EFFECTS OF TEMPERATURE ON GROWTH,LIGHT ABSORPTION,AND QUANTUM YIELD IN DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE)1

The effects of growth temperature on the marine chlorophyte Dunaliella tertiolecta Butcher were studied to provide a more mechanistic understanding of the role of environmental factors in regulating bio-optical properties of phytoplankton. Specific attention was focused on quantities that are relevant for modeling of growth and photosynthesis. Characteristics including chlorophyll a (chl z)-specific light absorption (a*_ph(λ)), C:chl a ratio, and quantum yield for growth (φ_μ) varied as functions of temperature under conditions of excess light and nutrients. As temperature increased over the range examined (12°-28°C), intracellular concentrations of chl a increased by a factor of 2 and a*_ph(λ) values decreased by more than 50% at blue to green wavelengths. The lower values of a*_ph(λ) were due to both a decrease in the abundance of accessory pigments relative to chl a and an increase in pigment package effects arising from higher intracellular pigment concentrations. Intracellular pigment concentration increased as a consequence of higher cellular pigment quotas combined with lower cell volume. At high growth temperatures, slightly more light was absorbed on a per-cell-C basis, but the dramatic increases in growth rate from μ= 0.5 d^?1 at 12° C to μ= 2.2 d^?1 at 28°C were primarily due to an increase in φ_μ (0.015–0.041 mol C (mol quanta)^?1). By comparison with previous work on this species, we conclude the effects of temperature on a*_ph(λ) and φ_μ are comparable to those observed for light and nutrient limitation. Patterns of variability in a*_ph(λ)and φ_μ as a function of growth rate at different temperatures are similar to those previously documented for this species grown at the same irradiance but under a range of nitrogen-limited conditions. These results are discussed in the context of implications for bio-optical modeling of aquatic primary production by phytoplankton.     

A SALT-INDUCED CHLOROPLAST PROTEIN IN DUNALIELLA SALINA (CHLOROPHYTA)1

The unicellular green alga Dunaliella salina Teod, is halophilic and wall-less. The cell acclimates to osmotic stresses by accumulation or degradation of glycerol. To investigate other mechanisms involved in its physiological recovery following hyperosmotic shocks, protein profiles from cells grown in various salinities were compared. A 13-kDa protein (P13) accumulated when cells were subjected to drastic hyperosmotic shock. Front our results with antibiotic-treated cells and purified chloroplasts, we believe that this component results from de novo translation in chloroplasts. The solubility of P13 was strongly promoted by Triton X-100. Its accumulation was correlated with the recovery of photosynthesis.     

AMMONIUM INDUCED PHOTOSYNTHETIC SUPPRESSION IN AMMONIUM LIMITED DUNALIELLA TERTIOLECTA (CHLOROPHYTA)1

Dunaliella tertiolecta (Butcher) was grown in chemostat culture over a wide range of ammonium limited growth rates. The addition of ammonium caused a rapid temporary suppression of photosynthetic carbon fixation. The magnitude of ammonium induced photosynthetic suppression increased with the severity of ammonium limitation. Cells growing at rates greater than ca. 80%μ_max exhibited no photosynthetic suppression in response to additions of the limiting nutrient. The duration of photosynthetic suppression was related to the concentration of added ammonium. Immediately following the suppression, photosynthesis was enhanced with respect to the controls. The eventual degree of enhancement increased with the concentration of added ammonium. Steady-state cellular chlorophyll concentrations, photosynthetic rates, and assimilation numbers are reported.     

ASSESSING PHYSIOLOGICAL STRESS IN THALASSIOSIRA FLUVIATILIS (BACILLARIOPHYTA) AND DUNALIELLA TERTIOLECTA (CHLOROPHYTA) WITH DCMU-ENHANCED FLUORESCENCE1

Exponentially growing cultures of Thalassiosira fluviatilis Hustedt and Dunaliella tertiolecta Butcher were exposed to 4 min temperature shocks of 5° to 20°C above ambient (20°C). Photosynthetic carbon fixation, changes in in vivo fluorescence and fluorescence on the addition of the herbicide DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) were measured over the subsequent 24 h. The fluorescence ratio (R, DCMU-enhanced fluorescence/in vivo fluorescence) paralleled changes in photosynthesis over this period; both were significantly reduced (P < 0.05) by temperature shocks of +15° and +20° C, but +5° and +10° C treatments had no inhibitory effect on either relative to the control. The instantaneous response obtained with the fluorescence ratio indicates that the technique might be applicable to routine bioassay procedures and thus replace the time consuming methods now used for the estimation of ¹⁴C-incorporation and growth rates.     

GROWTH AND PHOSPHORUS UPTAKE BY THE TOXIC DINOFLAGELLATE ALEXANDRIUM CATENELLA (DINOPHYCEAE) IN RESPONSE TO PHOSPHATE LIMITATION1

Alexandrium catenella (Whedon et Kof.) Balech has exhibited seasonal recurrent blooms in the Thau lagoon (South of France) since first reported in 1995. Its appearance followed a strong decrease (90%) in phosphate (PO₄^3?) concentrations in this environment over the 1970–1995 period. To determine if this dinoflagellate species has a competitive advantage in PO₄^3?‐limited conditions in terms of nutrient acquisition, semicontinuous cultures were carried out to characterize phosphorus (P) uptake by A. catenella cells along a P‐limitation gradient using different dilution rates (DRs). Use of both inorganic and organic P was investigated from measurements of ³³PO₄^3? uptake and alkaline phosphatase activity (APA), respectively. P status was estimated from cellular P and carbon contents (Q_P and Q_C). Shifts in trends of Q_P/Q_C and Q_P per cell (Q_P·cell?1) along the DR gradient allowed the definition of successive P‐stress thresholds for A. catenella cells. The maximal uptake rate of ³³PO₄^3? increased strongly with the decrease in DR and the decrease in Q_P/Q_C, displaying physiological acclimations to PO₄^3? limitation. Concerning maximal APA per cell, the observation of an all‐or‐nothing pattern along the dilution gradient suggests that synthesis of AP was induced and maximized at the cellular scale as soon as PO₄^3? limitation set in. APA variations revealed that the synthesis of AP was repressed over a PO₄^3? threshold between 0.4 and 1 μM. As lower PO₄^3? concentrations are regularly observed during A. catenella blooms in Thau lagoon, a significant portion of P uptake by A. catenella cells in the field may come from organic compounds.     

CHARACTERISTICS OF PHOSPHORUS LIMITATION IN CHLAMYDOMONAS REINHARDTH (CHLOROPHYCEAE) AND ITS PALMELLOIDS1

Chlamydomonas reinhardtii Dang, was grown in a chemostat culture under phosphate limitation. The steady state concentration of phosphate was below the detection limit (< 1 μg P/L) in all runs. The cellular content of phosphorus (Q_p), polyphosphate (Q_pp) and chlorophyll a increased with increasing dilution rate, and the growth rate of the alga was described by Q_p as well as Q_pp in the Droop model. The ratio Q_pp/Q_p and the activity of alkaline phosphatase were maximal at high and low growth rates, respectively. Palmelloids of Chlamydomonas were found at high dilution rates (D > 0.12 h^?1) and became attached to the wall of the culture vessel. They differed from the vegetative stage in both chemical composition and growth rate. Their contents of phosphorus and chlorophyll a were low, as in the vegetative cells, which grew at a low growth rate, whereas the ration Q_pp/Q_p and the activity of alkaline phosphatase were comparable with those of fast growing vegetative cells. The growth rate of the palmelloids was 0.03 h^?1 whereas maximum growth rate (μ_m) for the vegetative cells was 0.21 h^?1.     

PHYSIOLOGICAL AND ULTRASTRUCTURAL RESPONSES OF CYCLOTELLA MENEGHINIANA (BACILLARIOPHYTA) TO LIGHT INTENSITY AND NUTRIENT LIMITATION1

Photosynthetic characteristics and chloroplast ultrastructure of Cyclotella meneghiniana Kütz. were quantified while the organism was simultaneously adjusting to light and nutrient stress. Cells were grown in batch culture at either low or high light intensity on medium with a nitrogen/phosphorus molar ratio of 2:1 as a control, or with nitrogen or phosphorus deleted from the medium to create nutrient deficiencies. Analysis of variance indicated that light intensity, nutrient deficiency and duration of nutrient deficiency all had significant effects on cell growth, chlorophyll (Chl) concentration/cell, cellular fluorescence capacity (CFC), chloroplast volume and thylakoid surface density. Because interactions existed among nutrient deficiency, extent of nutrient deficiency, and light intensity, all three must be considered together in order to describe accurately the physiology and chloroplast ultrastructure of the diatom. Significant correlations were found between the Chl/cell or CFC/cell and chloroplast volume and thylakoid surface density. Through an increase in Chi concentration, chloroplast volume and thylakoid surface density, the cells successfully adapted to the conditions of low light intensity even while under nutrient stress. In contrast, less Chl/cell, smaller chloroplast volume and less thylakoid surface density were found at high light intensity.     

BIOCHEMICAL AND CYTOLOGICAL EVIDENCE FOR THE STIMULATION OFCLATHRIN-COATED PIT (VESICLE) FORMATION BY EXOGENOUS FOLIC ACID IN DUNALIELLA SALINA (CHLOROPHYTA)1

The effects of two kinds of molecules, Pb²⁺ and folic acid, on the formation of clathrin-coated pits and vesicles were studied in the unicellular green alga, Dunaliella salina Teod. Measurable amounts of algal clathrin were obtained in the postmicrosomal fraction from cells treated with folic acid. In contrast, algal clathrin heavy chains were below the detection limit in the postmicrosomal fraction from control and lead nitrate-treated cells. Consistent with the biochemical evidence, electron microscopy showed more clathrin-coated pits and vesicles in folic acid-treated cells compared to control cells or cells treated with lead nitrate alone. Observations of folic acid/lead nitrate-treated cells confirmed the endocytosis of Pb²⁺ through clathrin-coated pits and vesicles. As additional evidence for clathrin in the folic acid-stimulated cells of Dunaliella salina, clathrin was isolated and, for the first time in algae, the calcium-dependent reconstitution of clathrin cages was successfully obtained in vitro.     

EFFECT OF NITROGEN AND PHOSPHORUS SUPPLY ON GROWTH AND TISSUE COMPOSITION OF ULVA FENESTRATA AND ENTEROMORPHA INTESTINALIS (ULVALES,CHLOROPHYTA)1

The chlorophyte macroalgae Ulva fenestrata (Postels and Ruprecht) and Enteromorpha intestinalis (Linnaeus) Link. were grown under various nutrient regimes in indoor semi-continuous and batch cultures. Tissue nitrogen contents ranged from 1.3–5.4% N (dry wt), whereas tissue P ranged from 0.21–0.56% P (dry wt). Growth in low nitrogen medium resulted in N:P ratios of 5–8, whereas growth in high nitrogen medium resulted in N:P ratios of 21–44. For U. fenestrata, tissue N:P < 16 was indicative of N-limitation. Tissue N:P 16–24 was optimal for growth and tissue N:P > 24 was indicative of P-limitation. Growth of U. fenestrata was hyperbolically related to tissue N but linearly related to tissue P. Phosphorus-limited U. fenestrata maintained high levels of tissue N, but N-limited algae became depleted of P. For E. intestinalis, tissue N remained at maximum levels during P-limitation whereas tissue P decreased to about 85% of maximal levels during N-limitation. Growth rates for U. fenestrata decreased faster during P-limitation than during N-limitation. Simultaneously, tissue P was depleted faster than tissue N. Our results suggest that comparing tissue N and P of macroalage grown in batch cultures is useful for monitoring the nutritional status of macroalgae.     

EFFECT OF LIGHT QUALITY AND INTENSITY ON GLYCEROL CONTENT IN DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE) AND THE RELATIONSHIP TO CELL GROWTH/OSMOREGULATION1

Dunaliella tertiolecta Butcher was grown at two intensities (33, 150μEin · m^?2· s^?1) of blue light and white light at 0.25, 0.50 and 1.00 M NaCl. Growth rates were used as an indication of the relative osmoregulatory ability of cells in the various treatments. There was no significant effect on growth rate due to various NaCl molarities. No significant difference in growth rate was found between blue- and white-light cultures at the high intensity, the average growth constant being 2.07 divisions/day. However, at the low intensity illumination, blue light produced a significant increase in growth rate; 1.42 vs. 0.93 divisions/day for blue light and white light grown cells respectively. The average glycerol content of exponentially dividing cells grown at 0.25, 0.50 and 1.00 M NaCl was 0.12, 0.41 and 1.12 mg/10⁸ cells, respectively, as measured by gas chromatography. The intracellular glycerol content was significantly reduced by blue light at both light intensities and at each NaCl molarity. However, high light intensity reduced cellular glycerol content more than the reduction effected by blue light. Glycerol accumulated in the medium throughout culture growth. Intracellular glycerol content also increased with cellular aging reaching 2.72 mg/10⁸ cells in stationary phase, low intensity 1.00 M NaCl cultures. A negative correlation between glycerol content and growth rate was found. Total inhibition of glycerol production could not be obtained by treatment with blue light. However, this negative correlation possibly indicates that D. tertiolecta expends energy producing an excess amount of glycerol over that required for osmoregulation, leading to a reduction in the growth rate for the organism.     

TEMPERATURE RESPONSES AND EVOLUTION OF THERMAL TRAITS IN CLADOPHOROPSIS MEMBRANACEA (SIPHONOCLADALES,CHLOROPHYTA)1

Temperature tolerances and relative growth rates were determined for different isolates of the tropical to warm temperate seaweed species Cladophoropsis membranacea (C. Agardh) Boergesen (Siphonodadales, Chlorophyta) and some related taxa. Most isolates of C membranacea survived undamaged at 18° C for at least 8 weeks. Lower temperatures (5°–15°C) were tolerated for shorter periods of time but caused damage to cells. All isolates survived temperatures up to 34° C, whereas isolates from the eastern Mediterranean and Red Sea survived higher temperatures up to 36°C. Growth occurred between 18° and 32° C, but an isolate from the Red Sea had an extended growth range, reaching its maximum at 35°C. Struvea anastomosans (Harvey) Piccone & Grunow, Cladophoropsis sundanensis Reinbold, and an isolate of C. membranacea from Hawaii were slightly less cold- tolerant, with damage occurring at 18°C. Upper survival temperatures were between 32° and 36° C in these taxa. Temperature response data were mapped onto a phylogenetic tree. Tolerance for low temperatures appears to be a derived character state that supports the hypothesis that C. membranacea originated from a strictly tropical ancestor. Isolates from the Canary Islands, which is near the northern limit of distribution, are ill adapted to local temperature regimes. Isolates from the eastern Mediterranean and Red Sea show some adaptation to local temperature stress. They are isolated from those in the eastern Atlantic by a thermal barrier at the entrance of the Mediterranean.     

ORGANIC CARBON RELEASE BY DUNALIELLA SALINA (CHLOROPHYTA) UNDER DIFFERENT GROWTH CONDITIONS OF CO2, NITROGEN,AND SALINITY1

Two strains of Dunaliella salina (Dunal) Teod., UTEX 1644 and UTEX 200, were cultured under different growth regimes, including 10 mM NO₃^? or NH₄⁺, 1.5 or 3.0 M NaCl, and low (0.035%) or high (5%) CO₂ in air. The release of ¹⁴C-labeled dissolved organic carbon (DOC), expressed as a rate and as a percentage of photosynthetic ¹⁴CO₂ assimilation, was subsequently determined. The percentage of DOC released was inversely related to cell density in the assay medium, but photosynthesis on a per-cell basis was not. Release of DOC was low, in the range of 1–5% of photosynthesis, but during acclimation to growth on NH₄⁺, it rose to 11%. The presence of NH₄⁺ rather than NO₃^? in the growth medium increased the rate of release by both strains, but the percentage release was stimulated only in UTEX 200 cells, because their photosynthetic rate was depressed by NH₄⁺. For UTEX 1644, high, as compared to low, CO₂-grown cells, had somewhat higher rates and percentages of DOC release, but release from UTEX 200 cells was unaffected by the growth-CO₂. The rate of DOC release by high CO₂-grown cells was not enhanced at a low concentration of dissolved inorganic carbon, indicating that the released material did not originate from the photorespiratory pathway. The effects of NaCl on DOC release varied with strain and growth conditions. For UTEX 200, the cells in NO₃^?, but not NH₄⁺, exhibited a doubling or more in percentage of release with a doubling in NaCl concentration, irrespective of growth-CO₂. With UTEX 1644 the low CO₂-grown cells showed the greatest enhancement in 3.0 M NaCl. Organic matter accumulated on the external surface of the cell membrane and constituted a well-defined cell-coat, which was more dense in NH₄⁺ than in NO₃^?-grown cells. Microtubules, which may play a role in maintaining cell shape, were observed just below the plasma membrane. From a practical viewpoint, the presence of organic material in the hypersaline ponds of salt-works is detrimental to salt production. When D. salina cells become abundant in such ponds, the attendant, continuous release of DOC may make a significant contribution to the problem.     

PHYSIOLOGICAL INDICATORS OF NUTRIENT DEFICIENCY IN CLADOPHORA (CHLOROPHYTA) IN THE CLARK FORK OF THE COLUMBIA RIVER,MONTANA1

Cellular nutrient concentrations and nutrient uptake rates of Cladophora glomerata (L.) Kuetzing were determined during summer and fall in 1989–1990 at a site on the upper Clark Fork of the Columbia River, Montana. Both physiological tests indicated that Cladophora growth is likely to be limited by nitrogen during late summer-early fall. Maximum uptake rates of ammonia-N and nitrate-N were 5935–6991 and 507–984 μg · g DW^?1· h^?1, respectively, during July–October when dissolved inorganic nitrogen (DIN) concentrations in the river were less than 10 μg · L^?1. During November-December, when DIN was 72–376 μg · L^?1, maximum ammonia-N uptake was 1137–1633 μg · g DW^?1· h^?1 and maximum nitrate-N uptake was 0–196 μg · g DW^?1· h^?1. Cellular nitrogen during summer–early fall was 0.78–1.80% of Cladophora dry weight, frequently at or below 1.1%, a level suggested as a critical minimum N concentration for maximum growth. In contrast, cellular P was 0.18–0.36% of dry weight, 3–6 times the suggested critical P concentration of 0.06%. Molar ratios of cellular N:P (< 16:1) and DIN: SRP (< 4:1) during late summer-early fall also indicated potential N limitation. Cellular N and P from Cladophora collected from a second site influenced by a municipal wastewater discharge in 1990 displayed similar seasonal trends. At both sites, seasonal fluctuations in DIN were closely tracked by changes in cellular N, Cellular P, however, increased through the growing season despite declining levels of SRP in the river.     

PHOSPHORUS LIMITATION AND CARBON METABOLISM IN A UNICELLULAR ALGA: INTERACTION BETWEEN GROWTH RATE AND THE MEASUREMENT OF NET AND GROSS PHOTOSYNTHESIS1

Chlamydomonas reinhardii Dangeard was grown in continuous culture under P limitation at a range of dilution rates. Carbon uptake measurements were performed using double isotope (¹²C/¹⁴C) techniques and the fluxes of carbon in the light and dark were analysed over the range of growth rates. ¹⁴C uptake was shown to be equal to gross photosynthesis only at maximum relative growth rates; at low relative growth rates ¹⁴C uptake approximated net photosynthesis. The altered pattern of C uptake was found to be due to the suppression of dark respiration in the light and the release of ¹⁴C0₂ from respiratory pathways at low relative growth rates. Metabolic channelling of ¹⁴C from photosynthetic pathways to respiratory pathways occurred at low growth rates as the specific activity of the respired CO₂ reached 45% of the input gas mixture. These data are discussed in the light of the controversy concerning the measurement of gross and net photosynthesis in natural populations and in the light of models of ¹⁴C uptake in single celled algae. Existing models are shown to be adequate for high relative growth rates but not for low relative growth rates under P limitation.     

EFFECT OF LOW TEMPERATURE ON THE STEREOISOMER COMPOSITION OF β-CAROTENE IN THE HALOTOLERANT ALGA DUNALIELLA BARDAWIL (CHLOROPHYTA)1

Dunaliella bardawil Ben-Amotz & Avron, a β-carotene-accumulating halotolerant alga, was analyzed for the effect of growth temperatures on its pigment content and on the stereoisomeric composition of β-carotene by the use of advanced liquid chromatography and photodiode array detection. Decreasing culture temperature from 30° to 10°C increased the β-carotene content twofold and the ratio of 9-cis to all-trans β-carotene fourfold, with no significant changes in the other cell pigments. The variation of total β-carotene content by temperature was correlated with the integral irradiance received by the algal culture during a cell division cycle, whereas the 9-cis stereoisomer increased over the amount expected by that integration. The massive accumulation of 9-cisβ-carotene within the β-carotene globules is interpreted as indicating that the oily 9-cis stereoisomer protects against the crystallization of all-trans β-carotene at low temperatures.     

TEMPERATURE DEPENDENCE OF GROWTH AND PHOSPHORUS UPTAKE IN TWO SPECIES OF VOLVOX (VOLVOCALES,CHLOROPHYTA)1

The growth of Volvox globator L. and Volvox aureus Ehr. was measured at five temperatures and nine phosphorus concentrations. Growth rates were hyperbolically related to phosphorus concentrations for all temperatures using a Monod growth model. Optimal growth rates of 1.17 and 1.00 doublings d^?1 were obtained at 20°C for V. globator and V. aureus, respectively. Neither species grew at 5°C. The half-saturation constants for growth, K_s, were lower for V. aureus. Phosphorus uptake by both species was also dependent upon external phosphorus concentrations and temperature. At all temperatures, maximum phosphorus uptake (μmol P colony^?1 min^?1) was similar for both species; however, the half-saturation constants for uptake showed significant differences between the species. Comparisons of the kinetic constants for growth and phosphorus uptake suggest that V. aureus will outcompete V. globator under phosphorus limited, conditions.     

GROWTH AND BRANCHED HYDROCARBON PRODUCTION IN A STRAIN OF BOTRYOCOCCUS BRAUNII (CHLOROPHYTA)1

A strain Botryococcus braunii Kütz. that produces high levels of branched hydrocarbons (botryococcenes) was grown under different environmental conditions to investigate the relationship between growth and hydrocarbon production. Carbon dioxide concentration had the most significant influence on growth; 0.3% CO₂-enriched cultures demonstrated a minimum mass doubling time of ca. 40 h, compared to ca. 6 days for ambient air cultures grown on the same buffered growth medium. The botryococcene fraction, which consisted of 10 identified compounds (C_nH_2n-10; n = 30–34), usually constituted ca. 25–40% of the culture dry weight under various growth regimes, including nitrogen- and/or phosphate-deficiencies. CO₂ enrichment initially favored the production of the lower botryococcenes (C₃₀–C₃₂), whereas relatively slow-growing ambient air cultures accumulated C₃₃ and C₃₄ compounds. Colony color changed in response to different light intensities. High light increased the carotenoid/chlorophyll ratio, which resulted in orange colonies. Cultures exposed to low light intensity appeared green. This change in coloration was reversible over a period of a few days, and at no time were the linear hydrocarbons characteristic of the other form of the alga detected. Ostensible colony color is not, therefore, a reliable indicator of qualitative hydrocarbon content. Sequential solvent extraction experiments indicated that up to ca. 7% of the botryococcene fraction was intracellular and that the remainder was located within the colonial matrix. The internal (cellular) pool principally consisted of C₃₀–C₃₂ botryococcenes, whereas the external (colonial matrix) pool contained >99% of the C₃₃ and C₃₄ compounds, in addition to large amounts of the lower botryococcenes. These results, taken in conjunction with other data, are compatible with the hypothesis that the C₃₀ botryococcene is the precursor, presumably via methylation, of the higher botryococcenes.     

PHOTOSYNTHETIC CAPACITY AND LUXURY UPTAKE OF CARBON DURING PHOSPHATE LIMITATION IN PEDIASTRUM DUPLEX (CHLOROPHYCEAE)1

When cells of Pediastrum duplex Meyen experience phosphorus depletion, their capacity for carbon fixation declines, but sizes and carbon contents of the cells increase several-fold, an apparent instance of “luxury uptake” of carbon. Maximum rates of uptake of phosphate increase during the same period, and are consequently correlated with the enlarged surface area of the cells. Thus the disadvantage of increased cell volume, which may accelerate sinking speed in nature, is offset by the increased capacity of the cells for nutrient uptake.     

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.     

RELATIONSHIPS BETWEEN NITRATE REDUCTASE ACTIVITY AND RATES OF GROWTH AND NITRATE INCORPORATION UNDER STEADY-STATE LIGHT OR NITRATE LIMITATION IN THE MARINE DIATOM THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE)1

Although activity of the enzyme nitrate reductase (NR) can potentially be used to predict the rate of nitrate incorporation in field assemblages of marine phytoplankton, application of this index has met with little success because the relationship between the two rates is not well established under steady-state conditions. To provide a basis for using NR activity measurements, the relationships among NR activity, growth rate, cell composition, and nitrate incorporation rate were examined in cultures of Thalassiosira pseudonana (Hustedt)Hasle and Heimdal, growing a) under steady-state light limitation, b) during transitions between low and high irradiance (15 or 90 μmol quanta.m^?2.s^?1), and c) under steady-state nitrate limitation. Using a modified assay for NR involving additions of bovine serum albumin to stabilize enzyme activity, NR activity in light-limited cultures was positively and quantitatively related to calculated rates of nitrate incorporation, even in cultures that were apparently starved of selenium. During transitions in irradiance, growth rates acclimated to new conditions within 1 day; through the transition, the relationship between NR activity and nitrate incorporation rate remained quantitative. In nitrate-limited chemostat cultures, NR activity was positively correlated with growth rate and with nitrate incorporation rates, but the relationship was not quantitative. NR activity exceeded nitrate incorporation rates at lower growth rates (<25% of nutrient-replete growth rates), but chemostats operating at such low dilution rates may not represent ecologically relevant conditions for marine diatoms. The strong relationship between NR activity and nitrate incorporation provides support for the idea that NR is rate-limiting for nitrate incorporation or is closely coupled to the rate-limiting step. In an effort to determine a suitable variable for scaling NR activity, relationships between different cell components and growth rate were examined. These relationships differed depending on the limiting factor. For example, under light limitation, cell volume and cell carbon content increased significantly with increased growth rate, while under nitrate limitation cell volume and carbon content decreased as growth rates increased. Despite the differences found between cell composition and growth rate under light and nitrate limitation, the relationships between NR activity scaled to different compositional variables and growth rate did not differ between the limitations. In field situations where cell numbers are not easily determined, scaling NR activity to particulate nitrogen content may be the best alternative. These results establish a strong basis for pursuing NR activity measurements as indices of nitrate incorporation in the field.