Carbon and nitrogen dynamics during growth and degradation of phytoplankton under natural surface irradiance

Under conditions of natural irradiance, the development and decline of a flagellate-dominated phytoplankton population was followed in a coastal North Atlantic pond over a 3 d period in summer 1986. Irradiance negatively affected phytoplankton biomass estimated as chlorophyll a, which decreased during the day at photosynthetically available radiation (PAR) levels above 600 to 1000 mol m^-2s^-1; chlorophyll a increased at PAR values below this threshold. In addition, an inverse relationship was found between changes in chlorophyll a and changes in dissolved inorganic nitrogen, indicating synthesis of nitrogenous biomass mainly at night and degradation mainly during the day, with intense exchanges of material between the particulate and dissolved nitrogen fractions. The natural abundance of ¹³C in particulate matter increased initially, and then remained constant, and was controlled mainly by the ratio -carboxylases activity: ribulose biphosphate carboxylase activity. The hypothesis that the latter enzyme is broken down under high irradiance and is partly responsible for increases in external dissolved nitrogen was rejected.     

A new method for estimating phytoplankton growth rates and carbon biomass

A new method is described for the determination of phytoplankton growth rates and carbon biomass. This procedure is easy to apply and utilizes the labeling of chlorophyll a (chl a) with ¹⁴C. Pure chl a is isolated using two-way thin-layer chromatography, and the specific activity of chl a carbon is determined. Data from laboratory cultures indicate that the specific activity of chl a carbon becomes nearly equal to that of total phytoplankton carbon in incubations lasting 6 to 12 h and can be used to calculate phytoplankton growth rates and carbon biomass. Application of the method to the phytoplankton community in an eutrophic estuary in Hawaii indicates that the cells are growing with a doubling time of about 2 d and that about 85% of the particulate carbon consists of phytoplankton carbon.     

Marine diatoms grown in chemostats under silicate or ammonium limitation. III. Cellular chemical composition and morphology of Chaetoceros debilis,Skeletonema costatum,and Thalassiosira gravida

Three marine diatoms, Skeletonema costatum, Chaetoceros debilis, and Thalassiosira gravida were grown under no limitation and ammonium or silicate limitation or starvation. Changes in cell morphology were documented with photomicrographs of ammonium and silicate-limited and non-limited cells, and correlated with observed changes in chemical composition. Cultures grown under silicate starvation or limitation showed an increase in particulate carbon, nitrogen and phosporus and chlorophyll a per unit cell volume compared to non-limited cells; particulate silica per cell volume decreased. Si-starved cells were different from Si-limited cells in that the former contained more particulate carbon and silica per cell volume. The most sensitive indicator of silicate limitation or starvation was the ratio C:Si, being 3 to 5 times higher than the values for non-limited cells. The ratios Si:chlorophyll a and S:P were lower and N:Si was higher than non-limited cells by a factor of 2 to 3. The other ratios, C:N, C:P, C:chlorophyll a, N:chlorophyll a, P:chlorophyll a and N:P were considered not to be sensitive indicators of silicate limitation or starvation. Chlorophyll a, and particulate nitrogen per unit cell volume decreased under ammonium limitation and starvation. NH₄-starved cells contained more chlorophyll a, carbon, nitrogen, silica, and phosphorus per cell volume than NH₄-limited cells. N:Si was the most sensitive ratio to ammonium limitation or starvation, being 2 to 3 times lower than non-limited cells. Si:chlorophyll a, P:chlorophyll a and N:P were less sensitive, while the ratios C:N, C:chlorophyll a, N:chlorophyll a, C:Si, C:P and Si:P were the least sensitive. Limited cells had less of the limiting nutrient per unit cell volume than starved cells and more of the non-limiting nutrients (i.e., silica and phosphorus for NH₄-limited cells). This suggests that nutrient-limited cells rather than nutrient-starved cells should be used along with non-limited cells to measure the full range of potential change in cellular chemical composition for one species under nutrient limitation.Contribution No. 943 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Dynamique du phytoplancton et matière organique particulaire dans la zone euphotique de l'Atlantique Equatorial

At two fixed stations in the Equatorial Atlantic Ocean (0°–4° W), the physical, chemical and biological properties of the euphotic layer were determined for 14 d (Station A: 5–18 February, 1979) and 13 d (Station B: 20 October–7 November, 1979), respectively. The stability of the water column allowed comparison of 3 different “systems”: (i) a well-illuminated and nitrate-depleted mixed layer; (ii) a chlorophyll maximum layer (chl a _max) in the thermocline which is poorly illuminated (6.3% of surface irradiance); (iii) a well-illuminated but nitrate-rich (>0.9 μg-at l^-1) mixed layer. In each layer the particulate organic carbon (COP), nitrogen (NOP) and phosphorus (POP) contents were measured and compared with the phytoplankton biomass. In the chlorophyll maximum layer, the phytoplankton biomass contributed significantly to the total particulate organic matter (between 55 and 75%). In the nitrate-depleted mixed layer, the results varied according to whether the regression technique [COP=f(chl a)] was used, or the chl a synthesis during the incubation of the samples. With the former technique, the phytoplankton carbon (C _p) content appeared minimal, because the y intercept, computed using all the data of the water column, was probably overestimated for this layer. POP would be more associated with living protoplasm than with carbon and nitrogen in the three layers. In the chlorophyll a maximum layer it constitutes a valuable detritus-free biomass measurement, since 80% of the POP consist of phytoplankton phosphorus. The assimilation numbers (NA=μg C μg chl a ^-1 h^-1) were high in all three layers, but the highest values were recorded in the nitrate-depleted mixed layer (NA=15 μg C μg chl a ^-1 h^-1). In the chlorophyll maximum layer, light would be a limiting factor during incubation: between 10²⁵ and 8.10²⁴ quanta m^-2 d^-1 NA and light are positively correlated independant of nitrate concentration. The growth rates of phytoplankton (μ) were estimated and compared to the maximum expected growth rate. Our main conclusion was that despite very low biomass and nutrient content, the mixed layer was in a highly dynamic state, as evidenced by high rates of phytoplankton growth and short nutrient turnover times (1 d or less for PO-P₄ in the mixed layer versus 3 d in the thermocline). The presence of nitrate in the water column allows the development of a higher phytoplankton biomass but does not increase growth rate.     

Kinetics of light-intensity adaptation in a marine planktonic diatom

The marine planktonic diatom Thalassiosira weisflogii was grown in turbidostat culture under both continuous and 12 hL: 12 hD illumination regimes in order to study the kinetics of adaptation to growth-irradiance levels. In both illumination regimes adaptation to a higher growth-irradiance level was accompanied by an increase in cell division rates and a decrease in chlorophyll a cell^-1. The rates of adaptation for both processes, derived from first order kinetic analysis, equaled each other in each experiment. The results suggest that during the transition from low-to-high growth-irradiance levels chlorophyll a is diluted by cell division and is not actively degraded. Introduction of a light/dark cycle lowered the rate of adaptation. In transitions from high-to-low growth-irradiance levels there was a sharp drop in growth rates and a slow increase in chlorophyll a cell^-1 under both continuous and intermittent illumination. In the 12 hL:12hD cycle there was a circadian rhythm in chlorophyll a cell^-1, where cellular chlorophyll contents increased during the light cycle and decreased during the dark cycle. This circadian rhythm was distinctly different from light intensity adaptation. For kinetic analysis of light intensity adaptation in a 12 hL: 12 hD cycle, the circadian periodicity was separated from the light intensity response by subjecting the data to a Kaiser window optimization digital filter. Kinetic parameters for light-intensity adaptation were resolved from the filtered data. The kinetics of lightintensity adaptation of marine phytoplankton are discussed in relation to their spatial variations and time scales of mixing.This research was performed at Brookhaven National Laboratory under the auspices of the United States Department of Energy under Contract No. DE-AC02-76 CH00016     

Effect of UV-B radiation on the marine diatoms Lauderia annulata and Thalassiorsira rotula grown in different salinities

The marine diatoms Lauderia annulata Cleve and Thalassiosira rotula Meunier were grown at different salinities (20, 35 and 45) and exposed to different levels of midultraviolet, UV-B) 439, 717 and 1230 J m^-2 d^-1, weighted) for 2 d. A low UV-B dose (439 J m^-2 d^-1) usually caused a slight increase in biomass production (dry weight) compared to non-UV-B irradiated cells. Enhanced UV-B radiation (717 J m^-2 d^-1) depressed protein and pigment content (chlorophyll a, chlorophyll c ₁+c₂ and carotenoids), especially in algae grown at 20 or 35 salt concentration of the nutrient solution. The effect of UV-B radiation (717 J m^-2 d^-1) on the pattern and concentration of amino acids was species-dependent. Aspartic acid was reduced in all tested diatoms. A drastic increase in glutamine and a reduction in glutamic acid pools could be observed in L. annulata samples, but no significant variation of the impact of UV-B was found in dependence on the salt concentration of the nutrient medium. T. rotula cells grown at 35 S showed an increase of glutamic acid and a decrease of glutamine levels after UV-B radiation. The results are discussed in relation to the impact of UV-B upon carbon and nitrogen metabolism.     

Comparison of chlorophyll a and carotenoid pigments as predictors of phytoplankton biomass

Chlorophyll a concentration was compared with carotenoid concentration as a predictor of seasonal changes in phytoplankton biomass within Bedford Basin, Nova Scotia, Canada (1976–1977). For all seasons, predictions of biomass from different measures of chlorophyll a were poor and were not improved when chlorophyll a was measured accurately by chromatography. Chlorophyll a and a carotenoid (fucoxanthin) were highly correlated and equally good predictors of total biomass, but neither was related to changes in peridinin concentration. Correlations between specific carotenoids and diatom or dinoflagellate biomass indicate that carotenoids may be useful to describe changes in biomass composition. For all pigments measured, predictions of biomass were hampered when large dinoflagellate cells were present, which biased estimates of total cell volume. Regardless of species composition or cell density, dinoflagellate biomass contributed on the average 68% of the total cell volume measured each day compared with only 14% for diatoms and 17% for flagellates, the most abundant taxa.     

Carbon and nitrogen metabolism by Monochrysis lutheri: Measurement of growth-rate-dependent respiration rates

Dark respiration rates were measured and carbon-excretion rates calculated for a nitrate-limited population of the marine chrysophyte Monochrysis lutheri grown in continuous culture at 20°C on a 12 h light-12 h dark cycle of illumination and over a series of 4 growth rates. A significant (P<0.05) positive correlation was found between dark respiration rate and growth rate. From a simple linear fit to the data, the respiration rate at maximum growth rate was estimated to be roughly 10.5% of the maximum gross-carbon-production rate, and more than three times higher than the extrapolated respiration rate at zero net-growth rate. Carbon-excretion rates showed no significant correlation with growth rate, and averaged less than 5% of the maximum gross-carbon-production rate. Mean cell nitrogen to carbon ratios were correlated in a virtually linear manner (r=0.994) with growth rate, and at a given growth rate were consistently higher than nitrogen to carbon ratios for the same species grown on continuous light. A comparison of carbon and nitrogen quotas as a function of growth rate for M. lutheri and other species suggests that the increase of cellular nitrogen at high growth rates under nitrate-limited growth conditions may be associated with the storage of cellular protein or amino acids rather than the presence of an inorganic nitrogen reservoir. The maximum nitrate uptake rate per cell during the day changed very little over the range of growth rates studied, and was comparable to the maximum uptake rate found for cells grown on continuous light. However, the cell nitrogen quota increased steadily with growth rate, causing a reduction in the maximum specific-uptake rate of nitrate during the day at high growth rates. The dark nitrate-uptake capacity of the population was clearly exceeded by the supply rate at the two higher growth rates, leading to a buildup of nitrate during the night which amounted to as much as 21% of the particulate nitrogen in the growth chamber by morning.Hawaii Institute of Marine Biology Contribution No. 478.     

Persistent diel rhythms in the chlorophyll fluorescence of marine phytoplankton species

The diel periodicities of in-vivo chlorophyll fluorescence and DCMU-enhanced chlorophyll fluorescence of 47 marine phytoplankton species were examined for 2 d in a 14 h L:10 h D light: dark cycle and then in continous light for another 2 to 3 d. Almost all phytoplankton species exhibit a more rapid increase in in-vivo fluorescence and DCMU-enhanced fluorescence during the light phase than during the dark phase. About one-half of the species examined exhibited persistent diel rhythms in continous light, indicating the operation of a biological clock. No phylogenetic or habitat related trends as to which species exhibited persistent rhythms were apparent. Of the phyla of eukaryotic phytoplankton adequately examined, none lacked biological clocks. Contrary to past hypotheses, some phytoplankton species maintain a persistent diel rhythm in a constant environment while reproducing at a rate greater than one division per day.     

Diel variation in chlorophyll a,carbohydrate and protein content of the marine diatom Skeletonema costatum

Skeletonema costatum (Greville) Cleve isolated from Narragansett Bay, USA, was incubated at 3 light intensities (ca. 0.008, 0.040 and 0.075 ly min^-1) under a 12 h light: 12 h dark (12L:12D) photoperiod at 2°, 10° and 20°C. Cellular chlorophyll a increased at intensities less than ca. 0.040 ly min^-1; increases occured within one photoperiod at temperatures above 10°C. Cellular carbohydrate increased with light intensity at all temperatures; increases during the photophase were due to net production of the dilute acid-soluble fraction. Cellular protein increased during the photoperiod at 10° and 20°C; there was little difference in cellular protein among all cultures after one photoperiod. The rate at which cellular chlorophyll a increased in response to a decrease in light suggests that diel variation in cellular chlorophyll a is temperature-dependent in S. costatum. Protein: carbohydrate ratios ranged from ca. 0.5 to 2.0 over a diel cycle; ratios increased at lower intensities and higher temperatures. The diel range in protein:carbohydrate ratios equals that in cultures developing nitrogen deficiency; thus, use of this ratio as an index to phytoplankton physiological state must account for diel light effects.     

Cell cycle and toxin production in the benthic dinoflagellate Prorocentrum lima

Profiles of diarrhetic shellfish poisoning (DSP) toxins produced throughout the growth cycle and the cell cycle of the toxigenic marine dinoflagellate Prorocentrum lima were studied in triplicate unialgal batch cultures. Cells were pre-conditioned at 18 ± 1 °C, under a photon flux density (PFD) of 90 ± 5 μmol m⁻² s⁻¹ on a 14 h light:10 h dark photoperiod. In exponential growth phase, cultures were synchronized in darkness for 17 d. After dark synchronization, cultures were transferred back to the original photoperiod regime. Cells were harvested for DSP toxin analysis by LC-MS (liquid chromatography with mass spectrometry), and double-stranded (nuclear) DNA was quantified by flow cytometry. The cell populations became asynchronous within approximately 3 d after transition from darkness to the 14 h light:10 h dark photoperiod. This may be due to the prolonged division cycle (5 to 7 d) that is not tightly phased by the photoperiod. Unlike other planktonic Prorocentrum spp., cytokinesis in P. lima occurred early in the dark and ceased by “midnight”. Cellular levels of the four principal DSP toxins, okadaic acid (OA), OA C8-diol-ester (OA-D8), dinophysistoxin-1 (DTX1) and dinophysistoxin-4 (DTX4), ranged from 0.37 to 6.6, 0.02 to 1.5, 0.04 to 2.6, and 1.8 to 7.8 fmol cell⁻¹, respectively. No toxin production was evident during the extended period of dark synchronization nor during the initial period when NH₄ was consumed as the major nitrogen source. Soon after the cells were returned to the 14 h light:10 h dark cycle and they began to take up NO₃, cellular levels of all four toxins gradually increased. This increase in DSP toxins usually occurred in the light, marked by a rise in DTX4 levels that preceded an increase in the cellular concentration of OA and DTX1 (delayed by 3 to 6 h). Thus, DTX4 synthesis is initiated in the G1 phase of the cell cycle and persists into S phase (“morning” of the photoperiod), whereas OA and DTX1 production occurs later during S and G2 phases (“afternoon”). No toxin production was measured during cytokinesis, which happened early in the dark. The evidence indicates that toxin synthesis is restricted to the light period and is coupled to cell cycle events. Received: 3 September 1998 / Accepted: 30 March 1999     

Seasonal changes of benthic bacteria in a seagrass bed (Posidonia oceanica) of the Ligurian Sea in relation to origin,composition and fate of the sediment organic matter

Variations in number and biomass of benthic bacteria were examined in the surface sediments of a Mediterranean seagrass bed [Posidonia oceanica (L.) Delile] in the Gulf of Marconi (northwestern Mediterranean Sea) from 1990 to 1991. The annual dynamics of benthic bacterial density and biomass were compared to changes in elemental (organic C and total N) and biochemical (lipids, proteins, carbohydrates) composition of sediment organic matter, as well as to microphytobenthic biomass, dissolved inorganic nutrients and ATP. Bacterial densities exhibited marked seasonal variations (5.12 to 322.7x10⁸ cells g^-1 sediment dry wt) with highest values in late spring. Bacterial standing stocks (15.8 to 882.33 g C g^-1 of sediment dry wt) were high. Bacterial biomass did not correlate with organic C, total N or to specific biochemical components, but correlated significantly with chlorophyll a, ATP and porewater phosphate concentrations. There is evidence that benthic bacteria were responding to variations of algal biomass. Bacterial biomass accounted, on average, for 30% of total living carbon (calculated on the basis of the ATP concentrations) and 8.4% of total organic carbon.     

Diurnal and circadian rhythms in the turbidity of growing Skeletonema costatum cultures

Skeletonema costatum (Grev.) Cleve grown in batch culture at low light intensity under a 14 h light: 10 h dark photocycle showed exponential cell proliferation (1.1 doublings d^-1) without significant phasing of the cell division by the light: dark cycle. The growth in carbon concentration was, however, restricted to the light period. The turbidity of the culture closely followed the carbon oattern, and was not affected by the increase in the cell number during the dark period. It was found that a trustule suspension had only approximately 1% of the turbidity of the corresponding intact algae. Culture turbidity was therefore regarded as a biomass parameter similar to the carbon concentration, without direct correlation to the timing of the cell division. The short-time variations in the turbidity of growing algal cultures were further studied in a cage culture turbidostat. The growth rate (based on turbidity) increased rapidly during the first half of the light period, decreased slightly towards the evening and was zero throughout the dark period. When transformed to continuous light, the growth of the culture continued to show damped oscillations for up to 1 wk, but with a period of 26.7 h instead of 24 h. The same circadian rhythm was observed in chlorophyll content, and is thus possibly a reflection of a freely oscillating internal biological clock. The cage culture turbidostat was found to be a suitable device for studies of the photocycle related regulation of biosynthesis in S. costatum.     

Biomass and production in polar planktonic and sea ice microbial communities: a comparative study

Algal and bacterial biomass and production were measured in the plankton, platelet ice and congelation ice communities at one station in McMurdo Sound, Antarctica during September and October 1986. Bacterial abundances and particulate organic carbon and nitrogen were 10 to 100 times greater in the plankton than in the sea ice, whereas the chlorophyll a concentrations in the plankton and sea ice microbial communities (SIMCO) were similar Rates of both light-limited and light-saturated photosynthesis and daily primary production were 2 to 6 times greater in the plankton than in the SIMCO. Bacterial growth rates ranges from 0.7 to 1.5 d^-1 in all three communities; however, because of the greater bacterial biomass in the plankton, bacterial production was 15 to 20 times higher there than in the SIMCO. These results suggest that during the early austral spring, planktonic production contributes significantly to total production in ice-covered environments.     

Nocturnal synthesis and diurnal degradation of phytoplankton biomass in surface waters

Natural populations of phytoplankton were collected near the Bay of Bourgneuf, France, in spring 1982, and were subjected to natural surface irradiance outdoors. They exhibited exponential growth on time scales of a week, but significant decreases in biomass indicators such as chlorophyll a and particulate nitrogen were observed during daytime. At night, these decreases were more than compensated by increases in the same biomass variables, which could double over 12 h of darkness. These features are characteristic of phytoplankton populations in surface waters which cannot escape high irradiances, and may be representative of situations in incubation bottles held at fixed depths near the surface. Under such conditions, a decrease in biomass during daytime should not necessarily be interpreted as irreversible damage unless growth measurements are carried out over the following night hours to check for possible recovery.     

Factors affecting the photosynthetic capacity of laboratory cultures of the diatom Phaeodactylum tricornutum

The light-saturated photosynthetic capacity of cultures of Phaeodactylum tricornutum Bohlin grown under different conditions has been measured. In batch cultures grown in a regime of alternating light and dark periods, the photosynthetic capacity reaches a maximum before the end of the exponential phase of growth, and declines thereafter. In cultures illuminated at 0.7 mW (milliwatt)/cm², there is a 75% falloff in photosynthetic capacity per cell over an 8 day period following the time of maximum photosynthetic capacity. At 1.75 mW/cm², the corresponding fall-off is 85% over a 4 day period. Cultures exposed to a prolonged period of darkness (up to 16 days at 18°C) maintain a high photosynthetic capacity. Incubation in darkness also protects the cells from the deleterious effects of high temperature (28°C) upon photosynthetic capacity. The various fluctuations of photosynthetic capacity occur without any accompanying major changes in the concentration of chlorophyll a. Evidence from estimations of total protein and of the gross pattern of photosynthetic assimilation under different conditions suggests that the changes in photosynthetic capacity are largely controlled by the enzymic component of the photosynthetic machinery. By carefully controlling the conditions of dark incubation, the photosynthetic capacity can be reduced to a very low level without significantly affecting chlorophyll a concentration. Since the effect on photosynthetic capacity is reversible, it is possible to study aspects of chloroplast development without the complication of an associated synthesis of chlorophyll.     

The spring phytoplankton bloom in Lindåspollene,a land-locked Norwegian fjord. II. Biomass and activity of net- and nanoplankton

From February 24 to April 24, weekly samples were collected at fixed depths at one station in Lindåspollene, a land-locked Norwegian fjord. Adenosine triphosphate (ATP), chlorophyll a, phaeophytin, ¹⁴C assimilation, and respiratory activity [electron transport system (ETS) activity] were measured in the net- (>30 m) and nanoplankton. Netplankton contained on the average 48% of the total chlorophyll a and 56% of the ATP, but contributed only 7% to the total carbon assimilation and 11% to the ETS activity. The assimilation numbers for net- and nanoplankton ranged from 0 to 1.2 and from 1.5 to 13.2, respectively. At the oxygen/hydrogen sulphide interface, high concentrations of ATP, but not of chlorophyll a, were found in the nanoplankton fraction. Netplankton algae grew actively only in the first phase of the bloom, and nanoplankton predominated later, apparently due to low nutrient concentrations. During the bloom, Skeletonema costatum made up the main part of the biomass. The number of cells in the chains decreased throughout the bloom, possibly reflecting the lowered silicate content. It appeared that only nanoplankton were grazed by zooplankton, while netplankton sank to the bottom and represented input to the benthos.     

Pools of chlorophyll and live planktonic diatoms in aphotic marine sediments

Chlorophyll a and numbers of live pelagic diatoms were recorded from sediment depth profiles at 11 stations in the oligotrophic Øresund, Denmark, in late-June. Extraction efficiency of chlorophyll a analysed fluorometrically did not differ significantly between paired samples of frozen-thawed and fresh sediment. The depth profiles of chlorophyll a could be explained by a diagenetic model involving two different chlorophyll pools: one reactive pool declining exponentially with core depth, and one non-reactive pool, of about 1 µg Chl ml^-1 wet sediment, being constant with depth. The number of live diatoms, quantified by the dilution-extinction method, and expressed in terms of most probable number (MPN), declined from an average of about 300,000 g^-1 in the surface sediment to zero values at a depth of 13 cm. The number of live cells was significantly correlated with the sediment chlorophyll a, and the profiles of live cells as well as reactive chlorophyll followed the same exponential decline with core depth, suggesting that the main source of chlorophyll in the sediment was live pelagic diatoms. Taxonomic composition of diatoms in the sediment, dominated by the pelagic genera Chaetoceros, Thalassiosira and Skeletonema, matched the species composition in the water column 3 months earlier during the spring bloom. Regular recordings of the phytoplankton community in the water column showed that only these specific bloom species could be the source of the sediment content of diatoms and chlorophyll a. Further, the ratios between live cells and chlorophyll a were similar in the sediment and in the spring bloom. A conservative estimate of depth-integrated pools of diatoms in the sediment suggested that about 44% of the total phytoplankton biomass during the spring bloom was still present as live cells in the sediment after 3 months. This indicates that the spring bloom input to the sediment is not degraded immediately by the benthic fauna.     

Implications of salinity fluctuation for growth and nitrogen metabolism of the marine diatom Ditylum brightwellii in comparison with Skeletonema costatum

In 1987 effects of salinity fluctuations on growth of Ditylum brightwellii (West) Grunow, isolated from the Eastern Scheldt estuary (SW Netherlands) in 1981, were studied. D. brightwellii was grown in a 12 h light: dark cycle at constant salinity in brackish media. Ammonium-limited cultures were subjected to a salinity fluctuation. By decreasing the salinity to 4.8 photosynthesis and cell division were inhibited; cells were deformed. Protein and carbohydrate contents increased slightly, dark respiration was stimulated and cellular levels of glucose decreased at low salinity; this indicated a possible role of sugars in osmoregulation. Ammonium was accumulated in cultures, amino acids may have been stored; the role of the vacuole as a storage compartment was discussed. Both the ammonium uptake capacity and the affinity for ammonium decreased. Nitrogen limitation was relieved in the transient state. [With the activity of the nitrogen assimilation enzymes glutamine synthetase (GS) and glutamate synthase (GOGAT) being uninhibited by lower salinity.] Recovery from hypo-osmotic stress during a salinity increase was initiated by stimulated photosynthesis; chlorophyll a increased, but persistant contractions of cytoplasm (with chloroplasts) may have delayed cell growth. The glutamate dehydrogenase (GDH) activity decreased further whereas the cellular level of alanine increased in the presence of large ammonium pools; this may indicate a temporary activity of ADH (alanine dehydrogenase). Skeletonema costatum (Greville) Cleve, recovered faster from hypoosmotic stress than did D. brightwellii. Due to an osmotic shock from 13.6 to 7.1 S both species excreted amino acids and glucose; S. costatum accumulated more glucose, D. brightwellii accumulated more amino acids. S. costatum may with the competition for nitrogen in waters with an unstable salinity; it will replace D. brightwellii.Contribution no. 427 Delta Institute for Hydrobiological Research, Yerseke, The Netherlands     

Impact of a temporal salinity decrease on growth and nitrogen metabolism of the marine diatom Skeletonema costatum in continuous cultures

In 1987 effects of salinity fluctuations on growth of the centric diatom Skeletonema costatum (Greville) Cleve, isolated from the brackish Krammer estuary (SW Netherlands) in 1981, were investigated. Continuous cultures (12 h light: dark cycle) of S. costatum were adapted to constant salinity in natural (16.1) and synthetic (13.5) media. For several days the ammonium-limited cultures were exposed to a salinity fluctuation (minimum 4.8). Decreasing salinity caused an inhibition of photosynthesis, dark respiration and cell growth. Cellular pools of glucose decreased. While the carbohydrate content remained constant, the protein content increased slightly. Net carbon fixation was more inhibited than nitrogen assimilation. Ammonium accumulated during a salinity decrease; a total decline of the overcapacity of ammonium uptake was noticed and nitrogen limitation was relieved. Amino acid pools decreased, probably as a result of excretion (osmoregulation). The enzymes invoilved in ammonium assimilation showed an increased activity. Cellular activities were resumed during a salinity increase. Chlorophyll a increased; photosynthesis, ammonium uptake and growth were stimulated. The ammonium uptake capacity recovered completely; glutamic acid accumulation and increased glutamate-dehydrogenase (GDH) activity indicated supplementary ammonium assimilation via GDH. The activities of glutamine synthetase/glutamate synthase (GS/GOGAT) and GDH stabilized, and the cells returned to steady state under ammonium limitation.Communication no. 426 Delta Institute for Hydrobiological Research, Yerseke, The Netherlands