SHORT‐TERM AND LONG‐TERM EFFECTS OF TEMPERATURE ON PHOTOSYNTHESIS IN THE DIATOM THALASSIOSIRA PSEUDONANA UNDER UVR EXPOSURES1

Temperature is expected to modify the effects of ultraviolet radiation (UVR) on photosynthesis by affecting the rate of repair. We studied the effect of short‐term (1 h) and long‐term (days) acclimation to temperature on UVR photoinhibition in the diatom Thalassiosira pseudonana Hasle et Heimdal. Photosynthesis was measured during 1 h exposures to varying irradiances of PAR and UVR + PAR at 15, 20, and 25°C, the latter corresponding to the upper temperature limit for optimal growth in T. pseudonana. The exposures allowed the estimation of photosynthesis–irradiance (P–E) curves and biological weighting functions (BWFs) for photoinhibition. For the growth conditions used, temperature did not affect photosynthesis under PAR. However, photoinhibition by UVR was highly affected by temperature. For cultures preacclimated to 20°C, the extent of UVR photoinhibition increased with decreasing temperature, from 63% inhibition of PAR‐only photosynthesis at 25°C to 71% at 20°C and 85% at 15°C. These effects were slightly modified after several days of acclimation: UVR photoinhibition increased from 63% to 75% at 25°C and decreased from 85% to 80% at 15°C. Time courses of photochemical efficiency (Φ_PSII) under UVR + PAR were also fitted to a model of UVR photoinhibition, allowing the estimation of the rates of damage (k) and repair (r). The r/k values obtained for each temperature treatment verified the responses observed with the BWF (R² = 0.94). The results demonstrated the relevance of temperature in determining primary productivity under UVR exposures. However, the results suggested that temperature and UVR interact mainly over short (hours) rather than long (days) timescales.     

Effects of temperature, salinity, irradiance and diurnal periodicity on growth and photosynthesis in the diatom Nitzschia americana: light-limited growth

The effects of variations in temperature (10, 15, 20, 25, 30C)and salinity (8, 15, 20, 26, 32 p.p.t.) on cell size and ratesof photosynthesis and population growth were evaluated in axenic,light-limited (30 µE m^–2 s^–1) cultures ofan estuarine clone of the diatom ^{Nitzschia americana}. Experimentalconditions were chosen to reflect the range of natural conditionswhich occur in the clone's native environment, the Cape FearRiver Estuary, Nqrth Carolina. Rates of light-limited grossphotosynthesis; or photosynthetic efficiency (PSE), were determinedfrom short-term (1 h) ¹⁴C incubations. Diurnal variation inPSE was analyzed using ¹⁴C samples taken during times of estimatedmaximum and minimum rates of diurnal photosynthesis. The salinity-dependenttemperature response of PSE is characterized by a gradual increasein rates up to a temperature optimum at –25C, beyondwhich rates rapidly decline to zero at an upper lethal limit(30–40C). A similar pattern was observed in populationgrowth rates as a function of salinity and temperature. Independentof temperature, optimum salinity for growth was 26 p.p.t. Amaximum growth rate of 2.4 div d^–1 was measured at 25Cand 26 p.p.t. The effect of non-optimum salinity is a reductionin growth rates relative to a predicted temperature-dependentmaximum. Salinity-dependent patterns of variation in cell volume,in general, mirrored the response of population growth suchthat cultures with relatively high growth rates were dominatedby small cells. Significant diurnal variation was observed inPSE; maximum diurnal rates were generally 1.5–3.5 timesgreater than minimum diurnal rates.     

STUDIES ON THE AUTECOLOGY OF THE MARINE DIATOM THALASSIOSIRA NORDENSKIÖLDIICLEVE. 1. THE INFLUENCE OF DAYLENGTH,LIGHT INTENSITY,AND TEMPERATURE ON GROWTH1

The marine diatom Thalassiosira nordenskiöldii Clave was grown at 48 different combinations of daylength (9:15, 12:12, 15:9 LD), light intensity (0.011, 0.027, 0.066, 0.100 ly/min [g cal/cm²/min]), and temperature (0, 5, 10, 15 C). Growth occurred at all combinations of light and temperature except at 15 C at the highest light level. Maximum growth (K = 1.8 doublings/day) occurred at 10 C under the 15:9 LD cycle. At 15 C the maximum rate was 1.7 doublings/day but occurred at the shortest day-length (9:15 LD). The maxima at 5 and 0 C were 1.32 and 0.67 doubling/day, respectively. At 0 C growth was similar over a wide range of light intensities (K = 0.6–0.65), with, maximum growth being attained at a much lower light intensity than at 5 C. Above 5 C there was a decrease in the light intensity at which maximum growth occurred and excessive light became inhibitory to growth. At 15 C the light intensity at which maximum growth occurred was greater with shorter day-lengths. The temperature optimum was 10 C at 15:9 and 15 C at 9:15 LD. The chlorophyll a content of the cells was greatest under low light intensities and short daylengths, while temperature had a variable effect. The response of Thalassiosira in the laboratory contrasts with, its apparent preference for low temperatures in nature (0–5 C). The experiments suggest that the termination of the bloom of Thalassiosira in Narragansett Bay and elsewhere is not solely temperature dependent.     

Acclimation of thermotolerant algae to light and temperature interaction1

Here, we explore the responses of photosynthesis and related cellular processes in the thermotolerant microalga Micractinium sp. acclimated to limiting and saturating irradiances combined with elevated temperatures, using a novel computer-controlled multi-sensor system. This system allows for the monitoring of online values of oxygen exchange during photosynthesis and respiration with high accuracy. Micractinium sp. cells showed maximum growth and net oxygen production rates under the optimal temperature of 25°C regardless of the light acclimation conditions. Our results show that the upper thermal threshold for Micractinium sp. photosynthesis and growth ranges between 35°C and 40°C. This microalga exhibited stable photosynthetic efficiency and effective non-photochemical quenching (NPQ) under saturating light, and was more susceptible to temperature change when acclimated to limiting light levels. These results demonstrate that the acclimation of thermotolerant microalgae to saturating light helps to enhance the thermal tolerance of PSII. This feature results from enhanced heat stability of PSII photochemistry and oxygen evolution.     

Long-Term Temperature Acclimation of Photosynthesis in Steady-State Cultures of the Polar Diatom <Emphasis Type="Italic">Fragilariopsis cylindrus</Emphasis>

Cultures of the obligate psychrophilic diatom Fragilariopsis cylindrus (Grunow) were grown for 4 months under steady-state conditions at −1 °C and +7 °C (50 μmol photons m⁻² s⁻¹) prior to measurements in order to investigate long-term acclimation of photosynthesis to both temperatures. No differences in maximum intrinsic quantum yield of PS II (F_V/F_M) and relative electron transport rates could be detected at either temperature after 4 months of acclimation. Measurements of photosynthesis (relative electron transport rates) vs. irradiance (P vs. E curves) revealed similar values for relative light utilization efficiency (α = 0.57 at −1 °C, α = 0.60 at +7 °C) but higher values for irradiance levels at which photosynthesis saturates (E_K) at −1 °C and, therefore, higher maximum photosynthesis (P_MAX = 54 (relative units) at −1 °C, P_MAX = 49 at +7 °C). Nonphotochemical quenching (NPQ) measurements at 385 μmol photons m⁻² s⁻¹ indicated higher (37%) NPQ for diatoms grown at −1 °C compared to +7 °C, which was possibly related to a 2-fold increase in the concentration of the pigment diatoxanthin and a 9-fold up-regulation of a gene encoding a fucoxanthin chlorophyll a,c-binding protein. Expression of the D1 protein encoding gene psbA was ca. 1.5-fold up-regulated at −1 °C, whereas expression levels of other genes from Photosystem II (psbC, psbU, psbO), as well as rbcL, the gene encoding the Rubisco large subunit were similar at both temperatures. However, a 2-fold up-regulation of a plastid glyceraldehyde-P dehydrogenase at −1 °C indicated enhanced Calvin cycle activity. This study revealed for the first time that a polar diatom could efficiently acclimate photosynthesis over a wide range of polar temperatures given enough time. Acclimation of photosynthesis at −1 °C was probably regulated similarly to high light acclimation.     

The interacting effect of prolonged darkness and temperature on photophysiological characteristics of three Antarctic phytoplankton species

Photophysiological characteristics of the Southern Ocean phytoplankton species Phaeocystis antarctica, Geminigera cryophila, and Chaetoceros simplex were assessed during 7 weeks of darkness and subsequent recovery after darkness at 4 and 7°C. Chlorophyll a fluorescence and maximum quantum efficiency of PSII decreased during long darkness in a species-specific manner, whereas chlorophyll a concentration remained mostly unchanged. Phaeocystis antarctica showed the strongest decline in photosynthetic fitness during darkness, which coincided with a reduced capacity to recover after darkness, suggesting a loss of functional photosystem II (PSII) reaction centers. The diatom C. simplex at 4°C showed the strongest capacity to resume photosynthesis and active growth during 7 weeks of darkness. In all species, the maintenance of photosynthetic fitness during darkness was clearly temperature dependent as shown by the stronger decline of photosynthetic fitness at 7°C compared to 4°C. Although we lack direct evidence for this, we suggest that temperature-enhanced respiration rates cause stronger depletion of energy reserves that subsequently interferes with the maintenance of photosynthetic fitness during long darkness. Therefore, the current low temperatures in the coastal Southern Ocean may aid the maintenance of photosynthetic fitness during the austral winter. Further experiments should examine to what extent the species-specific differences in dark survival are relevant for future temperature scenarios for the coastal Southern Ocean.     

Phytoplankton primary production in a eutrophic cooling water pond

The seasonal variation of phytoplankton photosynthesis was measured with ¹⁴C-method in a warmed ice-free pond in central Finland. Simultaneously with in situ measurements the photosynthesis was also measured in an incubator with different water temperatures and constant light (ca. 16 W m^–2). The total annual photosynthesis was 57.2 C m^–2 a^–1. The portion of the winter and spring production of the annual photosynthesis was 18.4%, that of the autumn production ws 17.4%. Thus 64.3% of the total annual phytoplankton photosynthesis occurred in the three summer months. The range of the daily integrated photosynthesis per unit area was 1.9—563 mg C m^–2d^–1. The photosynthetic rate per unit chlorophyll a varied in situ from 0.94 to 33.1 mg C (mg chl. a)^–1 d^–1. The highest value was measured in the beginning of July and the lowest in mid-January. The photosynthetic rate increased in situ exponentially with increasing water temperature. In the incubator the highest photosynthetic rate values were also found in July and August (at+20 °C) when the phytoplankton population was increasing and the minimum values occurred after every diatom maximum both in spring and autumn. Light was a limiting factor for photosynthesis from September to Mid-January, low water temperature was a limiting factor from late January through May. The efficiency of the photosynthesis varied between 0.1 and 0.7% of P.A.R. According to the incubator experiments the Q₁₀ values for the photosynthesis were 2.45 and 2.44 for the winter population between 1 and 10° C and for the summer population between 5 and 15° C, respectively, but the Q₁₀ values decrease at the higher temperatures. The main effect of the warm effluents on the yearly photosynthesis was the increase of production in spring months due to the lack of ice cover. However, the increase of total annual phytoplankton photosynthesis was only ca. 10–15%, because the water temperature was during the spring months below 10° C.     

RATES OF PHOTOSYNTHESIS AND RESPIRATION OF THE MOSS BRYUM SANDBERGII AS INFLUENCED BY LIGHT INTENSITY AND TEMPERATURE

Using differential respirometry and air enriched to 3% CO₂ (v/v), the rates of photosynthesis and dark respiration of the moss Bryum sandbergii were measured as influenced by temperature and light intensity. The optimal temperature for net (apparent) photosynthesis was between 24 to 30 C; however, the photosynthesis/respiration ratio was about 11 to 27 between 4 to 24 C and dropped to lower values at 34 C., which indicates a wide temperature tolerance for this moss in short-term experiments. The maximum temperature for photosynthesis was about 41 C and the minimum was below –5 C. At 20 C light saturation was approached at 6.2 mw cm^–2 (ca. 700 ft-c) but not completely reached at 12 mw cm^-2; the light compensation point was estimated to be 0.4 mw cm^-2 (ca. 40 ft-c). At 4 C light saturation and the compensation point were at lower levels and apparently solarization occurred at 12 mw cm^-2. Light intensity had little or no apparent effect on dark respiration. However, respiration increased with temperature over various ranges extending from –5 to 39 C with temperature quotients of about 2.5 to 1.2. The significance of these characteristics is discussed with respect to the ecological relationships of the species.     

Influence of low temperatures on the growth and photosynthetic activity of industrial chicory, <Emphasis Type="Italic">Cichorium intybus</Emphasis> L. partim

The cold stress effect on early vigour and photosynthesis efficiency was evaluated for five industrial chicory varieties with contrasting early vigour. The relationships between the growth and physiological parameters were assessed. The varieties were examined at three growth temperatures: 16 (reference), 8 (intermediate) and 4 °C (stress). The effect was measured using physiological processes (growth, photosynthesis, chlorophyll a fluorescence), and pigment content. The analysis of the measured growth parameters (dry leaf and root mass, and leaf area) indicated that temperature had a significant effect on the varieties, but the overall reaction of the varieties was similar with lowering temperatures. The photosynthesis and chlorophyll a fluorescence measurements revealed significant changes for the photosynthesis (maximum net photosynthesis, quantum efficiency, light compensation point and dark respiration) and chlorophyll a fluorescence parameters (photochemical and non-photochemical quenching) with lowering temperatures for Hera and Eva, two extremes in youth growth. No significant differences could be found between the extremes for the different temperatures. The pigment content analysis revealed significant differences at 4 °C in contrast to 16 and 8 °C, especially for the xanthophyll/carotenoid pool, suggesting a protective role. Subsequently, the relationship between the physiological processes was evaluated using principal component analysis. At 4 °C, 2 principal components were detected with high discriminating power for the varieties and similar classification of the varieties as determined in the growth analysis. This provides a preview on the possible relationships between photosynthesis and growth for industrial chicory at low temperatures.     

Effect of Temperature on Diurnal Changes in CO2 Exchange in Intact Cucumber Plants

Multifactorial experiments were performed to study the diurnal dynamics of CO₂ exchange in intact cucumber plants (Cucumis sativus L.). Based on experimental data, we analyzed the models of net photosynthesis, night respiration, and biomass accumulation. This analysis allowed us to resolve the growth component of respiration and to determine the diurnal temperature pattern that is optimal for biomass accumulation. It was found that the most profound transformation of assimilates into the biomass occurs under the maximum ratio of growth respiration to maintenance respiration. Under the experimental conditions used, this requirement was fulfilled at a temperature of 25°C during the photoperiod (optimum of net photosynthesis) and at subsequent gradual cooling to a hardening temperature (13°C by the end of the night).     

The Influence of Cu on Photosynthesis and Growth in Diatoms

Cu in ionic form in a balanced medium affects the rate of photosynthesis and growth in the diatom Nitzschia palea in about the same way as in the green alga Chlorella pyrenoidosa. Remarkable differences are found, however. Small concentrations of Cu influence the rate of photosynthesis considerably more in the diatom than in the green alga whereas the opposite is the case concerning growth. The latter is most likely a consequence of excretion of organic matter by the diatom in presence of Cu whereas this does not take place in Chlorella. Some of the excreted organic matter may bind Cu and thus make the medium suitable for growth. Although pre-treatment with Cu in the dark has no influence on the subsequent rate of photosynthesis in the light, organic matter is excreted in the dark immediately after the addition of Cu. The influence of Cu on the rate of photosynthesis varies by a factor of about 30 according to the stage of growth in a synchronized culture.     

Photosynthetic Rates, Gross Patterns of Carbon Dioxide Assimilation and Activities of Ribulose Diphosphate Carboxylase in Marine Algae Grown at Different Temperatures

Studies of the marine green flagellate Dunaliella tertiolecta have confirmed and extended previous observations of Steemann Nielsen and his colleagues. Algae, grown at 12°C, assimilated carbon dioxide under light-saturated conditions more rapidly than did those grown at 20°C; for both, the assimilation rate being higher at 20°C than at 12°C. Cells grown at the lower temperature contained higher concentrations of soluble protein, higher activities of ribulose diphosphate carboxylase and showed an enhanced relative rate of protein synthesis during the photosynthetic assimilation of carbon dioxide. This appears to represent true adaptation since it allowed the growth rate at 12°C to be almost the same as that at 20°C. Studies of the marine diatom Phaeodactylum tricornutum have not revealed the same picture of temperature adaptation. Cultures grown at 5°C had significantly higher rates of photosynthesis than did those grown at 10°C, but the same was not true when algae grown at 10°C were compared with those grown at 20°C. In this organism, growth at the lower temperatures reduced its ability to photosynthesize at 20°C. Cells grown at the lower temperatures contained more protein than did those grown at 20°C; this was particularly marked in cells growing at 5°C, a temperature which reduced the growth rate. The relative rate of protein synthesis was higher in Phaeodactylum grown at lower temperatures; but this difference was most marked when the measurements were made at 20°C.     

INTERACTIVE EFFECTS OF IRRADIANCE AND TEMPERATURE ON THE PHOTOSYNTHETIC PHYSIOLOGY OF THE PENNATE DIATOM PSEUDO‐NITZSCHIA GRANII (BACILLARIOPHYCEAE) FROM THE NORTHEAST SUBARCTIC PACIFIC1

This study examined how light and temperature interact to influence growth rates, chl a, and photosynthetic efficiency of the oceanic pennate diatom Pseudo‐nitzschia granii Hasle, isolated from the northeast subarctic Pacific. Growth rates were modulated by both light and temperature, although for each irradiance tested, the growth rate was always the greatest at ～14°C. Chl a per cell was affected primarily by temperature, except at the maximum chl a per cell (at 10°C) where the effects of light were noticeable. At both ends of the temperature gradient, cells displayed evidence of chlorosis even at low light intensities. Chl fluorescence data suggested that cells at 8°C were significantly more efficient in their photosynthetic processes than cells at 20°C, despite having comparable concentrations of chl. Cells at low temperature showed photosynthetic characteristics similar to high‐irradiance‐adapted cells. The decline of growth rates beyond the optimum growth temperature coincided with the cell's inability to accumulate chl in response to increasing temperature. The decline in photosynthetic ability at 20°C was likely due to a combination of high‐temperature stress on cellular membranes and a decline in chl. Our results highlight the important interactions between light and temperature and the need to incorporate these interactions into the development of phytoplankton models for the subarctic Pacific.     

Effect of nitrate-nitrogen limitation on photosynthesis of the diatom Phaeodactylum tricornutum Bohlin (Bacillariophyceae)

Abstract Nitrate limited growth of the diatom Phaeodactylum tricornutum in chemostat cultures produced marked changes in biochemical composition and a six-fold reduction in the specific growth rate. This was associated with a reduction in the carbon and chlorophyll a specific light saturated rates, with little effect on light limited photosynthesis. Variations in specific growth rate were quantitatively related to carbon specific net photosynthesis and maximum chlorophyll a specific light saturated rates were positively correlated with cell nitrogen contents. The correlation between nitrogen content and photosynthesis for P. tricornutum and the differential effect of nitrogen supply on the light response curve of photosynthesis is qualitatively and quantitatively similar to published results for terrestrial vascular plants. There was little change in the photon (quantum) yield of photosynthesis which was not significantly different from 0.125mol O₂ mol photon^-1 the theoretical upper limit based on the Z scheme, even under severe nitrate deficiency. The capacity to maintain a high photon yield under nitrate limitation is discussed in relation to the nitrogen requirements of the stromal and membrane components of the photosynthetic apparatus.     

Elevated growth temperatures reduce the carbon gain of black spruce [Picea mariana (Mill.) B.S.P.]

We explored the effect of high‐growth temperatures on a dominant North American boreal tree, black spruce [Picea mariana (Mill.) B.S.P.]. In 2004 and 2005, we grew black spruce at either 22 °C/16 °C day/night temperatures [low temperature (LT)] or 30°/24 °C [high temperature (HT)] and determined how temperature affected growth, leaf morphology, photosynthesis, respiration and thermotolerance. HT spruce were 20% shorter, 58% lighter, and had a 58% lower root : shoot ratio than LT trees. Mortality was negligible in the LT treatment, but up to 14% of HT seedlings died by the end of the growing season. HT seedlings had a higher photosynthetic temperature optimum, but net photosynthesis at growth temperatures was 19–35% lower in HT than LT trees. HT seedlings had both a lower apparent maximum ribulose‐1,5‐bisphosphate carboxylation capacity (V_cmax) and a lower apparent maximum electron transport rate (J_max) than LT trees, indicating reduced allocation to photosynthetic components. Consistently, HT needles had 26% lower leaf nitrogen content than LT needles. At each measurement temperature, HT seedlings had 20–25% lower respiration rates than LT trees; however, this did not compensate for reduced photosynthetic rates at growth temperature, leading to a greater ratio of dark respiration to net carbon dioxide assimilation rate in HT trees. HT needles had 16% lower concentrations of soluble sugars than LT needles, but similar starch content. Growth at high temperatures increased the thermotolerance of black spruce. HT trees showed less PSII inhibition than LT seedlings and no increase in electrolyte leakage when briefly exposed to 40–57 °C. While trees that develop at high temperatures have enhanced tolerance for brief, extreme heat events, the reduction in root allocation indicates that seedlings will be more susceptible to episodic soil drying and less competitive for belowground resources in future climates of the boreal region.     

Comparison of temperate and tropical rainforest tree species: photosynthetic responses to growth temperature

Little is known about the differences in physiology between temperate and tropical trees. Australian rainforests extend from tropical climates in the north to temperate climates in the south over a span of 33° latitude. Therefore, they provide an opportunity to investigate differences in the physiology of temperate and tropical trees within the same vegetation type. This study investigated how the response of net photosynthesis to growth temperature differed between Australian temperate and tropical rainforest trees and how this correlated with differences in their climates. The temperate species showed their maximum rate of net photosynthesis at lower growth temperatures than the tropical species. However, the temperate species showed at least 80% of maximum net photosynthesis over a 12-16°C span of growth temperature, compared with a span of 9-11°C shown by the tropical species. The tropical species showed both larger reductions in maximum net photosynthesis at low growth temperatures and larger reductions in the optimum instantaneous temperature for net photosynthesis with decreasing growth temperature than the temperate species. The ability of the temperate species to maintain maximum net photosynthesis over a greater span of growth temperatures than the tropical species is consistent with the greater seasonal and day-to-day variation in temperature of the temperate climate compared with the tropical climate.     

Biogeochemical processes in the algal-bacterial mats of the Urinskii alkaline hot spring

The structure and production characteristics of microbial communities from the Urinskii alkaline hot spring (Buryat Republic, Russia) have been investigated. A distinctive characteristic of this hot spring is the lack of sulfide in the issuing water. The water temperature near the spring vents ranged from 69 to 38.5°C and pH values ranged from 8.8 to 9.2. The total mineralization of water was less than 0.1 g/liter. Temperature has a profound effect on the species composition and biogeochemical processes occurring in the algal-bacterial mats of the Urinskii hot spring. The maximum diversity of the phototrophic community was observed at the temperatures 40 and 46°C. A total of 12 species of cyanobacteria, 4 species of diatoms, and one species of thermophilic anoxygenic phototrophic bacteria, Chloroflexus aurantiacus, have been isolated from mat samples. At temperatures above 40°C, the filamentous cyanobacterium Phormidium laminosum was predominant; its cell number and biomass concentration comprised 95.1 and 63.9%, respectively. At lower temperatures, the biomass concentrations of the cyanobacterium Oscillatoria limosa and diatoms increased (50.2 and 36.4%, respectively). The cyanobacterium Mastigocladus laminosus, which is normally found in neutral or slightly acidic hydrothermal systems, was detected in microbial communities. As the diatom concentration increases, so does the dry matter concentration in mats, while the content of organic matter decreases. The concentrations of proteins and carbohydrates reached their maximum levels at 45–50°C. The maximum average rate of oxygenic photosynthesis [2.1 g C/(m² day)], chlorophyll a content (343.4 mg/m²), and cell number of phototrophic microorganisms were observed at temperatures from 45 to 50°C. The peak mass of bacterial mats (56.75 g/m²) occurred at a temperature of 65–60°C. The maximum biomass concentration of phototrophs (414.63 × 10^?6 g/ml) and the peak rate of anoxygenic photosynthesis [0.42 g C/(m² day)] were observed at a temperature of 35–40°C.     

EFFECTS OF DESICCATION AND ILLUMINATION ON PHOTOSYNTHESIS AND PIGMENTATION OF AN EDAPHIC POPULATION OF TRENTEPOHLIA ODORATA (CHLOROPHYTA)1

The photosynthetic rates of Trentepohlia odorata (L.) Martius growing on wall surfaces in Singapore changed throughout the day with a maximum in midmorning and decreasing thereafter during the day. Optimum temperature for photosynthesis was 25° C. Different levels of air humidity also affected photosynthetic rates with low relative humidity reducing the rates and efficiency of photosynthesis. Our results suggested that T. odorata was able to maximize its rate of photosynthesis before photoinhibitory light levels were reached and that its growth might be dependent on high levels of atmospheric relative humidity, which may serve as a source of water supply for the alga.     

Adaptation and acclimation of growth and photosynthesis of five Antarctic red algae to low temperatures

Temperature requirements for growth, photosynthesis and dark respiration were determined for five Antarctic red algal species. After acclimation, the stenothermal species Gigartina skottsbergii and Ballia callitricha grew at 0 or up to 5 °C, respectively; the eurythermal species Kallymenia antarctica, Gymnogongrus antarcticus and Phyllophora ahnfeltioides grew up to 10 °C. The temperature optima of photosynthesis were between 10 and 15 °C in the stenothermal species and between 15 and 25 °C in the eurythermal species, irrespective of the growth temperature. This shows that the temperature optima for photosynthesis are located well below the optima from species of other biogeographical regions, even from the Arctic. Respiratory rates rose with increasing temperatures. In contrast to photosynthesis, no temperature optimum was evident between 0 and 25 °C. Partial acclimation of photosynthetic capacity to growth temperature was found in two species. B. callitricha and Gymnogongrus antarcticus acclimate to 0 °C, and 5 and 0 °C, respectively. But acclimation did in no case lead to an overall shift in the temperature optimum of photosynthesis. B. callitricha and Gymnogongrus antarcticus showed acclimation of respiration to 5 °C, and P. ahnfeltioides to 5 and 10 °C, resulting in a temperature independence of respiration when measured at growth temperature. With respect to the acclimation potential of the species, no distinction can be made between the stenothermal versus the eurythermal group. (Net)photosynthetic capacity:respiration (P:R) ratios showed in all species highest values at 0 °C and decreased continuously to values lower than 1.0 at 25 °C. In turn, the low P:R ratios at higher temperatures are assumed to determine the upper temperature growth limit of the studied species. Estimated daily carbon balance reached values between 4.1 and 30.7 mg C g⁻¹ FW day⁻¹ at 0 °C, 16:8 h light/dark cycle, 12–40 μmol m⁻² s⁻¹. Received: 4 November 1999 / Accepted: 7 March 2000