the warm-temperate coast of southern Spain: A review**
F´elix L. Figueroa
∗& Iv´an G´omez
1Departamento de Ecolog´ıa, Facultad de Ciencias, Universidad de M´alaga, Campus Universitario de Teatinos s/n, E-29071 M´alaga, Spain
1Alfred-Wegener-Institut für Polar-und Meeresforschung, Wattenmeerstation Sylt, Hafenstr. 43, D-25992 List/Sylt,
Germany
(∗Author for correspondence; e-mail [email protected])
Received 8 January 2000; revised 17 January 2001
Key words:Atlantic, photoinhibition, photosynthesis, Mediterranean, red algae, Spain, UV radiation
Abstract
A review is presented of the physiological mechanisms developed by benthic macroalgae to cope with deleterious wavelengths, particularly UV radiation. Photoinhibition of photosynthesis, is a photoprotective mechanism in various species studied in southern Spain. Incubations in outdoor systems and transplantation experiments under natural radiation allowed to led to understanding of some the photoprotective strategies used by red algae. Under conditions of enhanced UV-B radiation, algae in shallow sites show marked photoinhibition and rapid recov-ery of photosynthesis (dynamic photoinhibition), whereas algae from deeper locations can suffer photodamage (chronic photoinhibition). The expression of this photoprotective strategy by intertidal species represents an effi-cient physiological adaptation to tolerate deleterious irradiance, when low tides coincide with the onset of solar radiation. Subtidal species can be also exposed to high doses of UV radiation. This is particularly evident in clear, Mediterranean waters, where light (including UV-B) can reach to 10 m depth. The implications of photoacclimation processes for macroalgal ecology in warm-temperate littorals and the possible consequences for outdoor cultivation are outlined in terms of environmental UV variability.
Abbreviations:ETR – electron transport rate; Fmmaximal fluorescence; Fo– initial fluorescence; Fs – current
steady-state fluorescence; Fv– variable fluorescence of plant pre incubated in darkness; Fv/Fm– maximal quantum
yield;F/Fm’ – effective quantum yield; LED – red-light emitting diode; PAM – pulse amplitude modulated; PAR
– photosynthetically active radiation (400–700 nm); UV-A – ultraviolet-A (315–400 nm); UV-B – ultraviolet-B (280–315 nm)
Introduction
The stratospheric ozone depletion and the consequent increase in the penetration of ultraviolet-B (UV-B) radiation (reviewed in Madronich et al., 1995) has
** This paper was presented in the International Symposium ‘Cultivation and Uses of Red Algae’ held in Puerto Vasas, Chile in November 1999 and organized by A. Buschmann, J. Correa and R. Westermeier. The symposium was supported financially by the FONDAP Program in Oceanography and Marine Biology (Chile).
prompted a considerable interest in evaluating its ef-fects on different aspects of plant biology. In aquatic systems, impairments of growth and primary pro-ductivity of different phytoplankton assemblages by short wavelengths (280–315 nm), especially in south-ern oceans, have broadly been documented, since C fixation in oceanic waters is crucial in the regulation of global climate changes (Helbling et al., 1992; Neale et al., 1994; Arrigo 1994, Holm-Hansen et al., 1994). However, the contribution of coastal marine
macro-phytes in the global C fixation and the effects of UV radiation on primary production of these organisms have not been well examined (Larkum & Wood, 1993; Dring et al., 1996; Franklin & Forster, 1997). Re-cent studies reveal that macroalgae may suffer DNA damage (Pakker et al., 2000, Wiencke et al., 2000) and various changes in the photosynthetic apparatus (Dring et al., 1996; Hanelt et al., 1997; Hanelt, 1998; Bischof et al., 1998), and in the particular case of red alga, of the light harvesting pigments (Talarico & Maranzana, 2000). In general, the magnitude of these alterations and their consequences are species-specific and related to the suite of morphological features as well as to patterns of vertical distribution.
In regions like the southern Mediterranean coast of Spain, macroalgae are exposed to very high doses of UV radiation (Häder & Figueroa, 1997). The high irradiance and the transparency of the shallow wa-ter in this region intuitively suggest that macroalgae have developed efficient photoprotective mechanisms to tolerate light stress. Studies on the effects of natural UV radiation on photosynthetic performance of red al-gae from this region (Häder et al.1997; Figueroa et al., 1997; Flores-Moya et al,. 1998; Gómez & Figueroa, 1998; Jiménez et al., 1998) have shown that the oc-currence of photoinhibition of photosynthesis under high solar radiation depends on daily changes in irra-diance, position on the shore, vertical light attenuation or a combination of all these factors. Despite this, no general photoacclimation responses of these organ-isms under different scenarios of UV climate could be predicted. Taking into account the distinct origin and the morpho-functional divergences of these species, a common photoadaptive strategy is unlikely.
The present report gives a overview of photo-ecological processes related to solar radiation in mac-roalgae from southern Spain, especially the red algae. The review summarises experiments on photosyn-thetic performance under different conditions of en-hanced UV radiation using UV cut-off filters andin situ measuring of solar radiation carried during the previous five years. The significance of UV effects and/or photoprotection under current solar radiation is discussed on the basis of physiological mechanisms, e.g. photoinhibition of photosynthesis, whose expres-sion would have implications for ecological fitness and primary productivity. An account of the current methodology used to measure aquatic light fields and photochemical processes is presented in order to em-phasize the importance of monitoring physiological status of plants under changing scenarios of natural
solar radiation. Because some of the genera are ex-ploited commercially, the technical approaches and results described may give valuable new tools for the monitoring and evaluation of factors affecting stand-ing stocks in the natural habitat and of cultivated seaweeds from warm-temperate regions.
Geographical context and characterization of natural light climate
The Mediterranean coast of southern Spain (Andalus-ian coast) represents the northern margin of the Al-boran sea which extends from Tarifa (Strait of Gibral-tar) to the Cabo de Gata in Almería (Figure 1). Hydro-graphically, the upper water mass (up to 150 m depth) constitutes the Atlantic component characterized by temperatures higher than 20◦C in summer and salin-ities lower than 36.5. Therefore, in the Alborán sea, there is an essentially Atlantic influence on the photic zone and especially the littoral assemblages. Although this region is regarded as an oligotrophic system, in some zones such as the east side of Gibraltar and off coast of Málaga, upwelling events caused by remo-bilization of deeper water masses may occur. Due to its transitional character and the remarkable hydro-graphic processes occurring in this region, the west and east coasts of southern Spain differ in water char-acteristics which affect composition and ecological functions of the benthic biota.
Measurements of underwater light conditions were carried out in order to relate the outdoor responses to the UV scenarios frequently found in natural hab-itats. Light penetration through the water column (generally 1 or 0.1% of surface irradiance) depends basically on the optical characteristics (Jerlov, 1976; Kirk, 1994). However, in the case of benthic mac-roalgae, local hydrographic and climatic conditions may be important factor affecting underwater light fields and consequently the environmental threshold for e.g., photosynthesis and germination capacity of spores and gametes (Piazena & Häder, 1997, Wiencke et al., 2000).
Water column profiles were made using a computer-controlled PUV–500 radiometer (Biospher-ical Instruments Inc., USA), which gives readings of cosine-corrected downwelling UV radiation at four bands, 305, 320 340, and 380 nm and a broad band of PAR (400–700 nm). Measurements were performed by submerging the instrument slowly through the wa-ter column to depths of 45 m. Simultaneously, spectral
Figure 1. Study sites along the coast of southern Spain where measurements of chlorophyll fluorescence kinetics and photoinhibition of photosynthesis under solar radiation were carried out.
irradiance in the range between 300 and 800 nm at each depth was recorded using a spectroradiometer (LI-COR 1800-UW, Licor Inc., USA) at different intervals between 0 (just below surface) and 32 m depth. Periodic intercalibrations were performed on the equipment, including the sensors.
Diffuse vertical attenuation coefficients of down-ward irradiance (Kd) were determined using following
formula (Kirk, 1994):
Kd= ln [Ed(z2)/ Ed(z1)]∗(z1-z2)− 1
where Ed(z1)and Ed(z2)are the irradiances at depth z1 and z2. Logarithmic dependencies of light attenuation
on water depths were calculated by non-linear regres-sion over a depth profile of several meters. Optical stratification of the water body is visible if the data deviate from its logarithmic regularity. Kdvalues were
calculated for the whole PAR range and for mono-chromatic radiation measurements of the underwater spectroradiometers.
Figure 2 shows typical depth profiles of light penetration at localities along the southern coast of Spain. The heterogeneous hydrographic characterist-ics, which cause differential attenuation coefficients (Kd) of incident solar radiation along this coast,
res-ult in marked variations in underwater UV-B climates. For example, the coast of Tarifa (Gibraltar, Cádiz: Figure 2A), which is strongly influenced by turbid, At-lantic waters and characterized by high concentrations of particulate material and local upwelling events, has significantly higher Kdvalues than the Mediterranean
waters of Cabo de Gata (Almería; Figure 3B). In the case of the penetration of UV-B measured at 305 nm (Kd,305), values of attenuation measured in
differ-ent localities indicates generally lower than 0.8 m−1. Whereas in the clear waters of Cabo de Gata (Type I in the Jerlov clasification), Kdat 05 nm does not exced
0.2 m−1in summer, in Marina del Este and Fuengirola (Málaga), values can be higher than 0.8 m−1. In Cabo de Gata, PAR reaches depths close to 72 m (1% sur-face irradiance), while 1% of sursur-face levels of UV-B measured at 305 nm is found at depths close to 19 m (summer) and 10 m (winter).
Figure 2. Experimental systems designed to measure physiological responses of macroalgae: A) outdoor incubation system; B) arrangement in field used in transplantation experiments (see text for details).
General methodology
Outdoor incubations as a tool for determination of photoinhibition
Algae were exposed to natural solar radiation in an outdoor system placed on a roof at the campus of the Universidad de Málaga (36◦47 N, 0.4◦19 W) (Figure 3a). The system consisted of three 0.5-L cu-vettes with constant bubbling of air. The cucu-vettes were placed in a water bath at 16–20◦C and covered with three different cut-off filters to provide the following UV conditions:
a) PAR + UV-A + UV-B, by use of Ultraphan 295 (Digefra GmbH, Munich, Germany)
b) PAR + UV-A , by use of Folex 320 (Folex GmbH, Dreieich, Germany)
c) PAR alone, by use of Ultraphan 395 (Digefra GmbH, Munich, Germany)
This experimental design presents various advantages: Firstly, the exposure to natural ratios of PAR to UV radiation, which are difficult to achieve using arti-ficial sources, permits a more realistic extrapolation to the situation in the field. Secondly, it is possible to standardise conditions for macroalgal growth, a situation normally not found in the natural habitat, where unpredictable environment (e.g. water motion or heterogeneous light exposure) makein situ experi-mentation difficult. Finally, a permanent availability of plant material that permits an exhaustive monitoring of
the physiological responses at different time intervals along the day.
In general, daily changes in Chl fluorescence (de-scribed below) were performed from 0830 to 1900 h at intervals of 2 h. In order to examine the recovery capacity after short term exposures to UV radiation, photosynthesis was analysed in plants exposed for 2– 3 h to solar radiation at midday. To induce recovery of photosynthesis, subsequently plants were covered with a light shading screen to reduce incident PAR to values lower than 100 µmol photon m−2s−1 and measured at different time intervals.
Transplantation experiments in the field
Measurements of chlorophyll fluorescence of algae in the natural habitat were carried out in samples just col-lected from shallow waters (<0.5 m depth). In some cases, algae were covered using the filters described previously and incubated for some hours or during a daily cycle to observe the hourly changes in photosyn-thetic activity. In studies of transplantation, generally plants were collected from depths down 10 m and maintained for 2 to 4 days at a depth of 0.5 m using a rope system suspended by buoys anchored to the bottom by mean of stones (Figure 3b). The incub-ations units were cylinders or boxes enveloped with the different foils as described above which contained the algae. After the exposure period, plants were
re-Figure 3. Underwater profiles of UV radiation measured at 305, 320, 340, 380 nm (UV) and 400–700 nm (PAR) using an underwater radiometer PUV500 at two coastal sites in southern Spain: A) Tarifa (Gibraltar Strait); B) Cabo de Gata (Almeria). C) Comparative penetration of UV-B (305 nm) at sites along the southern Spain coast.
moved from the cylinders and immediately measured for PAM fluorescence.
PAM chlorophyll fluorescence as an indicator of photosynthetic efficiency
Pulse amplitude modulated (PAM) chlorophyll fluor-escence associated with PSII was primarily developed to assess stress-dependent changes in photosynthesis of higher plants (Schreiber et al., 1986). The applic-ation of PAM fluorometry to macroalgae is relatively recent and it has become a useful tool for evaluation photosynthesis under different environmental condi-tions and in the laboratory under artificial lamps (Hen-ley et al., 1991; Schreiber et al., 1995; Hanelt, 1996; Häder & Figueroa, 1997). Chlorophyll fluorescence can function as an indicator of the different func-tional levels of photosynthesis such as photon capture by light harvesting pigments, primary light reactions, thylakoid electron transport reactions, dark-enzymatic stroma reactions and slow regulatory feedback pro-cesses (Büchel & Wilhem, 1993). The photochemical principles include two different types of competing de-excitation processes: (1) photochemical energy con-version at the photosystem II (PSII) centres and (2) non-photochemical dissipation of excitation energy at the antenna and reaction centre levels.
Figure 4 shows the different components of fluorescence and energy flux of dark- and light-acclimatised plants. Dark-light-acclimatised algae are used to determine the maximal or optimal quantum yield (Fv/Fm) and under actinic light, the effective quantum
yield (F/Fm’). The parameter Fv represents the
dif-ference between the maximal (Fm) and the minimal
(Fo) fluorescence yield of a dark-acclimatised sample
when all PSII reaction centres fully open (Fo state)
or closed (Fm state). When the sample is transferred
to actinic light, the maximal yield (Fm’) decreases
because part of the reaction centres remains reduced and the electron sink in the dark-reaction of photo-synthesis is partially saturated. The effective quantum yield (F/Fm’) was used by the first time by Genty
et al. (1989) to asses the overall quantum yield of photochemical energy conversion and can be correl-ated to the quantum yield of carbon assimilation (see below). Other fluorescence parameters are the pho-tochemical quenching (qP) and non-phopho-tochemical quenching (qN).
Figure 4. Fluorescence parameters and major photochemical events measured using pulse amplitude modulated chlorophyll fluores-cence associated to PSII. Optimal quantum yield (Fv/Fm), effect-ive quantum yield (F/Fm’), photochemical quenching (qP) and non-photochemical quenching (qN).
Special case red algae
Taking into account the differences in antenna com-ponents between macroalgae and higher plants, an optimisation of the PAM instrumentation has been needed to accurately meet the lower fluorescence emission of red macroalgae (Hanelt, 1996). For ex-ample, the presence of phycobilisomes in the light harvesting system associated to PSII in red macroal-gae results in generally lower fluorescence values than that measured in green and brown algae (Buchel & Wilhelm, 1993). Two instruments used generally for chlorophyll fluorescence determination in red mac-roalgae, PAM–2000 and Diving-PAM, are equipped with red light-emitting diodes (LED) with maximum emission at 655 nm. The LED radiation crosses through a short pass-filter (λ>670 nm) and the pho-todetector is protected by a long pass-filter (λ>700 nm). This configuration is very efficient in green plants including green macroalgae, however, does not works well in red macroalgae due to a low excitation of PSII under red light. Therefore, a modified protocol has been developed in which additional pulses of red ac-tinic and far-red light are given in order to ensure the
itions than green or brown algae (Buchel & Wilhelm, 1993). However, the contribution of state transition to non photochemical quenching compared to other quenching such as energy dependent quenching, qE or thermal energy dissipation, qI, is difficult to evaluate. When a light pulse is applied to determine quantum yield in a red alga, a state transition II is very ef-ficiently generated by the phosphorilation of LHCII proteins. This energy can then be transferred directly from PSII to PSI due to the proximity of both pho-tosystems during this state which results in a decrease in quantum yield, cyclic electron transport by PSI and ATP production (but not NADPH). Similarly, the con-tribution of state transitions in the ATP production in red algae has not fully been evaluated. During dy-namic photoinhibition, a decrease the quantum yield of PSII takes place, so the excess absorbed energy is rendered harmless by thermal dissipation. At sub-inhibitory fluence rates, thermal energy dissipation is mainly controlled by light-induced formation of a thylakoid pH gradient (energy dependent quenching, qE) which accelerates the rate of energy supplied to the Calvin cycle. At higher photoinhibitory fluence rates, photoinactivation due to photoinhibition opens an additional path of thermal energy dissipation (qI), thus optimising the rate of photochemical dissipation and diminishing the rate of photodamage.
Measurements of electron transport and estimation of photosynthetic capacity
The relationship between effective quantum yield of fluorescence and carbon assimilation and/or oxygen production has been demonstrated in higher plants (Genty et al., 1989) and in both micro- (Flameling & Kromkamp, 1998) and macroalgae (Hanelt et al., 1994). This relation validates chlorophyll fluorescence as an indirect determination of photosynthesis and can be used thus for estimation of primary productivity. In order to compare oxygen or carbon flux with elec-tron transport determined from fluorescence measure-ments, the following expression has been developed (Schreiber et al., 1986):
ETR = (F/Fm’)∗IPAR∗A∗0.5
where F/Fm’ is the effective quantum yield at a
given irradiance (IPAR), A is the absorptance of the
algal thallus and a factor 0.5, which comes from the
porate 1 CO2 molecule comes from PSII. Thus
4e-must be transported for every CO2 assimilated or O2
produced but the values of ETR/4e- are not neces-sarily identical for CO2 fixation rate or O2evolution
rate (Schreiber & Neubauer, 1990). At sub-inhibitory irradiances, this linear relationship between oxygen evolution and electron transport rate from fluorescence ratios can decrease: oxygen production is saturated whereas electron transport is still increasing (Flamel-ing & Kromkamp, 1998).
In several red algae no linear relationship between oxygen and Chlafluorescence has been shown, e.g. in
Palmaria palmataunder saturating irradiances (Hanelt & Nultsch, 1995), or in Porphyra leucostictaunder high CO2levels (1%) (Mercado et al., 1999). The case
ofP. leucostictameasured at high irradiances serves to exemplify the major links between both estimates of photosynthesis (Figure 5). In this species, gross photo-synthesis (GP) determined as oxygen evolution is sat-urated at a lower irradiance (125µmol m−2s−1) than that electron transport rate determined from chloro-phyll fluorescence usingF/Fm’ (373µmol m−2s−1).
Thus, the initial slope of the P-I curve (α) regarded as an indicator of photosynthetic efficiency, show also differences between both methods. However, a good correlation between gross photosynthesisvsETR may be defined: it has clearly two phases. Firstly, a lin-ear course in the range between 0–20µmol electrons m−2s−1, whereas between 20–50µmol electrons m−2 s−1, the curve becomes non-linear. Taking into ac-count only the first linear phase, oxygen evolution from ETR fluorescence data can be calculated using the following formula:
GP = a + b∗ETR
This algorithm can be useful to estimate carbon as-similation from fluorescence data but there is restric-ted to relatively low irradiances (down to 200 µmol photon m−2 s−1). For the whole irradiances range (200–1500 µmol photon m−2 s−1), the GP-ETR re-lationship adjusts to a hyperbolic tangent function:
GP = GPmax∗[tanh (α∗ETR/GPmax)]
Whereαis the initial slope of the function GPvsETR; α could be used as a good parameter to character-ize the relation between oxygen and electron fluxes in macroalgae, since it changes according to the en-vironmental conditions. In general, the use of these
Figure 5.Photosynthesisvsirradiance (P-I) curves and photosyn-thetic parameters of the intertidal red alga Porphyra leucosticta estimated using A) O2 evolution and B) relative electron trans-port rate ETR calculated from chlorophyll fluorescence and C) relationship between both data sets.
functions can lead to reliable estimates of carbon as-similation within experimental assays of only 15–30 min. For example, possible depletion of carbon or ni-trate during the incubation time (generally 2–3 hours) in standard O2 evolution chambers could be avoided
by the rapid determination of ETR by chlorophyll fluorescence.
By use of the function GP-ETR, the number of electrons from PSII necessary to produce one O2
mo-lecule can be determined. In the estimation of ETR is assumed that the half of the 8 electrons involved in the formation of one O2comes from PSII and the
other half from PSI. Thus the general assumption (4 e- per O2molecule) agrees closely with the value
ob-tained experimentally inPorphyra leucosticta(5 e- per O2). In any case, the contribution of each PS difficulty
may be exactly estimated since photons from different wavelengths are absorbed in a different way by the two photosystems.
Photoinhibition of photosynthesis
Daily cycles of chlorophyll fluorescence kinetics
Under a daily course of solar radiation, macroal-gae usually present photoinhibition at midday, i.e., a measurable decrease in photosynthetic activity, and normally, a recovery phase in the afternoon, when a decrease in incident irradiance to sub-inhibitory levels occurs. Two different kind of photoinhibition has been defined, dynamic and chronic photoinhibition (Os-mond, 1994). In general, sun-adapted algae exhibit dynamic photoinhibition, i.e., a reversible photopro-tective mechanism consisting in a down-regulation of the PSII in order to handle excess energy increasing thermal energy dissipation. In contrast, when shade-adapted algae are transferred to a high irradiance environment e.g. shallow waters, chronic photoinhib-ition becomes evident. This phenomenon is charac-terized by photodamage of PSII reaction centres and subsequent proteolysis of the D1 protein (Critchley & Rusell, 1994). Thus, the expression of photodamage occurs when the rate of degradation of D1 proteins exceeds the rate of repair (Aro et al., 1993). The extent of chronic photoinhibition can be calculated using the functions that describe photo- and non-photochemical quenching (1-qP; qN). The slope of the function 1-qP or qN vs irradiance indicates which mechanisms are operating in the photosynthetic apparatus at any time.
In terms of its ecological relevance for survival under changing conditions of light in the natural hab-itat, photoinhibition of photosynthesis in macroalgae (like in higher plants) has been proposed as a strategy of photoprotection against high irradiance (Osmond, 1994; Hanelt, 1996). The susceptibility to photoinhib-ition depends on the irradiance dose, season, water transparency and the presence of algal canopies. In
high zenith angles, which has been documented for the majority of the red algae studied so far (Häder et al. 1997). Even algae harvested from rock pools, nor-mally exposed to extreme solar radiation, show signs of photoinhibition after prolonged periods (Figueroa et al., 1997; Häder & Figueroa, 1997; Gómez & Figueroa 1998). Deep-water red algae and those ad-apted to shaded environments are inhibited even faster when exposed to direct solar radiation (Jiménez et al., 1998).
Although red algae from southern Spain can show a diversity of responses to high irradiances, the most severe photoinhibition in intertidal individuals occurs when low tide coincides with the intense solar radi-ation at noon. This is especially stressful in summer, when irradiances higher than 2000µmol photon m−2 s−1 can occur. Thus, many of the reported responses of the red algal assemblages of this region differ from that measured in red algae from cold-temperate loca-tions, e.g. North Sea (Dring et al., 1996; Yakovleva et al., 1998; Bischof et al., 2000). For example, the very high irradiances at which algae are exposed cause differential acclimation potential to solar radi-ation at spatial micro-scale. In two intertidal species ofGelidium, photo-acclimation was correlated to their different growth light environments along a small spa-tial scale, i.e. Gelidum latifoliumgrowing on rocky shores exposed to full solar radiation showed more efficient metabolic adjustments (dynamic photoinhib-ition) to cope with solar radiation thanGelidum ses-quipedale, a species inhabiting shaded crevices at the same vertical position in the shore and suffers chronic photoinhibition (Gómez & Figueroa, 1998).
Recovery of photosynthesis in the afternoon begins when fluence rates begins to decrease but remain still at saturating levels. In general, the recovery shows two phases, a fast phase at first followed by a slow phase one associated to repair via D1 protein turnover (Hu-ner et al., 1993; Leitsch et al., 1994). Hanelt (1998) using a mathematical model showed that the half-life time (τ) of the inhibition and recovery phases, i.e. the time necessary to reach half maximal response, is clearly related to the depth distribution of the species. Algae collected close to the water surface show a fast reaction of both, photoinhibition and recovery, hence, have a lowτ. With increasing depth, the reactions be-come slower and consequentlyτincreases. Jiménez et al. (1998) showed that the degree of photoinhibition
Figure 6. A) Spectral irradiance at the surface at M´alaga during three different months. B) weighted spectral irradiance of UV ra-diation using DNA damage and chloroplast photoinhibition action spectra (see text for details).
and recovery in different red macroalgae from South-ern Spain was also related to the light exposure history. The algae subjected to very high irradiances had cor-respondingly higher photoinhibition and recovery than that algae inhabiting shaded or deeper sites. In the intertidal red algaePorphyra leucosticta(Figueroa et al., 1997), Asparagopsis armataandFelmanophycus rayssae(Jiménez et al., 1998), the recovery of photo-synthesis occurs immediately after a decrease of only 10–20% of solar radiation in the afternoon.
In terms of tolerance to UV radiation and PAR, the enhanced capacity for dynamic photoinhibition and subsequent recovery observed in red algae from
south-Table 1. Percentage photoinhibition and recovery of photosynthesis measured as effective quantum yield of fluorescence (‘F/Fm’) under solar radiation in the natural environment. Plants were measured after exposure to different conditions of UV radiation by use of UV-blocking filters: PAR+UVA+UVB (PAB) and PAR alone (P). The percentages relate to an initial control (plants not exposed) in the morning
Species Type Photoinhibition Recovery Daily dose Habitat/ Reference
of % % (∗) kJ m−2 Depths/
radiation Season
Porphyra leucosticta PAB 41 100 PAR = 8800 Mediterranean Figueroa et al. (1997)
P 11 100 UV-A = 980 0–0.1 m
UV-B = 14 March 96
Rissoella verruculosa PAB 68 45 PAR = 11500 Mediterranean Flores-Moya et al. (1998)
P 68 50 UV-A = 1500 0–0.1 m
UV-B = 35.2 June-97
Gelidium sesquipedale PAB 52 66.7 PAR = 3094.8 Mediterranean G´omez & Figueroa (1998)
P 66 74 UV-A = 476.4 0–0.5 m
UV-B = 8.6 September 97
Gelidium latifolium PAB 88 100 PAR = 3094.8 Mediterranean G ´omez & Figueroa (1998)
P 79 83.6 UV-A = 476.4 0–0.5 m
UV-B = 8.6 September 97
Corallina elongata PAR = 8500 Atlantic Häder et al. (1997)
shade-type PAB 58 86.4 UV-A = 900 0–0.3 m
sun-type PAB 21 100 UV-B = 18 September 95
Asparagopsis armata PAR = 8500 Atlantic Jim´enez et al. (1998)
shade-type PAB 32.2 100 UV-A = 900 0.1–0.4 m
sun-type PAB 2.5 100 UV-B = 18 September 95
Gelidium sesquipedale PAB 29.1 96.5 Felmanophycus rayssiae PAB 71.6 100
Laurencia pinnatifida PAB 57.0 81.8 PAR = 9980 Mediterranean Häder et al. (1998)
Falkenbergia PAB 32.7 99.5 UV-A = 1100 0–0.1 m
UV-B = 23 September 96
(∗) Integrated daily irradiance measured at the surface.
ern Spain underlies clear sun-adaptation mechanisms (Table 1). The inhibition of photosynthesis expressed as a decline in effective quantum yield in algae grown under full solar radiation ranged between 71.6% in
Felmanophycus rayssae and 2.5% in Asparagopsis armata(Jiménez et al., 1998), whereas 100% recovery was observed in various species. OnlyRissoella ver-ruculosa, an endemic Mediterranean species, shown relatively low recovery values (Flores-Moya et al., 1998). A comparison between Porphyra leucosticta
and Rissoella verruculosa, which show comparable zonation pattern at intertidal sites, suggest different photo-protective strategies. In the eulittoral red algae
Gelidium sesquipedale from shaded crevices to full solar radiation, the recovery is much more slower than
Gelidium pusillum, an alga found commonly at ex-posed sites (Gómez & Figueroa, 1998). In general, seasonal changes in intensity of solar irradiation could affect the degree of photoinhibition in the
investig-ated algae: algae measured in summer have enhanced photoinhibition than samples collected in autumm or spring. In fact, spectral light intensity between 300 and 700 nm measured in June/July and February at Málaga significantly differ (Figure 6a, unpublished data). Es-timations using weighting functions for DNA damage and chloroplast photoinhibition (Setlow, 1974; Jones & Kok, 1966) indicate that, during summer, levels of short wavelengths, especially UV-B, may have po-tentially more accentuated detrimental effects on the organisms than in winter (Figure 6b, c).
Morphology and thallus structure, particularly light absorption properties and pigment composition, could also affect rates of photoinhibition. For example,
Porphyra leucostictaattains a thin, mono-layered thal-lus which permits a rapid and homogenous light trans-mission towards the light harvesting complexes. In contrast, in Rissoella, which has a thicker, leathery thallus structure, some scattering of photons through
Species Exposure time
2 days 4 days
PAB % UV P PAB % UV P
Udotea petiolata(Chlorophyta) 72±3.8 8 64±3.9 28±2.3 18 10±0.9 Codium bursa(Chlorophyta) 69±3.9 32 37±4.3 53±4.9 36 17±2.0 Cladostephus spongiousus(Phaeophyceae) 68±6.0 33 35±2.8 53±4.3 36 17±1.6 Zonaria tourneforti(Phaeophyceae) 81±5.7 17 64±5.2 72±6.7 40 32±2.7 Peyssonnelia rubra(Rhodophyta) 65±4.8 30 35±3.6 48±3.9 33 15±0.9 Lithophyllum incrustans(Rhodophyta) 70±6.7 34 36±2.3 46±4.8 38 8±1.5 Sphaerococcus coronopifolius(Rhodophyta) 69±6.5 39 30±3.6 55±3.7 40 15±2.3 Integrated daily irradiance:
0.5 m (PAR = 8323 kJm−2; UV-A = 934 kJm−2; UV-B = 33.2 kJm−2). 15 m (PAR = 2685 kJm−2; UV-A = 136 kJm−2; UV-B = 1.59 kJm−2).
the cell layers (self-shading) could take place. This appears to be evident under UV-B radiation: short wavelengths accounted by about 30% of the whole photoinhibition in Porphyra, whereas in Rissoella, solar spectrum including UV-B did not increase pho-toinhibition. Thus, the recovery kinetics gives insights into the photo-adaptive strategies of macroalgae and their light-stress tolerance capacity: algae with dy-namic photoinhibition during enhanced solar radiation and a rapid recovery in the afternoon may there-fore have competitive advantages in relation to algae without any efficient photoprotective mechanism.
Effects of UV radiation
High irradiances of PAR are regarded to be the most important wavelength components in this decline of photosynthetic rates at midday (Hanelt, 1996; Häder & Figueroa, 1997, Franklin & Forster, 1997). How-ever, it has been shown that UV-B (280–315 nm) and UV-A (315–400 nm) radiation have an important role in the photoinhibition and further recovery pro-cesses in intertidal and subtidal red macroalgae, even though their energy contribution relative to PAR is much lower (Larkum & Wood, 1993; Wood, 1989; Dring et al., 1996; Hanelt et al., 1997; Figueroa et al., 1997; Gómez & Figueroa, 1998). Recent studies indicate that UV-B, under certain environmental con-ditions of PAR, can even have beneficial effects on photobiological processes in macroalgae: in the brown algaDictyota dichotoma from southern Spain UV-B
promotes recovery subsequent to photoinhibition of photosynthesis (Flores-Moya et al., 1999).
Table 2 shows the contribution of UV radiation to the whole photoinhibition of photosynthesis of several macroalgae common to the Mediterranean locality, Cabo de Gata, charaterized by very clear waters and deep UV-B penetration (Figure 3). These data have not previously been published and serve to compare diverse algal groups in terms of their susceptibility to current levels of solar radiation. Photoinhibition was determined as the decrease in effective quantum yield (F/Fm’) after vertical transplantations at
mid-day during daily cycles. Algae were transplanted from 15 m to 0.5 m and subsequently maintained for two and four days under full solar radiation and PAR only. In general, photoinhibition of photosynthesis was greater under full solar radiation than that under PAR and no obvious differences were seen among al-gae belonging to different algal divisions. In species of red algae,Lithophyllum incrustansandSphaerococcus coronopifolium, and the green algaCodium bursa, UV radiation accounted for more than 30% of the total photoinhibition. Interestingly, the photoinhibition was lower in the 4thday after transplantation than after two days indicating a possible acclimation of plants to the shallow conditions, especially under PAR alone. This may be explained by relatively lower depth differences in PAR light climate compared to UV radiation: integ-rated daily irradiances of UV-A and UV-B increased 7 and 21 times from 15 to 0.5 m depth, respectively, whereas PAR increased by only a factor of 3 (see Figure 3).
Consequences for primary production
Apart from the direct deleterious effects that short wavelengths can have on specific targets in the pho-tosynthetic apparatus of algae, C assimilation and related pathways can be altered under UV solar stress as a consequence of the metabolic adjustments. A decline in photosynthesis at noon could also be correl-ated to negative CO2balance in the red algaRissoella verruculosaexposed to high natural solar irradiance (Flores-Moya et al., 1998). Apparently this does not necessarily affect the daily C balance, as light during the early morning and in the afternoon is high enough to supply energy requirements for C fixation: O2
pro-duction considerably exceeds O2consumption due to
dark respiration. It is uncertain whether C assimilation compensates for energy losses due to dissipation by PSII in algae living under normally low light condi-tions. In terms of ecological success, the ability to tolerate environmental light stress via metabolic ad-justments of the photosynthetic apparatus may be re-lated to increases in the chance of survival rather than increase in biomass. It must be emphasised that pho-toadaptation accounts for a part of the whole suite of physiological mechanisms allowing intertidal algae to survive. Other factors of course influence macroalgal zonation- and production on the shore.
The ability to tolerate enhanced UV radiation may be advantageous to maintain productivity in changing light environmental. However, the down-regulation of the photosynthetic apparatus in order to dissip-ate excess energy reduces photosynthetic rdissip-ates dur-ing various hours durdur-ing noon. Thus, estimations of daily production that include this factor are prefer-able when photosynthetic parameters are used (Mag-nusson, 1997). On the other hand, the fact that in short-term experiments photoinhibition of photosyn-thesis becomes less accentuated over the time as a result of acclimation it may be expected that long-term primary production by marine macrophytes does not dramatically decrease compared to algae that not show photoinhibition as has been demonstrated in short-term experiments (Häder & Figueroa, 1997; Franklin & Forster, 1997).
Photoinhibition of photosynthesis also appears to be a general phenomenon for cultivated species, if current levels of solar radiation are not artificially manipulated. Whereas in microalgal culture systems, photoinhibition has largely been recognised as a im-portant detrimental factor affecting production (Von-shak & Guy, 1988), few accurate determinations of
photosynthetic capacity have been carried out. Stud-ies of the photosynthetic responses of different pig-mented morphs of the cultivated red alga Euchema denticulatumshowed changes in O2-based
photosyn-thetic parameters, which could be associated with differences in photoacclimation-histories in the field (Dawes, 1992). However, no detailed monitoring of physiological accommodation on a daily basis has been carried for outdoor cultivated seaweeds under outdoor conditions. Recently, Aguirre-Wobeser et al. (2000) showed that the red algaChondrus crispus cul-tivated under outdoor conditions (100% of the solar radiation) in Baja California suffered marked photoin-hibition. It was proposed that photoinhibition could be reduced by applying shading screens or by increasing the biomass within the tanks. These findings con-firm that rapid, highly sensitive methodologies for the measurement of fluorescence kinetics in seaweeds at a mesocosm level can be reproduced in larger scale sys-tems, providing valuable insight into the physiological status of the plants in culture.
Acknowledgements
The authors are grateful for financial support from the Ministerio de Educación y Cultura, Spain (CICYT AMB97-1021-C02-01), European Union (Environ-mental and Climate Programme, ENV4-CT96-0188 and FEDER, 1FD97-0824) and Organising Commitee of ‘Phycologia 99’, Chile. Thanks also to B. Viñe-gla, E. Pérez-Rodríguez, C. Gallego, P. Sánchez and C. Maestre for their help. We are indebted to the Delegación Provincial de la Consejería de Medio Am-biente de la Junta de Andalucia for the use of logistic facilities in the Natural Park ‘Cabo de Gata-Níjar’.
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