PHYCOBILIN CONTENT OF THE CONCHOCELIS PHASE OF ALASKAN PORPHYRA (BANGIALES,RHODOPHYTA) SPECIES: RESPONSES TO ENVIRONMENTAL VARIABLES1

Variations of pigment content in the microscopic conchocelis stage of four Alaskan Porphyra species were investigated in response to environmental variables. Conchocelis filaments were cultured under varying conditions of irradiance and nutrient concentrations for up to 60 d at 11°C and 30 psu salinity. Results indicate that conchocelis filaments contain relatively high concentrations of phycobilins under optimal culture conditions. Phycobilin pigment production was significantly affected by irradiance, nutrient concentration, and culture duration. For Porphyra abbottiae V. Krishnam., Porphyra sp., and Porphyra torta V. Krishnam., maximal phycoerythrin (63.2–95.1 mg · g dwt^?1) and phycocyanin (28.8–64.8 mg · g dwt^?1) content generally occurred at 10 μmol photons · m^?2 · s^?1, f/4–f/2 nutrient concentration after 10–20 d of culture. Whereas for Porphyra hiberna S. C. Lindstrom et K. M. Cole, the highest phycoerythrin (73.3 mg · g dwt^?1) and phycocyanin (70.2 mg · g dwt^?1) content occurred at 10 μmol photons · m^?2 · s^?1, f nutrient concentration after 60 d in culture. Under similar conditions, the different species showed significant differences in pigment content. P. abbottiae had higher phycoerythrin content than the other three species, and P. hiberna had the highest phycocyanin content. P. torta had the lowest phycobilin content.     

PRODUCTIVITY OF THE FILAMENTOUS ALGA PITHOPHORA OEDOGONIA (CHLOROPHYTA) IN SURREY LAKE,INDIANA1

Biomass, akinete numbers, net photosynthesis, and respiration of Pithophora oedogonia were monitored over two growing seasons in shallow Surrey Lake, Indiana. Low rates of photosynthesis occurred from late fall to early spring and increased to maximum levels in late spring to summer (29–39 mgO₂·g^?1 dry wt·h^?1). Areal biomass increased following the rise in photosynthesis and peaked in autumn (163–206g dry wt·m^?2). Photosynthetic rates were directly correlated with temperature, nitrogen, and phosphorus over the entire annual cycle and during the growing season. Differences in photosynthetic activity and biomass between the two growing seasons (1980 and 1981) were apparently related to higher, early spring temperatures and higher levels of NO₃-N and PO₄-P in 1981. Laboratory investigations of temperature and light effects on Pithophora photosynthesis and respiration indicated that these processes were severely inhibited below 15°C. The highest P_max value occurred at 35°C (0.602 μmol O₂·mg^?1 chl a·min^?1). Rates of dark respiration did not increase above 25°C thus contributing to a favorable balance of photosynthetic production to respiratory utilization at high temperatures. Light was most efficiently utilized at 15°C as indicated by minimum values of I_k(47 μE·m^?2·s^?1) and I_c (6 μE·m^?2·s^?1). Comparison of P. oedogonia and Cladophora glomerata indicated that the former was more tolerant of temperatures above 30°C. Pithophora's tolerance of high temperature and efficient use of low light intensity appear to be adaptive to conditions found within the dense, floating algal mats and the shallow littoral areas inhabited by this filamentous alga.     

Temperature response of photosynthetic light‐ and carbon‐use characteristics in the red seaweed Gracilariopsis lemaneiformis (Gracilariales,Rhodophyta)

The red seaweed Gracilariopsis is an important crop extensively cultivated in China for high‐quality raw agar. In the cultivation site at Nanao Island, Shantou, China, G. lemaneiformis experiences high variability in environmental conditions like seawater temperature. In this study, G. lemaneiformis was cultured at 12, 19, or 26°C for 3 weeks, to examine its photosynthetic acclimation to changing temperature. Growth rates were highest in G. lemaneiformis thalli grown at 19°C, and were reduced with either decreased or increased temperature. The irradiance‐saturated rate of photosynthesis (P_max) decreased with decreasing temperature, but increased significantly with prolonged cultivation at lower temperatures, indicating the potential for photosynthesis acclimation to lower temperature. Moreover, P_max increased with increasing temperature (~30 μmol O₂ · g^?1FW · h^?1 at 12°C to 70 μmol O₂ · g^?1FW · h^?1 at 26°C). The irradiance compensation point for photosynthesis (I_c) decreased significantly with increasing temperature (28 μmol photons · m^?2 · s^?1 at high temperature vs. 38 μmol photons · m^?2 · s^?1 at low temperature). Both the photosynthetic light‐ and carbon‐use efficiencies increased with increasing growth or temperatures (from 12°C to 26°C). The results suggested that the thermal acclimation of photosynthetic performance of G. lemaneiformis would have important ecophysiological implications in sea cultivation for improving photosynthesis at low temperature and maintaining high standing biomass during summer. Ongoing climate change (increasing atmospheric CO₂ and global warming) may enhance biomass production in G. lemaneiformis mariculture through the improved photosynthetic performances in response to increasing temperature.     

Effects of salinity,light intensity and sediment on growth,pigments, agar production and reproduction in Gracilaria tenuistipitata from Songkhla Lagoon in Thailand

The effects of salinity, light intensity and sediment on Gracilaria tenuistipitata C.F. Chang & B.M. Xia on growth, pigments, agar production, and net photosynthesis rate were examined in the laboratory under varying conditions of salinity (0, 25 and 33 psu), light intensity (150, 400, 700 and 1000 µmol photons m^?2 s^?1) and sediment (0, 0.67 and 2.28 mg L^?1). These conditions simulated field conditions, to gain some understanding of the best conditions for cultivation of G. tenuistipitata. The highest growth rate was at 25 psu, 700 µmol photons m^?2 s^?1 with no sediments, that provided a 6.7% increase in weight gain. The highest agar production (24.8 ± 3.0 %DW) was at 25 psu, 150–400 µmol photons m^?2 s^?1 and no sediment. The highest pigment contents were phycoerythrin (0.8 ± 0.5 mg g^?1FW) and phycocyanin (0.34 ± 0.05 mg g^?1 FW) produced in low light conditions, at 150 µmol photons m^?2 s^?1. The highest photosynthesis rate was 161.3 ± 32.7 mg O₂ g^?1 DW h^?1 in 25 psu, 400 µmol photons m^?2 s^?1 without sediment in the short period of cultivation, (3 days) and 60.3 ± 6.7 mg O₂ g^?1 DW h^?1 in 25 psu, 700 µmol photons m^?2 s^?1 without sediment in the long period of cultivation (20 days). The results indicated that salinity was the most crucial factor affecting G. tenuistipitata growth and production. This would help to promote the cultivation of Gracilaria cultivation back into the lagoon using these now determined baseline conditions. Extrapolation of the results from the laboratory study to field conditions indicated that it was possible to obtain two crops of Gracilaria a year in the lagoon, with good yields of agar, from mid‐January to the end of April (dry season), and from mid‐July to the end of September (first rainy season) when provided sediment was restricted.     

A COMPARISON OF PHOTORESPONSE AMONG TEN DIFFERENT KARENIA BREVIS (DINOPHYCEAE) ISOLATES1

Many laboratories have solely used the Wilson isolate to physiologically characterize the harmful algal bloom (HAB) dinoflagellate Karenia brevis (C. C. Davis) G. Hansen et Moestrup. However, analysis of one isolate may lead to misinterpretations when extrapolating measurements to field populations. In this study, pulse‐amplitude‐modulated chlorophyll fluorometer (PAM‐FL) relative electron transport rate (ETR), F_v/F_m, and chl were compared with traditional techniques, such as ¹⁴C photosynthesis versus irradiance (P–E) curves, DCMU [3‐(3′,4′‐dichlorophenyl)‐1,1‐dimethyl urea] F_v/F_m, and extracted chl. The DCMU and PAM‐FL values of F_v/F_m (r² = 0.51) and chl (r² = 0.58) were in good agreement. There was no correlation between ¹⁴C and PAM‐FL α, P_max, and β parameters because PAM‐FL ETR was only a relative measurement. The PAM‐FL techniques were then used to investigate P–E curves, quantum yield of PSII (F_v/F_m), and chl from 10 K. brevis isolates to determine whether one or all isolates would better represent the species. Comparisons were made with a radial photosynthetron, which allowed for controlled conditions of light and temperature. Isolate α, P_max, and β varied between 0.097 and 0.204 μmol e^? · m^?2 · s^?1 · (μmol quanta · m^?2 · s^?1)^?1, 80.41 and 241 μmol e^? · m^?2 · s^?1, and 0.005 and 0.160 μmol e^? · m^?2 · s^?1 · (μmol quanta · m^?2 · s^?1)^?1, respectively. Either carbon limitation and/or bacterial negative feedback were implicated as the cause of the P–E parameter variability. Furthermore, these results directly contradicted some literature suggestions that K. brevis is a low‐light‐adapted dinoflagellate. Results showed that K. brevis was more than capable of utilizing and surviving in light conditions that may be present on cloudless days off Florida.     

PRELIMINARY EVALUATION OF THE BIOREMEDIATION POTENTIAL OF PORPHYRA: PHOTOSYNTHETIC PRODUCTION BY BLADES AND CONCHOCELIS

Given their rapid growth and nutrient assimilation rates, Porphyra spp. are good candidates for bioremediation. The production potential of two northeast U.S. Porphyra species currently in culture (P. purpurea and P. umbilicalis) was evaluated by measuring rates of photosynthesis (as O₂ evolution) of samples grown at 20° C. Gametophytes of P. umbilicalis photosynthesized at rates that were 80% higher than those of P. purpurea over 5–20° C at both sub‐saturating and saturating irradiances (37 and 289 μmol photons m^?2 s^?1). Porphyra umbilicalis was both more efficient at low irradiances (higher alpha) and had a higher P_max than did P. purpurea (23.0 vs. 15.6 μmol O₂ g^?1 DW min^?1), suggesting that P. umbilicalis is a better choice for mass culture where self‐shading may be severe. The photosynthesis‐irradiance relationship for the Conchocelis stage of P. purpurea was also examined. Tufts of filaments, grown at 10, 15, and 20° C, were assayed at growth temperatures at irradiances ranging from 0–315 μmol photons m^?2 s^?1. Tufts were slightly more productive at 15° than at 10° C, but only ca. 4–6% as productive as gametophytes. Maximum rates of net photosynthesis were reduced by 66–74% in tufts grown at 20° C (only about 2% of gametophytes). The Conchocelis stage, however, need not limit mariculture operations; once Conchocelis cultures are established, they can be maintained over the long‐term as ready sources of spores for net seeding.     

THE SHORT‐TERM EFFECT OF IRRADIANCE ON THE PHOTOSYNTHETIC PROPERTIES OF ANTARCTIC FAST‐ICE MICROALGAL COMMUNITIES1

Although sea‐ice represents a harsh physicochemical environment with steep gradients in temperature, light, and salinity, diverse microbial communities are present within the ice matrix. We describe here the photosynthetic responses of sea‐ice microalgae to varying irradiances. Rapid light curves (RLCs) were generated using pulse amplitude fluorometry and used to derive photosynthetic yield (Φ_PSII), photosynthetic efficiency (α), and the irradiance (E_k) at which relative electron transport rate (rETR) saturates. Surface brine algae from near the surface and bottom‐ice algae were exposed to a range of irradiances from 7 to 262 μmol photons · m^?2 · s^?1. In surface brine algae, Φ_PSII and α remained constant at all irradiances, and rETR_max peaked at 151 μmol photons · m^?2 · s^?1, indicating these algae are well acclimated to the irradiances to which they are normally exposed. In contrast, Φ_PSII, α, and rETR_max in bottom‐ice algae reduced when exposed to irradiances >26 μmol photons · m^?2 · s^?1, indicating a high degree of shade acclimation. In addition, the previous light history had no significant effect on the photosynthetic capacity of bottom‐ice algae whether cells were gradually exposed to target irradiances over a 12 h period or were exposed immediately (light shocked). These findings indicate that bottom‐ice algae are photoinhibited in a dose‐dependent manner, while surface brine algae tolerate higher irradiances. Our study shows that sea‐ice algae are able to adjust to changes in irradiance rapidly, and this ability to acclimate may facilitate survival and subsequent long‐term acclimation to the postmelt light regime of the Southern Ocean.     

LIGHT AND TEMPERATURE AS FACTORS REGULATING SEASONAL GROWTH AND DISTRIBUTION OF ULOTHRIX ZONATA (ULVOPHYCEAE)1

Ulothrix zonata (Weber and Mohr) Kütz. is an unbranched filamentous green alga found in rocky littoral areas of many northern lakes. Field observations of its seasonal and spatial distribution indicated that it should have a low temperature and a high irradiance optimum for net photosynthesis, and at temperatures above 10°C it should show an increasingly unfavorable energy balance. Measurements of net photosynthesis and respiration were made at 56 combinations of light and temperature. Optimum conditions were 5°C and 1100 μE·m^?2·s^?1 at which net photosynthesis was 16.8 mg O₂·g^?1·h^?1. As temperature increased above 5° C optimum irradiance decreased to 125 μE·m^?2·s^?1 at 30°C. Respiration rates increased with both temperature and prior irradiance. Light-enhanced respiration rates were significantly greater than dark respiration rates following irradiance exposures of 125 μE·m^?2·s^?1 or greater. Polynomials were fitted to the data to generate response surfaces. Polynomial equations represent statistical models which can accurately predict photosynthesis and respiration for inclusion in ecosystem models.     

EFFECT OF SEASONAL SEA ICE BREAKOUT ON THE PHOTOSYNTHESIS OF BENTHIC DIATOM MATS AT CASEY,ANTARCTICA1

Photosynthesis of marine benthic diatom mats was examined before and after sea ice breakout at a coastal site in eastern Antarctica (Casey). Before ice breakout the maximum under‐ice irradiance was between 2.5 and 8.2 μmol photons·m^?2·s^?1 and the benthic microalgal community was characterized by low E_k (12.1–32.3 μmol photons·m^?2·s^?1), low relETR_max (9.2–32.9), and high alpha (0.69–1.1). After breakout, 20 days later, the maximum irradiance had increased to between 293 and 840 μmol photons·m^?2·s^?1, E_k had increased by more than an order of magnitude (to 301–395 μmol photons·m^?2·s^?1), relETR_max had increased by more than five times (to 104–251), and alpha decreased by approximately 50% (to 0.42–0.68). During the same time interval the species composition of the mats changed, with a decline in the abundance of Trachyneis aspera (Karsten) Hustedt, Gyrosigma subsalsum Van Heurck, and Thalassiosira gracilis (Karsten) Hustedt and an increase in the abundance of Navicula glaciei Van Heurck. The benthic microalgal mats at Casey showed that species composition and photophysiology changed in response to the sudden natural increase in irradiance. This occurred through both succession shifts in the species composition of the mats and also an ability of individual cells to photoacclimate to the higher irradiances.     

COMPARISON OF DEVELOPMENT AND PHOTOSYNTHETIC GROWTH FOR FILAMENT CLUMPS AND REGENERATED MICROPLANTLET CULTURES OF AGARDHIELLA SUBULATA (RHODOPHYTA, GIGARTINALES)

Two axenic, in vitro liquid suspension cultures were established for Agardhiella subulata (C. Agardh) Kraft et Wynne, and their growth characteristics were compared. This study illustrated how reliable routes for the development of suspension cultures of macrophytic red algae of terete thallus morphology can be achieved for biotechnology applications. Undifferentiated filament clumps of 2–8 mm diameter were established by induction of callus-like tissue from thallus explants, and lightly branched microplantlets of 2–10 mm length were established by regeneration of filament clumps. The filament clumps were susceptible to regeneration. Adventitious shoot formation was reliably induced from 40% to 70% of the filament clumps by gentle mixing at 100 rev min^?1 on an orbital shaker. The specific growth rate of the microplantlets was higher than the filament clumps in nonagitated well plate culture (4%–6% per day for microplantlets vs. 2%–3% per day for filament clumps) at 24° C and 8–36 μmol photons·m^?2·s^?1 irradiance (10:14 h LD cycle) when grown on ASP12 artificial seawater medium at pH 8.6–8.9 with 20%–25% per day medium replacement. Oxygen evolution rate vs. irradiance measurements showed that relative to the filament clumps, microplantlets had a higher maximum specific oxygen evolution rate (P_o,max= 0.181 ± 0.035 vs. 0.130 ± 0.023 mmol O₂·g^?1 dry cell mass·h^?1), but comparable respiration rate (Q_o= 0.040 ± 0.013 vs. 0.033 ± 0.017 mmol O₂·g^?1 dry cell mass·h^?1), compensation point (I_c= 3.8 ± 2.4 vs. 5.7 ± 1.2 μmol photons·m^?2·s^?1), and light intensity at 63.2% of saturation (I_k= 17.5 ± 3.9 vs. 14.9 ± 2.6 μmol photons·m^?2·s^?1). The microplantlet culture was more suitable for suspension culture development than the filament clump culture because it was morphologically stable and exhibited higher growth rates.     

Temperature‐dependent phagotrophy and phototrophy in a mixotrophic chrysophyte

The roles of temperature and light on grazing and photosynthesis were examined for Dinobryon sociale, a common freshwater mixotrophic alga. Photosynthetic rate was determined for D. sociale adapted to temperatures of 8, 12, 16, and 20°C under photosynthetically active radiation light irradiances of 25, 66, and 130 μmol photons · m^?2 · s^?1, with concurrent measurement of bacterial ingestion at all temperatures under medium and high light (66 and 130 μmol photons · m^?2 · s^?1). Rates of ingestion and photosynthesis increased with temperature to a maximum at 16°C under the two higher light regimes, and declined at 20°C. Although both light and temperature had a marked effect on photosynthesis, there was no significant difference in bacterivory at medium and high irradiances at any given temperature. At the lowest light condition (25 μmol photons · m^?2 · s^?1), photosynthesis remained low and relatively stable at all temperatures. D. sociale acquired the majority of carbon from photosynthesis, although the low photosynthetic rate without a concurrent decline in feeding rate at 8°C suggested 20%–30% of the carbon budget could be attributed to bacterivory at low temperatures. Grazing experiments in nutrient‐modified media revealed that this mixotroph had increased ingestion rates when either dissolved nitrogen or phosphorus was decreased. This work increases our understanding of environmental effects on mixotrophic nutrition. Although the influence of abiotic factors on phagotrophy and phototrophy in pure heterotrophs and phototrophs has been well studied, much less is known for mixotrophic organisms.     

SHORT‐TERM EFFECT OF TEMPERATURE ON THE PHOTOKINETICS OF MICROALGAE FROM THE SURFACE LAYERS OF ANTARCTIC PACK ICE1

Microalgae growing within brine channels (85 psu salinity) of the surface ice layers of Antarctic pack ice showed considerable photosynthetic tolerance to the extreme environmental condition. Brine microalgae exposed to temperatures above ?5°C and at irradiances up to 350 μmol photons·m^?2·s^?1 showed no photosynthetic damage or limitations. Photosynthesis was limited (but not photoinhibited) when brine microalgae were exposed to ?10°C, provided the irradiance remained under 50 μmol photons·m^?2·s^?1. The highest level of photosynthetic activity (maximum relative electron transport rate [rETR_max]) in brine microalgae growing within the surface layer of sea ice was at approximately 18 μmol electrons·m^?2·s^?1, which occurred at ?1.8°C. Effective quantum yield of PSII and rETR_max of the halotolerant brine microalgae exhibited a temperature‐dependent pattern, where both parameters were higher at ?1.8°C and lower at ?10°C. Relative ETR_max at temperatures above ?5°C were stable across a wide range of irradiance.     

The influence of light on copper‐limited growth of an oceanic diatom,Thalassiosira oceanica (Coscinodiscophyceae)

Thalassiosira oceanica (CCMP 1005) was grown over a range of copper concentrations at saturating and subsaturating irradiance to test the hypothesis that Cu and light were interacting essential resources. Growth was a hyperbolic function of irradiance in Cu‐replete medium (263 fmol Cu′ · L^?1) with maximum rates achieved at 200 μmol photons · m^?2 · s^?1. Lowering the Cu concentration at this irradiance to 30.8 fmol Cu′ · L^?1 decreased cellular Cu quota by 7‐fold and reduced growth rate by 50%. Copper‐deficient cells had significantly slower (P < 0.0001) rates of maximum, relative photosynthetic electron transport (rETR_max) than Cu‐sufficient cells, consistent with the role of Cu in photosynthesis in this diatom. In low‐Cu medium (30.8 fmol Cu′ · L^?1), growth rate was best described as a positive, linear function of irradiance and reached the maximum value measured in Cu‐replete cells when irradiance increased to 400 μmol photons · m^?2 · s^?1. Thus, at high light, low‐Cu concentration was no longer limiting to growth: Cu concentration and light interacted strongly to affect growth rate of T. oceanica (P < 0.0001). Relative ETR_max and Cu quota of cells grown at low Cu also increased at 400 μmol photons · m^?2 · s^?1 to levels measured in Cu‐replete cells. Steady‐state uptake rates of Cu‐deficient and sufficient cells were light‐dependent, suggesting that faster growth of T. oceanica under high light and low Cu was a result of light‐stimulated Cu uptake.     

Photosynthesis of Marine Macroalgae from Antarctica: Light and Temperature Requirements

The photosynthetic performance of macroalgae isolated in Antarctica was studied in the laboratory. Species investigated were the brown algae Himantothallus grandifolius, Desmarestia anceps, Ascoseira mirabilis, the red algae Palmaria decipiens, Iridaea cordata, Gigartina skottsbergii, and the green algae Enteromorpha bulbosa, Acrosiphonia arcta, Ulothrix subflaccida and U. implexa. Unialgal cultures of the brown and red algae were maintained at 0°C, the green algae were cultivated at 10°C. I_K values were between 18 and 53 μmol m^?2 s^?1 characteristic or low light adapted algae. Only the two Ulothrix species showed higher I_K values between 70 and 74 μmol m^?2 s^?1. Photosynthesis compensated dark respiration at very low photon fluence rates between 1.6 and 10.6 μmol m^?2 s^?1. Values of α were high: between 0.4 and 1.1 μmol O₂ g^?1 FW h^?1 (μmol m^?2 s^?1)^?1 in the brown and red algae and between 2.1 and 4.9 μmol O₂ g^?1 FW h^?1 (μmol m^?2 s^?1)^?1 in the green algal species. At 0°C P_max values of the brown and red algae ranged from 6.8 to 19.1 μmol O₂ g^?1 FW h^?1 and were similarly high or higher than those of comparable Arctic-cold temperate species. Optimum temperatures for photosynthesis were 5 to 10°C in A. mirabilis, 10°C in H. grandifolius, 15°C in G. skottsbergii and 20°C or higher in D. anceps and I. cordata. P: R ratios strongly decreased in most brown and red algae with increasing temperatures due to different Q₁₀ values for photosynthesis (1.4 to 2.5) and dark respiration (2.5 to 4.1). These features indicate considerable physiological adaptation to the prevailing low light conditions and temperatures of Antarctic waters. In this respect the lower depth distribution limits and the northern distribution boundaries of these species partly depend on the physiological properties described here.     

Photosynthetic characteristics of two Cylindrospermopsis raciborskii strains differing in their toxicity

We studied the growth and photosynthetic characteristics of a toxic (CS506) and a nontoxic strain (CS509) of the bloom‐forming cyanobacterium Cylindrospermopsis raciborskii grown under identical experimental conditions. When exposed to light‐saturating growth conditions (100 μmol photons · m^?2 · s^?1), values for maximal photosynthetic capacity (P_max) and maximum quantum yield (F_v/F_m) indicated that both strains had an equal ability to process captured photons and deliver them to PSII reaction centers. However, CS506 grew faster than CS509. This was consistent with its higher light requirement for saturation of photosynthesis (I_k). Greater shade tolerance of CS509 was indicated by its higher ability to harvest light (α), lower photosynthetic light compensation point (I_c), and higher chlorophyll a to biovolume ratio. Strain‐specific differences were found in relation to non‐photochemical quenching, effective absorption cross‐sectional area of PSIIα‐centers (σPSIIα), and the antenna connectivity parameter of PSIIα (J_conPSIIα). These findings highlighted differences in the transfer of excitation from phycobilisome/PSII to PSI, on the dependence on different pigments for light harvesting and on the functioning of the PSII reaction centers between the two strains. The results of this study showed that both performance and composition of the photosynthetic apparatus are different between these strains, though with only two strains examined we cannot attribute the performance of strain 506 to its ability to produce cylindrospermopsins. The emphasis on a strain‐specific light adaptation/acclimation is crucial to our understanding of how different light conditions (both quantity and quality) can trigger the occurrence of different C. raciborskii strains and control their competition and/or dominance in natural ecosystems.     

Photosynthetic characteristics of charophytes from tropical lotic ecosystems

Responses of photosynthetic rates, determined by oxygen evolution using the light and dark bottles technique, to different temperatures, irradiances, pH, and diurnal rhythm were analyzed under laboratory conditions in four charophyte species (Chara braunii Gmelin, C. guairensis R. Bicudo, Nitella subglomerata A. Braun and Nitella sp.) from lotic habitats in southeastern Brazil. Parameters derived from the photosynthesis versus irradiance curves indicated affinity to low irradiances for all algae tested. Some degree of photoinhibition, [β= ‐(0.30–0.13) mg O2 g^?1 dry weight Ir¹ (μmol photons m^?2 s^?1)^?1], low light compensation points (I_c= 4–20 μmol photons m^?2 s^?1) were found for all species analyzed, as well as low values of light saturation parameter (I_k) and saturation (I_s) 29–130 and 92–169 μmol photons m^?2 s^?1, respectively. Photoacclimation was observed in two populations of N. subglomerata collected from sites with different irradiances, consisting of variations in photosynthetic parameters (higher values of a, and lower of I_k and maximum photosynthetic rate, P_max, in the population under lower irradiance). The highest photosynthetic rates for Chara species were observed at 10–15°C, while for Nitella the highest photosynthetic rate was observed at 20–25°C, despite the lack of significant differences among most levels tested. Rates of dark respiration significantly increase with temperature, with the highest values at 25°C. The results from pH experiments showed highest photosynthetic rates under pH 4.0 for all algae, suggesting higher affinity for inorganic carbon in the form of carbon dioxide, except in one population of N. subglomerata, with similar rates under the three levels, suggesting indistinct use of bicarbonate and carbon dioxide. Diurnal changes in photosynthetic rates revealed a general pattern for most algae tested, which was characterized by two peaks: the first (higher) during the morning (07.00–11.00) and the second (lower) in the afternoon (14.00–17.00). This suggests an endogenous rhythm determining the daily variations in photosynthetic rates.     

LIGHT DEPENDENCE OF GROWTH AND PHOTOSYNTHESIS IN PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

Phaeodactylum tricornutum Bohlin was maintained in exponential growth over a range of photon flux densities (PFD) from 7 to 230 μmol·m^?2s^?1. The chlorophyll a-specific light absorption coefficient, maximum quantum yield of photosynthesis, and C:N atom ratio were all independent of the PFD to which cells were acclimated. Carbon- and cell-specific, light-satuated, gross photosynthesis rates and dark respiration rates were largely independent of acclimation PFD. Decreases in the chlorophyll a-specific, gross photosynthesis rate and the carbon: chlorophyll ratio and increases of cell- or carbon-specific absorption coefficients were associated with an increase in cell chlorophyll a in cultures acclimated to low PFDs. The compensation PFD for growth was calculated to be 0.5 μmol·m^?2s^?1. The maintenance metabolic rate (2 × 10^?7s^?1), calculated on the basis of the compensation PFD, is an order of magnitude lower than the measured dark respiration rate(2.7 × 10^?6mol O₂·mol C^?1s^?1). Maintenance of high carbon-specific, light-saturated photosynthesis rates in cells acclimated to low PFDs may allow effective use of short exposures to high PFDs in a temporally variable light environment.     

INFLUENCE OF SALINITY AND TEMPERATURE ON GROWTH AND PHOTOSYNTHESIS IN THE EXTREMOPHILIC CHLOROPHYTE,NANNOCHLORIS SP.

Major, K. M. & Henley, W. J. Department of Botany, Oklahoma State University, Stillwater, OK 74078-3013 USA Preliminary data suggest Nannochloris sp., isolated from the Great Salt Plains National Wildlife Refuge, is a true extremophile. This alga is able to withstand salinities ranging from 0 to 150 ç and temperatures up to 45°C. To test the hypothesis that acclimation to high salinity confers tolerance to high temperature, experimental cultures were acclimated to salinities of 25 and 100 ç and/or temperatures of 23 and 38°C; irradiance (500 mol photons m^-2 s^-1) was saturating for both growth and photosynthesis. Cells acclimated to low salt and low temperature exhibited high photosynthetic performance in terms of both light-saturated photosynthesis (P_max; 45.0 fmol O₂ cell^-1 h^-1) and light-harvesting efficiency (0.103 fmol O₂ cell^-1 h^-1/mol photons m^-2 s^-1). However, high-salinity cells exhibited values for net P_max (18.1 fmol O₂ cell^-1 h^-1), (0.107 fmol O₂ cell^-1 h^-1/mol photons m^-2 s^-1) and growth rates (ca. 0.4 d^-1) that were equal to, or higher than, those of low-salinity cells when acclimated to high temperature. Both the amount of light required to achieve net photosynthesis (I_c) and that required to achieve light-saturated photosynthesis (I_k) were lower in high-salinity cells than those exhibited by low-salinity cells grown at high temperature; reductions in I_c and I_k were primarily due to increases in light-harvesting efficiency. We propose that an increase in growth temperature might release Nannochloris sp. from energy constraints associated with osmolyte production and low-temperature effects on enzyme activity. These data are consistent with effects of short-term temperature stress on Chl a fluorescence kinetics in this alga.     

GROWTH AND PHOTOSYNTHESIS OF ULVA ROTUNDATA (CHLOROPHYTA) AS A FUNCTION OF TEMPERATURE AND SQUARE WAVE IRRADIANCE IN INDOOR CULTURE1

Temperature and photon flux density (PFD) vary independently in estuaries, e.g. high PFD may occur at any temperature, so it is necessary to consider synergistic effects of these factors on algal growth. Because natural PFD is highly variable and daylength changes confound seasonal temperature cycles, it is easier to interpret factorial experiments in controlled laboratory conditions. Clonal Ulva rotundata Blid. (Chlorophyta) has been studied extensively in outdoor culture. In this study it was maintained indoors under square wave photoperiods at five PFDs and three temperatures. Growth rate, photqsynthetic light response (P-I) curves, and photosystem II chlorophyll fluorescence properties were measured at the growth temperature following acclimation. Interactions between PFD and growth temperature were strongly indicated in all physiological parameters measured. Greatest PFD response occurred at the highest temperature, and the largest temperature response occurred at the highest PFD. Light-saturated photosynthesis (P_m) dark respiration (R_d), and light-limited quantum yield (Φ_m) were sufficient to describe acclimation status. The light-saturation parameter (I_k) was redundant and potentially misleading. Although U. rotundata exhibits a great amplitude of photoacclimation, it apparently has little capacity for temperature acclimation compared to the kelp, Laminaria saccharina, for which published data indicate similar photosynthetic rates over a broad range of growth temperatures. Diurnal variation of P_m and R_d at a growth PFD of ～ 1700 ± 200 μmol photons · m^?2· s^?1 was similar to the pattern observed previously in outdoor culture, suggesting endogenous control of these parameters. Quantum yield and the ratio of variable to maximum chlorophyll fluorescence (F_v/F_m), which were depressed in midday sunlight exceeding ～ 1500 μmol photons · m^?2· s^?1, were relatively invariant through the day in indoor culture, indicating that these parameters are controlled primarily by instantaneous PFD. Growth and fluorescence data are also presented for some other macroalgae for comparative purposes.     

Thermal acclimation and photoacclimation of photosynthesis in the brown alga Laminaria saccharina

Thermal acclimation and photoacclimation of photosynthesis were compared in Laminaria saccharina sporophytes grown at temperatures of 5 and 17 °C and irradiances of 15 and 150μmol photons m^?2 s^?1. When measured at a standard temperature (17°C), rates of light-saturated photosynthesis (P_max) were higher in 5 °C-grown algae (c. 3.0 μmol O₂ m^?2 s^?1) than in 17 °C-grown algae (c. 0.9 μmol O₂ m_-2 s_-1). Concentrations of Rubisco were also 3-fold higher (per unit protein) in 5 °C-grown algae than in algae grown at 17 °C. Light-limited photosynthesis responded similarly to high temperature and low light Photon yields (α) were higher in algae grown at high temperature (regardless of light), and at 5 °C in low light, than in algae grown at 5 °C in high light Differences in a were correlated with light absorption; both groups of 17 °C algae and 5 °C low-light algae absorbed c. 75% of incident light, whereas 5 °C high-light algae absorbed c. 55%. Increased absorption was correlated with increases in pigment content PSII reaction centre densities and the fucoxanthin-Chl ale protein complex (FCP). Changes in a were also attributed, in part, to changes in the maximum photon yield of photosynthesis (0_max). PSI reaction centre densities were unaffected by growth temperature, but the areal concentration of PSI in low-light-grown algae was twice that of high-light-grown algae (c. 160.0 versus 80.0 nmol m^?2). We suggest that complex metabolic regulation allows L, saccharina to optimize photosynthesis over the wide range of temperatures and light levels encountered in nature.