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     

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.     

The uptake of silicic acid by rat liver mitochondria.

1. To gain insight into a putative role for mitochondria in silicon metabolism, mitochondrial uptake (by which it is meant the removal from the medium) of silicic acid [Si(OH)4] was studied under conditions minimizing SI(OH)4 polymerization. 2. Measurements of mitochondrial respiration and swelling indicated indirectly a significant uptake of Si(OH)4 as a weak acid, but this was not confirmed when 31Si(OH)4 was used as a tracer. 31Si(OH)4 occupied a mitochondrial volume similar to that of 3H2O and was relatively unaffected by mitochondrial energy status and by the pH gradient across the mitochondrial inner membrane. 3. Uptake was directly proportional to Si(OH)4 concentration in the range 0-3 mM. 4. The uptake consisted of two components: under all conditions examined, the greater quantity, amounting to 1-2nmol of Si(OH)4/mg of mitochondrial protein, was bound, a major portion of it external to the inner membrane, with the lesser quantity free within the matrix space. 5. Equilibration of 31Si(OH)4 between medium and matrix was a slow process, having a half-time of approx. 10 min at 22 degrees C. 6. Mersalyl and N-ethylmaleimide inhibited the uptake by preferentially lowering the amount of Si(OH)4 bound. Their action was somewhat variable, depending on the precise nature of the suspending medium, and suggesting that the bound material may represent polymerized forms of Si(OH)4. 7. It is concluded that Si(OH)4 may penetrate the mitochondrial inner membrane by a simple diffusion mechanism.     

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.     

Using stable isotope biogeochemistry to study marine mammal ecology

Stable isotope analysis (SIA) has emerged as a common tool in ecology and has proven especially useful in the study of animal diet, habitat use, movement, and physiology. SIA has been vigorously applied to the study of marine mammals, because most species live in habitats or undergo large migrations/movements that make them difficult to observe. Our review supplies a complete list of published SIA contributions to marine mammal science and highlights informative case examples in four general research areas: (1) physiology and fractionation, (2) foraging ecology and habitat use, (3) ecotoxicology, and (4) historic ecology and paleoecology. We also provide a condensed background of isotopic nomenclature, highlight several physiological considerations important for accurate interpretation of isotopic data, and identify research areas ripe for future growth. Because it is impossible to conduct controlled laboratory experiments on most marine mammal species, future studies in marine mammal ecology must draw on isotopic data collected from other organisms and be cognizant of key assumptions often made in the application of SIA to the study of animal ecology. The review is designed to be accessible to all audiences, from students unfamiliar with SIA to those who have utilized it in published studies.     

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 stable isotope data to advance marine food web modelling

Reviews in Fish Biology and Fisheries - Marine ecosystem models that incorporate fisheries and climate change are essential for forecasting and guiding sustainable ecosystem management decisions. A...     

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.     

A new method to measure plant water uptake and transpiration simultaneously

A new weighing lysimeter system is described measuring transpirationand water uptake simultaneously on one plant, growing in waterculture. The measurements may be made for short time intervals(min) making it possible to monitor quick responses to changingenvironmental conditions. Fresh weight change, a combinationof growth and water status alterations in the plant, may becalculated from transpiration and water uptake. The system consistsof two communicating vessels filled with nutrient solution;each placed on an electronic balance. One of these vessels carriesthe plant and is connected to the other by a flexible tube.Water uptake will cause an equal decrease of the solution levelin each vessel. The weight decrease of the vessel with no plantprovides a measure of water uptake, the total weight decreaseon both balances represents transpiration. Test observations showed that measurements of transpirationand water uptake in a greenhouse can be made to an accuracyof about 0.03 g min^–1 plant^–1. With fluctuatingradiation, a clearly radiation-dependent transpiration was measuredon a tomato plant in a greenhouse. These measurements showeda delay between transpiration and water uptake. Consequently,fresh weight also fluctuated with radiation. An immediate decreasein transpiration was measured upon closure of a screen in thegreenhouse, accompanied by an increase in fresh weight. Fromlate afternoon until sunrise a constant fresh weight increasewas measured; first at a relatively high rate probably due togrowth and recovering from water deficits, thereafter at a constantrate probably only due to growth. Key words: Transpiration, water uptake, water relations, tomato, lysimeter     

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.     

A stable isotope method for the quantification of N-acetyl-5-aminosalicylic acid in plasma and urine

The synthesis of a number of N-acyl-5-aminosalicylic acids and their derivatives is described. These compounds allow the sensitive and specific mass spectrometric determination of unlabelled or deuterium-labelled N-acetyl-5-aminosalicylic acid in biological samples. An in vivo study shows that the labelled compound, administered to rats, is excreted isotopically unchanged to at least 97%, and that no significant deacetylation/acetylation mechanism exists for this metabolite N-acetyl-5-aminosalicylic acid of salicylazosulphapyridine.     

A stable isotope method using a [(2)H(5)]glycerol bolus to measure very low density lipoprotein triglyceride kinetics in humans.

We have developed a method using a bolus of [(2)H(5)]glycerol to determine parameters of VLDL-triglyceride (VLDL-TG) turnover and have compared the data to that obtained using simultaneously a bolus of [2-(3)H]glycerol in six young normolipidemic men. No measurable enrichment was found after 12 h for [(2)H(5)]glycerol; therefore, we could only perform a monoexponential analysis of its data. No differences in fractional catabolic rate (FCR) were seen when comparing the multicompartmental modeling of [2-(3)H]glycerol data (modeled over 48 h) either to the monoexponential analyses of the [2-(3)H]glycerol or that of the [(2)H(5)]glycerol data. The two monoexponential approaches were highly correlated (r = 0.96 for FCR), however, FCR was 18% higher with the [(2)H(5)]glycerol than with the [2-(3)H]glycerol data (P < 0.003). In all six subjects, a 10-h infusion of [1-(13)C]acetate was started at the same time as the glycerol boluses were given. In two men we were able reliably to detect VLDL-TG-fatty acid enrichment. The measurement of FCR in these two subjects using the mass isotopomer distribution analysis (MIDA) approach was in good agreement (within 10%) with FCRs determined with the labeled glycerol methods.In conclusion, our results have shown that results obtained with the [(2)H(5)]glycerol bolus were highly correlated with those obtained with the [2-(3)H]glycerol, but the FCRs were slightly higher with the former. We have also demonstrated that FCRs determined from monoexponential modeling were in good agreement with those determined from the multicompartmental modeling of the TG-glycerol data.     

A theoretical description of nitrate uptake kinetics in marine phytoplankton based on bisubstrate kinetics

Using well known equations for bisubstrate enzyme kinetics, a model for nitrate uptake in marine phytoplankton is generated. The model attempts to include both extracellular nitrate concentrations and incident light intensities as substrates required by the cells to take up nitrate. An affinity constant, K_LN, is defined which relates nitrate uptake kinetics to light intensities and extracellular nitrate concentrations simultaneously.     

Nitrogen uptake and transformation in a midwestern U.S. stream: A stable isotope enrichment study

This study presents a comprehensive analysis ofnitrogen (N) cycling in a second-order forestedstream in southern Michigan that has moderatelyhigh concentrations of ammonium (mean,16 g N/L) and nitrate (17 g N/L). Awhole-stream ¹⁵NH₄ ⁺ addition wasperformed for 6 weeks in June and July, and thetracer ¹⁵N was measured downstream inammonium, nitrate, and detrital and livingbiomass. Ancillary measurements includedbiomass of organic matter, algae, bacteria andfungi, nutrient concentrations, hydrauliccharacteristics, whole-stream metabolism, andnutrient limitation assays. The resultsprovide insights into the heterotrophic natureof woodland streams and reveal the rates atwhich biological processes alter nitrogentransport through stream systems.Ammonium uptake lengths were 766–1349 m anduptake rates were 41–60 g N m^–2min^–1. Nitrate uptake could not bedetected. Nitrification rates were estimatedfrom the downstream increase in¹⁵N-enriched nitrate using a simulationmodel. The ammonium was removed bynitrification (57% of total uptake),heterotrophic bacteria and fungi associatedwith detritus (29%), and epilithic algae(14%). Growth of algae was likely limited bylight rather than nutrients, and dissolvedO₂ revealed that the stream metabolism washeterotrophic overall (P:R = 0.2). Incubationsof detritus in darkened chambers showed thatuptake of ¹⁵N was mostly heterotrophic.Microbial N in detritus and algal N inepilithon appeared to reach isotopic steadystate with the dissolved ammonium, but theisotopic enrichment of the bulk detritus andepilithon did not approach that of ammonium,probably due to a large fraction of organic Nin the bulk samples that was not turning over. The actively cycling fraction of total N inorganic compartments was estimated from theisotopic enrichment, assuming uptake ofammonium but not nitrate, to be 23% forepilithon, 1% for fine benthic organic matter,5% for small woody debris, and 7% for leaves. These percentages agree with independentestimates of epilithic algal biomass, whichwere based on carbon:chlorophyll ratios in bulksamples and in algal fractions separated bydensity-gradient centrifugation in colloidalsilica, and of microbial N in the detritus,which were based on N released by chloroformfumigations.     

A practical method for cell-free protein synthesis to avoid stable isotope scrambling and dilution

During recent years, the targets of protein structure analysis using nuclear magnetic resonance spectroscopy have become larger and more complicated. As a result, a complete and precise stable isotope labeling technique has been desired. A cell-free protein synthesis system is appropriate for this purpose. In the current study, we achieved precise and complete ¹⁵N and ²H labeling using an Escherichia coli cell extract-based cell-free protein synthesis system by controlling the metabolic reactions in the system with their chemical inhibitors. The addition of aminooxyacetate, d-malate, l-methionine sulfoximine, S-methyl-l-cysteine sulfoximine, 6-diazo-5-oxo-l-norleucine, and 5-diazo-4-oxo-l-norvaline was quite effective for precise amino acid-selective ¹⁵N labeling even for aspartic acid, asparagine, glutamic acid, and glutamine, which generally suffer from severe isotope scrambling and dilution when using the conventional cell-free system. For ²H labeling, the back-protonation of the H^α and H^β positions, which commonly occurred in the conventional system, was dramatically suppressed by simply adding aminooxyacetate and d-malate to the cell-free system except for the H^α positions in methionine and cysteine.