174
impacted by summer heat stress (Zimmerman et al. 2017). The experiment also
demonstrated a logarithmic response to CO 2 in terms of shoot proliferation, size,
growth, and sugar accumulation that was fundamentally consistent with short-term
laboratory experiments performed with other eelgrass populations from cool ocean
climates and other seagrass species from tropical and temperate environments.
These experimental results further validate model predictions of the combined
effects of climate warming and ocean carbonation on eelgrass distributions in the
Chesapeake region (Zimmerman et al. 2015). In addition to reproducing the negative effects of warm summer temperatures on eelgrass distributions, the model demonstrated that CO 2 increases projected for the next century should stimulate
photosynthesis sufficiently to offset the negative effects of thermal stress on eelgrass growing in the Chesapeake region. When combined, the experimental and
modeling results suggest that increased CO 2 availability can serve as a quantitative
antagonist to counter the negative impact of climate warming on seagrass growth
and survival.
The combined impacts of light and CO 2 availability on photoacclimation
responses in eelgrass are now being unraveled. Carbon limitation of photosynthesis
appears to be a common feature of most seagrass species (Invers et al. 2001; Jiang
et al. 2010; Koch et al. 2013) and helps explain the characteristically heavy stable
carbon isotope values (δ
13
C = −15 to 0) that have been used to trace seagrass carbon
through marine food webs (Hemminga and Mateo 1996). McPherson et al. (2015)
developed a mathematical model parameterized by laboratory experiments in order
to predict the impacts of substrate availability on carbon isotope fractionation in
seagrasses. Model predictions of δ
13
C were most sensitive to DIC and flow, but were
less sensitive to DIC source [CO 2(aq) vs. HCO 3
− ], indicating that carbon limitation of
Rubisco was the primary driver of seagrass δ
13
C. Accurate model predictions of
specific δ
13
C values reported for a variety of seagrass taxa from different environments provided a systems-level understanding of the environmental control of carbon isotope composition of seagrasses. The mathematical relationships embodied in
this model will become increasingly important for predicting the response of these
ecosystem engineers to local processes that affect light availability and flow, as well
as global impacts of climate warming and ocean acidification.
A long-term experiment conducted with Zostera marina L. (eelgrass) grown in
controlled outdoor aquaria revealed predictive increases in absolute growth, plant
size, and flowering that could be traced to increased rates of light-saturated photosynthetic and sucrose formation under CO 2 enrichment (Celebi 2016; Zimmerman
et al. 2017). In contrast to these increases in whole-plant performance characteristics, photosynthetic and photoprotective pigment content in eelgrass leaves was
downregulated, suggesting an important role for Rubisco in balancing redox state in
the chloroplast, which regulates expression of light-harvesting complexes
(Backhausen and Scheibe 1999; Pfannschmidt 2003; Hanke et al. 2009; Hüner et al.
2012). These regulatory mechanisms are generally thought to be controlled by the
redox state of Q A (a plastoquinone) in the thylakoid membrane that depends on the
continuity of electron transport under various limiting conditions (Pfannschmidt
2003; Pfannschmidt and Yang 2012). Thus, in addition to capturing energy for
R.C. Zimmerman
impacted by summer heat stress (Zimmerman et al. 2017). The experiment also
demonstrated a logarithmic response to CO 2 in terms of shoot proliferation, size,
growth, and sugar accumulation that was fundamentally consistent with short-term
laboratory experiments performed with other eelgrass populations from cool ocean
climates and other seagrass species from tropical and temperate environments.
These experimental results further validate model predictions of the combined
effects of climate warming and ocean carbonation on eelgrass distributions in the
Chesapeake region (Zimmerman et al. 2015). In addition to reproducing the negative effects of warm summer temperatures on eelgrass distributions, the model demonstrated that CO 2 increases projected for the next century should stimulate
photosynthesis sufficiently to offset the negative effects of thermal stress on eelgrass growing in the Chesapeake region. When combined, the experimental and
modeling results suggest that increased CO 2 availability can serve as a quantitative
antagonist to counter the negative impact of climate warming on seagrass growth
and survival.
The combined impacts of light and CO 2 availability on photoacclimation
responses in eelgrass are now being unraveled. Carbon limitation of photosynthesis
appears to be a common feature of most seagrass species (Invers et al. 2001; Jiang
et al. 2010; Koch et al. 2013) and helps explain the characteristically heavy stable
carbon isotope values (δ
13
C = −15 to 0) that have been used to trace seagrass carbon
through marine food webs (Hemminga and Mateo 1996). McPherson et al. (2015)
developed a mathematical model parameterized by laboratory experiments in order
to predict the impacts of substrate availability on carbon isotope fractionation in
seagrasses. Model predictions of δ
13
C were most sensitive to DIC and flow, but were
less sensitive to DIC source [CO 2(aq) vs. HCO 3
− ], indicating that carbon limitation of
Rubisco was the primary driver of seagrass δ
13
C. Accurate model predictions of
specific δ
13
C values reported for a variety of seagrass taxa from different environments provided a systems-level understanding of the environmental control of carbon isotope composition of seagrasses. The mathematical relationships embodied in
this model will become increasingly important for predicting the response of these
ecosystem engineers to local processes that affect light availability and flow, as well
as global impacts of climate warming and ocean acidification.
A long-term experiment conducted with Zostera marina L. (eelgrass) grown in
controlled outdoor aquaria revealed predictive increases in absolute growth, plant
size, and flowering that could be traced to increased rates of light-saturated photosynthetic and sucrose formation under CO 2 enrichment (Celebi 2016; Zimmerman
et al. 2017). In contrast to these increases in whole-plant performance characteristics, photosynthetic and photoprotective pigment content in eelgrass leaves was
downregulated, suggesting an important role for Rubisco in balancing redox state in
the chloroplast, which regulates expression of light-harvesting complexes
(Backhausen and Scheibe 1999; Pfannschmidt 2003; Hanke et al. 2009; Hüner et al.
2012). These regulatory mechanisms are generally thought to be controlled by the
redox state of Q A (a plastoquinone) in the thylakoid membrane that depends on the
continuity of electron transport under various limiting conditions (Pfannschmidt
2003; Pfannschmidt and Yang 2012). Thus, in addition to capturing energy for
R.C. Zimmerman