High-temperature tolerance of photosynthesis in the red alga Chondrus crispus

Chondrus crispus (Stackhouse) is a perennial red seaweed, common in intertidal and shallow sublittoral communities throughout the North Atlantic Ocean. In the intertidal zone, C. crispus may experience rapid temperature changes of 10 to 20C° during a single immerison-emerision cycle, and may be exposed to temperatures that exceed the thermal limits for long-term survival. C. crispus collected year-round at Long Cove Point, Chamberlain, Maine, USA, during 1989 and 1990, underwent phenotypic acclimation to growth temperature in the laboratory. This phenotypic acclimation enhanced its ability to withstand brief exposure to extreme temperature. Plants grown at summer seawater temperature (20°C) were able to maintain constant rates of lightsaturated photosynthesis at 30°C for 9 h. In contrast, light-saturated photosynthetic rates of plants grown at winter seawater temperature (5°C) declined rapidly following exposure to 30°C, reached 20 to 25% of initial values within 10 min, and then remained constant at this level for 9 h. The degree of inhibition of photosynthesis at 30°C was also dependent upon light intensity. Inhibition was greatest in plants exposed to 30°C in darkness or high light (600 mol photons m^-2s^-1) than in plants maintained under moderate light levels (70 to 100 mol photons m^-2s^-1). Photosynthesis of 20°C-acclimated plants was inhibited by exposure to 30°C in darkness or high light, but the degree of inhibition was less than that exhibited by 5°C-grown plants. Not only was light-saturated photosynthesis of 20°C plants less severely inhibited by exposure to 30°C than that of 5°C plants, but the former also recovered faster when they were returned to growth conditions. The mechanistic basis of this acclimation to growth temperature is not clear. Our results indicate that there were no differences between 5 and 20°C-grown plants in the thermal stability of respiration, electron transport associated with Photosystems I or II, Rubisco or energy transfer between the phycobilisomes and Photosystem II. Overall, our results suggest that phenotypic acclimation to seawater temperature allows plants to tolerate higher temperatures, and may play an important role in the success of C. crispus in the intertidal environment.     

Interactions between light and temperature on the physiological ecology ofGracilaria tikvahiae (Gigartinales: Rhodophyta)

Main effects and interactions of light and temperature on rates of growth (), net photosynthesis (P_s), and dark respiration (R) of the red seaweedGracilaria tikvahiae were investigated in outdoor, nutrient-replete continuous-flow seawater culture chambers. Below 15°C,G. tikvahiae did not grow and between 15° and 30°C, both main effects and interactions of light and temperature on and P_s were significant, which explains the occurrence of this alga as a summer annual in its northern range. Temperature interacted with light (I) through its influence on the vs I and P_s vs I curves. The initial slope of the vs I curve, , the light saturation intensity, I_s, and maximum growth rate, _max, were all significantly lowerat 15°C compared to 20°, 25°, or 30°C. Maximum values of _max, the P_s:R ratio and the net photosynthesis:gross photosynthesis ratio (P_s:P_g) all occurred at 25°C, suggesting that this is the best temperature for growth ofG. tikvahiae. Values for P_max increased up to 30°C, indicating that the temperature for maximum growth and net photosynthesis are not the same forG. tikvahiae. Significant photoinhibition of growth and photosynthesis at full incident sunlight (I₀) occurred at 15°C but not at 20°, 25°, or 30°C. Steele's equation fit the 15°C vs I data best, whereas the hyperbolic tangent function fit the 20°, 25°, and 30°C data best. Main effects and interactionof light intensity and temperature on rates of R were also significant (P<0.001). R was highly intercorrelated with and P_s (0.86r0.94), indicating that R inG. tikvahiae is primarily regulated by growth rate and not temperatureper se. Environmental factors that regulate growth, such as light intensity, exert a great influence on R inG. tikvahiae.     

Growth,photosynthesis and carbon metabolism in the temperate marine diatom Skeletonema costatum adapted to low temperature and low photon-flux density

The temperate diatom Skeletonema costatum (Grev.) Cleve was grown in low temperature and/or low light conditions. The cultures were acclimatized for at least three months before experiments were begun. Our data indicate that the initial slope of the photosynthesis vs irradiance curve () is controlled predominantly by light history and the light-saturated photosynthesis (P _max) by temperature. The number of divisions per day decreased with decreasing light intensity, but was identical for cultures grown at 3° or 18°C. The metabolic pathways of inorganic carbon fixation were not fundamentally affected by low temperature or low light intensity, but both these factors increased labelling of C₃ compounds, synthesized by the Calvin-Benson cycle, and decreased that of phosphoenolpyruvate (PEP) and other metabolites. This indicates an enhancement of ribulose-1,5-bisphosphate (RuBP) carboxylase activity, which is the first step in the C₃ pathway (3-phosphoglycerate and sugar phosphate synthesis); this may optimize cell functions. At low temperatures, a seven-fold increase in RuBP carboxylase activity per cell was observed. S. costatum is able to adapt to low irradiance by increasing and decreasing I _k (the ratio of P _max:, light intensity at onset of light saturation), and to low temperature by increasing its cellular chlorophyll a and RuBP carboxylase content. However, in the latter case, adaptation is not optimal. This study revealed two main features: (1) there is evidence that RuBP carboxylase has a key function in adjustment to high rates of photosynthesis at suboptimal temperatures or irradiances; (2) adaptive mechanisms are dynamic processes and the role of the time scale in physiological adaptation should be considered.     

Temperature ecotypes near the southern boundary of the kelp Laminaria saccharina

Effects of temperature on survival, growth, and photosynthesis were compared for two USA populations of Laminaria saccharina Lamour. One population was located in New York State, near the southern latitudinal boundary of the species in the western North Atlantic. This southern boundary population was exposed to ambient temperatures 20°C for about 6 wk each summer. The second population was located in Maine, toward the center of the latitudinal range of the species, and was rarely exposed to temperatures>17°C. sporophytes from the New York (NY) population exhibited greater tolerance of high temperature than plants from the Maine (ME) site. Juvenile sporophytes from the two sites had similar rates of survivorship and growth at temperatures below 20°C, but showed different responses at 20°C in laboratory experiments. NY plants survived and grew for 6 wk at 20°C. ME plants showed negative growth during wk 2 and 100% mortality during wk 3. NY and ME plants held in situ at the NY site during June to September, 1985, also exhibited differential survivorship when ambient temperatures exceeded 20°C. Results of photosynthesis and dark respiration measurements on NY and ME plants grown at various temperatures suggested that the high-temperature tolerance of NY plants was attributable to their ability to maintain positive daily net C-fixation at 20°C. The high-temperature tolerance of the NY plants appeared to be due to genetic adaptation and is probably crucial to the persistence of the species near its southern boundary.     

Effect of phosphate and ammonium levels on photosynthetic and respiratory responses of the red alga Gracilaria verrucosa

Gracilaria verrucosa (Hudson) Papenfuss exposed to nutrient enriched media (0.1 mM PO₄; 1.0 mM NH ₄ ⁺ ) by pulse feeding 2 h every third day for a period of 5 wk at 20°C and 25–30 salinity showed significantly higher rates of photosynthesis regardless of photon flux density correlated with increased pigment levels. Algae in nonenriched media showed significantly higher levels of soluble carbohydrates and decreased levels of phycoerythrin and chlorophyll a. Photosynthetic and respiratory responses to temperature 15°, 25°, 30°C and salinity (15, 25, 30 S) combinations indicate broad tolerances by both nutrient enriched and non-nutrient enriched algae. Photosynthetic and respiratory rates were highest at the high temperatures. Pulse-fed algae had significantly higher photosynthetic rates than non-nutrient enriched plants at all temperature and salinity combinations. Non-nutrient enriched algae had significantly higher respiratory rates than nutrient enriched algae at only 30°C and 15. The respiratory rates of both nutrient enriched and non-nutrient algae decreased under combinations of higher temperatures and salinities. G. verrucosa, grown without nutrients, has lower tolerances to environmental stresses.     

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.     

Optimal growth and maximal survival temperatures of Atlantic Laminaria species (Phaeophyta) in culture

The effects of temperature on growth rate of rapidly-growing cultured macrosporophytes of 9 isolates of Atlantic Laminaria comprising 4 species have been investigated. No significant population variation was observed within species despite wide variations in temperature between the original collecting sites. L. saccharina showed a broad temperature optimum in the 10°–15°C range, whereas L. longicruris had a sharp optimum at 10°C. L. digitata and L. hyperborea grew more slowly, with only slightly sub-optimal growth over a wide temperature range, but with peaks at 10°C (L. digitata) and 15°C (L. hyperborea). The maximum survival temperatures of individual male and female vegetatively-growing gametophytes were ascertained for these species plus the Arctic L. solidungula, and were as follows: L. saccharina and L. longicruris, 23°C; L. digitata (male), 23°C; L. digitata (female), 22°C; L. hyperborea, 21°C; L. solidungula, 18°C. The lack of within-species differences demonstrates that the success of the genus in areas with different temperature regimes is brought about by phenotypic plasticity of individuals rather than the selection of temperature races or ecotypes.     

Photoinhibition and recovery of the kelp Laminaria saccharina at optimal and superoptimal temperatures

Effects of high irradiance on photosynthetic characteristics were examined in sporophytes of the kelp Laminaria saccharina Lamour. from 1992 to 1994. Exposure to high irradiance (700 mol photons m^-2s^-1) for 1 h at optimal temperature (12°C) caused a 40 to 60% decline in photosynthetic efficiency (alpha), quantum yield, and the ratio of variable to maximum chlorophyll fluorescence (F_v/F_m), an indicator of Photosystem II efficiency. Although the photoinhibition effects were partly attributable to protective mechanisms, a concurrent increase in minimal fluorescence (F_o) indicated damage to Photosystem II reaction centers. The magnitude of photoinhibition was proportional to irradiance and duration; however, F_v/F_m was significantly reduced after exposure to irradiances as low as 40 to 50 mol photons m^-2s^-1 for 1 h, or to 700 mol photons m^-2s^-1 for only 5 min. In contrast, photosynthetic capacity (P_max) was affected only at much higher irradiance. Superoptimal temperatures up to 24°C did not exacerbate high-light effects. At 25°C, however, alpha and P_max were more susceptible to photoinhibition than at lower temperatures. Recovery from photoinhibition was examined by following F_v/F_m and F_o for 24 h after exposure to high light. Recovery of F_v/F_m was fastest during the first 1 to 3 h, and slowed or ceased after 6 to 12 h, while recovery of F_o was relatively constant over 12 h. Dithiothreitol, which blocks formation of energy-dissipating xanthophylls, reduced both the initial rate and extent of recovery. Chloramphenicol, which blocks chloroplast-encoded protein synthesis, had little effect on initial rates of recovery, but stopped recovery after 3 h. Thus, L. saccharina appears to rely on the xanthophyll cycle to protect the photosynthetic apparatus, and reversal of this protective mechanism causes the rapid initial recovery in F_v/F_m. Longterm recovery depends on repair of damaged reaction centers. Both the rate and extent of recovery were temperature-dependent. The initial rate was higher at 18 to 22°C than at 12°C, but the extent of recovery over 24 h declined with increasing temperature. High temperatures, therefore, appear to enhance protective mechanisms, but disrupt repair processes. L. saccharina from Long Island Sound, an ecotype adapted to low light and high temperature, showed slightly but consistently greater effects of photoinhibition than plants from the Atlantic coast of Maine, but exhibited faster recovery at superoptimal temperatures.     

Photosystem II photoinhibition and altered kinetics of photosynthesis during nutrient-dependent high-light photoadaptation in Gonyaulax polyedra

Gonyaulax poledra Stein was transferred at different cell densities from increasingly nutrient-limited low-light (LL, 80 E m^-2 s^-1) batch-cultures to high-light (HL, 330 E m^-2 s^-1) growth conditions. Several age-dependent differences in HL-adaptation strategies were apparent. Short-term (3h) susceptibility to photosynthetic photoinhibition increased with culture age, with light-limited rates of photosynthesis exhibiting greater photosuppression than light-saturated rates at all stages of growth. These shortterm changes were not accompanied by photobleaching of chlorophyll but were directly related to age-dependent photoinactivation of Photosystem II electron-transport rates. The capacity of electron transport by Photosystem I was only slightly affected. Prolonged exposure of LL log-phase cells to HL conditions did induce photobleaching of chlorophyll associated with increased cell volume, a transient decrease of organic carbon and nitrogen content, enhanced cellular-, carbon-and chlorophyll-based rates of light-saturated photosynthesis (P _max) and suppressed cellular rates of light-limited photosynthesis. As a result, the density of LL log-phase cells doubled and their cellular photosynthetic performance nearly tripled within 1 d of HL exposure while cellular respiratory demands remained unchanged. By contrast, prolonged HL incubation of LL stationary populations induced a transitory burst in cell division and a large reduction in cell volume, leading to a short-term increase in volume-based organic carbon and nitrogen content. Despite reduced cell volume and lowered carbon demand, the cellular-, carbon-and chlorophyll-based rates of P _max in nondividing populations fell by 64, 48 and 27%, respectively, over a 4 d exposure to HL, while light-limited rates were almost fully suppressed within 1 d and chlorophyll a content was reduced by 56%. As a result, the photosynthetic performance of LL-aged cells declined immediately under HL conditions. Addition of inorganic nutrients to LL stationary cultures at the time of HL transfer led to immediate and complete suppression of photosynthesis and cell lysis within 1 d. Addition of nutrients following transfer to HL induced cell responses intermediate to those described for LL log and aged cells exposed to HL. Results support the view that declining nutrient-status impairs HL photoadaptive responses in phytoplankton populations and that the rate and pattern of photoadaptive responses may be used as physiological growth indicators in field studies. The study was conducted from March 1981 to May 1983.     

Photoacclimation of Ulva rotundata (Chlorophyta) under natural irradiance

Two vegetative clones (designated 11/85 and 7/86 in accordance with month/year of collection) of the green macroalga Ulva rotundata were collected in the vicinity of Beaufort, North Carolina, USA. Each was grown in an outdoor continuous-flow system in summer (20°C) of 1986 and late winter (10° to 17°C) of 1987, in irradiances ranging from 9 to 100% of full sunlight, with and without NH ₄ ⁺ enrichment. Continuous enrichment of influent estuarine water (dissolved inorganic nitrogen 2 M, N:P5) to 8–12 M NH ₄ ⁺ had only a slight effect on growth rate. Temperature changes of 2 to 3°C had a much greater effect. Prolonged exposure to a given daily irradiance resulted in acclimation, exposure to a given daily irradiance resulted in acclimation, indicated by faster growth of conditioned plants relative to those transferred from a different irradiance. Most of the difference in growth rates between transferred and control plants was attributed to differences in thallus absorptance. Growth was photoinhibited above 40% sunlight at temperatures below 15°C, but not above 20°C. Following interday irradiance transfers, thallus percent dry weight changed in a manner that suggests different response times for photosynthesis and cell division.     

Seasonal effects of light,temperature, nutrient concentration and salinity on the physiology and growth of Caulerpa paspaloides (Chlorophyceae)

Caulerpa paspaloides (Bory) Greville were collected during the winter and summer (1978 to 1979) from the Florida Keys, USA. Thalli collected during the winter photosynthesized more efficiently at low light intensities (I_c<1, I_k=38 Exm^-2xs^-1) than did thalli collected in the summer (I_c=13, I_k=111 Exm^-2xs^-1). Summer thalli exhibited higher P_max values (2.20 mgO₂xg^-1 dry wtxh^-1) than winter thalli (1.70 mg O₂xg^-1 dry wtxh^-1). Rates of rhizome elongation and frond initiation were strongly inhibited by winter temperatures. The maximum lethal temperature for summer thalli was 37° to 38°C as measured by both growth and photosynthesis. No evidence of nitrogen or phosphorus limitation was found. Relatively minor reductions in salinity (3S) resulted in significant increases in rhizome apex motality. Results indicate that low winter temperatures are responsible for reduced winter growth rates previously reported for the Key Largo population. Increased photosynthetic efficiency at low light intensities and preferential maintenance of rhizome elongation over frond initiation appear to allow this tropical macroalga to optimize growth and survival during the winter.     

Autecology and clonal variability of the marine centric diatom Thalassiosira rotula (Bacillariophyceae) in response to light,temperature and salinity

The influence of 49 combinations of salinity (10–40 S, at 5 S intervals) and temperature (0°–30°C, at 5C° intervals) on the maximum daily division rate (K) and 18 combinations of light intensity (six levels) and temperature (5°, 15°, and 25°C) on photosynthesis, cell division, and chlorophyll a was examined using two clones of Thalassiosira rotula Meunier isolated from the upwelling area of Baja California (clone C8) and from Narragansett Bay, Rhode Islands (clone A8). Physiological differences appear to characterize these to clones with regard to their temperature tolerance (C8 5°–30°C, A8 0°–25°C), maximum growth rate (C8 K=2.9, A8 K=2.4), chlorophyll a content, and in the rates of growth and photosynthesis in response to light intensity and temperature. Optimum salinity for both clones (25–30 S) was generally independent of temperature, while chlorophyll a content decreased with temperature. T. rotula is a cosmopolitan paractic species; experimental studies indicate that it is eurythermal and moderately euryhaline. Comparison of five additional Narragansett Bay isolates of T. rotula reveal minimal spacial or temporal variability in genetically determined physiological characteristics within this local population.     

Freezing rate and duration determine the physiological response of intertidal fucoids to freezing

Differential thermal analysis (DTA) was used to measure the freezing temperature of nine species of red brown intertidal macroalgae from the coast of Maine, USA in 1991. Using slow and rapid cooling rates approximating those found in the field for Ascophyllum nodosum (L.) Le Jol. we found that, for a given rate, the freezing points of all species were similar: -7.06 to -8.02°C for slow cooling (ca. 0.25°C min^-1) and -3.42 to -4.56°C for rapid cooling (ca. 5.0°C min^-1). In the low shore species, Fucus evanescens C. Ag., photosynthesis was inhibited to a greater extent when plants were frozen or thawed rapidly than after slow freezing or thawing. However, in the upper shore species, F. spiralis (L.), photosynthesis recovered rapidly and completely regardless of freezing rate. Rapidly frozen F. evanescens also experienced greater loss of plasmalemmal integrity, evidenced by a greater loss of cellular contents on re-immersion, than those frozen slowly. Light-limited photosynthesis following freezing was more severely inhibited than light-saturated photosynthesis. Respiration was generally enhanced immediately after freezing, but then declined to rates below those of unfrozen controls within 2 h following re-immersion, with control rates of respiration being achieved after a 24 h recovery period. Our data suggest that the physiological consequences of winter emersion at sub-zero temperatures may vary widely between individual plants of freezing-susceptible species, due to the wide variations in freezing rate associated with microhabitat effects.     

Effects of salinity and possible interactions with temperature and pH on growth and photosynthesis of <Emphasis Type="Italic">Halophila johnsonii</Emphasis> Eiseman

The effects of salinity, temperature, and pH variations on growth, survival, and photosynthetic rates of the seagrass Halophila johnsonii Eiseman were examined. Growth and survival responses to salinity were characterized by aquarium experiments in which plants were exposed to seven different salinity treatments (0, 10, 20, 30, 40, 50, and 60 psu) during 15 days. Photosynthetic behavior was assessed for short-term salinity exposures (1 or 20 h) by incubation experiments in biological oxygen demand (BOD) bottles and by measuring photosynthesis versus irradiance (PI) responses in an oxygen electrode chamber. In the bottle experiments the possible effects of interactions between salinity and temperature (15, 25, and 35°C) or pH (5, 6, 7, and 8.2) were also examined. Growth and survival of H. johnsonii were significantly affected by salinity, with maximum rates obtained at 30 psu. Salinity also altered the parameters of the PI curves. Light-saturated photosynthesis (P _max) and the photosynthetic efficiency at subsaturating light (α) increased significantly up to an optimum of 40 psu, decreasing again at the highest salinities. Dark respiration rates and compensating irradiance (I _c) showed minimum values at 40 and 50 psu, while light-saturation point (I _k) was maximum at 30–50 psu. An interaction between salinity and temperature was not found although an increase of temperature alone produced an increase in α, P _max, respiration rates, and I _k. An interaction between salinity and pH was only found in the P _max response: P _max increased with pH=5 at 30 psu. In addition, reducing the pH increased α significantly. In the BOD bottles experiment a significant reduction in the dark respiration with decreasing pH was observed, but the opposite trend was observed in the photosynthetic rate. These results suggest that the endemic seagrass H. johnsonii could be negatively affected by hypo- or hypersalinity conditions, although salinity changes did not seem to alter the tolerance of this species to other environmental factors, such as temperature or pH.     

Photosynthetic performance of Laminaria solidungula measured in situ in the Alaskan High Arctic

Photosynthetic performance in the kelp Laminaria solidungula J. Agardh was examined from photosynthesis irradiance (P-I) parameters calculated from in situ ¹⁴C uptake experiments, using whole plants in the Stefansson Sound Boulder Patch, Alaskan Beaufort Sea, in August 1986. Rates of carbon fixation were determined from meristematic, basal blade, and second blade tissue in young and adult sporophytes. Differences in saturating irradiance (I _k, measured as photosynthetically active radiation, PAR), photosynthetic capacity (P _max), and relative quantum efficiency () were observed both between young and adult plants and between different tissue types. I _k was lowest in meristematic tissue (20 to 30 E m^–2 s^–1) for both young and adult plants, but consistently 8 to 10 E m^–2 s^–1 higher in young plants compared to adults in all three tissues. Average I _k for non-meristematic tissue in adult plants was 38 E m^–2 s^–1. Under saturating irradiances, young and adult plants exhibited similar rates of carbon fixation on an area basis, but under light limitation, fixation rates were highest in adult plants for all tissues. P _max was generally highest in the basal blade and lowest in meristematic tissue. Photosynthetic efficiency () ranged between 0.016 and 0.027 mol C cm^–2 h^–1/E m^–2 s^–1, and was highest in meristematic tissue. The relatively lower I _k and higher exhibited by L. solidungula in comparison to other kelp species are distinct adaptations to the near absence of light during the eight-month ice-covered period and in summer when water turbidity is high. Continuous measurement of in situ quantum irradiance made in summer showed that maximum PAR can be less than 12 E m^–2 s^–1 for several days when high wind velocities increase water turbulence and decrease water transparency.The Univeristy of Texas Marine Science Institute Contribution No. 695     

Spatial heterogeneity of photosynthesis and the effect of temperature-induced bleaching conditions in three species of corals

Heterogeneity in photosynthetic performance between polyp and coenosarc tissue in corals was shown using a new variable fluorescence imaging system (Imaging-PAM) with three species of coral, Acropora nobilis, Cyphastrea serailia and Pocillopora damicornis. In comparison to earlier studies with fibre-optic microprobes for fluorescence analysis, the Imaging-PAM enables greater accuracy by allowing different tissues to be better defined and by providing many more data points within a given time. Spatial variability of photosynthetic performance from the tip to the distal parts was revealed in one species of branching coral, A. nobilis. The effect of bleaching conditions (33°C vs. 27°C) was studied over a period of 8 h. Marked changes in fluorescence parameters were observed for all three species. Although a decline in _PSII (effective quantum yield) and Yi (the first effective quantum yield obtained from a rapid light curve) were observed, P. damicornis showed no visual signs of bleaching on the Imaging-PAM after this time. In A. nobilis and C. serailia, visual signs of bleaching over the 8 h period were accompanied by marked changes in F (light-adapted fluorescence yield), NPQ (non-photochemical quenching) and E _k (minimum saturating irradiance), as well as _PSII and Yi. These changes were most marked over the first 5 h. The most sensitive species was A. nobilis, which after 8 h at 33°C had reached a _PSII value of almost zero across its whole surface. Differential bleaching responses between polyps and coenosarc tissue were found in P. damicornis, but not in A. nobilis and C. serailia. NPQ increased with exposure time to 33°C in both the latter species, accompanied by a decreasing E _k, suggesting that the xanthophyll cycle is entrained as a mechanism for reducing the effects of the bleaching conditions.Communicated by L. Hagerman, Helsingør     

Photosynthetic response to temperature and desiccation of the intertidal alga Mastocarpus papillatus

When exposed to air during low tide, intertidal macroalgae experience a terrestrial environment and often encounter extreme levels of heating and desiccation. Two aspects of photosynthesis may be influenced by this increase in temperature and decrease in water content during exposure to air: (1) the rate of aerial photosynthesis itself, and (2) the rate at which aquatic photosynthesis recovers upon immersion in water at high tide. This laboratory study examines the effect of air temperature and desiccation on photosynthesis of the intertidal macroalga Mastocarpus papillatus Kützing. Plants were collected at Hopkins Marine Station, California, USA (36°37N; 121°54W) between July and December 1990. When apical tips were exposed to 15 to 25°C air for 2 h, photosynthesis was rapidly recovered upon reimmersion in seawater. Recovery was delayed, but complete, when tissue was exposed to 30°C air, but did not occur after exposure to 35°C air. Desiccation did not influence either the rate or the ultimate level of recovery upon reimmersion. Photosynthesis in air generally decreased with increasing desication, with no net photosynthesis occurring below 25% relative water content. Net photosynthesis of hydrated thalli increased with air temperature from 15 to 30°C, then decreased at 35°C. Dark respiration of hydrated thalli increased over the entire temperature range. This study indicates that thallus heating and desiccation during periods of exposure to air can potentially influence the total carbon budget of M. papillatus.     

Nitrogen-fixation by cyanobacteria associated withCodium fragile (Chlorophyta): Environmental effects and transfer of fixed nitrogen

N₂-fixation associated with the green macroalgaCodium fragile subsp.tomentosoides (van Goor) Silva from Long Island, New York, USA, was attributable to several species of endophytic cyanobacteria. Rates of N₂-fixation ranged from 0.03 to 3.2µg N g^–1 dry wt h^–1 in freshly collected plants from several sites. Growth of the cyanobacteria appeared to be light-limited, due to the transmission of only 5 to 10% of incident light through the pigmented surface-layer of the macroalga. Daily irradiance was the most important factor determining both abundance of cyanobacterial cells and rate of N₂-fixation. The rate was also affected by instantaneous irradiance, and increased twofold from dark to ambient surface irradiance. Rates were reduced at low temperature (8°C) but showed no temperature effect between 12° and 26°C. External concentrations of dissolved inorganic nitrogen (DIN) up to 20µM did not influence N₂-fixation rate, but long-term exposure to 60µmol l^–1 d^–1 of NH ₄ ⁺ caused a reduction in the rate. InC. fragile grown under high daily irradiance and low external DIN concentration, ~50% of the assimilated-N was attributable to N₂-fixation. However, chlorophyllb extracted from plants grown with¹⁵N₂ showed an atom % excess¹⁵N of less than 0.1, suggesting that only a small proportion of the bacterially fixed-N was transferred to the seaweed. The association betweenC. fragile and its endophytic cyanobacteria appears to be based primarily on microhabitat suitability, rather than mutual metabolic dependence. It is doubtful that N₂-fixation by cyanobacteria is important to the ecological success of this seaweed species.     

Response of gametophytes of Ecklonia radiata (Laminariales) to temperature in saturating light

Gametophytes of Ecklonia radiata (C.Ag.)J.Ag. from two New Zealand locations with different field temperature ranges were exposed to temperatures of 5° to 26°C in saturating light. Plants from Goat Island Bay (Lat. 36° 16S, Long. 174°48E) grew in 9.3° to 25°C and reproduced in 9.3° to 24°C. There was no growth at 8°C and plants died at 26°C. Plants from the cooler location, Houghton Bay (Lat. 41°20S, Long. 174°40E), grew from 8° to 24°C and reproduced up to 15°C but not at 21.5°C. The plants did not grow at 6°C and died at 26°C. The timing of the first cell division and subsequent growth rate were retarded close to the upper and lower tolerance limits. Reproduction was a broad optimum of roughly 12° to 20°C. Within this range, fertile female gametophytes grown at lower temperatures had fewer, larger cells and thus fewer potential ova than those grown at higher temperatures.     

Time-course of photoadaptation in the photosynthesis-irradiance relationship of a dinoflagellate exhibiting photosynthetic periodicity

Cultures of the marine dinoflagellate Glenodinium sp. were light-shifted and rates of photoadaptation determined by monitoring changes in cell volume, growth rate, pigmentation, parameters of the photosynthesisirradiance (P-I) curves and respiration. To approximate physiological conditions of field populations, cells were cultured on an alternating light-dark cycle of 12hL:12hD, which introduced a daily periodicity of photosynthesis. One result of the present study was to demonstrate how specific parameters of the P-I relationship influenced by periodicity of the light: dark cycle are distinguished from photosynthetic parameters influenced by changes in light level. Under steady-state conditions, rates of both light-saturated (P_max) and light-limited photosynthesis changed in unison over the day; these changes were not related to pigmentation, and displayed their maxima midday. This close relationship between P_max and the slope (a) of the cellular P-I curves in steadystate conditions was quickly adjusted when growth illumination was altered. Rates of light-limited photosynthesis were increased under low light conditions and the periodicity of cellular photosynthesis was maintained. The short-term responses of the P-I relationship to changing light level was different, depending on (1) whether the light shift was from high to low light or vice versa, and (2) whether the high light levels were sufficient to promote maximal photosynthesis rates. Major increases in the photosynthetic carotenoid peridinin, associated with a single type of light-harvesting chromo protein in the chloroplast, was observed immediately upon shifting high light cultures to low light conditions. Following pigment synthesis, significant increases in rates of light-limited photosynthesis were observed in about one-tenth the generation time, while cellular photosynthetic potential was unaffected. it is suggested that general results were consistent with suggested that general results were consistent with earlier reports that the major photoadaptive strategy of Glenodinium sp. is to alter photosynthetic unit (PSU) size. Photoadaptive response times to high light were light-dependent, but appeared to be shower than photoadaptive responses to low light. If light intensities were bright enough to maximize growth rates, photosynthetic response times were on the order of a generation period and pigmentation fell quickly as cells divided at a faster rate. If light-intensities were not sufficient to maximize growth rates, then pigment content did not decline, while rates of light-limited photosynthesis declined quickly. In all cases, photoadaptation was followed best by monitoring fast changes in half saturation constants for photosynthesis, rather than fluctuating changes in pigmentation. Results compared well with time-course phenomena reported for other groups of phytoplankton. Overall, results suggest phytoplankton can bring about photo-induced changes in photosynthesis very quickly and thus accommodate widely fluctuating light regimes over short periods of time.