137
consequence, it has been viewed as a wasteful process hindering photosynthetic
productivity in C3 plants (Zhu et al. 2004). Nevertheless, photorespiration has been
proven to be crucial for photosynthetic organisms as mutants lacking key photorespiratory enzymes often exhibit poor performance, stress symptoms, and lethality
under present- day atmospheric condition (Bauwe et al. 2010). Moreover, a number
of studies suggest that photorespiration acts as safety valve adjusting redox homoeostasis under stress conditions (Voss et al. 2013) or a source of intermediate metabolites for other metabolic pathways of the plant (Novitskaya et al. 2002).
Previous studies have reported photorespiratory activities in certain species
based on evidence from
14
C pulse-chase experiments showing O 2 -enhanced
14
C
incorporation into photorespiratory intermediates (Burris et al. 1976; Andrews and
Abel 1979), inhibited photosynthetic rates in the presence of high O 2 (Downton
et al. 1976), O 2 -stimulated respiration in the light (Hough 1976), activities of glycolate metabolizing enzymes (Tolbert 1976), and enhanced photosynthetic rates under
low O 2 condition (Buapet et al. 2013a). Nevertheless, it has been suggested that
HCO 3
− utilization systems in seagrass might be able to efficiently maintain high
CO 2 availability around Rubisco fixation site thus suppressing photorespiration
(Beer 1989). As a result, the role of photorespiration, as well as other alternative
electron sinks, has long been ignored. A few recent studies, however, have shown
that photorespiration in the temperate seagrasses, Z. marina and Ruppia maritima,
is enhanced by carbon limitation (Buapet et al. 2013a). Since carbon availability
occasionally becomes limiting in stagnant shallow coastal waters (Buapet et al.
2013b), the impact of photorespiration on seagrass primary production might be
more prominent in the natural setting.
Photosynthetic organisms have flexible electron transports. Linear electron flow
is the principal pathway in which the electrons are transported from PSII to PSI and
eventually used to generate NADPH via ferredoxin-NADP
+
-oxidoreductase. Also,
plants have other alternative electron flow pathways, e.g., pseudocyclic electron
flow or the Mehler reaction and cyclic electron flow around PSI (Allen 2003). In
pseudocyclic electron flow or the Mehler reaction, molecular oxygen acts as an
electron acceptor instead of NADP
+
, forming superoxide which is subsequently
detoxified into water (Heber 2002). Two pathways, PGR5- and NDH-dependent
pathways, have been identified to be involved in cyclic electron flow around PSI
(Johnson 2011). These alternative electron flow and linear electron flow pathways
appear to operate simultaneously. The rates, however, depend largely on the plant
growing conditions (Allen 2003). Studies have shown conflicting conclusions
regarding contribution and physiological role of each alternative electron flow pathway (Cornic and Briantais 1991; Biehler and Fock 1996; Lovelock and Winter
1996; Badger et al. 2000; Foyer and Noctor 2000; Proctor and Smirnoff 2011;
Driever and Baker 2011; Kramer and Evans 2011). It is proposed that these pathways serve as photoprotection by generating trans-thylakoid pH gradient, thus
inducing energy dissipation by non-photochemical quenching (Johnson 2011;
Johnson et al. 2014), while helping adjusting ATP/NAPDH production (Peng et al.
2009; Kramer and Evans 2011). Recent studies have reported suboptimal growth
rates in cyclic electron flow mutants, while plants lacking the Mehler reaction were
7 Photobiology of Seagrasses: A Systems Biology Perspective
consequence, it has been viewed as a wasteful process hindering photosynthetic
productivity in C3 plants (Zhu et al. 2004). Nevertheless, photorespiration has been
proven to be crucial for photosynthetic organisms as mutants lacking key photorespiratory enzymes often exhibit poor performance, stress symptoms, and lethality
under present- day atmospheric condition (Bauwe et al. 2010). Moreover, a number
of studies suggest that photorespiration acts as safety valve adjusting redox homoeostasis under stress conditions (Voss et al. 2013) or a source of intermediate metabolites for other metabolic pathways of the plant (Novitskaya et al. 2002).
Previous studies have reported photorespiratory activities in certain species
based on evidence from
14
C pulse-chase experiments showing O 2 -enhanced
14
C
incorporation into photorespiratory intermediates (Burris et al. 1976; Andrews and
Abel 1979), inhibited photosynthetic rates in the presence of high O 2 (Downton
et al. 1976), O 2 -stimulated respiration in the light (Hough 1976), activities of glycolate metabolizing enzymes (Tolbert 1976), and enhanced photosynthetic rates under
low O 2 condition (Buapet et al. 2013a). Nevertheless, it has been suggested that
HCO 3
− utilization systems in seagrass might be able to efficiently maintain high
CO 2 availability around Rubisco fixation site thus suppressing photorespiration
(Beer 1989). As a result, the role of photorespiration, as well as other alternative
electron sinks, has long been ignored. A few recent studies, however, have shown
that photorespiration in the temperate seagrasses, Z. marina and Ruppia maritima,
is enhanced by carbon limitation (Buapet et al. 2013a). Since carbon availability
occasionally becomes limiting in stagnant shallow coastal waters (Buapet et al.
2013b), the impact of photorespiration on seagrass primary production might be
more prominent in the natural setting.
Photosynthetic organisms have flexible electron transports. Linear electron flow
is the principal pathway in which the electrons are transported from PSII to PSI and
eventually used to generate NADPH via ferredoxin-NADP
+
-oxidoreductase. Also,
plants have other alternative electron flow pathways, e.g., pseudocyclic electron
flow or the Mehler reaction and cyclic electron flow around PSI (Allen 2003). In
pseudocyclic electron flow or the Mehler reaction, molecular oxygen acts as an
electron acceptor instead of NADP
+
, forming superoxide which is subsequently
detoxified into water (Heber 2002). Two pathways, PGR5- and NDH-dependent
pathways, have been identified to be involved in cyclic electron flow around PSI
(Johnson 2011). These alternative electron flow and linear electron flow pathways
appear to operate simultaneously. The rates, however, depend largely on the plant
growing conditions (Allen 2003). Studies have shown conflicting conclusions
regarding contribution and physiological role of each alternative electron flow pathway (Cornic and Briantais 1991; Biehler and Fock 1996; Lovelock and Winter
1996; Badger et al. 2000; Foyer and Noctor 2000; Proctor and Smirnoff 2011;
Driever and Baker 2011; Kramer and Evans 2011). It is proposed that these pathways serve as photoprotection by generating trans-thylakoid pH gradient, thus
inducing energy dissipation by non-photochemical quenching (Johnson 2011;
Johnson et al. 2014), while helping adjusting ATP/NAPDH production (Peng et al.
2009; Kramer and Evans 2011). Recent studies have reported suboptimal growth
rates in cyclic electron flow mutants, while plants lacking the Mehler reaction were
7 Photobiology of Seagrasses: A Systems Biology Perspective