Influence of ultraviolet-radiation on chlorophyll fluorescence and growth in different life-history stages of three species of Laminaria (phaeophyta)

Zoospores, gametophytes, young sporophytes and discs cut from mature sporophytes of Laminaria digitata, L. hyperborea and L. saccharina were exposed in the laboratory to UV-radiation, with a spectral composition and irradiance similar to natural sunlight, for periods ranging from 15 min to 8 d, and were then returned to white light. Germination of zoospores and the growth of gametophytes were reduced after exposures to UV longer than 1 h, whereas UV had little effect on the growth of young or mature sporophytes unless exposure continued for more than 48 h. The variable fluorescence (F _v:F_m) of all stages was strongly reduced immediately after short exposures to UV, but recovered almost completely within 24 h. However, exposure of gametophytes to UV for >4 h resulted in little or no recovery of F _v:F_m, whereas >16 h of UV were required to produce this result in young sporophytes, and >48 h in mature sporophytes. Thus, sensitivity to UV-radiation decreased from gametophytes to sporophytes, and with increasing age of sporophytes, but, in gametophytes, growth appeared to be a more sensitive indicator of UV-damage than F _v:F_m after 24 h recovery. The responses to UV of the zoospores and gametophytes of all three species were similar, but both growth and fluorescence measurements suggested that the sporophytes of L. saccharina were more sensitive to UV than those of the other two species.     

Effects of light exposure on the retention of kleptoplastic photosynthetic activity in the sacoglossan mollusc <Emphasis Type="Italic">Elysia viridis</Emphasis>

The effects of light exposure on the photosynthetic activity of kleptoplasts were studied in the sacoglossan mollusc Elysia viridis. The photosynthetic activity of ingested chloroplasts was assessed in vivo by non-destructively measuring photophysiological parameters using pulse amplitude modulation (PAM) fluorometry. Animals kept under starvation were exposed to two contrasting light conditions, 30 μmol photons m⁻² s⁻¹ (low light, LL), and 140 μmol photons m⁻² s⁻¹ (high light, HL), and changes in photosynthetic activity were monitored by measuring the maximum quantum yield of photosystem II (PSII), F _v/F _m, the minimum fluorescence, F _o, related to chlorophyll a content, and by measuring rapid light-response curves (RLC) of relative electron transport rate (rETR). RLCs were characterised by the initial slope of the curve, α_RLC, related to efficiency of light capture, and the maximum rETR level, rETR_m,RLC, determined by the carbon-fixation metabolism. Starvation induced the decrease of all photophysiological parameters. However, the retention of photosynthetic activity (number of days for F _v/F _m > 0), as well as the rate and the patterns of its decrease over time, varied markedly with light exposure. Under HL conditions, a rapid, exponential decrease was observed for F _v/F _m, α_RLC and rETR_m,RLC, F _o not showing any consistent trend of variation, and retention times ranged between 6 and 15 days. These results suggested that the retention of chloroplast functionality is limited by photoinactivation of PSII reaction center protein D1. In contrast, under LL conditions, a slower decrease in all parameters was found, with retention times varying from 15 to 57 days. F _v/F _m, α_RLC and rETR_m,RLC exhibited a bi-phasic pattern composed by a long phase of slow decrease in values followed by a rapid decline, whilst F _o decayed exponentially. These results were interpreted as resulting from lower rates of D1 photoinactivation under low light and from the gradual decrease in carbon provided by photosynthesis due to reduction of functional photosynthetic units.     

Capability of dynamic photoinhibition in Arctic macroalgae is related to their depth distribution

The capability of several macroalgal species to protect photosynthesis from excessive irradiance by dynamic photoinhibition was investigated relative to their depth distribution in summer 1995 in the Kongsfjord (79°N; 12°E, Ny Ålesund, Spitsbergen, Norway). Photoinhibition of photosynthesis was induced by exposure of algae from different water depths to a high photon fluence rate of 500?μmol?m^?2?s^?1 for 2?h. Changes in optimal quantum yield (F _v/F _m) were measured during the inhibition phase. Recovery of photosynthesis was subsequently induced by dim white light (10?μmol?m^?2?s^?1) and observed as changes in the variable fluorescence. With a newly developed mathematical model different parameters of the response kinetics of inhibition and recovery were calculated and related to the depth distribution of each algal species. It is shown that two components with slow and fast reaction kinetics, respectively, are involved in photoinhibition and recovery of photosynthesis. Their possible molecular bases are discussed. 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 investigated species. Algae collected close to the water surface show a fast reaction of both photoinhibition and recovery and, hence, have a low τ. With increasing depth the reactions become slower and τ increases. τ was highest in deep water algae. Further analysis of the reaction kinetics in Laminaria saccharina shows that the relative proportion of the two kinetics involved change with the collection depth. In contrast, a significant difference in the reaction rates of both kinetics was not observed.     

Morpho-functional patterns of photosynthesis and UV sensitivity in the kelp <Emphasis Type="Italic">Lessonia nigrescens</Emphasis> (Laminariales,Phaeophyta)

The photosynthetic responses of the south Pacific kelp Lessonia nigrescens of the coast of Valdivia, Chile (40°S), were investigated by exposing its different thallus parts, fronds, stipes and holdfasts, to UV radiation in the laboratory. Biologically effective doses (BED_{photoinhibition300}) between 400 and 800 kJ m⁻² were required for a 40% inhibition in photosynthesis under UVA+UVB radiation. At BED_{photoinhibition300} close to 250 kJ m⁻² (in treatments without UVB), the inhibition of photosynthesis did not exceed 20%. These UV doses were in the range of current daily doses measured in Valdivia on cloudless summer days. In general, exposure to UVB for periods longer than 12 h reduced photosynthesis, measured as maximal quantum yield (F _v/F _m) and electron transport. The fronds were the most UV-sensitive section of this alga, coinciding with the highest pigments contents and carbon fixation. Evidence of a photodamage was also seen. After a 48 h exposure to PAR+UVA+UVB, a decrease of F _v/F _m in the fronds was close to 41%, while in the stipes and holdfasts it was 12 and 18%, respectively. Although the thalli from the different size classes showed marked differences in their morphology and morphometry, no obvious differences in the UV tolerance of the fronds were detected. The results indicated that the UV-related responses are integrated in the suite of morpho-functional adaptations of the alga. Although the fronds are spatially more exposed to solar radiation than basal structures (stipes and holdfast), due their high turnover rate they may compensate better detrimental effects of UV. In contrast, stipes and the holdfast are key support structures characterized by low replacement rates and designed to confer hydrodynamic resistance to drag forces.     

Effects of four environmental variables on photosynthesis-irradiance relationships in Antarctic sea-ice microalgae

The effects of temperature, salinity, growth irradiance and diel periodicity of incident irradiance on photosynthesis-irradiance (P-I) relationships were examined in natural populations of sea-ice microalgae from McMurdo Sound in the austral spring of late 1984. Both P _m ^b (photosynthetic rate at optimum irradiance) and ^b (initial slope or P-I curve) were temperature-dependent reaching optimal rates at approximately +6° and +2°C, respectively. P-I relationships showed little difference at 20 and 33 S; however, no measurable photosynthesis by sea-ice microalgae was detected in a 60 S solution of brine collected from the upper layers of congelation ice. Although diel periodicity characteristic of the under-ice light field appeared to have little effect on P-I relationships, changes in growth irradiance had a profound effect. An increase in growth irradiance from 7 E m^-2 s^-1 (ambient) to 35 or 160 E m^-2 s^-1 resulted in a transient three-fold increase in P _m ^b and I _k (index of photoadaptation) during the first four days, followed by a sharp decline. The effects of these environmental factors on ice algal photosynthesis may influence the distribution of microalgae in sea-ice environments.     

Seasonal and depth related variation in the photosynthesis–irradiance response of<Emphasis Type="Italic"> Ecklonia radiata</Emphasis> (Phaeophyta,Laminariales) at West Island,South Australia

The photosynthesis–irradiance response of Ecklonia radiata (C. Agardh) J. Agardh, a common kelp in the temperate southern hemisphere, was investigated in situ throughout the year and across a depth profile at West Island, South Australia. Temperature and irradiance environment altered throughout the year, varying at 3 m between 14–20°C and 279–705 mol photons m^–2 s^–1. Photosynthetic capacity (P_m) varied throughout the year between 177–278 mol O₂ g^–1 dry wt h^–1 at 3 m and 133–348 mol O₂ g^–1 dry wt h^–1 at 10 m. The irradiance required for sub-saturation of photosynthesis (E_k) varied between 97–152 and 81–142 mol photons m^–2 s^–1 for 3 m and 10 m respectively, and the respiration rate varied between 15–36 and 13–20 mol O₂ g^–1 dry wt h^–1 for 3 m and 10 m. A clear seasonal change in photokinetic parameters was detected and provided strong evidence for a seasonal acclimation response. During winter an increase in the efficiency of light utilisation at low irradiance () was accompanied by a decrease in both E_k and that required for photosynthetic compensation. P_m also increased during the winter and autumn months and respiratory requirements decreased. These changes enable E. radiata to display an optimal photosynthetic performance throughout the year despite significant changes in the surrounding environment.Communicated by P.W. Sammarco, Chauvin     

Temperature acclimation of respiration and photosynthesis in the brown algaLaminaria saccharina

Sporophytes of the brown algaLaminaria saccharina (L.) Lamour grown at 15°C contained significantly more chlorophylla (chla) than did similar plants grown at 5°C. The increase in chla in 15°C plants was due to increased numbers of photosystem II reaction centes, and possibly to increased photosynthetic unit size, compared with 5°C plants. These changes were associated with increased values (photosynthetic efficiencies) in 15°C-grownL. saccharina relative to 5°C-grown plants. The changes in together with reduced respiration rates allowed 15°C-grownL. saccharina to achieve net photosynthesis and light-saturated photosynthesis at a lower photon fluence rate (PFR) than 5°C plants when both groups were assayed at the same temperature (15°C). The photon fluence rates necessary to reach the compensation point and achieve light-saturated photosynthesis (I _c andI _k , respectively) increased with increasing incubation temperature inL. saccharina grown at both 5 and 15°C. However, acclimation responses to growth temperature compensated for the short-term effect of temperature onI _c andI _k . Consequently, plants grown at 5 and 15°C were able to achieve similar rates of light-limited photosynthesis, and similarI _c andI _k values at their respective growth temperatures. These responses are undoubtedly important for perennial seaweeds such asL. saccharina, which frequently grow in light-limited habitats and experience pronounced seasonal changes in water temperature.Please address all correspondence and requests for reprints to I.R. Davison     

Short-term variability in the photosynthetic activity of microphytobenthos as detected by measuring rapid light curves using variable fluorescence

Short-term variability in the photosynthetic activity of microphytobenthos assemblages was studied by measuring chlorophyll fluorescence rapid light curves (RLC), using pulse amplitude modulated (PAM) fluorometry. Measurements carried out on undisturbed samples under dark–light cycles revealed large diel oscillations in both the initial slope of the RLC () and in the maximum relative electron transport rate (ETR_m). Short-term variations in RLC parameters were also observed, closely following changes in incident photon irradiance (E). Increases in irradiance were followed by decreases in and increases in ETR_m, resulting in significant correlations between the light-saturation parameter E_k and E. These results were interpreted as resulting from the onset of reversible energy-dissipating, non-photochemical quenching mechanisms and of compensatory high light-induced activation of carbon metabolism activity. Short-term RLC variability was shown to result mainly from physiological causes and to be detectable only by using short (10–20 s) light steps during RLC construction. Dark-adapted samples kept under constant conditions exhibited apparently endogenous rhythms in RLC parameters and in the maximum quantum yield, F_v/F_m, coincident with vertical migratory movements occurring during subjective photoperiods. These fluctuations appeared to result from the interaction between migratory rhythms and the physiological responses, and from the endogenous activation of processes affecting both the efficiency of energy transfer from light-harvesting antennae to the photosystem II (PSII) reaction centres or from non-radiative pathways (F_v/F_m, ) and the reactions downstream of PSII (ETR_m).Communicated by M. Kühl, Helsingør     

Photoinhibition of photosynthesis in a sun and a shade species of the red algal genus Porphyra

Gametophytes of two species of Porphyra collected around San Juan Island, Washington in 1986 and acclimated to low light conditions in culture showed different resistances to photoinhibition of photosynthesis. The intertidal species P. perforata J. Agardh exhibited photoinhibition at one-third the rate exhibited by the subtidal species P. nereocystis Anderson following treatments at 2000 mol photons m^-2 s^-1 under conditions of full hydration and optimal temperature. The greater resistance of P. perforata to photoinhibition could not be attributed to reduced photosynthetic pigment concentration, higher photosynthetic capacity, avoidance of light by chloroplast movement or to enhanced rates of photorespiration. Total carotenoid concentrations were similar in the two species. It is probable that the mechanisms of this resistance are operating at the level of the thylakoid membranes. Resistance to photoinhibition represents an adaptation of photosynthesis in P. perforata which may contribute to its persistance in the extreme environment of its intertidal habitat.     

Photosynthetic characteristics of Prochloron sp./ascidian symbioses

The prokaryotic green alga Prochloron sp. (Prochlorophyta) is found in symbiotic association with colonial didemnid ascidians that inhabit warm tropical waters in a broad range of light environments. We sought to determine the light-adaptation features of this alga in relation to the natural light environments in which the symbioses are found, and to characterize the temperature sensitivity of photosynthesis and respiration of Prochloron sp. in order to assess its physiological role in the productivity and distribution of the symbiosis. Colonies of the host ascidian Lissoclinum patella were collected from exposed and shaded habitats in a shallow lagoon in Palau, West Caroline Islands, during February and March, 1983. Some colonies from the two light habitats were maintained under conditions of high light (2 200 E m^–2 s^–1) and low light (400 E m^–2 s^–1) in running seawater tanks. The environments were characterized in terms of daily light quantum fluxes, daily periods of light-saturated photosynthesis (H_sat), and photon flux density levels. Prochloron sp. cells were isolated from the hosts and examined for their photosynthesis vs irradiance relationships, respiration, pigment content and photosynthetic unit features. In addition, daily P:R ratios, photosynthetic quotients, carbon balances and photosynthetic carbon release were also characterized. It was found that Prochloron sp. cells from low-light colonies possessed lower chlorophyll a/b ratios, larger photosynthetic units sizes based on both reaction I and reaction II, similar numbers of reaction center I and reaction center II per cell, lower respiration levels, and lower P_max values than cells from high-light colonies. Cells isolated from low-light colonies showed photoinhibition of P_max at photon flux densities above 800 E m^–2 s^–1. However, because the host tissue attenuates about 60 to 80% of the incident irradiance, it is unlikely that these cells are normally photoinhibited in hospite. Collectively, the light-adaptation features of Prochloron sp. were more similar to those of eukaryotic algae and vascular plant chloroplasts than to those of cyanobacteria, and the responses were more sensitive to the daily flux of photosynthetic quantum than to photon flux density per se. Calculation of daily minimum carbon balances indicated that, though high-light cells had daily P:R ratios of 1.0 compared to 4.6 for low-light cells, the cells from the two different light environments showed nearly identical daily carbon gains. Cells isolated from high-light colonies released between 15 and 20% of their photosynthetically-fixed carbon, levels sufficient to be important in the nutrition of the host. Q₁₀ responses of photosynthesis and respiration in Prochloron sp. cells exposed briefly (15–45 min) to temperatures between 15° and 45°C revealed a discontinuity in the photosynthetic response at the ambient growth temperatures. The photosynthetic rates were found to be more than twice as sensitive to temperatures below ambient (Q₁₀=3.47) than to temperatures above ambient (Q₁₀=1.47). The Q₁₀ for respiration was constant (Q₁₀=1.66) over the temperature range examined. It appears that the photosynthetic temperature sensitivity of Prochloron sp. may restrict its distribution to warmer tropical waters. The ecological implications of these findings are discussed in relation to published data on other symbiotic systems and free-living algae.     

Photoinhibition at low quantum flux densities in a marine dinoflagellate (Amphidinium carterae)

Growth and photosynthetic properties of the marine dinoflagellate Amphidinium carterae Hulbert were examined under continuous illumination in batch cultures at four different irradiances between 2 and 150 E m^-2 s^-1. The slope of both cell- and Chl a-based photosynthesis versus the irradiance curves was greatest for cells grown at 15 E m^-2 s^-1. The relative Chl a values cell^-1 were 1, 1.5 and 2 for cultures grown at 150, 80 and 15 E m^-2 s^-1, respectively. A low-temperature (-196°C) fluorescence technique was used to examine cells for photoinhibiton. Photoinhibition was greatest for cells grown at 150 E m^-2 s^-1. However, significant photoinhibition of this species was noted even at 80 E m^-2 s^-1. No significant difference in the fluorescence pattern was found between cells grown at 2 and 15 E m^-2 s^-1. Time course studies indicate that photoinhibition may occur within 2 h following exposure to 350 E m^-2 s^-1 in cells grown at 15 E m^-2 s^-1 and is reversible when light levels are lowered within 4 h. The ecological significance of phytoplankton unable to cope with excess photosynthetic excitation energy is discussed.     

外源Hg胁迫对3种烟草品种叶绿素含量和光合效应的影响

为研究不同浓度汞(Hg)对烟草光合作用的影响,选择福建省3个主栽烟草品种(翠碧1号、K326、云烟87)幼苗进行盆栽试验,设置0、1、2、4、8、16 mg·kg~(-1)共6个浓度组。结果表明:(1)3种烟草叶片叶绿素含量与对照比均无显著差异(P0.05),但F_v/F_m、F_v/F_o、PI_(ABS)随胁迫浓度增加均呈下降趋势,可见Hg胁迫下烟草叶绿素含量和光合强度没有必然的相关性。(2)在较高浓度(4 mg·kg~(-1))Hg胁迫下,OJIP曲线改变了形状,表明Hg胁迫对烟草叶片光合机构影响主要是PSⅡ及光系统反应中心受到损伤,光合电子传递过程受到抑制,这可能是烟草叶片光合效应显著下降的主要因素。(3)烟草叶片PSII单位反应中心比活性参数ABS/RC、TRo/RC、DIo/RC与胁迫浓度正相关,ETo/RC和RC/CSo与胁迫浓度呈负相关,在较高浓度Hg胁迫下烟草叶片通过耗散过剩的光能和提高多余能量消耗量来保护光合机构免受更大的伤害,表现出较强的自我调节能力。     

Diurnal measurements of F v/F m used to improve productivity estimates in macroalgae

The ratio of variable fluorescence to maximal fluorescence (F _v/F _m) was measured during a night and day cycle in five different macroalgae growing in the littoral zone at the Swedish west coast; the green algae Ulva lactuca, Cladophora sp. and Enteromorpha flexousa, the red alga Ceramium nodulosum and the brown alga Fucus vesiculosus. All the green algae and C. nodulosum showed both diurnal fluctuations in F _v/F _m during days of high light intensities and a significant negative correlation between F _v/F _m and photon irradiance (PI). An attempt has been made to improve calculations of macroalgal net growth based on carbon fixation (α_net) considering this diurnal change in F _v/F _m. By assuming that the negative linear correlation between F _v/F _m and the maximum quantum yield for photosynthesis (Φ) is proportional to that between F _v/F _m and α_net, it was possible to include the daily variation of α_net due to photoinhibition. To compensate for the variation of F _v/F _m, a representative value for each day was obtained by weighting all values of F _v/F _m over the diurnal period in relation to total PI. For all algal species there was a fairly good agreement between this representative value and the F _v/F _m value measured around noon. As the daily representative F _v/F _m value showed a negative linear correlation with the daily mean PI, it was possible to correct α_net for differences of daily mean PI. Received: 10 March 1997 / Accepted: 25 August 1997     

Growth patterns of Codium fragile ssp. tomentosoides in response to temperature,irradiance, salinity,and nitrogen source

Seasonal patterns of growth, reproduction, and productivity of Codium fragile spp. tomentosoides (van Goor) Silva were monitored at 3 locations in Rhode Island. Maximal growth occurred during the summer and was more significantly correlated with temperature than any other factor measured in this study. Multiple correlation models suggested an interaction between temperature, irradiance, and available nitrogen. Maximal reproduction occurred in late summer and early fall. The maximal productivity, based on harvested quadrats, was 2. 10 g dry weight m^-2 day^-1. A large amount (up to 87.3%) of the annual production entered the detrital food chain during the winter by fragmentation of the thallus. Culture studies examined the effects of temperature (6° to 30°C), irradiance (7 to 140 E m^-2 sec^-1), daylength (8 h light: 16 h dark to 24 h light: O h dark) and salinity (6 to 48) on growth. Differentiated thalli grew over a broad range of experimental conditions, with maximal growth at 24°C, 24 to 30 S, a minimal irradiance of 28 E m^-2 sec^-1, and 16 h daylength. The effect of increasing daylength was due to increased total daily irradiance rather than to a true photoperiodic effect. Undifferentiated sporelings survived and grew in a narrower range of environmental conditions than thalli. c. fragile spp. tomentosoides grew equally well with nitrate, nitrite, ammonium, and urea as a nitrogen source. The addition of NaHCO₃ stimulated growth at levels of 2.4 to 4.8 mM, suggesting an inorganic carbon limitation in static cultures. This study supports the hypothesis that the in situ seasonal growth pattern of c. fragile spp. tomentosoides is primarily due to the interaction of temperature and irradiance.     

Interactive effects of UV radiation and enhanced temperature on photosynthesis, phlorotannin induction and antioxidant activities of two sub-Antarctic brown algae

Lessonia nigrescens and Durvillaea antarctica, two large sub-Antarctic brown algae from the southern Chilean coast, were exposed to solar UV radiation in an outdoor system during a summer day (for 11 h) as well as to artificial UV radiation under controlled laboratory conditions at two temperatures (15 and 20 °C) for 72 h. Chlorophyll a fluorescence–based photoinhibition of photosynthesis was measured during the outdoor exposure, while electron transport rates, lipid peroxidation, antioxidant activity and content of phlorotannins were determined at different time intervals during the laboratory exposure. Under natural solar irradiances in summer, both species displayed well-developed dynamic photoinhibition: F _v/F _m values decreased by 70 % at noon coinciding with the levels of PAR >1,500 μmol m^?2 s^?1 and UV-B radiation >1 W m^?2 and recovered substantially in the afternoon. In treatments including UV radiation, recovery in D. antarctica started already during the highest irradiances at noon. The results from laboratory exposures revealed that (a) elevated temperature of 20 °C exacerbated the detrimental effects of UV radiation on photochemical parameters (F _v/F _m and ETR); (b) peroxidative damage measured as MDA formation occurred rapidly and was strongly correlated with the decrease in F _v/F _m, especially at elevated temperature of 20 °C; (c) the antioxidant activity and increases in soluble phlorotannins were positively correlated mainly in response to UV radiation; (d) phlorotannins were rapidly induced but strongly impaired at 20 °C. In general, short-term (2–6 h) exposures to enhanced UV radiation and temperature were effective to activate the photochemical and biochemical defenses against oxidative stress, and they continued operative during 72 h, a time span clearly exceeding the tidal or diurnal period. Furthermore, when algae were exposed to dim light and control temperature of 15 °C for 6 h, F _v/F _m increased and lipid peroxidation decreased, indicating consistently that algae retained their ability for recovery. D. antarctica was the most sensitive species to elevated temperature for prolonged periods in the laboratory. Although no conclusive evidence for the effect of the buoyancy of fronds was found, the interspecific discrepancies in thermo-sensitivity in the UV responses found in this study are consistent with various ecological and biogeographical differences described for these species.     

Photosynthesis and amelioration of desiccation in the intertidal saccate alga Colpomenia peregrina

Rates of gross photosynthesis for the intertidal saccate alga Colpomenia peregrina (Sauv.) Hamel were determined under submersed and emersed conditions. Maximal photosynthetic rates were lower than for most seaweeds but comparable with other saccate members of the genus. By fitting the data to a hyperbolic tangent function, maximal photosynthetic rates were estimated to be 5.29 mmol CO₂ m^-2 h^-1 under submersed conditions and 2.06 mmol CO₂ m^-2 h^-1 under emersed conditions. I_k for submersed thalli was 69.1 E m^-2 s^-1, wherea for emersed thalli it was 149.0 E m^-2 s^-1, or 2.2 times higher. At low tide in the field and under saturating irradiance, carbon from seawater retained within the thallus cavity was assimilated at 0.9 mmol CO₂ m^-2 h^-1. In the laboratory under emersed conditions, carbon from this source was taken up at 0.6 mmol CO₂ m^-2 h^-1 at 20°C and at 0.34 mmol CO₂ m^-2 h^-1. Retained seawater also greatly reduced drying under desiccating conditions. Experimental thalli from which seawater had been removed lost thallus water continuously throughout the drying period (120 min). On the other hand, control, thalli lost water for the first 15 min, after which no further water loss occurred. At the termination of the experiment, control thalli had lost 7.2% of their water, whereas experimental thalli had lost 39.2%. Desiccation affected the emersed photosynthetic rate of experimental and control thalli. Emersed photosynthetic rates for thalli dried for 15 min were higher than for fullyhydrated thalli. However, emersed photosynthesis of thalli dried for longer than 15 min was lower than fully-hydrated rates and was directly related to percent water loss. Utilizing data from this study, a model was constructed to determine total photosynthetic production of C. peregrina over a single daylight period. From these calculations it was determined that emersed photosynthesis can increase daily photosynthetic production of C. peregrina by 50%.     

Effect of feeding regime and irradiance on the photophysiology of the symbiotic sea anemone Aiptasia pulchella

To determine how the animal and algal components of the symbiotic sea anemone Aiptasia pulchella respond to changes in food availability and culture irradiance, sea anemones from a single clone were maintained at four irradiance levels (320, 185, 115, and 45 E m^-2 s^-1) and either starved or fed for 5 wk. Changes in protein biomass of sea anemones maintained under these conditions were not related to the productivity of zooxanthellae, since the protein biomass of fed A. pulchella decreased with increase in irradiance and there was no difference in protein biomass among starved sea anemones at the four irradiance levels. Except for the starved high-light sea anemones, the density of symbiotic zooxanthellae was independent of culture irradiance within both starved and fed. A. pulchella. Starved sea anemones contained over twice the density of zooxanthellae as fed sea anemones. Within both starved and fed individuals, chlorophyll per zooxanthella increased with decreasing culture irradiance while algal size remained constant (in fed sea anemones) at about 8.80 m diameter. Chlorophyll a: c ₂ ratios of zooxanthellae increased with decreasing culture irradiance in zooxanthellae from starved sea anemones but remained constant in zooxanthellae from fed sea anemones. As estimated from mitotic index data, the in situ growth rates of zooxanthellae averaged 0.007 d^-1 and did not vary with irradiance or feeding regime. Photosynthesis-irradiance (P-I) responses of fed A. pulchella indicated an increase in photosynthetic efficiency with decreasing culture irradiance. But there was no consistent pattern in photosynthetic capacity with culture irradiance. Respiration rates of fed sea anemones also did not vary in relation to culture irradiance. The parameter I _k , defined as the irradiance at which light-saturated rates of photosynthesis are first attained, was the only parameter from the P-I curves which increased linearly with increasing culture irradiance. The daily ratio of net photosynthesis to respiration for A. pulchella ranged from 1.6 to 2.8 for sea anemones maintained at the three higher irradiances, but was negative for those maintained at 45 E m^-2 s^-1. Since the final protein biomass was greatest for sea anemones maintained at the lowest irradiance, these results indicate that sea anemone growth cannot be directly related to productivity of zooxanthellae in this symbiotic association.     

Pigment content and photosynthetic rate of the fronds of Macrocystis pyrifera

The Macrocystis pyrifera (L.) C. Ag. frond is here described in terms of chlorophyll a, fucoxanthin, chlorophyll c and photosynthetic rate. Pigment concentrations increased back from the apical meristem reaching a maximum after 2 to 3 m. Pigment concentrations were then generally constant throughout most of the length of the frond, finally decreasing again in the oldest parts of the frond with the exception of the sporophylls. Pigment ratios remained relatively constant throughout. Maximum net photosynthetic rates on a given frond showed a decrease with tissue age on both an area basis (1040 down to 463 nmol O₂ cm^-2 h^-1) and on a chlorophyll a basis, which was shown as half-saturation constants (quantum irradiance) which dropped on an area basis from 85 mol m^-2 sec^-1 at 4.5 m above the holdfast to 26 mol m^-2 sec^-1 at 15.5 m. Young sporophytes transplanted from the sea floor to the surface (12 m) tended to decrease pigment content, while those transplanted to the bottom tended to increase all pigments, but especially fucoxanthin. Photosynthetic rates, however, changed little on a unit area basis. The results of these data are considered in the light of recent work on photosynthetic units, tissue age effects and general adaptations of the M. pyrifera frond to its light environment.     

Ecotypic differentiation in light-related traits of the kelp Laminaria saccharina

Light-related traits were compared for Laminaria saccharina Lamour. collected from three habitats in Maine and New York, USA, with different ambient light regimes. Light-level, expressed as a proportion of surface irradiance (I₀), ranged from 0.04 to 0.32 I₀ in the shallow habitat, but rarely exceeded 0.04 I₀ in the deep and turbid habitats. Juvenile sporophytes collected from each habitat in April, 1985, were grown at four acclimation light-levels (0.065, 0.12, 0.26, and 0.54 I₀) in a common-garden, laboratory experiment. Photosynthesis vs irradiance (PI) parameters, light-harvesting characteristics, and rates of carbon-assimilation and growth were determined for each group of plants. The results indicated that ecotypic differentiation had occurred among the three kelp populations. Photosynthetic capacity (P_max) and photosynthetic efficiency () were generally highest for plants from the turbid habitat, lowest for deep plants, and intermediate for shallow plants. These differences were largely attributable to variations in light-harvesting characteristics. The nature and magnitude of photoacclimation responses also differed among populations. Population differences in photosynthetic parameters resulted in different rates of C-assimilation and growth by plants from shallow, deep, and turbid habitats. Predictions of in situ growth rates indicated that the severity of light-limitation and, therefore, the adaptive significance of efficient light-utilization vary among the three populations. It is concluded that ecotypic differentiation in light-related traits is important to the broad environmetal range of L. saccharina.     

Diurnal changes in photosynthesis of Antarctic fast ice algal communities determined by pulse amplitude modulation fluorometry

The aim of this project was to determine both the diurnal changes in photosynthetic activity of Antarctic sea ice algae and also the protective mechanisms they use to mitigate the effects of in situ UV radiation. Changes in the diurnal photosynthetic parameters of fast ice algal communities at McMurdo Sound were measured in situ, using a custom designed monitoring pulse amplitude modulation fluorometer. The sea ice microalgae were able to adapt rapidly to either increasing or decreasing ambient irradiances. ΔF/F_m' values were between 0.2 and 0.51, while E_k varied between 2.1 and 18 μmol photons m^-2 s^-1. ΔF/F_m', E_k, and relative electron transfer rate (rETR) all varied sequentially over the course of a day. rETR and E_k were highest at midday at the highest irradiances, when there was apparent midday down regulation of photosynthesis, while ΔF/F_m' was highest at midnight. The effects of natural UV radiation on sea ice were examined, but it was not possible to detect the effect of either UVB or UVA and UVB on photosynthesis. This was considered to be largely because of the large spatial and temporal heterogeneity of the under ice community, changing irradiances throughout the day and the relatively small change caused by UV.