiency and limitation
of such carbon acquisition mechanisms differ among species and the environmental
conditions in which seagrasses grow (Hellblom et al. 2001; Uku et al. 2005). Recent
work has also shown that seagrass in the intertidal areas can utilize carbon from
both dissolved inorganic carbon pools and atmospheric CO 2 depending on the tidal
levels (Park et al. 2016). More negative δ13C values were observed in leaf tissues
of seagrasses from intertidal areas which suggests that these plants utilize atmospheric CO 2 directly during low tide exposure (Park et al. 2016).
Identifying seagrasses as C3 or C4 plants remains challenging. Although seagrasses are generally regarded as C3 plants, previous studies have shown contradictory results, and certain species appeared to have C3–C4 intermediate characteristics
(see review by Touchette and Burkholder 2000). The C4 Kranz anatomy identified
by chloroplast-rich bundle sheath cells is not observed in seagrasses although
several aquatic species of Hydrocharitaceae and certain species of Alismataceae are
able to operate Kranz-less single-cell C4 photosynthesis (Bowes et al. 2002).
Results from pulse-chase
14
C experiments have suggested that the photosynthetic
carbon fixation of several seagrass species such as Halodule wrightii, H. uninervis,
Syringodium filiforme, S. isoetifolium, Thalassia testudinum, T. hemprichii,
Thalassodendron ciliatum, Halophila spinulosa, and H. stipulacea follows the C3
pathway with phosphoglycerate being the first major stable organic compound and
malate and aspartate being a small fraction of the labeled substances (Andrews
and Abel 1979; Benedict et al. 1980; Beer and Waisel 1979; Beer et al. 1980;
Beer and Wetzel 1982).
7.1.3 Photorespiration and Alternative Electron Flows
Ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) is able to catalyze
both carboxylation and oxygenation reactions. Such dual function of Rubisco is
common to all photosynthetic organisms. The oxygenation of ribulose-1,5bisphosphate (RuBP) generates the toxic by-product 2-phosphoglycolate (2PG)
which is subsequently metabolized into the Calvin cycle intermediate, phosphoglycerate (PGA), via a series of reactions called photorespiration (Bauwe et al.
2010). Photorespiration takes place in three cell components: chloroplast, peroxisome, and mitochondria (Bauwe et al. 2010). This process also causes the loss of
carbon and nitrogen as CO 2 and NH 4 and consumes ATP and reducing equivalents.
It has been estimated that in C3 plants, photorespiration can lead to up to 30%
decrease in primary production under present-day atmospheric CO 2 :O 2 . As a
P. Buapet
HCO 3
− at the boundary layer. The newly formed CO 2 then diffuses into the cell
(Moroney et al. 1985; Mercado et al. 1997, 1998; Larsson and Axelsson 1999; Beer
and Rehnberg 1997; Invers et al. 1999; Borum et al. 2016; Ow et al. 2016). An additional system proposed in seagrasses involves the formation of CO 2 from HCO 3
− in
the diffusion boundary layer by lowering pH and shifting DIC equilibrium via protons extrusion (Hellblom et al. 2001; Uku et al. 2005). The efficiency and limitation
of such carbon acquisition mechanisms differ among species and the environmental
conditions in which seagrasses grow (Hellblom et al. 2001; Uku et al. 2005). Recent
work has also shown that seagrass in the intertidal areas can utilize carbon from
both dissolved inorganic carbon pools and atmospheric CO 2 depending on the tidal
levels (Park et al. 2016). More negative δ13C values were observed in leaf tissues
of seagrasses from intertidal areas which suggests that these plants utilize atmospheric CO 2 directly during low tide exposure (Park et al. 2016).
Identifying seagrasses as C3 or C4 plants remains challenging. Although seagrasses are generally regarded as C3 plants, previous studies have shown contradictory results, and certain species appeared to have C3–C4 intermediate characteristics
(see review by Touchette and Burkholder 2000). The C4 Kranz anatomy identified
by chloroplast-rich bundle sheath cells is not observed in seagrasses although
several aquatic species of Hydrocharitaceae and certain species of Alismataceae are
able to operate Kranz-less single-cell C4 photosynthesis (Bowes et al. 2002).
Results from pulse-chase
14
C experiments have suggested that the photosynthetic
carbon fixation of several seagrass species such as Halodule wrightii, H. uninervis,
Syringodium filiforme, S. isoetifolium, Thalassia testudinum, T. hemprichii,
Thalassodendron ciliatum, Halophila spinulosa, and H. stipulacea follows the C3
pathway with phosphoglycerate being the first major stable organic compound and
malate and aspartate being a small fraction of the labeled substances (Andrews
and Abel 1979; Benedict et al. 1980; Beer and Waisel 1979; Beer et al. 1980;
Beer and Wetzel 1982).
7.1.3 Photorespiration and Alternative Electron Flows
Ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) is able to catalyze
both carboxylation and oxygenation reactions. Such dual function of Rubisco is
common to all photosynthetic organisms. The oxygenation of ribulose-1,5bisphosphate (RuBP) generates the toxic by-product 2-phosphoglycolate (2PG)
which is subsequently metabolized into the Calvin cycle intermediate, phosphoglycerate (PGA), via a series of reactions called photorespiration (Bauwe et al.
2010). Photorespiration takes place in three cell components: chloroplast, peroxisome, and mitochondria (Bauwe et al. 2010). This process also causes the loss of
carbon and nitrogen as CO 2 and NH 4 and consumes ATP and reducing equivalents.
It has been estimated that in C3 plants, photorespiration can lead to up to 30%
decrease in primary production under present-day atmospheric CO 2 :O 2 . As a
P. Buapet