Community
MATERIALS AND METHODS Experimental Design
Sample collection and laboratory experiments were conducted during the Austral summer 2006. The sediments containing microalgae were collected from Brown Bay, Casey, Antarctica (Figure 1) and exposed to temperatures of –5ºC, –1.8ºC and 8ºC for the duration of four hours. The top 10 mm of the sediment samples were suspended in a beaker and the excess seawater removed. The concentrated sediment samples were then transferred into a transparent 20 ml glass vial. Filtered seawater was added to each vial and the vials were placed in three chambers of different irradiance in a temperature-controlled waterbath (Lauda 20 L, Brinkmann Lauda Inc. Konigshoven, Germany). The waterbath was filled with a mixture of methanol:water (1:4) to prevent the water from freezing. Triplicate sample vials were placed in each irradiance chamber. Irradiance intensities that were used in this experiment were 0 µmol photons m–2 s–1 and 450 µmol photons m–2 s–1, which were
provided by a halogen lamp (Thorn 300W, Borehamwood, UK). The irradiance of each chamber was adjusted by the addition of a number of layers of dark plastic filter. These filters also assisted in reducing the heat produced by the halogen lamps. The irradiance in each chamber
was measured with a LI-COR Quantum-Light Metre (Model L1-189). At the end of each incubation temperature, a replicate was transferred into a cuvette for photosynthetic analysis. A PAM fluorometer was used to determine the quantum yield at the end of each temperature treatment. RLCs were run to obtain values for effective quantum yield (∆F/Fm’), relative electron transport rate (rETRmax),
photosynthetic efficiency (α) and photoacclimation index (Ek). Before measurement, the samples were gently
shaken to ensure even mixing and to limit settlement, then placed inside the measuring cuvette of the Water PAM fluorometer.
Chlorophyll Fluorescence Measurement
The PAM fluorometer uses a weak measuring light (0.15 µmol photons m–2 s–1) to measure the minimum fluorescence
yield (Fo, open PSII reaction centres), while a saturating
pulse (>3000 µmol photons m–2 s–1 for 0.8 s) is used to
determine maximum fluorescence (Fm in the dark) when all
PSII reactions centres are closed. Maximum fluorescence (Fm) is achieved by exposing the dark adapted sample to a
pulse of intense irradiance. The maximum quantum yield of PSII is defined as:
Fv /Fm = (Fm – Fo ) / Fm … 1
To measure the Fv /Fm, benthic microalgae is dark adapted
for approximately 15 minutes. During this period, electron transport stops, thus eliminating the trans-thylakoid pH gradient and allowing the full reduction of the primary electron acceptor QA. A fifteen minute dark adaption
period has been suggested to be adequate by a number of studies as this is thought to be sufficient to result in a stable level of QA oxidation and hence a true measurement of
Fo (minimum fluorescence) can be obtained (Kromkamp
et al. 1998; Consalvey et al. 2005; McMinn & Hegseth 2004; McMinn et al. 2005a; McMinn et al. 2005b; Jordan et al. 2008). If a sample is exposed to additional in-situ actinic light, a similar saturating pulse will lead to a lower maximum fluorescence, Fm’, that provides a measure of the effective quantum yield (∆F/Fm’) and is derived by the following equation:
(Fm’– Fo’)/Fm’ where Fm’– Fo’= Fv
(Genty et al. 1989). … 2 Under most physiological conditions, ∆F/Fm’ is linearly correlated with the yield of carbon assimilation (Genty et al. 1989). Quantum yield of PSII is also a measure of the rate of charge separation in PSII (Bradbury & Baker 1981). Adverse environmental conditions such as high light with extreme temperatures can modulate cellular concentrations of RUBISCO that directly affect PSII. Therefore, a decrease in ∆F/Fm’ values indicates irreversible damage to PSII, which could lead to photoinhibition.
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RLCs were undertaken on all samples (Schreiber et al. 1997). A RLC is light treatment with eight consecutive 10 s intervals of actinic light of increasing intensity with an accompanying yield measurement at the end of each actinic exposure. Light emitting diodes provide the eight step-wise increments of actinic light at 0, 85, 125, 194, 289, 413, 517, 1046 and 1554 µmol photons m–2 s–1. Relative electron
transport rate (rETR) at a given irradiance is given by: rETR = ∆F/Fm’ × PAR … 3 RLCs provide a snapshot of the physiological state of a plant and its ability to adapt its photosynthetic apparatus to rapid changes in irradiance. Rapid light curves can be described using several characteristic parameters such as α (photosynthetic efficiency), rETRmax (relative electron
transport rate) and Ek (photoacclimation index), by fitting
the rETR and irradiance data to a single exponential decay function (McMinn & Hegseth 2004; McMinn et al. 2005b; Ralph & Gademann 2005). In this study, data were exported from WinControl (Walz, Effeltrich, Germany) into SPSS (SPPS 16.0 for Macintosh, SPSS Inc). Empirical data was mathematically fitted to an exponential decay function (Platt et al. 1980), using a Marquardt-Levenberg regression algorithm:
P = Pm (1–e–(αEd/Pm)) … 4
Pm is a scaling factor defined as the maximum potential
rETR (rETRmax) in the absence of photoinhibition and
represents the photosynthetic capacity at saturating light; α is the initial slope of the RLC before the onset of saturation (light limiting condition efficiency) and Ed is
the down-welling irradiance (400 nm – 700 nm). Ek is the
photoadaptive index or minimum point of light saturation (Falkowski and Raven 1997) and is described by the following equation:
Ek = rETRmax / α … 5
NPQ (non-photochemical quenching) is a measurement of the activity of the protective mechanism which is designed to protect against over-reduction of the photosynthetic electron transport chain by dissipation of excess absorbed light energy in the PSII antenna system as heat (Demmig- Adams & Adams 1992). NPQ was determined by the following equation:
NPQ = (Fm–Fm’) / Fm’ (Schreiber 2004) … 6
Analysis of the Rate of Recovery
The method of determining and analyzing the rate of recovery was adapted from McMinn & Hattori (2006).
Figure 1. Location of sampling area in Brown Bay, near Casey Station, Antarctica.
Southern Ocean Antartica Brown Bay Casey Casey 2 KM 0 Southern Ocean paper 10.indd 83 7/31/2010 9:42:47 PM
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The measurement of recovery from photoinhibition was made using the pre-installed routine RLC + recovery of the water-PAM (Walz, Effeltrich, Germany). After the last actinic light period of the RLC, the sample was left in the dark while Fv/Fm was determined after 30, 60, 180, 300
and 600 s. The rate of recovery from photoinhibition was calculated following Oliver et al. (2003):
Φt = ΦI + (ΦM – ΦI) (1 – e –rt ) … 7
where,
Φt is the given value of Fv /Fmat time t (s), ΦI is the initial value
of Fv /Fm before recovery measurements have commenced,
ΦM is the fully recovered value and r is an exponential rate
constant for the recovery of Fv /Fm from photoinhibition
(s–1). This formula assumes that the recovery rate of F v /Fm
is dependent on the degree of damage. Statistical Analysis
Mean and standard deviations were calculated from three independent replicates. To evaluate the effects of temperature and irradiances (fixed factor) on the photosynthetic parameters (ΔF/Fm’, α, rETRmax and Ek), NPQ and recovery,
a two-way analysis of variance (ANOVA) was used. Differences were accepted as significant at P<0.05 unless otherwise stated.
RESULTS
The effects of temperature and irradiance on all the photosynthetic parameters are presented in Figure 2a to 2f. Decrease in values was observed in a majority of the parameters at temperatures of –5ºC and 8ºC. These reductions caused by adverse temperature, however, did not enhance photoinhibition. Although significant differences (ANOVA, P<0.05) were observed in ∆F/Fm'
values, no photoinhibition occurred as the microalgae were able to recover after each treatment. At temperatures of –5 and –1.8ºC, higher values of ∆F/Fm’ were observed in the dark adapted samples (Figure 2a). In summary, ∆F/Fm' was
more affected by warmer temperatures than colder temperatures. In contrast to the ∆F/Fm' microalgae
incubated in ambient temperature (–1.8ºC) showed lesser ability to utilise the given irradiance by comparison to –5ºC and 8ºC. This was seen in the α values in Figure 2b. Effects of temperature were more significant (ANOVA, P = 0.0178) by comparison to between irradiances (ANOVA, P = 0.5771). Temperatures up to 8ºC were found to have a greater effect on rETRmax
(photosynthetic capacity) by comparison to –5ºC (Figure 2c) although, there were significant differences (ANOVA, P = 0.0016) between all temperatures. The photoacclimation index (Ek) showed a consistent trend
when higher values were obtained in high irradiance
adapted microalgae. Only the microalgae incubated in ambient temperature (–1.8ºC) and high irradiance showed the ability to adapt to the given experimental irradiance condition, where Ek values were almost similar to the
experimental irradiance (Figure 2d). Significant (ANOVA, P = 0.0013) effects between temperatures were observed in the Ek values. Figure 2e shows that the ability to dissipate
excess light energy via NPQ was affected by the adverse temperatures (–5ºC and –1.8ºC). However, in colder and ambient temperatures, microalgae showed a better ability to recover than at warmer temperature (Figure 2f). In summary the data showed that photosynthetic activity was still present at high irradiance and temperature indicating no severe photodamage had occurred.