Gerd Slawyk

Feasibility of using an automated colorimetric procedure for the determination of seawater nitrate in the 0 to 100 nm range examples from field and culture

An automatic colorimetric procedure for measuring nanomolar quantities of nitrate and nitrite in seawater has been improved in sensitivity. Application of this method toin situ investigations in the North Atlantic Ocean in January 1989 and algal culture (Skeletonema costatum) experiments in the laboratory demonstrated that one can record concentrations in the 0 to 100 nM range using classical Technicon AutoAnalyzer equipment. In this working range highest precision obtained was ±2.87 and ±1.24 nM for nitrate and nitrite, respectively.

Nitrogen Uptake and Assimilation by Marine Phytoplankton

The introduction of the 15N tracer technique into ocean research (1) has allowed the direct measurement of growth-limiting nutrient salts such as nitrate or ammonium, and has led to new insights into the mechanisms of control of primary productivity in the marine environment.

13C and 15N uptake by marine phytoplankton. IV: Uptake ratios and the contribution of nitrate to the productivity of Antarctic waters (Indian Ocean sector)

Abstract Significant latitudinal gradients in inorganic carbon and nitrate uptake were observed in the Indian sector of the Antarctic Ocean. Carbon uptake weas directly related to photosynthetically available radiation (PAR), while nitrate uptake was inversely related to PAR. This led to a direct relationship between carbon:nitrate uptake ratios and PAR both within and between stations. Field measurements as well as laboratory experiments on cultures of two diatoms isolated from the study area show that ammonium uptake decreases with time after addition of ammonium, under conditions of substrate saturation. This phenomenon, as well as other problems associated with measurements of ammonium uptake, makes it difficult to estimate the contribution of this compound to primary production from direct uptake measurements. More reliable estimates can be obtained from values of carbon:nitrate uptake ratios determined from double labeling experiments. These ratios are lower than the Redfield ratio south of the polar front, and indicate that nitrate can account for most of the primary production in offshore areas during the austral summer.

13C AND 15N UPTAKE BY MARINE PHYTOPLANKTON. I. INFLUENCE OF NITROGEN SOURCE AND CONCENTRATION IN LABORATORY CULTURES OF DIATOMS1

Two species of marine diatoms, Skeletonema costatum (Grev.) Cleve and Phaeodactylum tricornutum Bohlin were grown in batch and continuous cultures on four different nitrogen compounds (nitrate, nitrite, ammonium, urea). Carbon and nitrogen uptake were measured simultaneously with the stable isotopes 13C and 15N. Nitrogen uptake generally increased with N concentration in the medium, but no clear difference existed between the N sources. Carbon fixation was decreased for up to 5 h following the addition of the N compound. Nitrite generally had the greatest inhibitory effect on C uptake. Carbon‐to‐nitrogen uptake ratios decreased with increasing dissolved N concentration, becoming lower than one in nutrient‐limited cultures. In contrast, batch cultures exhibited C:N uptake ratios greater than one. These effects are essentially short‐term and differ from long‐term influences of the N source on the cellular chemical composition.

On the relationship between carbon-to-nitrogen composition ratios of the particulate matter and growth rate of marine phytoplankton from the Northwest African upwelling area

Abstract Carbon-to-nitrogen composition ratios of particulate matter ranged between 3.7 and 9.6. High ratios (7.8–9.6) were found at inshore stations located in cold (

Nitrate reductase activity as a function of in situ nitrate uptake and environmental factors of euphotic zone profiles

Abstract The activity of nitrate reductase in euphotic zone profiles from several oceanic areas has been compared with in situ nitrate uptake rates estimated by the 15 N technique. There were marked variations in the ratio of uptake to reduction and these have been related to the available light intensity, nitrate concentration, and the absolute value of nitrate uptake. It is shown that uptakereduction ratios greater than one are not due to dephasing between the two processes and that accumulation of nitrate inside the cells cannot account for the discrepancy. Nitrate uptake depends primarily on the external nitrate concentration, while nitrate reductase activity seems to be controlled by the intracellular nitrate level. The importance of those results as regards the significance of the nitrate reductase assay as an index of nitrate assimilation by marine phytoplankton is discussed.

GLUTAMINE SYNTHETASE ASE ACTYVITY IN IN CHAETOCEROS AFFINIS (BACILLARIOPHYCEAE): COMPARISON WITH OTHER ESTIMATES OF NITROGEN UTILIZATION DURING NUTRIENT PERTURBATION1

Glutamine synthftase (GS) activity was investigated in a nitratt limited continuous culture of the marine diatom Chaeloccros afTinis (Lauder) Hustedt before and after the perturbation of the culture medium with 10 μM of 15 N labelled nitrate. Parallel studies were carried out on nitrate reductase(NR). nitrate uptake and assimilation, and Ievels of cellular nitrogen containing compounds with the objective to determine the validity of the GS assay as a measure of nitrate utilization. Activities in N‐deficient cells, grown at steady state, correlated well with uptake and assimilation rates. In N‐sufftcient celts, however, during the nitrate pertirbation period, they accounted only for about 10% of the two latter rates, when ambient nitrate concentrations were high (0. 7‐10 μ). It is proposed that under these growth conditions an alternative pathway via glutamate dehydrogenase (GDH) was operative. At low ambient nitrate concentrations (0.1‐0.7 μM), GS activities, uptake and assimilation rates again balanced rather well. Thus, the data support the view that GDH activity is associated with high levels and GS with low levels of external or internal nitrogen.

Direct measurements of nanomolar nitrate uptake by the marine diatom Phaeodactylum tricornutum (Bohlin). Implications for studies of oligotrophic ecosystems

A new automated procedure for nanomolar nitrate analysis was applied to the study of uptake of low nitrate concentrations (< 100 ngat l−1) by phytoplankton. The precision of this analytical method (± 3 ngat l−1) made it possible to monitor the absorption of very low quantities of nitrate over short term periods by a low cell-density culture of the marine diatom Phaeodactylum tricornutum, where the levels of particulate nitrogen and chlorophyll were equivalent to those found in oligotrophic areas (0.5 µgat N l−1 and 0.4 µg l−1 respectively). By continuous monitoring of nitrate disappearance from the culture medium, we are able to describe accurately the transient uptake responses of the diatom after a spike addition of trace quantities of nitrate and thus to provide new information on the still largely unknown small-scale phenomenon of pulsed nitrate supply in the upper layer of stratified oceans and rapid uptake of these nitrate patches by phytoplankton.The results show that a N-limited culture of Phaeodactylum tricornutum is immediately capable of taking up trace quantities of nitrate (< 100 ngat l−1) at high rates (0.10–0.14 h−1) . These initial rates are one order of magnitude higher than the theoretical rates calculated from the Michaelis-Menten equation and are close to the level of Vmax (0.15 h−1) obtained when cells are exposed to saturating nitrate concentrations. This rapid initial uptake would be a considerable advantage in oligotrophic areas where nanomolar nitrate supply is thought to be episodic. The present results suggest that phytoplankton evolve adaptations to utilize the available nitrate at the spatial and temporal scales at which it occurs. On the other hand, we can consider this physiological adaptation as evidence of nitrate pulses in the field which would invalidate the steady-state approach to the oligotrophic ecosystems.

Quantitative determination of particulate organic N and C in marine-phytoplankton samples using mass-spectrometer signals from isotope-ratio analyses in 15N- and 13C-tracer studies

Abstract A gas-sampling device between an oven for Dumas combustion and an isotope mass spectrometer enables the determination of particulate organic N and C concentrations during isotopic 15 N and 13 C analysis of phytoplankton samples from dual-labelling experiments. A description of the interface is given with details of the operating procedure. The precision of the method was comparable with an elemental C-N analyser with a C.V. of 2.70% at the level of 5.17 μmol for N and 7.26% at the level of 48.77 μmol for C.

On the discrepancies between a colorimetric and isotopic method for measuring nitrate utilization in nutrient-depleted waters : implications for the design of experimental protocols in new production studies

Colorimetric analyses of nitrate disappearance from seawater have been compared with isotopic analyses of 15N-labelled nitrate incorporation into particulate matter. The slope (1.41) of a regression line calculated from 19 sample pairs gathered during 6 time-series experiments and 2 single end-point incubations showed that nitrate incorporation is positively related to changes in nitrate concentration but that it accounts only for 71% of nitrate disappearance. 15N-isotope dilution as a consequence of nitrification, if any, would not fully explain discrepancies between the two analytical procedures. A further possible mechanism responsible for the imbalance between nitrate-incorporation and -disappearance rates suggests losses of 15N label from plankton biomass to an unanalyzed pool (dissolved organic nitrogen?) which increase (up to 65%) with incubation time. The lack of 15N-mass balance calls for the need to consider additional nitrogen pools in 15N budgets of isotope experiments and not only substrate and biomass pools as has been done so far.