Photosynthetic electron transport in Dunaliella tertiolecta (Chlorophyceae) measured by fast repetition rate fluorometry: relation to carbon assimilation

A comparison of photosynthesis-irradiance response curves (P–Eresponse curves) obtained through fast repetition rate (FRR)fluorometry and radiocarbon (¹⁴C) tracer method was made inthe chlorophyte, Dunaliella tertiolecta, grown under differentirradiance conditions. In FRR-based P–E response curveexperiments, actinic light provided by white light-emittingdiodes (LEDs) was increased gradually from 0 to 1500 µmolquanta m^–2 s^–1 and the rate of photosyntheticelectron transport was determined at each light level. Short-termexperiments (20 min) of ¹⁴C-based P–E response curvewere carried out with an improved photosynthetron, which containswhite LEDs as the light source. Irrespective of growth irradiance,the ratios of FRR to ¹⁴C-based initial slopes were almost uniform.The ratios of FRR- to ¹⁴C-based maximum rates were 25–36%higher than those of FRR- to ¹⁴C-based initial slopes. The relationshipbetween electron transport and carbon assimilation was non-linearwith increasing discrepancy towards high actinic light. Thisnon-linear relationship between FRR- and ¹⁴C-based estimatesis primarily due to the effect of physiological processes stimulatedat high levels of light, such as cyclic electron flow and theMehler reaction. The results of this study indicate that theFRR fluorometry can be used as a good indicator of photosyntheticrates from low to middle light levels, but becomes increasinglyquestionable as the maximum photosynthetic rate is approached.The degree to which this relationship is further affected bynutrient-status warrants investigation.     

On the compatibility of carbon uptake rates calculated from stable and radioactive isotope data: implications for the design of experimental protocols in aquatic primary productivity

Basic differences between the ¹³C and the ¹⁴C techniques ofmeasuring carbon uptake by phytoplankton exist at the levelof the isotopic analysis. With the first method, a ratio ofisotopic abundances is measured on the sample, whereas an absoluteamount of isotope is estimated with the second method. If acarbon source other than the labeled one is utilized by thephytoplankton during incubation, the stable isotope method maybe biased. Specific uptake values will be underestimated bythis effect. It is, however, possible to obtain unbiased estimatesof the ¹³C-labeled carbon uptake by using an equation containinga term describing the final particulate carbon concentration.Only under this condition will carbon uptake rates derived from¹³C isotope data be always compatible with the ¹³C method ofmeasuring primary production ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP), Second International Workshop heldat the National Oceanographic Institute, Haifa, Israel in April–May1984.     

Photosynthetic oxygen production and community respiration in the Indian sector of the Antarctic Ocean during austral summer

Photosynthetic oxygen production by phytoplankton and community respiration in the Indian sector of the Antarctic Ocean were estimated from changes in oxygen concentrations in light and dark bottles. Gross production varied between 0.1 and 5.1 µmol O₂ l^-1 day^-1. In the same water, community respiration (the sum of oxygen consumption by heterotrophs and phytoplankton) was 0.4-3.6 µmol O₂ l^-1 day^-1, which accounted for 47-343% of the gross production. Algal and heterotrophic respirations were distinguished using some assumptions. These estimates showed that heterotrophic respiration accounted for most of the community respiration (70-91% depending upon the assumptions), indicating that heterotrophic respiration plays an important role in the mineralization of phytoplankton production in the surveyed sea area. Gross production rate correlated with chlorophyll a concentration, showing that the photosynthetic production rate of oxygen depends on the abundance of phytoplankton. Moreover, there was a significant relationship between gross production and community respiration rates. These regression equations suggested that negative net production occurred under the usually low concentration of chlorophyll observed in the Indian sector of the Antarctic Ocean. Hence, the net exchange of carbon dioxide due to biological processes through the sea surface seemed to be not as large as expected in the Antarctic Ocean, although the number of data were limited at this stage.     

Bioenergetics of Carbon Assimilation in Intact Chloroplasts: Coupling of Proton to Electron Transport at the Ratio H+/e=3 Is Incompatible with H+/ATP=3 in ATP Synthesis

During a transition from aerobic to largely anaerobic conditionslight-saturated carbon assimilation of intact chloroplasts wasnot decreased although both the transthylakoid proton gradientand ATP levels declined. After a dark period under anaerobiosis,illumination failed to initiate carbon assimilation. ATP increasedonly transiently in the light and then returned to the darklevel. Under such conditions, the addition of electron acceptorssuch as oxygen, oxalacetate or nitrite resulted in the increaseof ATP levels and carbon assimilation was initiated. Assimilationcontinued under anaerobiosis in the presence of reduced protongradients and reduced ATP levels after electron acceptors addedin addition to bicarbonate were reduced. Cyclic electron transport was inhibited when anaerobiosis didnot permit linear electron transport. It was induced in thissituation by micromolar concentrations of oxygen or when, underanaerobiosis, DCMU decreased PSII activity. Oxygen inhibitedcyclic electron transport by draining electrons from the cyclicpathway only when electron donation from PSII was weak. Theobservations give evidence of the delicate redox balance requiredfor cyclic electron transport. Since H⁺/e=3 in linear electron transport, the observationsof effective carbon reduction under a decreased transthylakoidproton gradient and decreased levels of ATP are incompatiblewith H⁺/ATP=2 or 3. They are compatible with H⁺/ATP=4. (Received May 1, 1995; Accepted October 3, 1995)     

Comparison of radioactive and stable isotope tracer techniques for measuring photosynthesis: 13C and 14C uptake by marine phytoplankton

The stable carbon isotope ¹³C has been used in the open oceanto estimate the inorganic carbon uptake by phytoplankton andthis technique has been compared with the ¹³C tracer method.An overall correlation coefficient of 0.806 and a regressionslope of 1.29 were calculated from 50 sample pairs gatheredduring three cruises in widely different oceanic areas rangingin production rates from 0.01 to 6 mgC m^–3 h^–1.However, significant differences between the two methods wereapparent for cruises located in nutrient-depleted areas. Possibleexplanations lie either in a volume effect, the high silicatecontent of the ¹⁴C solution which could stimulate the ¹⁴C uptakeor in errors associated with the particulate carbon measurementswhich are necessary to convert specific uptake rates to absoluteuptake rates and to yield compatible units for the comparison,in laboratory cultures the ¹⁴C technique overestimated the netparticulate carbon increase by — 16%. ⁺Present address: Laboratoire marin CNRS-IFREMER, L'Houmeau,17137 Nieul-sur-Mer, France. ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP) First International Workshop heldat the Limnological Institute, University of Konstanz, in April1982.     

Zooplankton grazing effects on 14C-based phytoplankton production measurements: a theoretical study

A model constructed to describe carbon dynamics of phytoplanktongrowth during ¹⁴C-tracer incubations gives the range of errorin estimating specific growth rates and productivity rates causedby zooplankton grazing during the incubation. Error increaseswith increasing incubation times and higher specific growthrates. The range of these errors can be given as a functionof the specific growth rate calculated from measurements. Atthe low calculated specific growth rates of the oligotrophicPacific Ocean, 0.2 d^–1, errors are 16%. Similar argumentssuggest that bacterial uptake of excreted organics would notcause large errors at low oligotrophic ocean growth rates. Thereare, however, other possible ways that ¹⁴C-based productivityestimates could be wrong.     

Diurnal variations in the contributions of autotrophic picoalgae and heterotrophic bacteria to planktonic production in an upland lake

An investigation of the diurnal variation in contributions toproduction of the autotrophic and heterotrophic components ofthe picoplankton community was carried out during August andSeptember in Llyn Padarn, a mesotrophic upland lake in NorthWales. The picoplankton was separated using 1 µm pore-sizedfilters into the smaller cell sized fraction (<1 µm),the majority of the bacteria and the larger cell sized picoalgae(<3>1 µm), together with some bacteria. The distributionof bacterial heterotrophic activity between these two fractionsof picoplankton was assessed by uptake of [¹⁴C]glucose and differentialfiltration. Thus, the absolute autotrophic production by picoalgaeand the heterotrophic contribution by bacteria to picoplanktoncommunity production via uptake of extracellular organic carbon(EOC) were determined. Rates of picoplankton community productionexhibited diurnal variation with maximum rates of 19.1 mg Cm^–3 h^–1 recorded at 18.00 h at 4 m depth in September.The bacterial contribution to picoplankton community productionincreased markedly between 15.00 and 18.00 h. Rates of absoluteautotrophic production varied less over 24 h than rates of accumulationin bacteria of ¹⁴C-labelled EOC released from the entire phytoplanktoncommunity. Bacteria contributed up to 86–98% of the neworganic carbon within the picoplankton community at the endof the day. The maximum rate of absolute autotrophic productionin the picoplankton was 1.6 mg C m^–3 h^–1 at 18.00h at 1 m in August, and the maximum rate of bacterial accumulationof new organic carbon was 18.5 mg C m^–3 h^–1 at 18.00h in September at 4 m depth. The diurnal pattern of picoplanktoncommunity production involved increasing rates during the daywith a maximum at 18.00 h. Autotrophic processes were dominantin the first 3–6 h of the light cycle and heterotrophicuptake of ¹⁴C-labelled EOC was the major component from 15.00h onwards. Bacterial uptake of newly released EOC by phytoplanktonwas rapid, comprised the majority of picoplankton production,particularly later in the day, and contributed a maximum of60% of the total pariculate primary production in plankton between15.00 and 18.00 h at 4 m in September with a mean contributionof between 6 and 26% over 24 h in these investigations. Theimportance of autotrophic processes in picoplankton communityproduction has been overestimated in previous investigations.Bacteria play a major role in transferring newly produced EOCrapidly from phytoplankton to the picoplankton community. Atthe end of the day, the majority of newly produced organic carbonis in bacterial cells and this production is significant inthe dynamics of carbon production within the entire planktoniccommunity.     

Photoinhibition and light-induced cyclic electron transport in ndhB(-) and psaE(-) mutants of Synechocystis sp. PCC 6803

The ndhB^– and psaE^– mutants of the cyanobacteriumSynechocystis sp. PCC 6803 are partly deficient in PSI-drivencyclic electron transport. We compared photoinhibition in thesemutants to the wild type to test the hypothesis that PSI cyclicelectron transport protects against photoinhibition. Photoinhibitorytreatment greatly accelerated PSI cyclic electron transportin the wild type and also in both the mutants. The psaE^–mutant showed rates of PSI cyclic electron transport similarto the wild type under all conditions tested. The ndhB^–mutant showed much lower rates of PSI cyclic electron transportthan the wild type following brief dark adaptation but exceededwild type rates after exposure to photoinhibitory light. Thewild type and both mutants showed similar rates of photoinhibitiondamage and photoinhibition repair at PSII. Photoinhibition atPSI was much slower than at PSII and was also similar betweenthe wild type and both mutants, despite the known instabilityof PSI in the psaE^– mutant. We conclude that photoinhibitorylight induces sufficient PSI-driven cyclic electron transportin both the ndhB^– and psaE^– mutants to fulfill anyrole that cyclic electron transport plays in protection againstphotoinhibition. ⁴ Corresponding author: E-mail, sherbert@uwyo.edu; Fax, +1-307-766-2851;Phone, +1-307-766-4353.     

Nitrogen uptake by heterotrophic bacteria and phytoplankton in Arctic surface waters

We estimated rates of heterotrophic bacterial and phytoplanktonuptake of nitrate, ammonium, and urea using ¹⁵N-labelled nitrogenand specific metabolic inhibitors of prokaryote and eukaryotenitrogen metabolism in the surface waters of the North Water(northern Baffin Bay) during autumn that were characterizedby the absence of cyanobacteria (comprising prochlorophytes).The percentage of nitrate + ammonium uptake by heterotrophicbacteria ranged between 44 and 78% of the measured total uptakeand was the highest when the phytoplankton biomass was relativelylow (<2 µg Chlorophyll a L^–1). Phytoplanktonaccounted for a larger fraction (e.g., 58–95%) of ureauptake than heterotrophic bacteria. When our results are combinedwith those from previous studies carried out in diverse temperateand polar areas, it appears that heterotrophic bacteria accountfor 25% (14–40%; median and interquartile range) of thetotal nitrate uptake in surface waters with chlorophyll biomass<2 µg L^–1. Estimates of new production computedfrom phytoplankton carbon uptake and f-ratios may be stronglyoverestimated in regions where nitrate uptake by heterotrophicbacteria is high and the biomass of phytoplankton is low.     

Estimation of Primary Productivity by Chlorophyll a in vivo Fluorescence in Freshwater Phytoplankton

Primary productivity in marine waters is widely estimated by the measurements of ¹⁴C incorporation, the underwater light climate, and the absorption spectra of phytoplankton. In bio-optical models the quantum efficiency of carbon fixation derived from ¹⁴C incorporation rates, the photosynthetically absorbed radiation derived from the underwater light climate, and the phytoplankton absorption spectra are used to calculate time- and depth-integrated primary productivity. Due to the increased sensitivity of commercially available fluorometers, chlorophyll a in vivo fluorescence became a new tool to assess the photosynthetic activity of phytoplankton. Since fluorescence data yield only relative photosynthetic electron transport rates, a direct conversion into absolute carbon fixation rates is not possible. Here, we report a procedure how this problem can be adressed in freshwater phytoplankton. We adapted a marine bio-optical model to the freshwater situation and tested if this model yields realistic results when applied to a hypertrophic freshwater reservoir. Comparison of primary productivity derived from ¹⁴C incorporation to primary productivity derived from Chl a fluorescence showed that the conversion of fluorescence data into carbon fixation rates is still an unsolved problem. Absolute electron transport rates calculated from fluorescence data tend to overestimate primary production. We propose that the observed differences are caused mainly by neglecting the package effect of pigments in phytoplankton cells and by non-carbon related electron flow (e.g., nitrogen fixation). On the other hand, the ¹⁴C incorporation rates can be artificially influenced by "bottle effects", especially near the water surface, where photoinhibition, photorespiration, and Mehler reaction can play a major role.     

Microplanktonic regeneration of ammonium and dissolved organic nitrogen in the upwelling area of the NW of Spain: relationships with dissolved organic carbon production and phytoplankton size-structure

Ammonium regeneration and dissolved organic nitrogen (DON) releasewere studied experimentally in the euphotic zone of shelf andoceanic waters of NW Spain in relation to coastal upwellingdynamics and the size-structure of phytoplankton communities.Incubations of plankton labelled with [¹⁵N]ammonium were madeduring four cruises, two of which also included size-fractionateddeterminations of chlorophyll a and primary production, andexperimental determinations of production rates of dissolvedorganic carbon (DOC) using ¹⁴C. Inorganic nitrogen concentrationswere mainly related to nitrate enrichment by upwelling pulses,while ammonium concentrations were generally low in all situations.Ammonium did not accumulate in the study area, suggesting adaily time scale coupling between regeneration and uptake. Incontrast, DON largely exceeded inorganic nitrogen in all situationsand generally increased from spring to autumn. Ammonium regenerationwas positively correlated with DON release and both rates showedthe largest variation in summer, with minimum values duringactive upwelling and maximum values when upwelling relaxed.Comparison of DON stocks and rates in different shelf areassuggests that DON release near the coast during summer was morepersistent in the water than DON release in off-shelf and oceanicareas. The carbon:nitrogen ratio of DOC and DON release rateswas highly variable, revealing a large excess of DOC comparedwith DON. This excess can be attributed to either an underestimateof total DON release (as only release from ammonium was measured)or to an enhanced production of carbon-rich organic substancesby diatoms in coastal areas. By considering a broad range oftrophic situations, this study reveals a fundamental differencebetween short term release of DOC and DON by plankton. Physiologicalprocesses (such as carbohydrate exudation by diatoms) seem tobe the cause of large DOC excess, whereas trophic processes(such as grazing) are more likely the cause of enhanced DONrelease.     

Photosynthesis and chlorophyll fluorescence in sunflower (Helianthus annuus L.) leaves as affected by phosphorus nutrition

The effects of phosphate concentration on plant growth and photosyntheticprocesses in primary leaves of young sunflower (Helianthus annuusL.) plants were examined. Plants were grown for 3 weeks on half-strengthHoagland's solution containing 0, 0.1, 0.5, 1.0, and 3.0 molm^–3 orthophosphate (Pi). It was shown that optimal photosynthesisand the highest light utilization capacity were achieved at0.5 mol m^–3 Pi in the growth medium, which was in goodagreement with the maximum content of organic phosphorus inthe leaves. Low phosphate in the medium inhibited plant growthrate. Phosphate deficiency appreciably decreased photosyntheticoxygen evolution by leaves, the efficiency of photosystem two(PSII) photochemistry and quantum efficiency of PSII electrontransport. High oxidation state of PSII primary electron acceptorQA, at 0.1 mol m^–3 Pi, however, indicates that photosyntheticelectron transport through PSII did not limit photosynthesisin Pi-deficient leaves. The results indicate that diminishedphotosynthesis under sub- and supra-optimal Pi was caused mainlyby a reduced efficiency of ribulose 1, 5-bisphosphate (RuBP)regeneration at high light intensities. These results suggestthat, under non-limiting C0₂ and irradiance, photosynthesisof the first pair of leaves could be diminished by both sub-and supra-optimal phosphorus nutrition of sunflower plants. Key words: Helianthus annuus L, phosphate nutrition, photosynthesis, photochemical efficiency     

Productivity, growth rates and cell size distributions of phytoplankton in the SW Tasman Sea: implications for carbon metabolism in the photic zone

Data are presented on primary productivity, cell size distributionsand the standing stocks of living and detrital paniculate organiccarbon (POC) in the waters of the SW Tasman Sea. Primary productivitywas measured by both standard 4- and 12-h incubations as wellas time-series incubations. Data are presented for ¹⁴C uptakeand loss in 12L/12D methods. The importance of time zero anddark controls is demonstrated. The uptake of ¹⁴C in the lightwas linear and the loss of label in the following dark periodranged from zero to 39%. The loss of label in the dark was correlatedwith the particle size distribution, being greatest in oligotrophicwaters dominated by small cells (25–30%) and least inspring bloom areas (0–20%) dominated by large diatoms.Kinetic data were strongly supportive of a major grazing impactby microphagous organisms. The data were an experimental confirmationof recent theoretical models of ¹⁴C uptake and grazing. Sizedistributions of phytoplankton and detritus were measured byHIAC and by microscopic counting. The phytoplankton consistedof a ubiquitous group of picoplankton, and variable contributionsfrom small flagellates and diatoms. The distribution of totalcell volume was dominated by large cells in spring bloom areas.Chlorophyll concentrations were strongly correlated with themean cell size of the phytoplankton. Comparison of the resultsof ¹⁴C uptake experiments with the results of experiments todetermine changes in POC, by counting particles, gave good correlation.In detail, the comparison of the methods revealed systematicerrors with the greatest discrepancy between the methods atlow apparent growth rates. The detritus showed constant sizedistributions in surface waters. The mean size of detritus particlesreduced rapidly with depth and declined in a way suggestingbiological reprocessing and removal by grazing. These resultsare discussed in the context of the patterns of carbon metabolismin the photic zone, the role of living and detrital POC andthe balance of ‘new’ versus ‘regenerated’production in surface waters.     

Characterization of Photosynthetic 14Carbon Assimilation by Potamogeton lucens L.

Photosynthetic assimilation of exogenous ¹⁴CO₂ and H¹⁴CO₃^–by the aquatic angiosperm Potamogeton lucens L. is reported.Equivalent maximum rates of assimilation (1.5 µmol s^–1m^–2) were obtained in the presence of saturating levelsof ¹⁴CO₂ (1.0 mol m^–3, pH 5.3) or H¹⁴CO₃^– (1.5 molm^–3, pH, 9.2). Under subsaturating ¹⁴CO₂ levels, bothgaseous diffusion and H¹⁴CO₃^– transport were shown tooperate simultaneously, such that maximal photosynthetic rateswere established. An induction lag of approximately 3 min was observed when exogenous¹⁴CO₂ was assimilated. A longer lag of approximately 12 minwas required, however, before linear assimilation rates wereestablished when H¹⁴CO₃ acted as the carbon source. The light-activatedH¹⁴CO₃^– transport system was found to be quite labile.A brief (5 min) dark treatment returned the system to the inactivestate. Bicarbonate transport was shown to be competitively inhibitedby CO₃^2–ions. The possibility is discussed that this formof inhibition may be common to many HCO₃^– assimilators. Preliminary polar cation transport studies (from lower to upperleaf surface) indicated an almost exact one to one relationshipbetween the rates of Na⁺ influx and efflux and H¹⁴CO₃^–assimilation. The possible relationship(s) between these transportprocesses and the requirement for electrical neutrality is brieflydiscussed.     

A COMPARISON OF PHOTOSYNTHETIC ELECTRON TRANSPORT RATES IN MACROALGAE MEASURED BY PULSE AMPLITUDE MODULATED CHLOROPHYLL FLUOROMETRY AND MASS SPECTROMETRY

The relationship between whole chain photosynthetic electron transport and PSII activity was investigated in Porphyra columbina (Montagne) (Rhodophyta), Ulva australis (Areschoug) (Chlorophyta), and Zonaria crenata ( J. Agardh) (Phaeophyta). Mass spectrometric measurements of gross O₂ evolution and gross O₂ uptake were combined with simultaneous measurement of pulse-modulated chl fluorescence under a range of irradiances and inorganic carbon (C_i) concentrations. At light-limiting irradiance, a good correlation between gross O₂ evolution and the electron transport rate (ETR) calculated from chl fluorescence ((F_m′− F_s)/F_m′) was found in the optically thin species (Ulva and Porphyra). The calculated ETR was equivalent to the theoretical electron requirement in these species but overestimated gross O₂ evolution in the thicker species Zonaria. In saturating light, especially when C_i availability was low, ETR overestimated gross O₂ evolution in all species. Excess electron flow could not be accounted for by an increase in gross O₂ uptake; thus neither Mehler-ascorbate-peroxidase reaction nor the photosynthetic carbon oxidation cycle were enhanced at high irradiance or low C _i. Alternative explanations for the loss of correlation include cyclic electron flow around PSII that may be engaged under these conditions or nonphotochemical energy quenching within PSII centers. The loss of correlation between ETR and linear photosynthetic electron flow as irradiance increased from limiting to saturating or at low C_i availability and in the case of optically thick thalli limits the application of this technique for measuring photosynthesis in macroalgae.     

Arundo donax L. (Poaceae) — a C3 Species with Unusually High Photosynthetic Capacity

Gas exchange characteristics, chlorophyll a fluorescence and leaf water potential were investigated in the giant reed, Arundo donax, under natural conditions in an estuarine mangrove swamp in Durban, South Africa. Maximum photosynthetic CO₂ uptake ranged between 19.8 and 36.7 μmol m^?2 s^?1, depending on irradiance, and appeared to be regulated by leaf conductance. There was no saturation of CO₂ uptake or electron transport through PSII (ETR) with increasing irradiance up to 2500 μmol photons m^?2 s^?1. A linear relationship between CO₂ uptake, corrected for respiration (A), and ETR has only been reported for C₄ species and C₃ species when photorespiration is eliminated. From this relationship, it was calculated that 8.5 electrons were transported through PSII for the fixation of one mole of CO₂. Predawn leaf water potential was about ?0.5 MPa and decreased to ?1.5 MPa on a cloudy day and to ?2.1 MPa on a clear day. Diurnal change in leaf water potential had little influence on leaf conductance and hence CO₂ uptake. The molar water use efficiency (WUE) ranged between 4.1 and 9.3 μmol mmol^?1. Percentage photorespiration was between 36 and 39%.     

Characteristics of Transport of L-Leucine and Glycine in Pea Protoplasts

The uptake of L-leucine and glycine into pea protoplasts wasstudied under various conditions. The uptake of both L-leucineand glycine was pH dependent with the optimal pH being 4.0 and5.0 for L-leucine and glycine, respectively. A kinetic studyof L-leucine uptake showed that uptake is multiphasic; K_m valuesof different phases were 1.1 mol m^–3, 33.3 mol m^–3and 100 mol m³. A similar analysis for glycine at a concentrationrange of 0.1–10 mol m^–3 also showed a multiphasictransport system for it. The uptake of L-leucine at lower concentrations(between0.1–2.0 mol m^–3) was energy dependent, since arsenate,azide, dinitrophenol and iodoacetate inhibited the uptake. However,the uptake of L-leucine was not inhibited by ouabain at anyconcentration of L-leucine employed. The uptake of glycine wasnot inhibited by any of these inhibitors suggesting that glycineuptake was not mediated by an active process. Key words: Pea protoplast, L-Leucine, Glycine transport, Active transport, Mediated transport     

Particulate and dissolved phytoplankton production of the Patos Lagoon estuary, southern Brazil: comparison of methods and influencing factors

Uptake rates of ¹⁴C (filtration and the acidification-bubblingmethod—ABM) were measured weekly in a shallow region ofthe Patos Lagoon estuary (3207'S, 5206'W) between March 1989and March 1990. Phytoplankton production varied seasonally,the lowest values occurring in the austral winter (June–August1989) and the highest rates during spring and summer (March1989; September 1989–March 1990). Particulate carbon productionvaried between 0.65 and 70.6 mg C m^–3 h^–1 and wasmostly associated with organisms <20 µm (mean = 73.4%).Dissolved organic carbon (DOC) released by phytoplankton variedbetween 0.1 and 89.3 mg C m^–3 h^–1 representing     

Carbon and nitrogen uptake response to light by phytoplankton during an upwelling event

The interactions between phytoplankton nutrition and the responseof carbon (C) and nitrogen (N) uptake to irradiance relationshipswere examined during September 1993 in Monterey Bay, California,an eastern boundary current upwelling regime. Measurements ofN uptake and C assimilation rates versus irradiance (V:I andP:I) experiments were performed using trace-level additionsof ¹⁵N-labeled NO₃^– and NH₄⁺, and ¹⁴C-labeled bicarbonateto water collected from a depth of {small tilde}30% of surfacephotosynthetic photon flux density (PPFD). An upwelled watermass was sampled consecutively, with hydrographic stations locatedat the upwelling site, 48 h later down the horizontal axis ofthe upwelling plume, and a final time (24 h later) with watersconsisting of a mixture of 5–6 day aged upwelled waterand warmer surface water from outside the plume. As the wateraged, a pro gressive shift in the rates of C and N utilizationoccurred, with C assimilation increasing while N uptake ratesdecreased. At the same time, NH₄⁺ dominated the nitrogenousnutrition in older upwelled water, even in the presence of highconcentrations of ambient NO₃^–. Dark-uptake rates forall substrates were uniformly low at all stations; NH₄⁺ uptakedemonstrated the least dependence on PPFD. The results of thisstudy demonstrate dramatic changes in the light-mediated responseof C and N uptake, resulting in assimilation ratios considerablydifferent from predicted values assuming phytoplankton C:N uptakerates will be proportional to Redfield C:N composition. Thesedata provide clear evidence of physiological changes in thenatural planktonic assemblage of this evolving upwelling ecosystem.     

ELEVATED CARBON DIOXIDE DIFFERENTIALLY ALTERS THE PHOTOPHYSIOLOGY OF THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE) AND EMILIANIA HUXLEYI (HAPTOPHYTA)1

Increasing anthropogenic carbon dioxide is causing changes to ocean chemistry, which will continue in a predictable manner. Dissolution of additional atmospheric carbon dioxide leads to increased concentrations of dissolved carbon dioxide and bicarbonate and decreased pH in ocean water. The concomitant effects on phytoplankton ecophysiology, leading potentially to changes in community structure, are now a focus of concern. Therefore, we grew the coccolithophore Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler and the diatom strains Thalassiosira pseudonana (Hust.) Hasle et Heimdal CCMP 1014 and T. pseudonana CCMP 1335 under low light in turbidostat photobioreactors bubbled with air containing 390 ppmv or 750 ppmv CO₂. Increased pCO₂ led to increased growth rates in all three strains. In addition, protein levels of RUBISCO increased in the coastal strains of both species, showing a larger capacity for CO₂ assimilation at 750 ppmv CO₂. With increased pCO₂, both T. pseudonana strains displayed an increased susceptibility to PSII photoinactivation and, to compensate, an augmented capacity for PSII repair. Consequently, the cost of maintaining PSII function for the diatoms increased at increased pCO₂. In E. huxleyi, PSII photoinactivation and the counter‐acting repair, while both intrinsically larger than in T. pseudonana, did not change between the current and high‐pCO₂ treatments. The content of the photosynthetic electron transport intermediary cytochrome b6/f complex increased significantly in the diatoms under elevated pCO₂, suggesting changes in electron transport function.