172
Seagrasses are also vulnerable to elevated summertime water temperature,
despite considerable physiological tolerance to short-term temperature elevation. At
least part of the vulnerability to extreme summer temperature results from the differential responses of photosynthesis and respiration. Eelgrass near the Chesapeake
Bay, for example, are commonly exposed to seasonal temperature excursions ranging from wintertime lows near 5°C to summertime highs exceeding 25°C. Prolonged
summer temperatures can approach 30°C during particularly warm years, causing
eelgrass meadows to die back throughout the southern Chesapeake Bay (Orth and
Moore 1983; Moore and Jarvis 2008). This thermal intolerance has been linked to
the differential effects of temperature on photosynthesis and respiration (Evans
et al. 1986; Zimmerman et al. 1989). In essence, the Q 10 response for eelgrass respiration is about 2.5, but only 1.98 for photosynthesis. Thus, temperature causes respiration rates to increase more dramatically than photosynthesis rates, making
eelgrass vulnerable to negative carbon balance above 25°C.
8.2.3 A Surprising Mechanism Responsible for High Light
Requirements
Much of the paradoxical vulnerability of this otherwise highly adapted group of
marine angiosperms to light limitation and thermal stress can be attributed to their
relatively poor ability to extract dissolved inorganic carbon (DIC) from seawater to
support photosynthesis. Seawater is an alkaline medium, and most of the DIC is
present in the form of bicarbonate ( HCO 3
− ≅ 2 mM). However, CO 2 is the exclusive
substrate for the carboxylation reaction of Rubisco, and the concentration of aqueous CO 2 [CO 2(aq) ] is sufficiently low (10 to 20 μM) that passive diffusion of CO 2
cannot satisfy the photosynthetic demand for inorganic carbon in most cases (Raven
2010). Consequently, many marine autotrophs possess carbon-concentrating mechanisms for extracting CO 2 from the more abundant pool of HCO 3
− in seawater
(Falkowski and Raven 2007; Raven and Beardall 2014).
Although seagrass leaves are capable of extracellular dehydration and direct
uptake of HCO 3
− (Al-Moghrabi et al. 1996; Beer and Rehnberg 1997), lightsaturated photosynthesis of many species is CO 2 limited. The instantaneous positive
response to increased CO 2(aq) demonstrates a constitutive ability for light harvesting,
electron transport, and carbon fixation at rates that vastly exceed the C-limited photosynthetic capacity of seagrass leaves in the present-day ocean (Durako 1993;
Zimmerman et al. 1995; Beer and Koch 1996; Invers et al. 2001). Higher rates of
photosynthesis resulting from elevated CO 2(aq) can reduce the light requirements for
daily carbon balance (Zimmerman et al. 1997) and promote the accumulation of
carbon reserves that increases vegetative shoot proliferation and flowering shoot
differentiation (Palacios and Zimmerman 2007). Coupling these experimentally
derived physiological responses with a geometrically explicit formulation of radiative transfer through submerged plant canopies led to the development of GrassLight
(Fig 8.2), a bio-optical model that permits quantitative exploration of the impacts of
R.C. Zimmerman
Seagrasses are also vulnerable to elevated summertime water temperature,
despite considerable physiological tolerance to short-term temperature elevation. At
least part of the vulnerability to extreme summer temperature results from the differential responses of photosynthesis and respiration. Eelgrass near the Chesapeake
Bay, for example, are commonly exposed to seasonal temperature excursions ranging from wintertime lows near 5°C to summertime highs exceeding 25°C. Prolonged
summer temperatures can approach 30°C during particularly warm years, causing
eelgrass meadows to die back throughout the southern Chesapeake Bay (Orth and
Moore 1983; Moore and Jarvis 2008). This thermal intolerance has been linked to
the differential effects of temperature on photosynthesis and respiration (Evans
et al. 1986; Zimmerman et al. 1989). In essence, the Q 10 response for eelgrass respiration is about 2.5, but only 1.98 for photosynthesis. Thus, temperature causes respiration rates to increase more dramatically than photosynthesis rates, making
eelgrass vulnerable to negative carbon balance above 25°C.
8.2.3 A Surprising Mechanism Responsible for High Light
Requirements
Much of the paradoxical vulnerability of this otherwise highly adapted group of
marine angiosperms to light limitation and thermal stress can be attributed to their
relatively poor ability to extract dissolved inorganic carbon (DIC) from seawater to
support photosynthesis. Seawater is an alkaline medium, and most of the DIC is
present in the form of bicarbonate ( HCO 3
− ≅ 2 mM). However, CO 2 is the exclusive
substrate for the carboxylation reaction of Rubisco, and the concentration of aqueous CO 2 [CO 2(aq) ] is sufficiently low (10 to 20 μM) that passive diffusion of CO 2
cannot satisfy the photosynthetic demand for inorganic carbon in most cases (Raven
2010). Consequently, many marine autotrophs possess carbon-concentrating mechanisms for extracting CO 2 from the more abundant pool of HCO 3
− in seawater
(Falkowski and Raven 2007; Raven and Beardall 2014).
Although seagrass leaves are capable of extracellular dehydration and direct
uptake of HCO 3
− (Al-Moghrabi et al. 1996; Beer and Rehnberg 1997), lightsaturated photosynthesis of many species is CO 2 limited. The instantaneous positive
response to increased CO 2(aq) demonstrates a constitutive ability for light harvesting,
electron transport, and carbon fixation at rates that vastly exceed the C-limited photosynthetic capacity of seagrass leaves in the present-day ocean (Durako 1993;
Zimmerman et al. 1995; Beer and Koch 1996; Invers et al. 2001). Higher rates of
photosynthesis resulting from elevated CO 2(aq) can reduce the light requirements for
daily carbon balance (Zimmerman et al. 1997) and promote the accumulation of
carbon reserves that increases vegetative shoot proliferation and flowering shoot
differentiation (Palacios and Zimmerman 2007). Coupling these experimentally
derived physiological responses with a geometrically explicit formulation of radiative transfer through submerged plant canopies led to the development of GrassLight
(Fig 8.2), a bio-optical model that permits quantitative exploration of the impacts of
R.C. Zimmerman