B .W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
177 Table 2
Photosynthetic suppression reported in seagrasses by a specific carbonic anhydrase CA inhibitor, acetazola- mide pH also indicated. Greater suppression of photosynthesis indicates higher reliance on CA in acquiring
inorganic carbon for photosynthesis. Data are given as means61 S.E. for temperate and tropical subtropical ˜
seagrass species as designated by Phillips and Menez, 1988 Species
Photosynthetic pH n
Source suppression
Temperate Posidonia australis
25 7.6–8.8
James and Larkum 1996 Posidonia oceanica
53 8.2–8.5
Invers et al. 1999 Zostera marina
60 8.2
Beer and Rehnberg 1997 Grand mean61 S.E.
46.0610.7 n 53 species
Tropical subtropical Cymodocea nodosa
35 8.2–8.5
Invers et al. 1999 ¨
Cymodocea rotundata 55
8.2 Bjork et al. 1997
¨ Cymodocea serrulata
50 8.2
Bjork et al. 1997 ¨
Enhalus acoroides 60
8.2 Bjork et al. 1997
¨ Halodule wrightii
50 8.2
Bjork et al. 1997 ¨
Halophila ovalis 25
8.2 Bjork et al. 1997
¨ Syringodium ioetifolium
45 8.2
Bjork et al. 1997 ¨
Thalassia hemprichii 40
8.2 Bjork et al. 1997
¨ Thalassodendron ciliatum
20 8.2
Bjork et al. 1997 Grand mean61 S.E.
42.264.5 n 59 species
made either through direct enzymatic measurements, or through indirect decline in photosynthesis following addition of a CA-specific inhibitor acetazolamide. Decreases
measured in photosynthesis following application of acetazolamide have ranged from 0
2
to as much as 75, suggesting that the degree at which seagrasses utilize HCO via
3
membrane-bound CA is highly variable Table 2.
3. Photosynthesis–irradiance relationships
Most of the research that has been conducted on seagrass physiology has focused on photosynthesis–irradiance P –I relationships Fig. 2, in efforts to determine light
levels needed to maintain healthy growth. Such curves have provided estimates for photosynthetic capacity P
, photosynthetic quantum efficiency a; moles of carbon
max
fixed per mole of PAR absorbed, saturating irradiance for photosynthesis I 5 P a,
k max
compensation irradiance I , and other variables Tables 3 and 4. The parameter I
c c
represents the light intensity at which oxygen production is equivalent to oxygen demand during respiration in photosynthetic tissues. Whole-plant respiratory oxygen
demand is higher than the respiratory demand of photosynthetic tissues only; thus, I
cp
represents the additional light required for whole-plant compensation irradiance Tomasko, 1993. Most of the available data for I , rather than I do not consider
c cp
belowground and non-photosynthetic tissues, and are of limited use in predicting whole-plant carbon balance Dunton and Tomasko, 1991; Tomasko, 1993; Burd and
178
B .W
. Touchette
, J
.M .
Burkholder
J .
Exp .
Mar .
Biol .
Ecol .
250 2000
169 –
205 Table 3
Photosynthetic–irradiance parameters reported for seagrass species including I compensation irradiance, I saturating irradiance, a light-limited slope or quantum
c k
efficiency; | a 5 P I , and growing and or measurement conditions. Data are given as means 6 1 S.E. for temperate and tropical subtropical species, or as
max k
ranges if means were not available. Grand means confined to consideration of I and I values, since they were expressed with common units across studies were
c k
calculated using midrange values for those cases
b
Species I
I a
Conditions Source
c k
22 21
22 21
a
mE m s
mE m s
Photosyn. units Temperate
P 21
21
Amphibolis antarctica 17–23
32–40 0.039–0.054; mg O mg
chl h Variable temp. 13–238C
Masini and Manning 1997
2 P
21 21
Amphibolis griffithii 20
70 0.035; mg O mg
chl h Gross P
Masini et al. 1995
2 max
P 21
21
Amphibolis griffithii 15–17
25–56 0.039; mg O mg
chl h Variable temp. 13–238C
Masini and Manning 1997
2 P
21 21
Posidonia australis 25
90 0.009; mg O mg
chl h Gross P
Masini et al. 1995
2 max
P 21
21
Posidonia australis 17–20
35–50 0.015–0.024; mg O mg
chl h Variable temp. 13–238C
Masini and Manning 1997
2 21
21
Posidonia oceanica 37
257 0.01; mg O g
dw h Yearly and tissue age means
Alcoverro et al. 1998
2 P
21 21
Posidonia sinuosa 24
55–59 0.016–0.019; mg O mg
chl h Gross P
Masini et al. 1995
2 max
P 21
21
Posidonia sinuosa 20–25
38–55 0.015; mg O mg
chl h Variable temp. 13–238C
Masini and Manning 1997
2 22
21
Zostera capricorni 45
182 0.018; mm O m
s Leaf segments, artif. seawater
Flanigan and Critchley 1996
2 21
21
Zostera marina No data
100–290 0.0035; mm O g
fw min Seasonal variations
Zimmerman et al. 1995
2 22
21
Zostera marina 7–13
40– 55 0.0020–0.0053; mM O dm
min Apex young intermed. leaf
Mazzella and Alberte 1986
2
Zostera marina 10
100 No data
Leaf segments Dennison and Alberte 1982
Zostera marina 28
230 No data
Whole shoots Drew 1979
P 21
21 2
Zostera marina 85
450 0.005–0.008; mg O g
dw h NO
enrichment Touchette 1999
2 3
B .W
. Touchette
, J
.M .
Burkholder
J .
Exp .
Mar .
Biol .
Ecol .
250 2000
169 –
205
179
21 21
Zostera marina 10– 15
65 0.002–0.004; mM O mg
chl min Variable temp. 15–358C
Marsh et al. 1986
2 21
21
´ Zostera marina
30– 35 250
0.008; mg C g dw h
Young leaf segments Jimenez et al. 1987
21 21
Zostera marina 12– 60
198–210 0.003; mm O mg
chl min Variable soil sulfide
Goodman et al. 1995
2 P
21 21
Zostera noltii 98–300
222–390 0.23–0.63; mg O g
AFD min Seasonal variations
Vermatt and Verhagen 1996
2 21
21
´ Zostera noltii
30– 35 350
0.008; mg C g dw h
Young leaf segments Jimenez et al. 1987
Grand mean61 S.E. 28.563.3
146.0638.8 n58 species
Tropical subtropical
21 21
Cymodocea nodosa | 0.01–43
26–230 0.005–0.63; mg O g
dw h Variable temp. 10–308C
Terrados and Ros 1995
2
Halodule uninervis 20–40
50 No data
Variable water depth Beer and Waisel 1982
P 22
21
Halodule wrightii 85
319 0.5–2.4; mm O g
dw h In situ
Dunton and Tomasko 1994
2 P
22 21
Halodule wrightii 81
319 0.5–2.4; mm O g
dw h In situ, yearly means
Dunton 1996
2
Halophila engelmannii 10–60
430–500 No data
Seasonal and salinity response Chan et al. 1987
Halophila stipulacea 20–40
100 No data
Variable water depth Beer and Waisel 1982
Syringodium filiforme 10–60
430–500 No data
Seasonal and salinity response Chan et al. 1987
Thalassia testudinum 10–60
430–500 No data
Seasonal and salinity response Chan et al. 1987
22
¨ Thalassodendron ciliatum
No data 1.5–5 W m
No data Plants from 0.5233 m depth
Parnik et al. 1992 Grand mean61 S.E.
38.567.6 284.6671.1
n57 species
a
Slope values a are ratios and, consequently, are dependent on units; therefore, units of photosynthesis are also provided note that light units were standard
´ ¨
across studies. Light values from Zostera noltii Jimenez et al., 1987 and Thalassodendron ciliatum Parnik et al., 1992 were not used to calculate the grand mean,
22
due to use of extreme light levels and different units Watts m , respectively. Note that AFD5ash-free dry weight; chl5chlorophyll; and dw5dry weight.
b
Superscript letter P indicates measurements using whole plants thus, considered the influence, or demand, of both above- and belowground tissues on light requirements to sustain the plants e.g., I , and I .
cp kp
180
B .W
. Touchette
, J
.M .
Burkholder
J .
Exp .
Mar .
Biol .
Ecol .
250 2000
169 –
205 Table 4
Photosynthetic parameters reported for seagrass species including P maximum photosynthesis, I
minimum light for maximum photosynthesis, and growing
max max
and or measurement conditions. Data are given as means 6 1 S.E. for temperate and tropical subtropical species, or as ranges if means were not available
b
Species P
I Conditions
Source
max max
22 21 a
mE m s
Temperate
P 21
21
Amphibolis antarctica 1–1.5 mg O g
h No data
Variable temp. 13–238C Masini and Manning 1997
2 P
21 21
Amphibolis griffithii 2.4 mg O mg chl
h No data
Gross P Masini et al. 1995
2 max
P 21
21
Amphibolis griffithii 1–3.5 mg O g
h No data
Variable temp. 13–238C Masini and Manning 1997
2 P
21 21
Posidonia australis 0.84 mg O mg chl
h No data
Gross P Masini et al. 1995
2 max
P 21
21
Posidonia australis 0.8–2.0 mg O g
h 400–700
Variable temp. 13–238C Masini and Manning 1997
2 21
21
Posidonia oceanica 7.7 mg O g
dw h 350
Yearly and tissue age means Alcoverro et al. 1998
2 P
21 21
Posidonia oceanica 2.2 mg C mg
dw h No data
Highest seasonal value, mid-leaf Modigh et al. 1998
P 21
21
Posidonia sinuosa 0.8–1.1 mg O mg chl
h No data
Gross P Masini et al. 1995
2 max
P 21
21
Posidonia sinuosa 0.6–1.2 mg O g
h 100–800
Variable temp. 13–238C Masini and Manning 1997
2 22
21
Zostera capricorni 4.2 mmol O m
s 450
Leaf segments, artif. seawater Flanigan and Critchley 1996
2 21
21
Zostera marina 0.5–1.7 mmol O gfw
min 200–900
Seasonal variations Zimmerman et al. 1995
2 22
21
Zostera marina 0.66 mM O dm
min 200
Apex of young intermed. leaf Mazzella and Alberte 1986
2 21
21
Zostera marina 1.2–1.5 mmol O g
dw min 100
Leaf segments Dennison and Alberte 1982
2 22
21
Zostera marina 2.0 mmol O dm
min 230
Whole shoots Drew 1979
2 P
21 21
2
Zostera marina 5–6.2 mg O g
dw h 600
NO enrichment
Touchette 1999
2 3
B .W
. Touchette
, J
.M .
Burkholder
J .
Exp .
Mar .
Biol .
Ecol .
250 2000
169 –
205
181
21 21
Zostera marina 0.40 mM O mg
chl min 75–150
Variable temp. 15–358C Marsh et al. 1986
2 21
21
Zostera marina 0.5 mmol O mg
chl min 700–900
Variable soil sulfide Goodman et al. 1995
2 P
– 1 21
Zostera noltii 71–236 mg O g
AFD min 150–900
Seasonal variations Vermatt and Verhagen 1996
2 21
21
´ Zostera noltii
3–6.5 mg C g dw h
3600 Young leaf segments
Jimenez et al. 1987
a
Grand mean61 S.E. 452631.6
n55 species Tropical subtropical
21 21
21
Cymodocea nodosa 3.0 mg O g
dw h No data
Saturating light, flow.0.64 cm s Koch 1994
2 21
21
Cymodocea nodosa 2.4–8 mg O g
dw h 100–400
Variable temp. 10–308C Terrados and Ros 1995
2 21
21
Halodule uninervis 0.12 mmol O mg
chl min No data
Variable depth Beer and Waisel 1982
2 P
21 21
Halodule wrightii 374 mmol O g
dw h 520
In situ Dunton and Tomasko 1994
2 P
21 21
Halodule wrightii 422 mmol O g
dw h 400–800
In situ, yearly means Dunton 1996
2 21
21
Halophila engelmannii 40–65 ppm O g
dw h No data
Seasonal and salinity response Chan et al. 1987
2 21
21
Halophila stipulacea 40 mmol O mg
chl min No data
Variable depth Beer and Waisel 1982
2
Syringodium filiforme No data
No data Seasonal and salinity response
Chan et al. 1987
21 21
21
Thalassia testudinum 3.2 mg O g
dw h No data
Artif. seawater; flow.0.25 cm s Kock 1994
2
Thalassia testudinum No data
No data Seasonal and salinity response
Chan et al. 1987
21 21
22
¨ Thalassodendron ciliatum
30–50 mmol CO kg dw s
20–80 W m Plants from 0.5–33 m depth
Parnik et al. 1992
2 a
Grand mean61 S.E. 4056155
n52 species
a
Grand means confined to consideration of I values since they were expressed in common units across studies were calculated using the midrange values for
max
´ ¨
those cases. Light values from Zostera noltii Jimenez et al., 1987 and Thalassodendron ciliatum Parnik et al., 1992 were not used to calculate the grand mean, due
22
to use of extreme light levels and different units Watts m , respectively. Note that AFD5ash-free dry weight; chl5chlorophyll; and dw5dry weight.
b
Superscript letter P indicates measurements using whole plants; thus, these data considered the influence or demand of belowground tissues on photosynthesis and light requirements e.g. I
.
max
182 B
.W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
Dunton, 2000. Caution should also be used in interpreting data on saturating irradiance for photosynthesis I , because seagrass photosynthesis has often been shown to
k
increase under light intensities greater than I Fig. 2; Tomasko, 1993.
k
Geographic comparisons of seagrass photosynthesis are difficult because of inconsis- tencies in units used for photosynthetic rates. From the available data, temperate-zone
seagrasses have lower I values than tropical subtropical species means61 standard
c 22
21
error [S.E.] as 28.563.3 and 38.567.6 mE m s
, respectively; Table 3, indicating that temperate seagrasses can utilize lower light levels for photosynthesis. Temperate-
zone seagrasses also have been reported to have lower I values than tropical
k 22
21
subtropical species means61 S.E. as 146.0638.8 and 284.6671.1 mE m s
, respectively; Table 3, which would be expected since available ambient light is lower in
temperate regions. In addition to the light intensity, the duration of the daily light period at which light
equals or exceeds the photosynthetic light saturation point H is important in seagrass
sat
growth and survival, especially for plants at or near the maximum depth distribution for the species in a given location Dennison and Alberte, 1982, 1985; Zimmerman et al.,
1995a. A parameter taken from phytoplankton studies, H , has been used to estimate
sat
seagrass productivity, mostly in research on Z . marina Herzka and Dunton, 1998.
Lower H values have been related to significant decreases in productivity and or
sat
increasing mortality in Z . marina Dennison and Alberte, 1985; Dennison, 1987;
Zimmerman and Alberte, 1991; Zimmerman et al., 1991. However, H is site- as well
sat
Fig. 2. Theoretical photosynthesis–irradiance P –I curve, illustrating maximum photosynthesis P ,
max
maximum photosynthetic irradiance I , the minimum irradiance that supports P
, compensation
max max
irradiance I , saturating irradiance I , and photosynthetic efficiency a. Photosynthetic efficiency is
c k
expressed as photosynthetic rate per mole of photons modified from Tomasko, 1993.
B .W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
183
as species-specific range of 3–12 h reported for Z . marina, with this high variability
likely resulting from differences in temperature, metabolic activity, and biomass distribution between C-sink and C-source tissues Zimmerman and Alberte, 1991. Thus,
use of H to predict productivity should not be extrapolated to multiple sites Dennison
sat
and Alberte, 1985; Zimmerman et al., 1989, 1991; Herzka and Dunton, 1998. The H
model assumes that productivity does not occur at light levels below I , thus
sat k
omitting light-limited photosynthesis from consideration Herzka and Dunton, 1997, 1998. Although the model has been used successfully to estimate productivity of Z
. marina, Herzka and Dunton 1998 demonstrated that it is more limited in estimating
productivity of the subtropical seagrass, Thalassia testudinum. For example, during a period of low irradiance due to light attenuation, the H
model predicted 0–37 of the
sat
production that was calculated from numerical integration of empirical data Herzka and Dunton, 1998. In this and other seagrass species with higher light requirements, the H
sat
model may not be applicable because of the potential for extended periods of light- limited photosynthesis.
4. Photoinhibition and photosuppression
