149
7.3.3.1 Alternative Electron Flows
It is challenging to fully understand the interconnectivity within the photosynthetic
network itself. Photosynthetic electron transport is highly flexible, and many alternative electron flow pathways have been identified. Since the role of these pathways
has long been understudied in seagrass systems, little is known about how they
operate and their relative contribution to seagrass primary production under changing environments. Although the capacity of alternative electron flows is generally
low in optimal conditions, these processes likely play a substantial role in redox
regulation under stress conditions (Ort and Baker 2002; Peltier and Cournac 2002).
An obvious link between photosynthetic carbon assimilation and alternative
electron flows is that both processes utilize photosynthetically derived electrons.
Relative contributions of each electron sink vary depending on the relative amounts
of CO 2 and O 2 surrounding the active site of Rubisco which is influenced by the
future scenarios of warm and high CO 2 conditions (Bloom 2015). CO 2 and
temperature impose antagonistic effects on carbon fixation and photorespiration. As
increased CO 2 availability favors carbon fixation, an increase in temperature promotes photorespiratory activity. The solubility of CO 2 decreases more than the solubility of O 2 with rising temperature, resulting in a lower dissolved CO 2 :O 2 ratio.
Also, the oxygenase activity of Rubisco is stimulated by increasing temperature to
a greater extent than the carboxylase activity. Photosynthetic carbon assimilation,
thus the production of carbohydrate, becomes less efficient under higher temperature (Ehleringer et al. 1997). In this condition, altered gene expression in photorespiratory pathways might be expected since plants have to increase the capacity to
metabolize phosphoglycolate generated from Rubisco oxygenation of RuBP
(Hodges et al. 2016; Timm et al. 2016). It has been suggested that suppressed photorespiration by CO 2 -enrichment contributed, in part, to an increase in effective
photochemical efficiency observed in the seagrasses Z. noltii (Alexandre et al.
2012). There is evidence of photorespiration and the Mehler reaction operating in
the seagrass Z. marina (Buapet et al. 2013a; Buapet and Björk 2016). However,
knowledge of rates and regulations of these alternative electron flow in relation to
photosynthesis as well as in response to the future high CO 2 and temperature is still
missing. Given that these processes might be upregulated under stress conditions, it
is of high importance to consider them in order to get accurate estimates of seagrass
primary production.
In addition to being diverted paths from carbon assimilation, alternative electron
flows might play photoprotective roles under unfavorable conditions. It has been
proposed that photorespiration, the Mehler reaction, chlororespiration, and cyclic
electron flow may serve as safety valves when light energy is in excess, thus mitigating photoinhibition (see review by Osmond and Grace 1995; Wingler et al. 2000;
Voss et al. 2013). The common function of these processes is maintaining the redox
balance of the photosynthetic electron transport chain, while the mode of action is,
however, slightly different. The proposed photoprotective roles of alternative electron flows have been supported by many studies in terrestrial systems. For example,
7 Photobiology of Seagrasses: A Systems Biology Perspective
7.3.3.1 Alternative Electron Flows
It is challenging to fully understand the interconnectivity within the photosynthetic
network itself. Photosynthetic electron transport is highly flexible, and many alternative electron flow pathways have been identified. Since the role of these pathways
has long been understudied in seagrass systems, little is known about how they
operate and their relative contribution to seagrass primary production under changing environments. Although the capacity of alternative electron flows is generally
low in optimal conditions, these processes likely play a substantial role in redox
regulation under stress conditions (Ort and Baker 2002; Peltier and Cournac 2002).
An obvious link between photosynthetic carbon assimilation and alternative
electron flows is that both processes utilize photosynthetically derived electrons.
Relative contributions of each electron sink vary depending on the relative amounts
of CO 2 and O 2 surrounding the active site of Rubisco which is influenced by the
future scenarios of warm and high CO 2 conditions (Bloom 2015). CO 2 and
temperature impose antagonistic effects on carbon fixation and photorespiration. As
increased CO 2 availability favors carbon fixation, an increase in temperature promotes photorespiratory activity. The solubility of CO 2 decreases more than the solubility of O 2 with rising temperature, resulting in a lower dissolved CO 2 :O 2 ratio.
Also, the oxygenase activity of Rubisco is stimulated by increasing temperature to
a greater extent than the carboxylase activity. Photosynthetic carbon assimilation,
thus the production of carbohydrate, becomes less efficient under higher temperature (Ehleringer et al. 1997). In this condition, altered gene expression in photorespiratory pathways might be expected since plants have to increase the capacity to
metabolize phosphoglycolate generated from Rubisco oxygenation of RuBP
(Hodges et al. 2016; Timm et al. 2016). It has been suggested that suppressed photorespiration by CO 2 -enrichment contributed, in part, to an increase in effective
photochemical efficiency observed in the seagrasses Z. noltii (Alexandre et al.
2012). There is evidence of photorespiration and the Mehler reaction operating in
the seagrass Z. marina (Buapet et al. 2013a; Buapet and Björk 2016). However,
knowledge of rates and regulations of these alternative electron flow in relation to
photosynthesis as well as in response to the future high CO 2 and temperature is still
missing. Given that these processes might be upregulated under stress conditions, it
is of high importance to consider them in order to get accurate estimates of seagrass
primary production.
In addition to being diverted paths from carbon assimilation, alternative electron
flows might play photoprotective roles under unfavorable conditions. It has been
proposed that photorespiration, the Mehler reaction, chlororespiration, and cyclic
electron flow may serve as safety valves when light energy is in excess, thus mitigating photoinhibition (see review by Osmond and Grace 1995; Wingler et al. 2000;
Voss et al. 2013). The common function of these processes is maintaining the redox
balance of the photosynthetic electron transport chain, while the mode of action is,
however, slightly different. The proposed photoprotective roles of alternative electron flows have been supported by many studies in terrestrial systems. For example,
7 Photobiology of Seagrasses: A Systems Biology Perspective