182
8.4 Implications of the Seagrass Paradox for the Future
of Seagrass Systems
The systems analysis described above leads us to wonder why seagrasses have not
followed the example of many of their algal counterparts in evolving more efficient
carbon-concentrating mechanisms to cope with relative scarcity of this important
inorganic carbon substrate and why they bothered to retain so much excess photosynthetic capacity in a low-CO 2 world. The paradoxically high reliance on CO 2(aq)
may be the legacy of the evolutionary origins of seagrasses in the high-CO 2
environment of the Cretaceous (Hartog et al. 1979; Retallack 2001). Further, the
possession of roots and rhizomes gives seagrasses a unique ability to colonize and
exploit the nutrient-rich, permanently flooded sediments of protected coastal shores
that may have provided an important refuge from competition for light and space
with the more photosynthetically efficient algae. Without competition from the
algae in oligotrophic waters of the preindustrial world, seagrasses may simply not
have experienced the selective pressure required to increase photosynthetic efficiency, or to reallocate resources invested in unnecessary photosynthetic capacity,
in these shallow, high light environments. Unfortunately, anthropogenic alteration
of the coastal ocean is probably occurring too rapidly to facilitate adaptive change
in this group of organisms that has persisted for 100 million years.
Despite their inefficiency in extracting CO 2 for photosynthesis in seawater, seagrasses possess marvelous physiological mechanisms to protect the leaves from
photoinhibition in high light environments. Carbon limitation imposes a relatively
low threshold for irradiance saturation of photosynthesis that should make seagrasses extremely vulnerable to photoinhibition. Yet, paradoxically, they thrive best
in high light environments. Acclimation processes that may protect seagrass leaves
involve dynamic xanthophyll cycling capable of modulating non-photochemical
quenching on time scales of minutes (Ralph et al. 2002) to the adjustment of lightharvesting pigment concentrations over relatively long time scales involving weeks
to months (Cummings and Zimmerman 2003; Celebi 2016). Carbon limitation of
photosynthesis combined with high O 2 concentration in the leaf epidermis also
appears to promote the dissimilatory photorespiration reaction of Rubisco (Buapet
et al. 2013; Celebi 2016). The photorespiratory pathway is often considered a wasteful relic of this enzyme’s evolution in a high-CO 2 world because it lowers the efficiency of photosynthesis by oxidizing carbon substrates from the Calvin cycle (Xue
et al. 2010), reducing net photosynthesis by 25% or more. In doing so, however,
photorespiration may provide a CO 2 -responsive “clutch” that facilitates the turnover
of Rubisco and maintains the flow of electrons generated by photosystem II, thereby
preventing photoinhibition in even the highest light environments.
It is ironic to consider that increasing the atmospheric concentration of CO 2 ,
perhaps the most significant global impact of anthropogenic climate change, may
offset some of the insults visited on seagrass populations by anthropogenic eutrophication and climate warming. Our ability to develop a predictive understanding of
the future trajectory of seagrass-based ecosystems to climate change will require a
R.C. Zimmerman
8.4 Implications of the Seagrass Paradox for the Future
of Seagrass Systems
The systems analysis described above leads us to wonder why seagrasses have not
followed the example of many of their algal counterparts in evolving more efficient
carbon-concentrating mechanisms to cope with relative scarcity of this important
inorganic carbon substrate and why they bothered to retain so much excess photosynthetic capacity in a low-CO 2 world. The paradoxically high reliance on CO 2(aq)
may be the legacy of the evolutionary origins of seagrasses in the high-CO 2
environment of the Cretaceous (Hartog et al. 1979; Retallack 2001). Further, the
possession of roots and rhizomes gives seagrasses a unique ability to colonize and
exploit the nutrient-rich, permanently flooded sediments of protected coastal shores
that may have provided an important refuge from competition for light and space
with the more photosynthetically efficient algae. Without competition from the
algae in oligotrophic waters of the preindustrial world, seagrasses may simply not
have experienced the selective pressure required to increase photosynthetic efficiency, or to reallocate resources invested in unnecessary photosynthetic capacity,
in these shallow, high light environments. Unfortunately, anthropogenic alteration
of the coastal ocean is probably occurring too rapidly to facilitate adaptive change
in this group of organisms that has persisted for 100 million years.
Despite their inefficiency in extracting CO 2 for photosynthesis in seawater, seagrasses possess marvelous physiological mechanisms to protect the leaves from
photoinhibition in high light environments. Carbon limitation imposes a relatively
low threshold for irradiance saturation of photosynthesis that should make seagrasses extremely vulnerable to photoinhibition. Yet, paradoxically, they thrive best
in high light environments. Acclimation processes that may protect seagrass leaves
involve dynamic xanthophyll cycling capable of modulating non-photochemical
quenching on time scales of minutes (Ralph et al. 2002) to the adjustment of lightharvesting pigment concentrations over relatively long time scales involving weeks
to months (Cummings and Zimmerman 2003; Celebi 2016). Carbon limitation of
photosynthesis combined with high O 2 concentration in the leaf epidermis also
appears to promote the dissimilatory photorespiration reaction of Rubisco (Buapet
et al. 2013; Celebi 2016). The photorespiratory pathway is often considered a wasteful relic of this enzyme’s evolution in a high-CO 2 world because it lowers the efficiency of photosynthesis by oxidizing carbon substrates from the Calvin cycle (Xue
et al. 2010), reducing net photosynthesis by 25% or more. In doing so, however,
photorespiration may provide a CO 2 -responsive “clutch” that facilitates the turnover
of Rubisco and maintains the flow of electrons generated by photosystem II, thereby
preventing photoinhibition in even the highest light environments.
It is ironic to consider that increasing the atmospheric concentration of CO 2 ,
perhaps the most significant global impact of anthropogenic climate change, may
offset some of the insults visited on seagrass populations by anthropogenic eutrophication and climate warming. Our ability to develop a predictive understanding of
the future trajectory of seagrass-based ecosystems to climate change will require a
R.C. Zimmerman