Photosynthetic carbon uptake by marine phytoplankton: comparison of the stable (13C) and radioactive (14C) isotope methods

The ¹⁴C and the ¹³C methods are routinely used for measuringphytoplankton production. However, few systematic comparisonsof estimates using the two approaches have been conducted. Thepresent comparison is based on 257 pairs of samples, representing1 year of monthly sampling at 10 depths in the euphotic zone.Data, obtained by the same operators following a standard protocol,were collected at three different stations on the Scotian Shelf(Northwestern Atlantic Ocean). Overall agreement between thetwo methods was good (r=0.827). However, relative differencesbetween the two sets of estimates were not randomly distributedin time and space. Three factors (photic depth, station andsampling date), identified to explain the observed differences,were included in a three-way analysis of variance (ANOVA) usingrelative differences as the covariate. Following this ANOVA,the whole set was split into three subsets. For the two subsetswhere the above identified factors did not have any significanteffect, the distribution of relative differences was narrowerthan these for the whole data set. Significant effects of thethree factors persisted for the third subset and relative differencesexhibited wide variations. Possible explanations for the observeddifferences include (i) the volume of incubation bottles, (ii)the incubation temperature and (iii) the absence of measurementsof dark uptake.     

Stable carbon isotope fractionation by marine phytoplankton during photosynthesis

Abstract. In the marine environment, the range of values of carbon isotope fractionation between particulate tissue of phytoplankton and inorganic carbon can be more than 20‰ (− 35‰ < δ¹³C < − 14‰). This review considers the influence of seawater temperature, lipid content of phytoplanktonic cells, kinetic fractionation, and carbon pathway on δ¹³C values observed at sea.
In order to study the contribution of carboxylases (RUBISCO and the β-carboxylases phosphoenolpyruvate carboxylase, phosphoenoplpyruvate carboxykinase and pyruvate carboxylase) to variations of particulate δ¹³C values at sea, we present results obtained simultenously on carboxylase activities and δ¹³C in various environmental conditions. The lowest δ¹³C values are clearly associated with predominance of ribulose-1.5-bisphosphate carboxylase activity, but it was more difficult to explain the high δ¹³C values. Different hypotheses are discussed.     

A stable isotope trace method to measure silicic acid uptake by marine phytoplankton.

A tracer method is described that uses the stable isotope ³⁰Si to measure rates of silicic acid uptake by diatom cultures and natural populations of marine phytoplankton. The method involves (i) incubation of organisms requiring silicic acid for growth in the presence of ³⁰Si-labeled silicic acid, (ii) collection of the resulting particulate silicon, (iii) conversion of the particulate silicon to BaSiF₆, (iv) determination of the ³⁰Si content of BaSiF₆ by solid sample mass spectrometry, and (v) calculation of the uptake rate from the ³⁰Si enrichment of the particulate matter during the incubation. The maximum overall error in the uptake rate measurement is ±10%.     

Base-line variations in stable isotope values in an Arctic marine ecosystem: effects of carbon and nitrogen uptake by phytoplankton

Floods are fundamental for the maintenance of floodplain biodiversity. As a result, well-functioning floodplains are characterized by a high spatio-temporal heterogeneity. Most floodplain-organisms need this shifting landscape mosaic to fulfil their environmental requirements and display a range of adaptations to survive floods. However, in temperate areas, where winter floods are common, extraordinary floods occurring in summer, a period of high physiological activity, may seriously impact the floodplain fauna. This is especially true for guilds characterized by low mobility, such as molluscs. Here we examined the immediate and longer-term response of Elbe grassland molluscs to the extreme 2002 Elbe summer flood in terms of abundance, diversity, and community composition by comparing pre- and post-flood data collected with identical methods. The flood favoured the colonization of aquatic species and led to a shift of the community towards a more hydrophilic composition. Both diversity and abundance increased significantly in the first year following the flood but decreased later gradually to the pre-flood levels. The high spatio-temporal habitat heterogeneity played an important part in the maintenance of mollusc diversity by increasing refuge opportunities and favouring the maintenance of various mollusc communities with different environmental requirements within the floodplain. Handling editor: S. I. Dodson     

Application of liposome and stable isotope tracer techniques to study polyunsaturated fatty acid biosynthesis in marine zooplankton

We investigated the ability of four species of marine zooplanktonto synthesize polyunsaturated fatty acids (PUFAs) during pulse-chasetracer experiments. Liposomes containing a deuterium labelledprecursor fatty acid, D₅-18:3n-3, were fed to female Calanusfinmarchicus, Calanoides acutus, Drepanopus forcipatus and calyptopuslarvae of Euphausia superba during ship-board experiments. Althoughall species of zooplankton readily ingested the liposomes andincorporated the D₅-18:3n-3 tracer into their somatic lipidpool, only negligible products of elongation and desaturation;D₅-18:4n-3, D₅-20:5n-3 and D₅-22:6n-3 were detected after 96h incubations. We conclude that the four species of marine zooplanktonexamined here are unable to synthesize PUFA at ecologicallysignificant rates and certainly not in amounts sufficient tosupport growth and reproductive processes.     

Water uptake by plants: perspectives from stable isotope composition

Stable isotope studies of hydrogen and oxygen stable isotope ratios of water within plants are providing new information on water sources, competitive interactions and water use patterns under natural conditions. Variation in the utilization of summer rain by aridland species and limited use of stream water by mature riparian trees are two examples of how stable isotope studies have modified our understanding of plant water relations. Analyses of xylem sap and tree rings have the potential of providing both short-term and long-term information on plant water use patterns.     

Using the stable isotope marker 13C to study extrafloral nectar uptake by parasitoids under controlled conditions and in the field

Parasitic wasps are prominent natural enemies of crop pests. They usually feed on floral resources during the adult stage (nectar, pollen, or honeydew). Extrafloral nectar is an alternative source of sugar easily accessible to adult parasitoids. We developed an original method of nectar labelling based on the injection of labelled sugar solution into the plant stem in order to analyse the nectar uptake by parasitoids (cotton wick method). This method was used to artificially enrich extrafloral cornflower, Centaurea cyanus L. (Asteraceae), nectar with the stable isotope ¹³C. We analysed (1) the transfer of ¹³C from the sugar solution into extrafloral nectaries, (2) the uptake of labelled nectar by parasitoids under laboratory conditions, and (3) the ability of the method to discriminate, in an oilseed rape (Brassica napus L., Brassicaceae) field, between labelled parasitoids (i.e., those who have fed on labelled cornflowers located adjacent to the field) and unlabelled parasitoids to track parasitoid movements from the margin into the field. The extrafloral nectar of all test plants was ¹³C‐labelled. Most (66%) of the parasitoids were identified as marked after 96 h of exposure to labelled plants in the laboratory. We could also detect labelled parasitoids inside the field, but the detection rate was only 1%. The experiments clearly demonstrate that the cotton wick method is appropriate to label extrafloral nectar and parasitoids feeding on this labelled nectar. Further research is needed on the amount of labelled extrafloral nectar required to obtain a sufficient marker level to track parasitoid movements in the field.     

Double isotope tracer method for measuring fractional zinc absorption: theoretical analysis

Several approaches for estimation of fractional zinc absorption (FZA) by calculating the ratio of oral to intravenous stable isotopic tracer concentrations (at an appropriate time) in urine or plasma after their simultaneous administration have been proposed in the last decade. These simple-to-implement approaches, often referred to as the double isotopic tracer ratio (DITR) method, are more attractive than the classical "deconvolution" method and the more commonly used single-tracer methods based on fecal monitoring and indicator dilution, after oral or intravenous tracer administration, respectively. However, the domain of validity of DITR for measuring FZA has recently been questioned. In this paper, we provide a theoretical justification of the validity of four different "approximate" formulations of the DITR technique by demonstrating mathematically that their accuracy is a consequence of the particular properties of zinc kinetics.     

Exponential growth rates of phytoplankton calculated from 14C uptake rates: a clarification

Phytoplankton growth rates can be estimated from measurementsof light-dependent [¹⁴C]bicarbonate uptake (photosynthesis)rates using the integrated (logarithmic) form of the exponentialgrowth law. A clanfication of this method is presented herein view of confusions in the recent literature.     

Growth-dependent stable carbon isotope fractionation by basidiomycete fungi: delta(13)C pattern and physiological process

We grew 11 basidiomycetes in axenic culture to characterize their physiological capacities to fractionate stable C isotopes. Generally, delta(13)C values of the fungal biomass were (i) enriched in (13)C relative to the growth medium, (ii) variable among the isolates, and (iii) dependent on the growth rate and growth stage of the fungi. We found a multiphasic dynamic of fractionation for Cryptoporus volvatus and Marasmius androsaceus during various growth stages. The first phase, P1, corresponded to the exponential growth stage and was characterized by an increasing enrichment in (13)C content of the fungal biomass relative to the growth medium ranging between 4.6 and 6.9 per thousand. The second phase, P2, exhibited a continual depletion in (13)C of the fungal biomass, with the delta(13)C values of the fungal biomass asymptotically returning to the delta(13)C value of the growth medium at inoculation. The expression of the various fractionation phases was dependent on the amount of low-concentration micronutrients and growth factors added to the growth medium. The onset of P2 occurred at reduced concentrations of these elements. All of the sugars in the growth medium (sucrose, maltose, and glucose) were utilized for growth, indicating that the observed fractionation was not an artifact derived from the preferential use of (13)C-rich maltose, which was found at low concentrations in the growth medium. In this study, we establish a framework with which to explore the impact of physiological fractionations by fungal interfaces on natural distributions of stable C isotopes.     

Estimation of N or C uptake rates by phytoplankton using 15N or 13C: revisiting the usual computation formulae

The uptake of N by phytoplankton is generally estimated usingthe ¹⁵N technique and, under some circumstances, the uptakeof C is estimated using ¹³C. Rigorous examination of formulaefor computing net transport rates leads to several interestingand even unexpected conclusions. These are that the ¹⁵N or ¹³Ctechnique formula for computing net transport rates () is identicalto that of the ¹⁴C technique, in spite of apparent dissimilaritieswhich reflect differences in equipment used for determiningnon-radioactive and radioactive isotopes; the so-called specificuptake rates (V) should not be used with natural samples, exceptas a step in the calculation of transport rates (); estimationof is unaffected by the presence/absence of non-phytoplanktonicpaniculate organic matter (POM) in the incubated sample; thepractice of adding the concentration of tracer to the denominatorof expression representing the concentration of tracer in thedissolved phase at the beginning of incubation should be discontinued;and the concentration of POM should be determined from the inoculatedsample at the end of incubation (or, alternatively, from a sampleincubated in parallel) and not from a water sample taken atthe beginning of the incubation.     

The effect of water motion on short-term rates of photosynthesis by marine phytoplankton

Phytoplankton respond to variations in light intensity as they are mixed through the water column. Changes in pigment content are characteristic of the relatively slow response of 'sun-shade' photoacclimation that occurs on timescales typical of mixing in the open ocean. In estuaries, the variations are much faster and induce correspondingly rapid changes in the activity (rather than abundance) of different components of the photosynthetic apparatus. These components modulate light harvesting and Calvin cycle activity, or protect the pigment bed from excess energy absorption. When the protective capacity is exceeded, photoinhibition occurs. All these mechanisms modulate the rate of photosynthesis in situ.     

Problems in estimating growth rates of marine phytoplankton from short-term14C assays

Growth rate estimates () of phytoplankton populations that were sampled from nitrogen-limited continuous cultures and then incubated for short durations in batch culture with added¹⁴C-HCO₃ ^– were significantly different than steady-state growth rates () for 3 of 5 marine phytoplankton species. Two diatoms,Thalassiosira weissflogii andChaetoceros simplex, displayed virtually identical growth rates (=) over a wide range of, whereas for a third diatom,Phaeodactylum tricornutum, was overestimated by an average of 40% compared to. In contrast, was underestimated by the¹⁴C technique for the two remaining species: up to 40% at a steady-state of 1.0 day^–1 for the chlorophyteDunaliella tertiolecta and up to 100% at of 1.4 day^–1 for the haptophytePavlova lutheri. For the latter two species the divergence between and appeared to increase with increasing steady-state. A simple model of labeled and total carbon flow between the aqueous phase and cellular biomass was constructed to demonstrate that respiration was negligible when=, but was significant when>. In the cases in which<, a rapid physiological alteration presumably took place once the steady state was disturbed and cells were placed in the incubation chambers, which perhaps was related to the nutritional state of the cultures at the time of sampling. Questions thus are raised regarding our ability to measure accurately primary productivity from shipboard experiments with confined samples of phytoplankton from nutrient-impoverished waters that probably are less hardy than the laboratory cultures used in these studies.     

Influence of solar ultraviolet radiation on photosynthesis and motility of marine phytoplankton

Abstract: The effects of solar ultraviolet radiation (UV) on carbon uptake, oxygen evolution and motility of marine phytoplankton were investigated in coastal waters at Kristineberg Marine Research Station on the west coast of Sweden (58° 30'N, 11° 30'E). The mean irradiances at noon above the water surface during the investigation period were: photosynthetic active radiation (PAR, 400–700 nm) 1670 μmol m⁻² s⁻¹; ultraviolet-A radiation (UV-A, 320–400 nm) 35.9 W m⁻² and ultraviolet-B radiation (UV-B, 280–320 nm) 1.7 W m⁻². UV-B radiation was much more attenuated with depth in the water column than were PAR and UV-A radiation. UV-B radiation could not be detected at depths greater than 100–150 cm. Inhibition of carbon uptake by UV-A and UV-B in natural phytoplankton populations was greatest at 50 cm depth and the effects of UV-B were greater than those of UV-A. At depths greater than 50 cm there was almost no effect of ultraviolet radiation on carbon uptake. PAR, UV-A and UV-B decreased oxygen evolution by the dinoflagellate Prorocentrum minimum . Inhibition of oxygen evolution was greater after 4 h than 2 h but it was not possible to distinguish the negative effects of the different light regimes. The motility of P. minimum was not affected by PAR, UV-A and UV-B. The importance of exposure of phytoplankton to different light regimes before being exposed to natural solar radiation is discussed.     

Patterns of dark14CO2 incorporation by natural marine phytoplankton communities

The rates of dark¹⁴CO₂ fixation by natural phytoplankton communities growing in eutrophic and oligotrophic waters were studied with short-term in situ experiments. Three aspects were investigated: (1) the time course incorporation of¹⁴CO₂ in darkness, (2) the depth variability in dark¹⁴CO₂ fixation, and (3) the variability in¹⁴CO₂ fixation within a year. The highest dark¹⁴CO₂ incorporation rates were observed during the first interval of incubation (20 min) after which they approached a constant rate with time. The observed differences in dark¹⁴CO₂ fixation rates between populations from different depths were associated with differences in species composition as well as with physiological differences caused by exposure to different illumination conditions prior to their exposure to darkness. Autocorrelation coefficients were computed for the analysis of variability of dark¹⁴CO₂ fixation rates within a year. It was suggested that dark¹⁴CO₂ incorporation might be a periodic phenomenon depending mainly on the productive capacity of the phytoplankton community.     

Discrimination between12C and13C by marine plants

Summary The natural abundance¹³C/¹²C ratios (as δ¹³C) of organic matter of marine macroalgae from Fife and Angus (East Scotland) were measured for comparison with the species' ability to use CO₂ and HCO ₃ ^- for photosynthesis, as deduced from previously published pH-drift measurements. There was a clear difference in δ¹³C values for species able or unable to use HCO ₃ ^- . Six species of Chlorophyta, 12 species of Phaeophyta and 8 species of Rhodophyta that the pH-drift data suggested could use HCO ₃ ^- had δ¹³C values in the range -8.81‰ to -22.55‰. A further 6 species of Rhodophyta which the pH-drift data suggested could only use CO₂ had δ¹³C values in the range -29.90‰ to-34.51‰. One of these six species (Lomentaria articulata) is intertidal; the other five are subtidal and so have no access to atmospheric CO₂ to complicate the analysis. For these species, calculations based on the measured δ¹³C of the algae, the δ¹³C of CO₂ in seawater, and the known¹³C/¹²C discrimination of CO₂ diffusion and RUBISCO carboxylation suggest that only 15–21% of the limitation to photosynthesisin situ results from CO₂ diffusion from the bulk medium to the plastids; the remaining 79–85% is associated with carboxylation reactions (and, via feedback effects, down-stream processes). This analysis has been extended for one of these five species,Delesseria sanguinea, by incorporating data onin situ specific growth rates, respiratory rates measured in the laboratory, and applying Fick's law of diffusion to calculate a boundary layer thickness of 17–24 μm. This value is reasonable for aDelesseria sanguinea frondin situ. For HCO ₃ ^- -using marine macroalgae the range of δ¹³C values measured can be accommodated by a CO₂ efflux from algal cells which range from 0.306 of the gross HCO ₃ ^- influx forEnteromorpha intestinalis (δ¹³C=-8.81‰) in a rockpool to 0.787 forChondrus crispus (δ¹³C=-22.55‰). The relatively high computed CO₂ efflux for those HCO ₃ ^- -users with the more negative δ¹³C values implies a relatively high photon cost of C assimilation; the observed photon costs can be accommodated by assuming coupled, energy-independent inorganic carbon influx and efflux. The observed δ¹³C values are also interpreted in terms of water movement regimes and obtaining CO₂ from the atmosphere. Published δ¹³C values for freshwater macrophytes were compared with the ability of the species to use CO₂ and HCO ₃ ^- and again there was an apparent separation in δ¹³C values for these two groups. δ¹³C values obtained for marine macroalgae for which no pH-drift data are available permit predictions, as yet untested, as to whether they use predominantly CO₂ or HCO ₃ ^-     

Diel periodicity in dark uptake of ammonium by natural populations of marine phytoplankton

We discovered the phenomenon that dark uptake of ammonium increasesat night and maximizes around midnight. The maximum values atmidnight are 2 or 3 times higher than the values in the daytime.