151
nitrogen deficiency symptoms under high CO 2 , i.e., decreased nitrogen content and
reduced shoot/root ratio (Takatani et al. 2014). Both carbon and nitrogen assimilation
compete for ATP, reducing equivalents and ferredoxin supplied by the light- dependent
reactions of photosynthesis. Enhanced photosynthetic carbon assimilation induced
by high CO 2 under light-limited condition decreases the amount of ATP and reducing
powers supplied to nitrogen assimilation and eventually leads to nitrogen limitation
(Bloom 2015). Additionally, a reduction in photosynthetic activity under long-term
exposure to high CO 2 is more pronounced under nitrogen limitation (Stitt and Krapp
1999; Sun et al. 2002; Sanz-sàez et al. 2010). Despite a growing number of studies
on the effect of increased CO 2 on seagrass primary production, simultaneous impact
on nitrogen metabolisms has been scarcely studied. Alexandre et al. (2012) investigated the effect of predicted future CO 2 level on carbon and nitrogen metabolisms in
Z. noltii in the mesocosm experiment. Here an increase in the photosynthetic activity
was observed, while nitrate uptake and nitrogen content in the leaves of CO 2 -enriched
plants were found to be significantly lower than controls. Thus no positive effect of
CO 2 enrichment on the growth rates of Z. noltii in these experiments is likely to be a
result of nitrogen limitation. This suggests that nitrogen might become limiting for
Z. noltii in the future CO 2 level. Similar declines in the leaf nitrogen content under
high CO 2 have been reported in T. hemprichii (Jiang et al. 2010) and T. testudinum
(Campbell and Fourqurean 2013a). This shift of limiting factor under high CO 2
might be the underlying reason for a limited or no positive effect of CO 2 enrichment
observed in many studies conducted on seagrasses (as discussed in the previous section). On the contrary, CO 2 enrichment did not affect nitrate uptake and nitrate reductase activity of H. uninervis, whereas it increased nitrate assimilation in T. hemprichii
grown under high nitrate level (Ow et al. 2016). Such interaction might as well affect
downstream pathways such as ammonium assimilation which also requires ATP and
NADPH. A Recent study by Pernice et al. (2016) has revealed a fast increase in
expression of glutamine synthetase (GS), a key enzyme for ammonium assimilation,
following exposure to a pulse of ammonium supply. This indicated that GS could be
used as a molecular marker of nitrogen assimilation and should be included in further
investigations of seagrass metabolisms. In-depth physiological studies are needed in
order to elucidate the connection between these two vital processes: photosynthesis
and nitrogen assimilation. This subject is of great ecological relevance as inorganic
nitrogen concentrations are commonly low in seagrass-dominated habitats (Lee and
Dunton 1999; McGlathery et al. 2001).
7.4 Further Prospects for Integrating Seagrass
Photophysiology with Systems Biology
Seagrasses are rapidly declining worldwide. Therefore, there is a critical need to
characterize the impacts of environmental changes on seagrasses, particularly those
associated with the future global change scenarios. Better mechanistic understanding of photosynthesis is necessary to assess how environmental constraints affect
seagrass primary production and its carbon budget. Although photosynthesis plays
7 Photobiology of Seagrasses: A Systems Biology Perspective
nitrogen deficiency symptoms under high CO 2 , i.e., decreased nitrogen content and
reduced shoot/root ratio (Takatani et al. 2014). Both carbon and nitrogen assimilation
compete for ATP, reducing equivalents and ferredoxin supplied by the light- dependent
reactions of photosynthesis. Enhanced photosynthetic carbon assimilation induced
by high CO 2 under light-limited condition decreases the amount of ATP and reducing
powers supplied to nitrogen assimilation and eventually leads to nitrogen limitation
(Bloom 2015). Additionally, a reduction in photosynthetic activity under long-term
exposure to high CO 2 is more pronounced under nitrogen limitation (Stitt and Krapp
1999; Sun et al. 2002; Sanz-sàez et al. 2010). Despite a growing number of studies
on the effect of increased CO 2 on seagrass primary production, simultaneous impact
on nitrogen metabolisms has been scarcely studied. Alexandre et al. (2012) investigated the effect of predicted future CO 2 level on carbon and nitrogen metabolisms in
Z. noltii in the mesocosm experiment. Here an increase in the photosynthetic activity
was observed, while nitrate uptake and nitrogen content in the leaves of CO 2 -enriched
plants were found to be significantly lower than controls. Thus no positive effect of
CO 2 enrichment on the growth rates of Z. noltii in these experiments is likely to be a
result of nitrogen limitation. This suggests that nitrogen might become limiting for
Z. noltii in the future CO 2 level. Similar declines in the leaf nitrogen content under
high CO 2 have been reported in T. hemprichii (Jiang et al. 2010) and T. testudinum
(Campbell and Fourqurean 2013a). This shift of limiting factor under high CO 2
might be the underlying reason for a limited or no positive effect of CO 2 enrichment
observed in many studies conducted on seagrasses (as discussed in the previous section). On the contrary, CO 2 enrichment did not affect nitrate uptake and nitrate reductase activity of H. uninervis, whereas it increased nitrate assimilation in T. hemprichii
grown under high nitrate level (Ow et al. 2016). Such interaction might as well affect
downstream pathways such as ammonium assimilation which also requires ATP and
NADPH. A Recent study by Pernice et al. (2016) has revealed a fast increase in
expression of glutamine synthetase (GS), a key enzyme for ammonium assimilation,
following exposure to a pulse of ammonium supply. This indicated that GS could be
used as a molecular marker of nitrogen assimilation and should be included in further
investigations of seagrass metabolisms. In-depth physiological studies are needed in
order to elucidate the connection between these two vital processes: photosynthesis
and nitrogen assimilation. This subject is of great ecological relevance as inorganic
nitrogen concentrations are commonly low in seagrass-dominated habitats (Lee and
Dunton 1999; McGlathery et al. 2001).
7.4 Further Prospects for Integrating Seagrass
Photophysiology with Systems Biology
Seagrasses are rapidly declining worldwide. Therefore, there is a critical need to
characterize the impacts of environmental changes on seagrasses, particularly those
associated with the future global change scenarios. Better mechanistic understanding of photosynthesis is necessary to assess how environmental constraints affect
seagrass primary production and its carbon budget. Although photosynthesis plays
7 Photobiology of Seagrasses: A Systems Biology Perspective