INDUCIBLE MECHANISMS FOR HCO3– UTILIZATION AND REPRESSION OF PHOTORESPIRATION IN PROTOPLASTS AND THALLI OF THREE SPECIES OF ULVA (CHLOROPHYTA)1

Thalli of Ulva reticulata Forskaal, Ulva rigida C. Ag., and Ulva pulchra Jaasund were incubated at different concentrations of dissolved CO₂. Incubation at a high CO₂ concentration resulted in decreased oxygen evolution rate and lower affinity for inorganic carbon at high pH conditions, i.e. the ability to use HCO₃^– as a carbon source was reduced. This effect was reversible, and plants regained this HCO₃^– uptake capacity when transferred to air concentrations of CO₂. The phytosynthetic oxygen evolution rate of plants grown at high CO₂ concentration was reduced by high O₂ concentrations, whereas thalli and protoplasts from cultures grown at air concentration were not affected. This is interpreted as a deactivation of the carbon-concentrating mechanism during conditions of high CO₂ resulting in high photorespiration when plants are exposed to high O₂ concentrations. Protoplasts were not affected by high O₂ to the same extent and were not able to utilize HCO₃^– from the medium. The algae were able to grow at very low CO₂ concentrations, but growth was suppressed when an inhibitor of external carbonic anhydrase was present. Assay of carbonic anhydrase activities showed that external and internal CA activities were lower in plants grown at a high CO₂ concentration compared to plants grown at a low concentration of CO₂. Possible mechanisms for HCO₃^– utilization in these Ulva species are discussed.     

DEVELOPMENT OF PROTOPLASTS OF ULVA FASCIATA (CHLOROPHYTA) FOR ALGAL SEED STOCK

The aim of this study was to isolate and cultivate protoplasts of the green alga Ulva fasciata Delile and subsequently induce them to form a microthallus suspension for algal seed stock. The protoplasts were covered with secreted mucilage following 6 h of culture when viewed with SEM. The mucilage fused to form thick layers during day 1 of culture. Microfibrillar cell walls were deposited into the thick layers of mucilage on the 5th day of culture. An average of about 10% of the freshly isolated protoplasts began to divide at 6–14 days. These protoplasts subsequently developed varied morphologies, depending on the time of collection during the year. Protoplasts isolated from U. fasciata collected in March to June developed frond thalli or microthalli when they were cultured in low or high densities (cells/area), respectively. The microthallus suspension was cultured for more than two years at 10–40 μ mol·m^{− 2} ·s^{− 1} . Frond thalli formed when the suspension was cultivated at 100–160 μ mol·m^{− 2} ·s^{− 1} . Therefore, microthallus suspension can serve as a seed stock of U. fasciata .     

GLYCOCONJUGATE ORGANIZATION OF ENTEROMORPHA (=ULVA) FLEXUOSA AND ULVA FASCIATA (CHLOROPHYTA) ZOOSPORES1

Ecologically successful algae that colonize natural and artificial substrates in the marine environment have distinct strategies for opportunistic dispersal and settlement. The objective of this research was to visualize molecular architecture of zoospores from Enteromorpha (=Ulva) flexuosa (Wulfen) J. Agardh and Ulva fasciata Delile that coexist but alternate in dominance on an intertidal bench. Multiple fluorescent lectins were used to stabilize and probe for diverse zoospore glycoconjugates (GC) that could be involved in cell and substrate interactions. Results from epifluorescence microscopy showed distinct cellular and extracellular polymeric substance (EPS) domains of GC relative to settlement morphologies. Glycoconjugates were similar for both species with (1) α‐d mannose and/or glucose moieties localized on flagella, the anterior domes and anterior regions, the plasma membranes, and EPS; (2) α‐fucose localized on flagella and anterior regions; (3) N or α,ß‐N acetylglucosamine localized on flagella, the anterior regions, and EPS; and (4) varied N‐acetylgalactosamine and/or galactose moieties localized on each domain for both species excluding the plasma membranes. Some differences in lectin binding were observed for each species at the flagella, the anterior domes, and the plasma membranes. Glycoconjugate distributions shifted with morphological changes that followed initial adhesion. TEM of E. flexuosa zoospore stages following carbohydrate‐stabilizing fixations and gold‐conjugated lectin probes resolved GC with α‐d mannose and/or glucose, and/or N‐acetylglucosamine at the plasma membrane, ER and diverse vesicles of the anterior pole, EPS, and discontinuous regions or knobs associated with flagellar surfaces. The distinct distribution and diversity of zoospore GC may be central to recognition and attachment on diverse substrata by these algae.     

POLYAMINE TRANSPORT IN THE SEAWEED ULVA RIGIDA (CHLOROPHYTA)1

The excessive growth of Ulva rigida C. Agardh, a green seaweed present in the Northern Adriatic Sea, is a problem for the inhabitants and the economy of the region. As information about hormonal control of growth in seaweeds is scarce, our aim was to investigate the presence of endogenous polyamines and their absorption by algal cells and to correlate the findings with terrestrial plants. Free polyamines (putrescine, spermidine, and spermine) were present endogenously in the algal thallus at concentrations ranging from 4 to 134 μM. Putrescine and spermidine were also present in the seawater in which the alga usually grows at concentrations between 0 and 0.9 μM. Uptake of labeled polyamines occurred, but it was inhibited by cations present in the seawater. Uptake was investigated also by incubation in distilled water. In this case, uptake displayed characteristics similar to those observed in higher plant systems. Uptake studies in seawater showed that polyamine accumulation in algal cells occurred and that it followed a concentration gradient and displayed linear kinetics. The mechanism proposed that of a passive uptake, as indicated also by the inability of metabolic inhibitors to block transport. There was evidence for polyamine binding to external cell sites, but polyamine uptake by protoplasts as well as polyamine translocation and secretion by the whole thallus was also demonstrated. Since cultured and actively growing thallus discs displayed a higher uptake ability than freshly collected ones, a role for polyamines in sustaining growth is discussed.     

EFFECT OF CURRENT VELOCITY ON THE DETACHMENT OF THALLI OF ULVA LACTUCA (CHLOROPHYTA) IN A NEW ZEALAND ESTUARY

Experiments were undertaken in a recirculating flume to determine the relationships among water velocity, thallus area, drag, and the probability of thallus breakage or detachment in the foliose green alga Ulva lactuca L. In all specimens tested to breaking point, thalli detached from their bivalve substrates as a result of stipe breakage rather than in midthallus or by holdfast detachment. There was no relationship between thallus size and drag at which detachment occurred. Rather, the probability of detachment was normally distributed about a mean drag of 0. 70 N (95% confidence limits 0.55–0.85 N). Average breaking stress of stipes was 345 kN.m^-2 (95% cl 250–485 kN.m^-2). Similar results were obtained in field experiments where the horizontal force required to detach thalli was measured directly as 0.93 N (95% cl 0.69–1.15 N). Drag coefficients of plants were not constant with water velocity but increased up to 0.4 m.s^-1, declining exponentially at velocities above this. Empirical relationships were established between coefficient of drag and Reynold's number and, hence, among drag, thallus area and water velocity. These relationships permitted estimation of mean water velocity at which plants of a given area would detach .     

ANALYSIS OF EXPRESSED SEQUENCE TAGS FROM THE GREEN ALGA ULVA LINZA (CHLOROPHYTA)1

There is a general lack of genomic information available for chlorophyte seaweed genera such as Ulva, and in particular there is no information concerning the genes that contribute to adhesion and cell wall biosynthesis for this organism. Partial sequencing of cDNA libraries to generate expressed sequence tags (ESTs) is an effective means of gene discovery and characterization of expression patterns. In this study, a cDNA library was created from sporulating tissue of Ulva linza L. Initially, 650 ESTs were randomly selected from a cDNA library and sequenced from their 5′ ends to obtain an indication of the level of redundancy of the library (21%). The library was normalized to enrich for rarer sequences, and a further 1920 ESTs were sequenced. These sequences were subjected to contig assembly that resulted in a unigene set of approximately 1104 ESTs. Forty‐eight percent of these sequences exhibited significant similarity to sequences in the databases. Phylogenetic comparisons are made between selected sequences with similarity in the databases to proteins involved in aspects of extracellular matrix/cell wall assembly and adhesion.     

INHIBITION OF GROWTH OF ULVA EXPANSA (CHLOROPHYTA) BY ULTRAVIOLET-B RADIATION1

We examined the effect of ultraviolet-B radiation (UV-B, 290–320 nm) on the growth rate of the intertidal marine alga Ulva expansa (Setch.) S. & G. (Chlorophyta). Segments of thallus collected from a natural population were grown in outdoor seawater tanks. Combinations of UV-B-opaque screens, UV-B-transparent screens, and UV-B lamps were used to investigate the effects of solar UV-B and solar plus supplemental UV-B on the growth of these segments. Growth was measured by changes in segment surface area, damp weight, and dry weight. Growth rates of segments were inhibited under both solar UV-B and solar plus supplemental UV-B treatments. Growth rates were also inhibited by high levels of photosynthetically active radiation, independent of UV-B fluence. These results indicate that increases in UV-B resulting from further ozone depletion will have a negative impact on the growth of this alga.     

PHOTOSYNTHETIC INORGANIC CARBON ACQUISITION IN AN ACID‐TOLERANT,FREE‐LIVING SPECIES OF COCCOMYXA (CHLOROPHYTA)1

The processes of CO₂ acquisition were characterized for the acid‐tolerant, free‐living chlorophyte alga, CPCC 508. rDNA data indicate an affiliation to the genus Coccomyxa, but distinct from other known members of the genus. The alga grows over a wide range of pH from 3.0 to 9.0. External carbonic anhydrase (CA) was detected in cells grown above pH 5, with the activity increasing marginally from pH 7 to 9, but most of the CA activity was internal. The capacity for HCO₃^? uptake of cells treated with the CA inhibitor acetazolamide (AZA), was investigated by comparing the calculated rate of uncatalyzed CO₂ formation with the rate of photosynthesis. Active bicarbonate transport occurred in cells grown in media above pH 7.0. Monitoring CO₂ uptake and O₂ evolution by membrane‐inlet mass spectrometry demonstrated that air‐grown cells reduced the CO₂ concentration in the medium to an equilibrium concentration of 15 μM, but AZA‐treated cells caused a drop in extracellular CO₂ concentration to a compensation concentration of 27 μM at pH 8.0. CO₂‐pulsing experiments with cells in the light indicated that the cells do not actively take up CO₂. An internal pool of unfixed inorganic carbon was not detected at the CO₂ compensation concentration, probably because of the lack of active CO₂ uptake, but was detectable at times before compensation point was reached. These results indicate that this free‐living Coccomyxa possesses a CO₂‐concentrating mechanism (CCM) due to an active bicarbonate‐uptake system, unlike the Coccomyxa sp. occurring in symbiotic association with lichens.     

PHOTOACCLIMATION AND GROWTH RATE RESPONSES OF ULVA ROTUNDATA (CHLOROPHYTA) TO INTRADAY VARIATIONS IN GROWTH IRRADIANCE1

A vegetative clone of the chlorophyte macroalga Ulva rotundata was maintained in an outdoor continuous flow system under nutrient sufficient conditions and various light regimes. Step changes between 9 and 100% incident irradiance (I^o) were employed to simulate cloud passage. Temporary (1–4 h) midday (I^o)perturbations evoked net changes in growth rate (μ) and chlorophyll (chl) content. Under I_o alternating at various periodicities from 15 min to 7 h, net μ was the average of the μ under steady state 9 and 100% I^o, regardless of periodicity. However, the μ in alternating light was considerably less than μ under steady state 55% I^o(? 9%+ 100%/2), as expected based on the nonlinear shape of the μ-I relationship. Unlike μ, chl content depended primarily on the total daily irradiance, probably clue to the slower response of chl compared to photosynthetic rate. On time scales ≥ one day, chl was linearly correlated with light-regulated daily μ under both steady state and intraday fluctuating irradiance, consistent with photosynthetic feedback regulation of chl concentration.     

INORGANIC CARBON LIMITATION OF PHOTOSYNTHESIS IN ULVA ROTUNDATA (CHLOROPHYTA)1

A computerized oxygen electrode Astern was used to make rapid and accurate measurements of photosynthetic light and dissolved inorganic carbon (DIC) response cures with a macroalga. Ulva rotundata Blid. was grown in an outdoor, continuous flow system in seawater under sunlight or 9% of sunlight at Beaufort, North Carolina. The light compensation points in the shade- and sun-grown plants, measured in seawater, were at photon flux densities (PFDs) of 16 and 27 μmol. Photons·m^?2·s^?1, respectively but the quantum yield of O₂ evolution was not significantly different. Rates of photosynthesis in seawater per unit area of thallus under saturating light and rates of dark respiration were about 1.5-fold higher in sun- than in shade-grown plants. The concentration of DIC in seawater (approximately 2 mM) limited photosynthesis at absorbed PFDs above 60–70 μmol photons·m^?2·s^?1 Addition of 20 mM inorganic carbon had no effect on quantum yield but caused about a 1.5-fold increase in the light-saturated photosynthetic rate in both shade- and sun-grown Ulva. The effect of DIC supplementation was greatest in plants grown in October and least in plants grown in June. The light- and DIC-saturated rate of photosynthesis in seawater was similar to the maximum rate obtained by exposing Ulva to 10% CO₂, in the gas phase. The carbon isotope values (δ¹³C, reflecting the ¹³C/¹²C ratio compared to a standard) of Ulva grown in the same seawater supply were dependent on light and agitation. Samples from Beaufort Inlet were more negative (δ¹³C value, ?20.03‰) than those grown in bright light with agitation (δ¹³C value, ?17.78‰ outdoors; ?17.23‰ indoors), which may indicate DIC supply limited carbon uptake in seawater.     

UNCOUPLING OF VARIOUS MEASURES OF GROWTH IN ULVA ROTUXDATA(CHLOROPHYTA) FOLLOWING A LARGE DECREASE IN IRRADIANCE1

A vegetative clone of the chlorophyte macroalga Ulva rotundata Blid. was maintained in an outdoor continuous flow system and subjected to a large decrease in irradiance. Specific growth rates based on changes in fresh (μ_FW) and dry weight (μ_DW) and surface area (μ_SA) were determined using precut disks over the 24 h following a post-sunset transfer from full sunlight (100% I₀) to 9% I₀ All three measures of growth rate were approximately equivalent for untransferred control plants at either limiting (9%) or saturating (100%)I₀. Transferred disks exhibited μ_FW and μ_SA which were slightly lower than 100%I₀ controls and much higher than 9% I₀ controls; μ_DW was nearly identical for transferred disks and 9% I₀ controls. Cell size was unchanged following transfer, indicating that surface area changes reflected a proportional increase in cell number. Cell division therefore continued at a high rate for one day following transfer of U. rotundata to irradiances which are subsaturating for photosynthesis (indicated by μ_Dw). Starch reserves were largely depleted, and the C/N ratio decreased during this period.     

GAMETOGENESIS AND GAMETE RELEASE OF ULVA MUTABILIS AND ULVA LACTUCA (CHLOROPHYTA): REGULATORY EFFECTS AND CHEMICAL CHARACTERIZATION OF THE “SWARMING INHIBITOR”1

Gametophytes of Ulva mutabilis Føyn and Ulva lactuca L. were artificially induced to form gametangia by removal of sporulation inhibitors. After this treatment, U. mutabilis gametes were ready for swarming on the third morning after induction, while U. lactuca gametangia needed 1–2 d longer for maturation. Release of gametes of U. lactuca was dependent solely upon exposure to the first light in the morning. Gametangia of U. mutabilis, however, also required sufficient dilution of the swarming inhibitor (SWI). SWI was excreted transiently by both Ulva species early during gametogenesis. While the SWI concentration in U. mutabilis medium remained above the inhibitory concentration until the gametangia were mature, the concentration of U. lactuca‐SWI dropped rapidly below this level. In the presence of sufficient SWI, mature gametes of U. mutabilis remained motionless within the gametangia despite light and open exit pores. However, using SEM, an additional seal was detected within these pores, which probably prevented premature swarming until dilution of SWI and exposure to light. Observations by time lapse microscopy and experiments with the myosin kinase inhibitor BDM suggest that the gametes may be either extruded by the gametangium or leave the exit pore by active gliding motion, driven by a myosin‐like motor protein. The SWIs were purified from both Ulva species, and mass spectral analysis showed their molecular masses (292 Da) were identical.     

PHOTOSYNTHETIC UTILIZATION OF INORGANIC CARBON IN THE ECONOMIC BROWN ALGA,HIZIKIA FUSIFORME (SARGASSACEAE) FROM THE SOUTH CHINA SEA1

The mechanism of inorganic carbon (C_i) acquisition by the economic brown macroalga, Hizikia fusiforme (Harv.) Okamura (Sargassaceae), was investigated to characterize its photosynthetic physiology. Both intracellular and extracellular carbonic anhydrase (CA) were detected, with the external CA activity accounting for about 5% of the total. Hizikia fusiforme showed higher rates of photosynthetic oxygen evolution at alkaline pH than those theoretically derived from the rates of uncatalyzed CO₂ production from bicarbonate and exhibited a high pH compensation point (pH 9.66). The external CA inhibitor, acetazolamide, significantly depressed the photosynthetic oxygen evolution, whereas the anion‐exchanger inhibitor 4,4′‐diisothiocyano‐stilbene‐2,2′‐disulfonate had no inhibitory effect on it, implying the alga was capable of using HCO₃^? as a source of C_i for its photosynthesis via the mediation of the external CA. CO₂ concentrations in the culture media affected its photosynthetic properties. A high level of CO₂ (10,000 ppmv) resulted in a decrease in the external CA activity; however, a low CO₂ level (20 ppmv) led to no changes in the external CA activity but raised the intracellular CA activity. Parallel to the reduction in the external CA activity at the high CO₂ was a reduction in the photosynthetic CO₂ affinity. Decreased activity of the external CA in the high CO₂ grown samples led to reduced sensitiveness of photosynthesis to the addition of acetazolamide at alkaline pH. It was clearly indicated that H. fusiforme, which showed CO₂‐limited photosynthesis with the half‐saturating concentration of C_i exceeding that of seawater, did not operate active HCO₃^? uptake but used it via the extracellular CA for its photosynthetic carbon fixation.     

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.     

DIFFERENTIATION OF ULVA MUTABILIS (CHLOROPHYTA) GAMETANGIA AND GAMETE RELEASE ARE CONTROLLED BY EXTRACELLULAR INHIBITORS1

Blade cells of Ulva mutabilis Føyn (Chlorophyta) excrete regulatory factors into their cell walls and into the environment. These factors are essential for the maintenance of the vegetative state. “Sporulation inhibitor-1a” (SI-1a) is a glycoprotein that was isolated from the culture medium of axenic Ulva growing as an undifferentiated callus. This protein was unusually stable to denaturing treatments and showed an extremely high apparent molecular mass (M_r) of 1–4 × 10⁷ daltons estimated by size exclusion chromatography. The glycosylation was not essential for activity. SI-1a suppressed gametogenesis completely at concentrations lower than 10^?14 M. When Ulva developed normally in the presence of their symbiotic bacteria, smaller forms of SI-1 accumulated in the medium (10⁴–10⁶ daltons). Sporulation inhibitors of the same size spectrum and with similar properties were also extracted from crude cell walls of nonaxenic Ulva. A class of different nonprotein sporulation inhibitors (SI-2) of very low M_r and yet unknown structure was isolated from the inner space between the two blade cell layers. Excretion of all SI-1 forms decreased with maturation of the thallus, whereas the overall concentration of SI-2 in the thallus stayed constant throughout the life cycle. The SI-2 affected different Ulva species whereas SI-1 was species-specific. Gametogenesis was induced upon removal of both Sporulation inhibitors from small single-layered fragments of mature blades. After a “determination phase” of 23–46 h, dependent on the time of induction within the cell cycle, the cells became irreversibly committed to differentiation and were no longer susceptible to SI-1 or SI-2. Subsequently, during a 28-h “differentiation phase,” 16 progametes were formed by synchronous genome doublings and cell divisions and differentiated into mature gametes. These became motile and were released from the gametangia when the concentration of a “swarming inhibitor” of low M_r, excreted mainly during the “determination phase,” declined below a threshold concentration. The biochemical properties of these regulatory factors and their effects on gametogenesis and gamete release are described.     

SEASONAL VARIABILITY OF PHYSICOCHEMICAL AND RHEOLOGICAL PROPERTIES OF ULVAN IN TWO ULVA SPECIES (CHLOROPHYTA) FROM THE BRITTANY COAST1

The seasonal variability in the extraction yield, physicochemical characteristics, and rheological properties of ulvan from two Ulva species contributing to Brittany “green tides” has been studied. These seaweeds were collected in the water column for Ulva armoricana Dion, de Reviers et Coat and on hard substrata for Ulva rotundata Bliding. The maximum ulvan extraction efficiency was not related to the maximum ulvan content in the seaweeds, but with the active growth period of the seaweeds. Ulvan chemical structure, macromolecular characteristics, and rheological properties were affected by both species and seasons. The proportion of high‐molecular‐weight ulvan was the major factor positively correlated with the gelling properties. Characteristics of ulvan from U. rotundata subjected to tides were more affected by seasons than ulvan from U. armoricana living in a more constant environment. These results point to several useful recommendations concerning Ulva sp. biomass collected with regard to ulvan characteristics and uses.     

THE PERFORMANCE OF SPRAY‐IRRIGATED ULVA LACTUCA (ULVOPHYCEAE,CHLOROPHYTA) AS A CROP AND AS A BIOFILTER OF FISHPOND EFFLUENTS1

The seaweed Ulva lactuca L. was spray cultured by mariculture effluents in a mattress‐like layer, held in air on slanted boards by plastic netting. Air‐agitated seaweed suspension tanks were the reference. Growth rate, yield, and ammonia‐N removal rate were 11.8% · d^?1, 171 g fresh weight (fwt) · m^?2 · d^?1, and 5 g N · m^?2 · d^?1, respectively, by the spray‐cultured U. lactuca, and 16.9% · d^?1, 283 g fwt · m^?2 · d^?1, and 7 g N · m^?2 · d^?1, respectively, by the tank U. lactuca. Biomass protein content was similar in both treatments. Dissolved oxygen in the fishpond effluent water was raised by >3 mg · L^?1 and pH by up to half a unit, upon passage through both culture systems. The data suggest that spray‐irrigation culture of U. lactuca in this simple green‐mattress‐like system supplies the seaweed all it needs to grow and biofilter at rates close to those in standard air‐agitated tank culture.     

TRANSIENT AMMONIUM UPTAKE IN THE MACROALGA ULVA LACTUCA (CHLOROPHYTA): NATURE,REGULATION, AND THE CONSEQUENCES FOR CHOICE OF MEASURING TECHNIQUE1

The nature of transient ammonium uptake by the macroalga Ulva lactuca L. was studied from the depletion of ammonium after single additions of ammonium to batch cultures. The experiments were carried out by the application of two different experimental setups: the “multiple flask” and the “perturbation” techniques. Uptake rate was nonlinear with time, and three distinct, succeeding phases of uptake were identified: 1) “surge” uptake, i.e. transiently enhanced uptake that lasted for a few hours only, followed by 2) “internally” controlled uptake, a relatively constant phase occurring at high substrate concentrations, and finally 3) the “externally” controlled uptake phase, which was substrate-dependent and occurred at low substrate concentrations. Surge uptake occurred over a broad range of substrate concentrations but was concentration-dependent and, so, equalled externally controlled uptake rates at substrate concentrations below 3–10 μM. The transient nature of ammonium uptake rate seemed related to rapid changes in small intracellular pools of inorganic nitrogen or amino acids rather than to changes in total N content of the algae. The transient nature of ammonium uptake has important implications for the measurement of uptake rates when either of the two standard methods, the multiple flask and the perturbation technique, are used, and I recommend that a combination of the two methods be used for future uptake experiments.     

EFFECT OF NITRATE CONCENTRATION ON THE RELATIONSHIP BETWEEN PHOTOSYNTHETIC OXYGEN EVOLUTION AND ELECTRON TRANSPORT RATE IN ULVA RIGIDA (CHLOROPHYTA)1

The electron transport rate (ETR) versus gross photosynthesis (GPS) relationship varies as a function of species, temperature, irradiance, and inorganic carbon levels, but less is known about the effect of nitrogen supply on this relationship. The objective of this study was to evaluate the effect of nitrate concentration on the ETR versus GPS relationship in Ulva rigida C. Agardh from the Mediterranean Sea. Chlorophyll content and tissue absorptance increased 2‐fold as nitrate in the media increased from 0 to 50 μM. Whereas internal N content increases 3‐fold at 50 μM, internal C increased slightly. Oxygen evolution and ETR, evaluated as in vivo chl fluorescence using pulse amplitude modulated fluorometry, in general saturated at irradiances above 100 μmol photons·m^?2·s^?1. Both maximum ETR and GPS values increased as nitrate concentration increased. In general, the ETR versus GPS relationship showed a linear response to increasing nitrate with little variance of the data. This relationship, however, became more variable at high irradiances and high nitrate concentrations. The ETR/GPS ratio was close to the theoretical value of 4 at low nitrate concentrations, and the ratio decreased exponentially when nitrate concentration in the media increased. The variations of ETR/GPS under different inorganic nitrogen supply are discussed in terms of the effect of nitrate on the photosynthesis and respiration relationship.