MORPHOLOGICAL CHANGES IN TOXIC AND NON-TOXIC MICROCYSTIS ISOLATES AT DIFFERENT IRRADIANCE LEVELS1

Light intensity (4.5–40.0 μEin m^?2 s^?1) and culture age had a pronounced effect on cell size and size range of a non-toxic axenic Microcystis isolate. The rate of cell volume increase (μm³ d^?1) was 1.03 × light intensity (μEin m^?2 s^?1) – 6.49. Average cell volume varied from 33 to 119 μm³, cells at higher light intensities being larger and having a larger size range. The effects on a toxic axenic Microcystis isolate were similar but less pronounced. Average cell volume ranged from 21–74 μm³. In general, cell size and especially size variability appear to be sensitive indicators of physiological state, with cells under stress conditions being larger and associated with a larger size range. The wide range of cell diameters observed at different irradiance levels (3.4–7.2 μm for the non-toxic and 1.8–6.4 μm for the toxic isolate), makes questionable the use of cell size as a taxonomic character without careful consideration of environmental conditions.     

INFLUENCE OF CELL VOLUME ON THE GROWTH AND SIZE REDUCTION OF MARINE AND ESTUARINE DIATOMS1

The maximal growth rate (μ_max) of 19 marine and estuarine diatoms decreased with increasing cell volume (V). The relationship between log μ_max (Y) and log V (X) was calculated. Statistical analyses showed that the slope of the equation was not significantly different from those obtained by other researchers and that the 95% confidence intervals of mean μ_max at cell volumes of 10³–10⁵μm³ were not significantly different from those cited in most studies. A new regression line for diatoms was calculated as follows: log μ_max= 0.47–0.14 log V; r =–0.69. The rate of size reduction per generation of the 19 diatom species ranged from 0.03 to 0.87 μm per generation. The rate increased with increasing cell length and cell volume and with decreasing maximum division rate. Statistical analyses showed that the rate was closely related to the cell volume and to the reciprocal of the growth rate. The relationships between maximal growth rate and cell volume and between rate of size reduction and cell volume showed that a diatom with a large volume had a smaller maximal growth rate and a larger rate of size reduction than a diatom with a small volume. The estimates using the equation for the regression line between the rate of size reduction and the reciprocal of maximum division rate indicated that a diatom with a high maximum division rate would need more generation equivalents for a certain size reduction than a diatom with a low maximum division rate, but the periods required for reduction would be approximately equal irrespective of maximum division rate.     

STRATEGIES AND KINETICS OF PHOTOACCLIMATION IN THREE ANTARCTIC NANOPHYTOFLAGELLATES1

Three Antarctic nanophytoflagellates (two cryptophyte species and a Pyramimonas sp.) were compared for their capacity to phiotoacclimate and for their kinetic responses in changing photic environments. Division rate, cell size cellular fluorescence, and chlorophyll a content were measured steady and transient states of semi-continuous cultures maintain at 1.0° C. Of all parameters tested, cell size was most affected by irradiance. Acclimation kinetics were modeled using a first-order equation. Rates of change in cell size following shifts in irradiance were comparable with rates of change in chemical composition reported for temperate algae. Response rates of cellular in vivo red and orange fluorescence were lower. In many cases, however, responses could not be described by the first-order kinetic model. Division rates remained high for approximately 3 days following a shift down in irradiance, after which new division rates were established. The nanoflagellates studied here appear to respond to small irradiance perturbations at low rates. However, they may fail to adapt and abrupt changes in photon flux density (PFD). When shade-adapted (25 μmol, m^?2, m^?2, s^?1) cells were exposed to high PFD (400 μmol, m^?2, s^?1) for 1–3 days, cell were incapable of readapting division rate and pigment content to the initial irradiance condition (25 μmol, m^?2, s^?1) for about 1 month following the shift-down step. The ecological role of the kinetics of photoacclimation in nanophytoflagellate growth performance in Antarctic ecosystems is discussed.     

SIZE DEPENDENCE OF GROWTH RATE,RESPIRATORY ELECTRON TRANSPORT SYSTEM ACTIVITY,AND CHEMICAL COMPOSITION IN MARINE DIATOMS IN THE LABORATORY1

The dependence of growth, electron transport system activity and chemical composition on the size of diatoms was examined during the exponential phase of growth. The six different marine centric species compared ranged in volume from 7.7 μm³ to 62 × 10⁵μm³. A size dependence was observed for growth, ¹⁴C uptake, respiration and the productivity index (¹⁴C/chl a). Although the size dependence of all parameters was similar, the results indicate that on a carbon basis, growth efficiency decreases with increasing size. The C/N and C/chl a ratios were not size dependent. The importance of the surface area to cell volume ratio, and the importance of carbon per unit volume in determining the observed size dependence are discussed.     

PHOTOSYNTHESIS AND CELL DIVISION BY ANTARCTIC MICROALGAE: COMPARISON OF BENTHIC,PLANKTONIC AND ICE ALGAE1

Irradiance-dependent rates of photosynthesis and cell division of six species of microalgae isolated from the benthos, plankton and sea ice microbial community in McMurdo Sound, Antarctica were compared. Microalgae isolated from different photic environments had distinct photosynthetic and growth characteristics. For benthic and ice algae, photosynthesis saturated at 6 to 20 μE.m^?2.s^?1 and was photoinhibited at 10 to 80 μE.m^?2.s^?1 while for the planktonic algae, saturation irradiances were up to 13 times higher and photoinhibition was not detected. The slope of the light-limited portion of the P-I relationship was up to 50 times greater for the benthic algae than for either the ice or planktonic algae suggesting that benthic algae used the low irradiances more efficiently for carbon uptake. Cell division was dependent on the incubation irradiance for all but one microalga examined. The dependence of division rates on irradiance was however much smaller than for carbon uptake, suggesting that cell division buffers the influence of short term variations of irradiance on cellular metabolism.     

PHOTOACCLIMATION OF PHOTOSYNTHESIS IRRADIANCE RESPONSE CURVES AND PHOTOSYNTHETIC PIGMENTS IN MICROALGAE AND CYANOBACTERIA1

The photosynthesis‐irradiance response (PE) curve, in which mass‐specific photosynthetic rates are plotted versus irradiance, is commonly used to characterize photoacclimation. The interpretation of PE curves depends critically on the currency in which mass is expressed. Normalizing the light‐limited rate to chl a yields the chl a‐specific initial slope (α^chl). This is proportional to the light absorption coefficient (a^chl), the proportionality factor being the photon efficiency of photosynthesis (φ_m). Thus, α^chl is the product of a^chl and φ_m. In microalgae α^chl typically shows little (<20%) phenotypic variability because declines of φ_m under conditions of high‐light stress are accompanied by increases of a^chl. The variation of α^chl among species is dominated by changes in a^chl due to differences in pigment complement and pigment packaging. In contrast to the microalgae, α^chl declines as irradiance increases in the cyanobacteria where phycobiliproteins dominate light absorption because of plasticity in the phycobiliprotein:chl a ratio. By definition, light‐saturated photosynthesis (P_m) is limited by a factor other than the rate of light absorption. Normalizing P_m to organic carbon concentration to obtain P_m^C allows a direct comparison with growth rates. Within species, P_m^C is independent of growth irradiance. Among species, P_m^C covaries with the resource‐saturated growth rate. The chl a:C ratio is a key physiological variable because the appropriate currencies for normalizing light‐limited and light‐saturated photosynthetic rates are, respectively, chl a and carbon. Typically, chl a:C is reduced to about 40% of its maximum value at an irradiance that supports 50% of the species‐specific maximum growth rate and light‐harvesting accessory pigments show similar or greater declines. In the steady state, this down‐regulation of pigment content prevents microalgae and cyanobacteria from maximizing photosynthetic rates throughout the light‐limited region for growth. The reason for down‐regulation of light harvesting, and therefore loss of potential photosynthetic gain at moderately limiting irradiances, is unknown. However, it is clear that maximizing the rate of photosynthetic carbon assimilation is not the only criterion governing photoacclimation.     

SEASONAL VARIATIONS OF PHOTOSYNTHETIC PIGMENTS, TOTAL C, N, AND P CONTENT, AND PHOTOSYNTHESIS IN PHYLLARIOPSIS PURPURASCENS: (PHAEOPHYTA) FROM THE STRAIT OF GIBRALTAR

Photosynthetic pigments, C, N, and P tissue composition, and photosynthetic rate were measured from April to October in the brown alga Phyllariopsis purpurascens (C. Agardh) Henry et South (Laminariales, Phaeophyta) growing at a 30-m depth in the Strait of Gibraltar. Ir-radiance reaching the population ranged from 13.5 to 27.5 mol.m^-2.mo^-1. The available light for this species, expressed as a percentage of the irradiance above the water, was 1.8%. Dissolved inorganic nitrogen forms, NO3-and NH₄+, were constant from April to October, whereas phosphate was depleted in August. Chlorophyll a decreased from 520.0 ± 165.0 to 199.6 ± 159.9 μg.g^-1 dry weight; in contrast, chlorophyll c and carotenoids did not change until September but increased threefold in October. C:N and N:P ratios changed in the same way and in the same range. They were constant until July but increased from 15–17 up to 42 (C:N) and from 14 to 40 (N:P) in October, suggesting a severe P limitation of growth of this species. The dark respiration rate and the light compensation point were constant from April to October (0.5 ± 0.1 μmol O₂. m^-2.s^-1 and 6.5 ± 0.2 μmol.m^-2. s^-1, respectively), whereas the maximum rate of apparent photosynthesis, light onset saturation parameter, and half saturation constant for light were maximum in April to May (3.7 μmol O₂. m^-2.s^-1and 40 and 41.5 μmol.m^-2. s^-1, respectively) and October (3.6 μmol O₂. m^-2.s^-1 and 50 and 53.7 μmol.m^-2. s^-1, respectively). They were minimum in August (1.2 μmol O₂.m^-2.s^-1 and 11.3 and 12 μmol.m^-2.s^-1, respectively). These minimum figures yielded a negative carbon budget in August and 0 in September, whereas it was positive the rest of the year. Photosynthetic efficiency, estimated by the ratio between maximum apparent photosynthesis and light half saturation constant, showed a strong agreement with productivity measured by means of an independent method. These results indicate that lamina expansion in this species is controlled by photosynthetic efficiency.     

THE EFFECT OF NUTRIENT LIMITATION AND ITS INTERACTION WITH LIGHT UPON THE PRODUCTS OF PHOTOSYNTHESIS IN MERISMOPEDIA TENUISSIMA (CYANOPHYCEAE)1

The metabolic fate of photosynthetically-fixed CO₂ was determined by labeling samples of Merismopedia tenuissima Lemmerman for 30 min with NaH¹⁴CO₃ and analyzing its incorporation into low molecular weight compounds, polysaccharide and protein. In N- and P-sufficient cultures, relative incorporation into protein increased as the irradiance used during the labeling period was decreased to 20 μE · m^-2 s^-1. This pattern was found for cells grown at irradiances of either 20 or 180 μE · m^-2· s^-1, although incorporation into protein was greater in cultures grown at the higher irradiance. In N-limited continuous cultures, relative incorporation into protein was low, independent of growth rate, and the same for samples tested at 20 or 180 μE · m^-2· s^-1 irradiance. In contrast, ¹⁴C incorporation into protein by P-limited cultures increased as growth rate increased, and at relative growth rates greater than 0.25, the incorporation was greater at 20 than at 180 μE · m^-2· s^-1. However, the total RNA content and maximum photosynthetic rate of the cultures was the same at all growth rates tested. The interaction between nutrient concentration and light intensity was studied by growing-limited continuous cultures at the same dilution rate, but different irradiances. Relative incorporation into protein was highest in cultures grown at 20 μE · m^-2· s^-1, in which the relative growth rate was 0.4. These results suggest that photosynthetic carbon metabolism may respond to relative growth rate μ/μ_max rather than to growth rate directly.     

Temperature × light interaction and tolerance of high water temperature in the planktonic freshwater flagellates Cryptomonas (Cryptophyceae) and Dinobryon (Chrysophyceae)

Using microcosm experiments, we investigated the interactive effects of temperature and light on specific growth rates of three species each of the phytoplanktonic genera Cryptomonas and Dinobryon. Several species of these genera play important roles in the food web of lakes and seem to be sensitive to high water temperature. We measured growth rates at three to four photon flux densities ranging from 10 to 240 μmol photon · m^?2 · s^?1 and at 4–5 temperatures ranging from 10°C to 28°C. The temperature × light interaction was generally strong, species specific, and also genus specific. Five of the six species studied tolerated 25°C when light availability was high; however, low light reduced tolerance of high temperatures. Growth rates of all six species were unaffected by temperature in the 10°C–15°C range at light levels ≤50 μmol photon · m^?2 · s^?1. At high light, growth rates of Cryptomonas spp. increased with temperature until the temperature optimum was reached and then declined. The Dinobryon species were less sensitive than Cryptomonas spp. to photon flux densities of 40 μmol photon · m^?2 · s^?1 and 200 μmol photon · m^?2 · s^?1 over the entire temperature range but did not grow under a combination of very low light (10 μmol photon · m^?2 · s^?1) and high temperature (≥20°C). Among the three Cryptomonas species, cell volume declined with temperature and the maximum temperature tolerated was negatively related to cell size. Since Cryptomonas is important food for microzooplankton, these trends may affect the pelagic carbon flow if lake warming continues.     

EFFECTS OF LIGHT INTENSITY ON DIVISION RATE,STIMULABLE BIOLUMINESCENCE AND CELL SIZE OF THE OCEANIC DINOFLAGELLATES DISSODINIUM LUNULA,PYROCYSTIS FUSIFORMIS AND P. NOCTILUCA12

Preadapted cultures were grown in a 12:12 LD cycle at a series of light intensities under cool-white, fluorescent lamps. Pyrocystis fusiformis Murray maintained high division rates at low light intensities at the expense of cell size. In contrast, Dissodinium lunula (Schuett) Taylor had relatively lower division rates at low light intensities with little concomitant decrease in size. The response of P. noctiluca Murray was intermediate between these two species. For all three, cell numbers did not increase above an intensity of 5–10 μEin·m^?2·sec^?1 and division rate was saturated at ca. 30, 60, and 60μEin·m^?2·sec^?1 for P. fusiformis, P. noctiluca, and D. lunula, respectively. The capacity for stimulable bioluminescence was saturated at light intensities of 0.15 μEin·m^?2·day in short-term (2-day) experiments. In cultures of P. fusiformis and P. noctiluca, maintained for at least one month at lower intensities than needed to saturate division rate, a decrease in the capacity for stimulable bioluminescence was accompanied by a reduction in cell size. Our results suggest that cell size and bioluminescent capacity may prove to be a potentially useful indication of the history of exposure of natural populations of Pyrocystis spp. to ambient intensities.     

RELATIONSHIPS BETWEEN PHOTOSYNTHETIC CAPACITY AND LEAF STRUCTURE IN SEVERAL SHADE PLANTS

Leaf structure, photosynthetic characteristics and related physiological parameters have been studied in three ornamental shade species: Fatsia japonica, Cissus rhombifolia (relatively light-tolerant plants), and Philodendron scandens (obligate shade plant). Species were grown in a shadehouse. Maximum photosynthetic photon flux density was 470 μmol m^-2 s^-1. Net rate of CO₂ uptake at light saturation (maximum Pn) in Fatsia was 6.90 ± 1.27 μmol m^-2 s^-1. In Cissus and Philodendron values were about 30% and 63% less respectively, than those measured in Fatsia. The nitrogen content, relative dry wt, specific leaf dry wt (SLDW), chlorophyll a/b ratio, and nitrogen to chlorophyll ratio were lower in Philodendron. However, leaf thickness in Philodendron (296 ± 17 μm) was about 54% and 160% higher, respectively, than in Fatsia and Cissus, and the ratio between mesophyll cell area and leaf surface area (A^mes/A) was nearly similar in the three species. However Philodendron exhibited a percentage of palisade parenchyma about three times lower than that observed in the two other species. The chloroplast number per mm of cell wall in transverse sections (chloroplast density) in the palisade parenchyma was fairly constant (about 65), irrespective of species. The “chloroplast density” in the spongy parenchyma of Philodendron was about 53% and 63%, respectively, of Fatsia and Cissus values. In Fatsia and Cissus chloroplast ultrastructure seems to change gradually and continuously from sun to shade type with the depth from the adaxial to abaxial surface. Special emphasis was given in order to determine the structural parameters best correlated with maximum Pn between the different species. In this way chloroplast number in transverse sections (chloroplast number) and the ratio between chloroplast area and leaf surface area (A^chl/A) were the parameters best correlated with maximum Pn, and stomatal frequency was also a good determinant of maximum Pn. However, leaf thickness, SLDW, and even A^mes/A ratio were weakly correlated with maximum Pn.     

LIGHT‐INDUCED MOTILE RESPONSES OF THE ESTUARINE BENTHIC DIATOMS NAVICULA PERMINUTA AND CYLINDROTHECA CLOSTERIUM (BACILLARIOPHYCEAE)1

Motility of estuarine epipelic (mud‐inhabiting) diatoms is an important adaptation to living in biofilms present within fine sediments. Motility allows cells to migrate within the photic zone in response to a wide range of environmental stimuli. The motile responses of two species of benthic diatoms to photon fluence rates and spectral quality were investigated. Cultures of Navicula perminuta (Grunow) in van Heurck and Cylindrotheca closterium (Ehrenb.) J. C. Lewin et Reimann both exhibited photoaccumulation at ～200 μmol · m^?2 · s^?1 and photodispersal from photon flux densities (PFDs) of ～15 μmol · m^?2 · s^?1. Photokinesis (changing cell speed) contributed toward photodispersal for both species, and red light (λ = 681–691 nm) was most effective at inducing this process. N. perminuta showed a phototactic (directional) response, with active movement in response to a light gradient. Although this response was exhibited in white light, these directional responses were only elicited by wavelengths from 430 to 510 nm. In contrast, C. closterium did not exhibit phototaxis under any light conditions used in this study. Motile benthic diatoms thus exhibit complex and sophisticated responses to light quantity and quality, involving combinations of photokinesis and phototaxis, which can contribute toward explaining the patterns of large‐scale cell movements observed in natural estuarine biofilms.     

TEMPERATURE AND IRRADIANCE EFFECTS ON GROWTH OF PITHOPHORA OEDOGONIA (CHLOROPHYCEAE) AND SPIROGYRA SP. (CHAROPHYCEAE)1

Growth responses of Pithophora oedogonia (Mont.) Wittr. and Spirogyra sp. to nine combinations of temperature (15°, 25°, and 35°C) and photon flux rate (50, 100, and 500 μmol·m^?2·s^?1) were determined using a three-factorial design. Maximum growth rates were measured at 35°C and 500 pmol·m^?2·s^?1 for P. oedogonia (0.247 d^?1) and 25°C and 500 μmol·m^?2·s^?1 for Spirogyra sp. (0.224 d^?1). Growth rates of P. oedogonia were strongly inhibited at 15°C (average decrease= 89%of maximum rate), indicating that this species is warm stenothermal. Growth rates of Spirogyra sp. were only moderately inhibited at 15° and 35°C (average decrease = 36 and 30%, respectively), suggesting that this species is eurythermal over the temperature range employed. Photon flux rate had a greater influence on growth of Spirogyra sp. (31% reduction at 50 pmol·m^?2·s^?1 and 25°C) than it did on growth of P. oedogonia (16% reduction at 50 μmol·m^?2·s^?1 and 35°C). Spirogyra sp. also exhibited much greater adjustments to its content of chlorophyll a (0.22–3.34 μg·mg fwt^?1) than did P. oedogonia (1.35–3.08 μg·mg fwt^?1). The chlorophyll a content of Spirogyra sp. increased in response to both reductions in photon flux rate and high temperatures (35°C). Observed species differences are discussed with respect to in situ patterns of seasonal abundance in Surrey Lake, Indiana, the effect of algal mat anatomy on the internal light environment, and the process of acclimation to changes in temperature and irradiance conditions.     

RELATIONSHIP BETWEEN PHOTOSYNTHESIS AND CELL SIZE OF MARINE DIATOMS12

Three photosynthetic parameters of 7 species of marine diatoms were studied using Na₂¹⁴CO₃ at 5–8 C using log phase axenic cultures. The cell volumes of the different species varied from 70 μm³ to 40 × 10⁵μm³. The present experiment is consistent with the interpretation that the initial slope α (mg C · [mg chl a]^?1· h^?1· w^?1· m²) of photosynthesis vs. light curves is controlled by self-shading of chlorophyll a in the cell. Pm, the rate of photosynthesis at light saturation (mg C · [mg cell, C]^?1· h^?1) and R, the intercept at zero light intensity (mg C · [mg cell C]^?1· H^?1) are both dependent on the ratio of surface area to volume of cell.     

INFLUENCE OF IRRADIANCE ON THE FATTY ACID COMPOSITION OF PHYTOPLANKTON1

Eight species of marine phytoplankton commonly used in aquaculture were grown under a range of photon flux densities (PEDs) and analyzed for their fatty acid (FA) composition. Fatty and composition changed considerably at different PFDs although no consistent correlation between the relative proportion of a single FA and μ or chl a · cell^?1 was apparent. Within an individual species the percentage of certain fatty acids covaried with PFDs, growth rate and/or chl a · cell^?1. The light conditions which produced the greatest proportion of the essential fatty acids was species specific. Eicosapentaenoic acid. 20:5ω3 increased from 6.1% to 15.5% of the total fatty acids of Chaetoceros simplex Ostenfield grown at PFDs which decreased from 225 μE · m^?2· s^?1 to 6 μE · m^?2· s^?1, respectively. Most species had their greatest proportion of 20: 5ω3 at low levels of irradiance. Conversely, docosahexaenoic acid, 22:6ω3, decreased from 9.7% to 3.6% of the total fatty acids in Pavlova lutheri Droop as PFD decreased. The percentage of 22:6ω3 generally decreased with decreasing irradiances. In all diatoms the percentage of 16:0 was significantly correlated with PFD, and in three of five diatoms, with growth rate (μ). Results suggest that fatty acid composition is a highly dynamic component of cellular physiology, which responds significantly to variation in PFD.     

PHYSIOLOGICAL VARIATION AND ELECTROPHORETIC BANDING PATTERNS OF GENETICALLY DIFFERENT SEASONAL POPULATIONS OF SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)1

Clones of Skeletonema costatum (Grev.) Cl. isolated from Narragansett Bay, R.I., during different seasons were grouped according to their electrophoretic banding patterns. The growth rates, pg chlorophyll · cell^?1, carbon uptake · cell^?1· h^?1, and carbon uptake · pg chl^?1· h^?1 were measured at 20°C, in a 14:10 h L:D cycle at 180 μE · m^?2· s^?1. Statistically significant sources of variation were found among groups of clones in growth rate, pg chl · cell^?1, and carbon uptake · pg chl^?1· h^?1. It was concluded that there is a significant relationship between the physiological characteristics of clones isolated from populations in different seasons and patterns of genetic variation inferred from the electrophoretic studies. However, genetic diversity detected by banding patterns tends to underestimate the total genetic diversity in natural populations. The groups of clones most common in summer bloom populations had significantly higher growth rates, lower values of pg chl · cell^?1, and higher rates of carbon uptake · pg chl^?1· h^?1 at 20°C than did the group of clones most common in winter bloom populations. However, differences among groups in these parameters at 20°C alone cannot account for the seasonal cycling of genetically variable populations of Skeletonema in Narragansett Bay. The range of growth rates among clones of this species is 0.1–5.0 divisions · d^?1 under a single set of temperature and light conditions. Chlorophyll concentrations range from 0.2–1.7 pg chl · cell^?1 and carbon uptake · pg chl^?1· h^?1 varies by a factor of 7 among clones. The range of physiological variation in this species means that it is difficult to use laboratory studies of single clones to analyze the responses of natural populations of Skeletonema.     

PHOTOSYNTHESIS OF THE RED ALGA GASTROCLONIUM COULTERI (RHODOPHYTA) IN RESPONSE TO CHANGES IN TEMPERATURE,LIGHT INTENSITY,AND DESICCATION1

Previously reported transplantation experiments in the field showed that Gastroclonium coulteri (Harvey) Kylin could survive above its normal intertidal range (0.0–0.5 m above MLLW), except during periods of daytime low tides in spring. Net photosynthetic rate measurements in the laboratory were performed to determine which physical factors might determine the upper boundary for this species in the intertidal zone. Maximum net photosynthesis occurred between 15 and 20° C, but remained positive between 4 and 35° C. The air temperature extremes observed in the field were 2° C (only seen once) and 26° C. Net photosynthesis increased as expected with light intensity to the highest value obtainable in the laboratory, 1400 μEin m^?2 s^?1. Plants collected from the field under higher light intensity (up to 2000 μEin m^?2 s^?2) also showed high rates of photosynthesis. Neither the temperature nor light levels observed in the field were directly damaging to photosynthesis. Desiccation, however, resulted in a sharp decrease in both photosynthesis and respiration. G. coulteri fully recovered from successive daily treatments of about 35% desiccation, but not from successive treatments of 50% desiccation. One exposure to 70% desiccation allowed no recovery of photosynthetic capacity.     

EFFECT OF TEMPERATURE AND IRRADIANCE ON GROWTH OF HAEMATOCOCCUS PLUVIALIS (CHLOROPHYCEAE)1

The growth characteristics of Haematococcus pluvialis Flotow were determined in batch culture. Optimal temperature for growth of the alga was between 25° and 28°C, at which the specific growth rate was 0.054 h^?1. At higher temperatures, no cell division was observed, and cell diameter increased from 5 to 25 μm. The saturated irradiance for growth of the alga was 90 μmol quanta · m^?2·s^?1; under higher irradiances (e.g. 400 μmol quanta·m^?2·s^?1) astaxanthin accumulation was induced. Growth rate, cell cycle, and astaxanthin accumulation were significantly affected by growth conditions. Careful attention should be given to the use of optimal growth conditions when studying these processes.     

LIGHT ACCLIMATION IN PORPHYRIDIUM PURPUREUM (RHODOPHYTA): GROWTH,PHOTOSYNTHESIS, AND PHYCOBILISOMES1

Acclimation to three photon flux densities (10, 35, 180 μE.m^?2.s^?1) was determined in laboratory cultures of Porphyridium purpureum Bory, Drew and Ross. Cultures grown at low, medium, and high PPFDs had compensation points of <3, 6, and 20 μE-m^?2.s^?1, respectively, and saturating irradiances in the initial log phase of 90, 115, 175 μE.m^?2.s^?1 and up to 240 μE.m^?2.s^?1 in late log phase. High light cells had the smallest photosynthetic unit size (phycobiliproteins plus chlorophyll), the highest photosynthetic capacity, and the highest growth rates. Photosystem I reaction centers (P700) per cell remained proportional to chlorophyll at ca. 110 chl / P700. However, phycobiliprotein content decreased as did the phycobilisome number (ca. 50%) in high light cells, where as the phycobilisome size remained the same as in medium and low light cells. We concluded that acclimation of this red alga to varied PPFDs was manifested by the plasticity of the photosystem II antennae with little, if any, effect noted on photosystem I.     

Modulation of photosynthesis and respiration in guard and mesophyll cell protoplasts by oxygen concentration

Stomatal movement is an energetic oxygen-requiring process. In the present study, the effect of oxygen concentration on mitochondrial respiratory activity and red-light-dependent photosynthetic oxygen evolution by Vicia faba and Brassica napus guard cell protoplasts was examined. Comparative measurements were made with mesophyll cell protoplasts isolated from the same species. At air saturated levels of dissolved oxygen in the protoplast suspension media, respiration rates by mesophyll protoplasts ranged from 6 to 10μmoles O₂ mg^?1 chl h^?1, while guard cell protoplasts respired at rates of 200–300 μmoles O₂ mg chl^?1 h^?1, depending on the species. Lowering the oxygen concentration below 50–60 mmol m^?3 resulted in a decrease in guard cell respiration rates, while rates by mesophyll cell protoplasts were reduced only at much lower concentrations of dissolved oxygen. Rates of photosynthesis in mesophyll cell protoplasts isolated from both species showed only a minor reduction in activity at low oxygen concentrations. In contrast, photosynthesis by guard cell protoplasts isolated from V. faba and B. napus decreased concomitantly with respiration. Oligomycin, an inhibitor of oxidative phos-phorylation, reduced photosynthesis in mesophyll cell protoplasts by 27–46% and in guard cell protoplasts by 51–58%. The reduction in both guard cell photosynthesis and respiration following exposure to low oxygen concentrations suggest close metabolic coupling between the two activities, possibly mediated by the availability of substrate for respiration associated with photosynthetic electron transport activity and subsequent export of redox equivalents.