Characteristics of the uptake system for L-lysine and L-arginine inPhaeodactylum tricornutum

Cells ofPhaeodactylum tricornutum Bohlin develop the ability to take up L-lysine when they are deprived of nitrogen (illuminated in nitrogen-free medium), carbon (incubated in darkness) or both. Cells with a developed uptake system take up and accumulate lysine in an unchanged form. Uptake occurs under either aerobic or anaerobic conditions and is dependent on the presence of sodium⁺ ions (K _s ^{Na +}=,ca. 10 mM). Some potassium⁺ ions are necessary for uptake, presumably within the cells, but with potassium⁺-replete cells, increasing K⁺ concentration depresses lysine uptake. The lysine-uptake porter also transports L-arginine.K _s values are about 1.5 M for lysine and 0.5 M for arginine. It is, however, possible that the uptake system developed by incubating cells in darkness differs from that produced in light; it shows a pronounced pH optimum at pH 8.5, whereas the activity of the light-developed system declines from pH 6.5 to pH 9.0 and correlates well with the concentration of lysine⁺. The uptake system developed in darkness may also have a higher affinity for lysine. Lysine uptake is not inhibited by 1 mM concentrations of nitrate, nitrate, ammonium, or urea nor by similar concentrations of amphoteric or acidic amino acids.     

Nitrogenous nutrition in the euphotic zone of the Central North Pacific Gyre

The uptake rates of the three nitrogen compounds ammonium, nitrate, and urea were measured in the oligotrophic North Central Pacific Gyre in August–September 1985. The measurements were performed by using ¹⁵N-labelled substrates and incubating for short-time periods (3 to 4 h) under simulated in situ conditions. Ambient concentrations of the nitrogenous nutrients were generally below 0.10 mol l^-1. The average total daily nitrogen uptake rate, integrated over the euphotic zone, was 12.5 mmol N m^-2 d^-1. Diel studies in the upper water mass resulted in a calculated phytoplankton growth rate of 1.3 d^-1. Ammonium was the dominating nutrient, accounting for on the average 54% of the total nitrogen uptake, while urea uptake represented 32% and nitrate 14%. Ammonium uptake rates at a coastal station off the Hawaiian Islands were very close to the rates found at the oceanic station. Organisms <3 m dominated the nitrogen assimilation, being responsible for about 75% of the ammonium uptake. The nitrogen uptake rates in this study seem to be higher than those found by earlier investigations in the area, but correlated well with other productivity measurements performed during the same cruise.     

Effect of nitrogen supply on nitrogen uptake,accumulation and assimilation in Porphyra perforata (Rhodophyta)

Porphyra perforata J. Ag. was collected from a rocky land-fill site near Kitsilano Beach, Vancouver, British Columbia, Canada and was grown for 4 d in media with one of the following forms of inorganic nitrogen: NO ₃ ^- , NH ₄ ⁺ and NO ₃ ^- plus NH ₄ ⁺ and for 10 d in nitrogen-free media. Internal nitrogen accumulation (nitrate, ammonium, amino acids and soluble protein), nitrate and ammonium uptake rates, and nitrate reductase activity were measured daily. Short initial periods (10 to 20 min) of rapid ammonium uptake were common in nitrogen-deficient plants. In the case of nitrate uptake, initial uptake rates were low, increasing after 10 to 20 min. Ammonium inhibited nitrate uptake for only the first 10 to 20 min and then nitrate uptake rates were independent of ammonium concentration. Nitrogen starvation for 8 d overcame this initial suppression of nitrate uptake by ammonium. Nitrogen starvation also resulted in a decrease in soluble internal nitrate content and a transient increase in nitrate reductase activity. Little or no decrease was observed in internal ammonium, total amino acids and soluble protein. The cultures grown on nitrate only, maintained high ammonium uptake rates also. The rate of nitrate reduction may have limited the supply of nitrogen available for further assimilation. Internal nitrate concentrations were inversely correlated with nitrate uptake rates. Except for ammonium-grown cultures, internal total amino acids and soluble protein showed no correlation with uptake rates. Both internal pool concentrations and enzyme activities are required to interpret changes in uptake rate during growth.     

Assessment of simultaneous uptake of nitrogenous nutrients (15N) and inorganic carbon (13C) by natural phytoplankton populations

The simultaneous uptake of nitrogenous nutrients and inorganic carbon was measured in shipboard incubations of natural phytoplankton populations, using tracer additions of ¹³C-bicarbonate and ¹⁵N-labelled nitrogenous substrates. From March 1991 through March 1992, three stations on the Scotian Shelf (eastern Canada) were sampled monthly at ten depths in the euphotic zone. Additions of labelled nitrogen compounds ranged between 0.5 and 98% of ambient concentrations. Most of the C/N (at/at) uptake ratios were lower than the Redfield ratio, suggesting that nitrogen was not limiting. The fixation of carbon with and without addition of nitrate, ammonium or urea was generally similar. Some samples presented significant differences in carbon uptake rate between the four treatments, but these differences were not related to nitrogen enrichment (percent or nitrogen species). Given these results, the double-labelling method appears to be a reliable tool for measuring the simultaneous uptake of carbon and nitrogen by natural phytoplankton.     

Partitioning of nitrogen and carbon in cultures of the marine diatom Thalassiosira fluviatilis supplied with nitrate,ammonium, or urea

Small or negligible differences in growth rates, average cell size, yields in cell numbers and total cell volumes were found in cultures of Thalassiosira fluviatilis inriched with nitrate, ammonium, or urea. Intracellular pools of unassimilated nitrate, nitrate, and ammonium were found in nutrient-rich conditions, but urea was not accumlated internally. Nitrogen assimilation into organic combination rather than nitrogen nutrient uptake was a critical rate-limiting step in nitrogen utilization. The free amino acid pool, protein, lipid-associated nitrogen, pigments, and total cell nitrogen were all highest in young or mature phase cells and decreased with age in senescent cells, whereas chitan, lipid, carbohydrate, and total cellular carbon all continued to increase during senescence. Dissolved organic nitrogen compounds accumulated in the medium only during senescence. C:N and lipid:protein were sensitive indicators of nitrogen depletion and age in T. fluviatilis.     

Nitrogen uptake kinetics in three year-classes of Laminaria groenlandica (Laminariales: Phaeophyta)

Nitrate and ammonium uptake rates were measured for three year-classes of the perennial macrophyte Laminaria groenlandica Rosenvinge, collected from nitrogen-depleted waters in Barkley Sound, British Columbia, Canada, in summer 1981. A time course of uptake rate revealed that ammonium uptake was high during the first hour and then decreased for all three year-classes; the opposite pattern was exhibited for the time course of nitrate uptake rate. Nitrate uptake rate increased linearly with nitrate concentration up to the highest level tested (60 M). The nitrate uptake rate of first-year plants was three times higher than second- and third-year plants; ammonium uptake rates showed similar patterns to those for nitrate. The interaction between nitrate and ammonium was examined for first-year plants. Nitrate and ammonium were taken up simultaneously and uptake rates were identical and equal to uptake rates when only nitrate or ammonium was present in the medium. Therefore, first-year plants are able to take up twice as much inorganic nitrogen per unit time when both nitrate and ammonium are present. First-year plants showed significant diel periodicity in ammonium uptake rates, whereas second- and third-year plants showed no periodicity in nitrate or ammonium uptake rates.     

In-situ measurements of nitrogenous nutrient uptake by kelp (Ecklonia maxima) and phytoplankton in a nitrate-rich upwelling environment

Nitrogen uptake by the kelp Ecklonia maxima Osbeck and phytoplankton was examined under different conditions of nutrient availability in a kelp bed off the Cape of Good Hope by measuring nutrient depletion in large plastic bags by the kelp and ¹⁵N uptake by phytoplankton. E. maxima took up nitrate and ammonia, but not urea, and showed only a weak preference for reduced nitrogen. Phytoplankton absorbed all three forms of nitrogen available, with a preference for ammonia and urea. Ambient nitrate concentration exhibited a marked and rapid decrease with northerly winds and an increase in response to offshore southerly winds. Nitrogen uptake by E. maxima was linearly related to ambient concentration and did not saturate even at nitrate concentrations >20g-at N l^-1, resulting in a significantly higher tissue nitrogen content under upwelling conditions. Nitrate imported by upwelling was the chief source of nitrogen utilised within the kelp bed. Locally regenerated nitrogen (ammonia and urea) was calculated to contribute only ca 4% of total nitrogen uptake during upwelling and 30% during the relaxation or downwelling phase.     

Utilization of L-lysine and L-arginine by the diatomPhaeodactylum tricornutum

The rate of growth ofPhaeodactylum tricornutum Bohlin with L-lysine as sole nitrogen source is about half that with ammonium or nitrate, and only one of the two nitrogen atoms in the lysine molecule appears to be used for growth. The organism cannot grow heterotrophically with lysine as carbon source. The rate of lysine metabolism is slow and most of that utilized is incorporated into protein, this process being faster in light than in darkness. The dark incorporation of lysine-carbon into protein is stimulated by addition of ammonium, whereas incorporation in light is unaffected. Arginine-carbon is also mainly incorporated into protein. Light has little effect on this incorporation, and addition of ammonium decreases it both in light and darkness. There is no appreciable conversion of the carbon of either amino acid to carbon dioxide. Although, under anaerobiosis, lysine is taken up and accumulated, negligible metabolism of lysine occurs.     

Effects of light intensity on aging of the dinoflagellate Gonyaulax polyedra

Gonyaulax polyedra Stein grown in increasingly nutrientlimited batch culture undergoes the following changes (collectively termed aging): there is a decline in the intracellular concentrations of carbon, nitrogen and photosynthetic pigments; nitrate reductase activity decreases; rates of respiration and photosynthesis fall; and cell division virtually ceases (accompanied in bright light by a decrease in the volume of individual cells). The effect of light intensity on these aging events was tested by growing cells in either bright or dim light. The bright light (330 E m^-2 s^-1) was enough to saturate photosynthesis and the dim light (80 E m^-2 s^-1) was low enough to induce significant shade adaptation of photosynthesis without lowering growth rate. At both light intensities, a decline in carbon and nitrogen content preceded or accompanied all other monitored changes, and the sequence of aging events was similar. However the onset of the decline in intracellular nutrients and photosynthetic rate in low-light cells was delayed by a least one cell division time (i.e., to twice the cell density) in comparison to cells under bright light. At both light levels, pigment-protein complexes of the photosynthetic apparatus began to break down after intracellular carbon and nitrogen had been depleted to a critically low level. The beginning of the drop in pigmentation signalled the end of log-phase growth. It is suggested that the greater pigmentation of low-light cells may represent a larger nutrient supply than found in bright-light cells and could increase the survival time of nutrient-stressed populations.     

Light-dependency of nitrate uptake by phytoplankton over the spring bloom in Auke Bay,Alaska

The effect of light intensity on nitrate uptake by natural populations of phytoplankton was examined by ¹⁵N traceruptake experiments during the spring (March–May 1987) in Auke Bay, Alaska. The data were fit to a rectangular hyperbolic model which included a term for dark uptake. Three types of curves described nitrate uptake as a function of light intensity. The first (Type I) had a low half-saturation light intensity (K _I), low chlorophyll-specific uptakes rates, no dark uptake and occasional photoinhibition. These were observed during a period of biomass decrease, accompanied by low daily light and strong wind, prior to the major bloom. The second type (Type II) had relatively high K _I, high chlorophyll-specific uptake rates, and no dark uptake. Type II curves were observed during most of the period prior to nitrate depletion in the surface waters. Types I and II both appeared prior to nitrate depletion in the water and reflected variations in the light history of the phytoplankton population. The third type (Type III) occurred in nitrate-deplete conditions, when nitrate uptake was less dependent on light intensity (i.e., high rates of dark uptake and lower K _I). Decreased light-dependency during this period was coupled with physiological nitrogen deficiency in the population. Comparing these parameters to those of photosynthetic carbon fixation, K _Ivalues of nitrate uptake were generally higher than those of photosynthesis prior to nitrate depletion, and lower during nutrient-deplete conditions.     

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.     

Effect of seawater velocity on inorganic nitrogen uptake by morphologically distinct forms of Macrocystis integrifolia from wave-sheltered and exposed sites

In conditions of low water motion (<0.06 ms^–1), the availability of essential nutrients to macroalgae, and thus their potential productivity, may be limited by thick diffusion boundary-layers at the thallus surface. The ability of macroalgae to take up nutrients in slow moving water may be related to how their blade morphology affects diffusion boundarylayer thickness. For the giant kelp, Macrocystis integrifolia Bory, morphological measurements indicate that blades of plants from a site exposed to wave action are thick, narrow and have a heavily corrugated surface. In contrast, blades from a site with a low degree of water motion are relatively thin, with few surface corrugations and large undulations along their edges. The aim of our work was to test the hypothesis that morphological features of M. integrifolia blades from a sheltered site allow enhanced inorganic nitrogen uptake at low seawater velocities compared to blades with a wave-exposed morphology. The rate of nitrate and ammonium uptake by morphologically distinct blades of M. integrifolia, from sites that were sheltered from and exposed to wave action, were measured in the laboratory at a range of seawater velocities (0.01 to 0.16 ms^–1), between March and May 1993. For both sheltered and exposed blade morphologies, nitrate and ammonium uptake rates increased with increasing seawater velocity, reaching a maximum rate at 0.04 to 0.06 ms^–1. Uptake parameters V _max (maximum uptake rate) and U _0.37 (the velocity at which the uptake rate is 37% of the maximum rate) were estimated using an exponential decay formula. These parameters were similar for both blade morphologies, at all seawater velocities tested. Additional measurements suggest that the nitrogen status of M. integrifolia blades from wavesheltered and exposed sites were similar throughout the experimental period, and thus nitrogen status did not affect the rate of nitrogen uptake in these experiments. on the basis of these results, we conclude that blade morphology does not enhance nitrogen uptake by M. integrifolia in conditions of low water motion. Potential effects of diffusion boundary-layers on kelp productivity are discussed.     

The rapid response of the marine diatom Skeletonema costatum to changes in external and internal nutrient concentration

A nitrogen-deficient batch culture of the marine diatom Skeletonema costatum, when resupplied with a mixture of nitrate and ammonium, showed an initial enhanced nitrate uptake rate leading to a large internal concentration (pool) of nitrate. Following this initial nitrate uptake event, nitrate uptake ceased, and nitrate assimilation was inhibited until the ammonium present was used. At this point, nitrate uptake resumed and nitrate assimilation began. No internal ammonium pool was observed during nitrate utilization, but a large nitrate pool remained throughout the utilization of external nitrate. The internal nitrate pool decreased rapidly after exhaustion of nitrate from the culture medium, but growth of cellular particulate nitrogen continued for about 24 h. A mathematical simulation model was developed from these data. The model cell consisted of a nitrate pool, ammonium pool, dissolved organic nitrogen pool, and particulate nitrogen. It was found that simple Michaelis-Menten functions for uptake and assimilation gave inadequate fit to the data. Michaelis-Menten functions were modified by inclusion of inhibitory and stimulatory feedback from the internal pools to more accurately represent the observed nutrient utilization.     

Regulation of growth in Laminaria digitata: use of in-vivo nitrate reductase activities as an indicator of nitrogen limitation in field populations of Laminaria spp.

The seasonal variation in growth rate of a population of Laminaria digitata (Huds.) Lamour growing at Arbroath, Scotland was studied between August 1981 and September 1982, and was found to follow the biphasic annual cycle typical of this genus. Growth rates were maximum (0.3 cm cm^-1 mo^-1) in early June and minimum (0.05 cm cm^-1 mo^-1) between September and January. An analysis of the relationship between the seasonal changes in environmental factors (inorganic nitrogen concentrations, irradiance and temperature) with those of growth rate and the accumulation or mobilisation of cellular reserves of carbohydrates and nitrate, indicated that growth was nitrogen-limited between June and October and light-limited (with a possible co-involvement of temperature) for the remainder of the year. These conclusions were supported by the seasonal changes in the ratio of actual: potential in-vivo nitrate reductase activities in L. digitata, thus confirming the suitability of this technique for monitoring the occurrence of nitrogen limitation in Laminaria spp. The seasonal changes in blade nitrate reductase activities closely followed those of growth rate, with maximum activities [0.3 mol NO ₃ ^- reduced g^-1 (wet wt) h^-1] being present in late May and minimum levels [0.01 mol NO ₃ ^- reduced g^-1 (wet wt) h^-1] occurring between November and March. The correlation observed between nitrate reductase activities and growth rate is consistent with the ability of Laminaria spp. to store excess inorganic nitrogen, available during winter and early spring, as NO ₃ ^- , and with the requirement to conserve enzyme protein during the summer period of nitrogen limitation.     

Interactions between nitrate and ammonium uptake: variation with growth rate,nitrogen source and species

The interaction between nitrate and ammonium uptake was examined as a function of preconditioning growth rate and nitrogen source by adding nitrate, ammonium, or both to nitrogen-sufficient,-deficient, and-starvedSkeletonema costatum (Grev.) Cleve and nitrogen-deficientChaetoceros debilis Cleve. By simultaneously measuring the internal accumulation of intermediates of nitrogen assimilation and the rates of nitrogen assimilation, the metabolic control of nitrogen uptake could be assessed. After the simultaneous addition of nitrate and ammonium to culture, both nitrate and ammonium uptake rates were decreased in comparison with the rates observed when each was added alone, although nitrate uptake was usually decreased more than ammonium uptake. Since both nitrate and ammonium uptake rates vary with time, preconditioning growth conditions, nitrogen sources present, and species, it was necessary to use several different indices to quantify inhibition. In general, ammonium inhibition of nitrate uptake inS. costatum was greatest in cultures preconditioned to ammonium and those at low growth rates, whereas ammonium uptake was inhibited most in cultures preconditioned to nitrate. In nitrogen-deficientC. debilis, nitrate uptake was more inhibited by ammonium, but uptake returned to normal rates more quickly than inS. costatum, whereas inhibition of ammonium uptake was similar. These results explain why the interaction between nitrate and ammonium uptake in the field can be so variable. Inhibition of uptake is not controlled by internal ammonium or total amino acids, nor is it related to the inability to reduce nitrate. Instead, inhibition must be determined in part by the external concentration of nitrogen compounds and in part by some intermediate(s) of nitrogen assimilation present inside the cell.Bigelow Laboratory Contribution No. 82022     

Effect of bryozoan colonization on inorganic nitrogen acquisition by the kelps Agarum fimbriatum and Macrocystis integrifolia

The effect of bryozoan colonization on inorganic nitrogen acquisition by Agarum fimbriatum Harv. and Macrocystis integrifolia Bory., collected from the west coast of Vancouver Island, British Columbia, Canada, was examined in laboratory experiments during June and July 1992. Pieces of kelp blades that were completely covered on one side by the bryozoans Lichenopora novae-zelandiae Busk or Membranipora membranacea, L., or uncolonized (clean treatment), were used to estimate the rate at which nitrate and ammonium were removed from the surrounding seawater. In addition, the rate of ammonium excretion by bryozoans isolated from their associated kelp was measured and also estimated from the results of the uptake experiments. Values obtained were used to estimate the contribution of ammonium excreted by bryozoans to the total amount of inorganic nitrogen available to the associated kelp. Both bryozoan species reduced the ability of the associated kelp to remove nitrate and ammonium from seawater but provided a source of ammonium to the kelp through excretion. The nitrogen status of colonized and clean kelp disks was determined from the ratio of total particulate carbon to total particulate nitrogen (C:N ratio). The C:N ratios for A. fimbriatum colonized with either L. novae-zelandiae or M. membranacea were similar (C:N=12 to 14), and differences between colonized and clean treatments were not significant. For A. fimbriatum, therefore, the C:N ratio indicates that this species was not nitrogen limited at the time of the present study. In contrast, both colonized and clean disks of M. integrifolia were nitrogen limited, but colonized disks (C:N=19) were significantly less limited by nitrogen than clean disks (C:N=29). Results are discussed in relation to the different environments inhabited by both kelp species and are consistent with the hypothesis that ammonium excreted by bryozoans was an important source of inorganic nitrogen to M. integrifolia, but not to A. fimbriatum, at the time of the study.     

Inorganic nitrogen metabolism in marine bacteria: Nitrate uptake and reduction in a marine pseudomonad

Growth experiments in batch cultures indicated that the uptake of nitrate by the marine pseudomonad PL1 was inhibited in the presence of ammonia provided that the ammonia concentration was higher than 1 mM. At ammonia concentrations of less than about 1 mM, however, both nitrate and ammonia were utilised simultaneously. The saturation constants for nitrate and ammonia uptake were both 2.6x10^-4 M, and similar to the Michaelis constants of nitrate reductase for nitrate (2.9x10^-4 M) and glutamine synthetase for ammonia (2x10^-4 M). Nitrate reductase activity linked to NADH was detected in chemostat-grown cultures with nitrate as nitrogen source, and in cultures containing limiting concentrations of nitrate and ammonia, ammonia or glutamate. Enzyme synthesis appeared to be repressed in cultures containing an excess of ammonia or glutamate. Chemostat cultures utilised ammonia or glutamate in preference to nitrate, while there was no marked preference between ammonia and glutamate.     

Effect of boundary layer transport on the fixation of carbon by the giant kelp Macrocystis pyrifera

The uptake of inorganic carbon into the thallus of Macrocystis pyrifera (L.) C. Ag. requires first that the inorganic carbon pass through the water medium to the plant surface. This transfer of inorganic carbon to the thallus must take place through a boundary layer. Experiments in water tunnels indicate that the boundary layer adjacent to the M. pyrifera blade may be turbulent in water speeds as low as 1 cm sec^-1. Photosynthetic output of the blade can be increased by a factor of 300% by increasing water speeds over the blade surface from 0 to 4 cm sec^-1. This is consistent with a decrease in the thickness of the boundary layer. Above 4 cm sec^-1, the assimilation of carbon was limiting. The assimilation of carbon is generally known to follow Michaelis-Menten-like kinetics. Combining the two uptake steps into an overall model of carbon uptake agrees well with photosynthetic data obtained from M. pyrifera under varying conditions of water speed and bicarbonate concentrations in the laboratory. The ecological and morphological consequences of these findings are discussed.     

Nutrient uptake kinetics and growth of zooxanthellae maintained in laboratory culture

Growth characteristics and nutrient uptake kinetics were determined for zooxanthellae (Gymnodinium microadriaticum) in laboratory culture. The maximum specific growth rate (_max) was 0.35 d^-1 at 27 °C, 12 hL:12 hD cycle, 45 E m^-2 s^-1. Anmmonium and nitrate uptake by G. microadriaticum in distinct growth phases exhibited Michaelis-Menten kinetics. Ammonium half-saturation constants (K_s) ranged from 0.4 to 2.0 M; those for nitrate ranged from 0.5 to 0.8 M. Ammonium maximum specific uptake rates (V_max) (0.75 to 1.74 d^-1) exceeded those for nitrate (0.14 to 0.39 d^-1) and were much greater than the maximum specific growth rate (0.35 d^-1), suggesting that ammonium is the more significant N source for cultured zooxanthellae. Ammonium and nitrate V_max values compare with those reported from freshly isolated zooxanthellae. Light enhanced ammonium and nitrate uptake; ammonium inhibited nitrate uptake which was not reported for freshly isolated zooxanthellae, suggesting that physiological differences exist between the two. Knowledge of growth and nutrient uptake kinetics for cultured zooxanthellae can provide insight into the mechanisms whereby nutrients are taken up in coral-zooxanthelae symbioses.Contribution No. 1515 from the University of Maryland Center for Environmental and Estuarine Studies, Chesapeake Biological Laboratory, Solomons, Maryland 20688-0038, USA     

Carbon-nitrogen relations at whole-cell and free-amino-acid levels during batch growth of Isochrysis galbana (Prymnesiophyceae) under conditions of alternating light and dark

Isochrysis galbana Parke, Strain CCAP 927/1, was grown in ammonium-limited batch culture under a 12 h light: 12 h dark illumination cycle. Samples were taken every 12 h over the 26 d period from lag phase through exponential into stationary phase (no net carbon fixation), with more frequent sampling at points of interest. Exponential cell-specific growth rate was 0.3 to 0.4d^-1. Cell division occurred during the dark phase, while cell volume increase, ammonium uptake, and pigment synthesis occurred during the light. Stationary phase cells were small, and the lag phase was long (5 d) even though the C:N ratio had returned from 18 to 6.5 within 2 d, followed by synthesis of chlorophyll a. Net chlorophyll synthesis ceased within 4 d of exhaustion of the nitrogen source. The chlorophyll c: chlorophyll a ratio remained constant during increasing nitrogen deprivation. Biovolume and carotenoids correlated with carbon biomass. Levels of chlorophyll a correlated poorly with carbon fixation and carbon biomass once the nitrogen source had been exhausted. Except after the addition of ammonium to nitrogen-deprived cells (refeeding), the content of intracellular glutamine and the glutamine: glutamate ratio were low during the dark phase, rising to a plateau within the first 1 h of illumination. Refeeding of cells which had only just exhausted the extracellular nitrogen source resulted in a much smaller increase in glutamine than refeeding of nitrogen-starved (stationary-phase) cells. Nitrogen biomass correlated with the presence of an unidentified intracellular amine.