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
Although most seagrasses are regarded as shade- or low light-adapted Ralph and Burchett, 1995, shallow-water or intertidal species may sustain photoinhibition from
high photon flux densities during low tides Ralph and Burchett, 1995. Photoinhibition is defined here as a reduction in photosynthetic rates in response to high light intensities,
whereas photosuppression is defined as a reduction in photosynthetic rates due to other processes such as toxicological e.g., herbicides, metals or physiological effects e.g.,
feedback inhibition. Photoinhibition is believed to be a photoprotective mechanism that depresses photosynthetic rates PSII and impairs both electron transport and photo-
phosphorylation, thus allowing excessive light energy to be dissipated as heat Krause and Weis, 1991; Hanelt et al., 1994. In seagrasses, this process appears to occur at light
22 21
22 21
intensities between 700 and 1600 mE m s
, most often at.1000 mE m s
, with maximal photoinhibition between 1200 and 1500 h Table 5; Dawes et al., 1987; Hanelt
et al., 1994. The increased energy dissipation in photoinhibition is generally associated with an
increase in zeaxanthin levels in plants, and or with a decrease in the number of active PS II centers Guenther and Melis, 1990; Adams and Demming-Adams, 1992; Hanelt et
al., 1994. Zeaxanthin increases following de-epoxidation of violaxanthin in the xanthophyll cycle, providing the mechanism for the energy dissipation Demming-
Adams and Adams, 1992; Adams et al., 1995; Flanigan and Critchley, 1996. Energy dissipation may also be accomplished through turnover of the D1 protein in the reaction
center of PSII. In high light, continuous D1 protein degradation replacement is believed to occur; but in extremely high light, repair of the reaction center via D1 protein
replacement occurs much more slowly than D1 protein degradation, thus producing a photoinhibitory response Ohad et al., 1984; Guenther and Melis, 1990; Krause and
Weis, 1991; Aro et al., 1993. However, in the seagrass Zostera capricorni, maximum
22 21
synthesis and degradation of D1 occurred at 350 mE m s
, much lower than the light
184 B
.W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205 Table 5
22 21
Irradiance-associated photoinhibition reported in seagrass species, including light intensities mE m s
and experimental and or culture conditions
Species Light
Conditions Source
Temperate Posidonia sinuosa
.1020 Young and basal leaves
Masini et al. 1995 Zostera capricorni
1100 Based on fluorescence
Flanigan and Critchley 1996
2
Zostera marina .1500
NO enrichment
Touchette 1999
3
´ Zostera marina
1200 Young leaves
Jimenez et al. 1987 Zostera marina
No inhibition Light levels.1400
Mazzella and Alberte 1987 ´
Zostera noltii No inhibition
Light levels.5900 Jimenez et al. 1987
Tropical subtropical Halophila engelmannii
700 Culture bottles
Dawes et al. 1987 Halophila ovalis
1000 After 120 min
Ralph and Burchett 1995 Halophila stipulacea
1000 Based on chl response
Drew 1979 Thalassia hemprichii
1600 Low tide
Hanelt et al. 1994
22 21
levels considered to photoinhibit this species 1100 mE m s
; Flanigan and Critchley, 1996. Moreover, D1 protein turnover was not proportional to irradiance, suggesting that
the D1 protein levels in this plant may be more influenced by pH and ATP levels in the thylakoid lumen. If so, then — at least in this seagrass species — D1 protein turnover
does not function in photoprotection via photoinhibition Critchley and Russell, 1994; Flanigan and Critchley, 1996.
Chlorophyll fluorescence techniques for example, PAM fluorimetry have enabled non-intrusive study of the behavior of photosystem II and electron transport Krause and
Weis, 1991; Beer et al., 1998; Ralph et al., 1998. Under typical temperature regimes, most chlorophyll a fluorescence is attributed to PSII, and can be used to gain
information about light conditioning, photosynthetic capacity, photosynthetic efficiency, and electron transport of PSII Krause and Weis, 1991; Ralph et al., 1998. Variable
fluorescence F is related to maximum and initial fluorescence F and F , respective-
v m
o
ly as: F 5 F 2 F Ralph et al., 1998. In ‘sun’ plants that are adapted to grow under
v m
o
high light, F remains relatively constant and F fluctuates Demmig and Bjorkman,
o m
1987; Franklin et al., 1992; Ralph and Burchett, 1995. In contrast, ‘shade’ plants that are adapted for growth in low-light conditions tend to fluctuate substantially in F — a
o
response that has been linked to photoinhibition and or other adverse affects on the PSII reaction centers Demmig and Bjorkman, 1987; Franklin et al., 1992; Ralph and
Burchett, 1995; Dawson and Dennison, 1996. The F F ratio photochemical
v m
efficiency is used to evaluate the physiological state including the extent of photo- inhibition of the photosynthetic apparatus in various plants, including some seagrasses
Table 6. A decrease in this ratio may be associated with environmental stressors that directly affect PSII efficiency Krause and Weis, 1991.
Seagrasses such as Halophila ovalis and Posidonia australis show variations in F
v
that have been interpreted to indicate photosuppression due to UV-B radiation in a photoinhibition-like response see below; Larkum and Wood, 1993. This UV-B
response suggests a lower electron flux through the oxidizing side of reaction center
B .W
. Touchette
, J
.M .
Burkholder
J .
Exp .
Mar .
Biol .
Ecol .
250 2000
169 –
205
185 Table 6
a
Photochemical efficiencies quantum yield, F F reported in seagrass species as an indication of physiological stress
v m
Species F F
F F Conditions
Source
v m
v m
Control Treatment
Light treatments Cymodocea serrulata
0.810 0.680
25 increase UV; 7 d Dawson and Dennison 1996
Halodule uninervis 0.860
0.470 25 increase UV; 7 d
Dawson and Dennison 1996
22 21
Halophila ovalis 0.650–0.700
0.200 1000 mE m
min ; 2 h
Ralph and Burchett 1995 Halophila ovalis
0.840 0.540
25 increase UV; 7 d Dawson and Dennison 1996
Halophila ovalis 0.700–0.800
No change Light deprivation
Longstaff et al. 1999 Syringodium isoetifolium
0.800 0.680
25 increase UV; 7 d Dawson and Dennison 1996
Zostera capricorni 0.840
0.710 25 increase UV; 7 d
Dawson and Dennison 1996 Other stressors
21
Halophila ovalis 0.780
0.700 .1 mg Cd l
; 24 h Ralph and Burchett 1998a
21
Halophila ovalis 0.780
0.150–0.650 1 mg Cu l
; 24 h Ralph and Burchett 1998a
21
Halophila ovalis 0.780
0.700–0.720 1 mg Pd l
; 48 h Ralph and Burchett 1998a
21
Halophila ovalis 0.780
0.300–0.700 1 mg Zn l
; 24 h Ralph and Burchett 1998a
Halophila ovalis 0.780
0.630–0.700 Variable oil exposure; 24 h
Ralph and Burchett 1998b Halophila ovalis
0.780 0.650–0.680
Variable oil dispersant; 24 h Ralph and Burchett 1998b
a
Halophila ovalis 0.750
0.600–0.740 Hyposaline 0–50 sw
Ralph 1998 Halophila ovalis
0.730 0.300–0.700
Hypersaline 150–250 sw Ralph 1998
a a
Zostera marina 0.730 ls
0.600 hs Variable salinity
Kamermans et al. 1999
23
Zostera marina 0.832–0.855
0.219 25 mg Irgarol 1051 dm
; 10 d Scarett et al. 1999
a
Values are indicated for plants in ambient conditions controls and various treatments. The data indicate a general decline in photochemical efficiencies under treatment conditions. Abbreviations: sw, seawater; ls, low salinity; hs, high salinity.
186 B
.W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
1
P680 in PSII, including the primary donor of P680 Z; reaction side of the D1 protein,
which would prevent Q from being reduced Larkum and Wood, 1993. Increases in F ,
o
apparently result, as well, from UV damage to the PSII reaction centers in the seagrasses Cymodocea serrulata, Halodule uninervis, Halophila ovalis, Syringodium isoetifolium
and Zostera capricorni Dawson and Dennison, 1996. F F ratios have been used in
v m
seagrasses to demonstrate photosuppression and PSII responses to UV radiation, light deprivation, and other stressors Table 6.
5. Carbohydrate metabolism
