Marine diatoms grown in chemostats under silicate or ammonium limitation. IV. Transient response of Chaetoceros debilis,Skeletonema costatum,and Thalassiosira gravida to a single addition of the limiting nutrient

The kinetic response of ammonium- or silicate-limited and ammonium- or silicatestarved populations of Chaetoceros debilis, Skeletonema costatum, and Thalassiosira gravida was determined by a single addition of the limiting nutrient to a steady-state culture and subsequent monitoring of the nutrient disappearance of the limiting and non-limiting nutrients at frequent time intervals. The kinetic response of nonlimited (nutrient) populations of these three species was also determined. Three distinct modes of the uptake of the limiting nutrient were observed for ammonium-or silicate-limited populations of these three species, surge uptake (V _s ), internally (cellular) controlled uptake (V _i ), and externally (ambient limiting nutrient concentration) controlled uptake (V _e ). Non-limited populations did not exhibit the three distinct segments of uptake, V _s , V _i and V _e . Estimates of the maximal uptake rate (V _max) and the Michaelis constant (K _s ) were obtained from nutrient-limited populations during the V _e segment of the uptake curve. Pooled values of V _e for the three ammonium-limited populations yielded V _max and K _s estimates of 0.16 h^-1 and 0.5 g-at NH₄–N l^-1. Kinetic data derived from the V _e segment of the uptake curve for silicate-limited populations yielded different values of V _max and K _s for each of the three species. In a number of parameters that were measured, T. gravida was clearly different from C. debilis and S. costatum and its recovery from nutrient starvation was the slowest. Recovery of all species from silicate limitation or starvation was slower than from ammonium limitation or starvation. Ammonium-starved populations maintained a maximal uptake rate at a substrate concentration an order of magnitude lower (0.1 g-at NH₄–N l^-1) than that observed for NH₄-limited populations (1.0 g-at NH₄–N l^-1). Adaptation to the severity of the nutrient limitation occurred as changes in the magnitude of cellular characteristics, such as short-term uptake potential (V _s ) and affinity for the substrate (K _s ). The consequence of these results are discussed in terms of another possible mechanism to explain changes in species composition and succession in nutrient-depleted environments.Contribution No. 944 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Interactions of inorganic nitrogen in the uptake and assimilation by marine phytoplankton

The effect of ambient ammonium concentration on the nitrate uptake rate of marine phytoplankton was investigated. These studies consisted of laboratory experiments using unialgal species and field experiments using natural phytoplankton communities. In laboratory experiments, ammonium suppressed the uptake rates of nitrate and nitrite. Approximately 30 min were required for ammonium to exhibit its fully inhibitory effect on nitrate uptake. At high ammonium concentration (>3 g-at/l), a residual nitrate uptake rate of approximately 0.006 h^-1 was observed. When the ambient ammonium concentration was reduced to a value less than 1 g-at/l, the suppressed nitrate uptake rate subsequently attained a value comparable to that observed before the addition of ammonium. A range of 25 to 60% reduction in the nitrate uptake rate of natural phytoplankton communities was observed at ambient ammonium concentrations of 1.0 g-at/l. A mechanism is proposed for the suppression of nitrate uptake rate by ammonium through feedback control of the nitrate permease system and/or the nitrate reductase enzyme system. The feedback control is postulated to be regulated by the level of total amino acids in the cell.Contribution No. 936 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA. This paper represents a portion of a dissertation submitted to the Department of Oceanography, University of Washington, Seattle, in partial fulfillment of the requirements for the Ph.D. degree.     

Marine diatoms grown in chemostats under silicate or ammonium limitation. I. Cellular chemical composition and steady-state growth kinetics of Skeletonema costatum

Skeletonema costatum was grown in chemostats under ammonium or silicate limitation to examine its growth kinetics and changes in cellular chemical composition at different steady-state growth rates. When the relationship between the effluent limiting substrate concentration and steady-state growth rates was examined, deviations from the simple hyperbolic form of the Monod growth equation were noted at low and high dilution rates. The data from the plot of growth rate and substrate concentration were divided into 4 regions and the relationship of these region to cell quota is discussed. Two physiological states were identified. All populations grown at D<0.05 h^-1, regardless of the size of the cells or the magnitude of Q, exhibited a maximal growth rate of approximately 0.05 h^-1, while populations grown at higher dilution rates (D>0.06 h^-1 to 0.14 h^-1). The maximal value of growth rate is obtained only in cultures grown at very high dilution rates where nutrient shift-up appears to occur, the cell quota approaches a maximum and the heterogeneous cell population becomes more homogeneous.Contribution No. 881 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA. This paper represents a portion of two dissertations submitted by P.J.H. and H.L.C. to the Department of Oceanography, University of Washington, Seattle, in partial fulfillment of the requirements for the Ph.D. degree.     

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     

Ammonia excretion by zooplankton and its significance to primary productivity during summer

Excretion rates of ammonia have been determined for zooplankton off the coasts of Washington and Oregon (USA). Rates varied from 0.16 to 0.60 g-at NH ₄ ⁺ -N/mg dry weight/day for most planktonic animals, and from 0.02 to 0.06 for jellyfishes. Ammonia concentration in seawater was low in offshore regions. Ammonia released by zooplankton was studied in relation to primary productivity during summer. It was found that, in the Columbia River plume offshore, excreted ammonia contributed about 90% of the total nitrogen requirements of observed production rates. The ammonia-N contribution was 36% in oceanic waters, and was relatively unimportant in the inshore region. The significance of eddy diffusivity in offshore waters and upwelling in inshore waters is also discussed.Contribution No. 747 from the Department of Oceanography, University of Washington, Seattle, USA.     

Effects of light intensity on nitrate and nitrite uptake and excretion by Chaetoceros curvisetus

Michaelis-Menten uptake kinetics were observed at all light intensities. With constant illumination, the V_max and K₁ in nitrate uptake over the natural light intensity range of 0 to 2000 E were 0.343 g-at NO₃–N(g)^-1 at protein-N h^-1 and 26 E, respectively. Nitrate uptake was inhibited at higher light intensities. The K_s for nitrate uptake did not vary as a function of light intensity remaining relatively constant at 0.62 g-at NO₃–N 1^-1. With intermittent illumination, the V_mzx for light intensity in nitrate uptake over a light intensity range of 0 to 5000 E was 0.341 g-at NO₃–N(g)^-1-at protein-N h^-1. No inhibition of nitrate uptake was observed at higher than natural light intensities. Chaetoceros curvisetus will probably never experience light inhibition of nitrate uptake under natural conditions.     

Growth of plankton algae as a function of N-concentration,measured by means of batch technique

Employing a special batch technique which involves extremely low concentrations of the alga Selenastrum capricornutum, it is shown that a growth rate of ca. 75% of optimum growth occurs at a concentration of 0.02 g-at N(NH₄ ⁺) l^-1. The low concentrations of this N-source in the oligotrophic parts of the oceans (ca. 0.06 g-at l^-1) thus give rise to a borderline deficiency. As the half-saturation constant (K _S) for growth was recorded at 0.05 g-at N(NO₃ ^-), this ion can hardly be of much importance at the low concentrations present in these parts of the oceans. It is wrong to assert that N is the primary limiting nutrient. Phosphate also is found at concentrations giving rise to a borderline deficiency.     

Persistent blooms of surf diatoms along the Pacific coast,USA

Productivity was studied in two diatom species, Chaetoceros armatum T. West and Asterionella socialis Lewin and Norris, which form persistent dense blooms in the surf zone along the Pacific coast of Washington and Oregon, USA. Past observations have shown that surf-diatom standing stock usually declines in summer along with concentrations of nitrate and ammonium. Using the ¹⁴C method, photosynthetic rates in natural surf samples were measured monthly for one year (October 1981 through September 1982) at a study site on the Washington coast. Also measured were temperature, salinity, dissolved nutrients, particulate carbon and nitrogen (used as estimates of phytoplankton C and N), and chlorophyll a. Assimilation numbers (P _max) were higher in summer (5 to 8 g C g^-1 chl a h^-1) than in winter (3 to 4gC). Specific carbon incorporation rates (µ_max) showed no obvious seasonality, mostly falling within the range of 0.09 to 0.13 g C g^-1 C(POC) h^-1. The discrepancy between the seasonal trends for chlorophyll-specific and carbon-specific rates reflects a change in the carbon-to-chlorophyll ratio. Because of seasonal differences in daylength and light intensity, daily specific growth rates () are thought to be higher in summer than in winter. Neither ammonium enrichment assays nor particulate carbon-to-nitrogen ratios provided convincing evidence for nitrogen limitation during summer, and the observed changes in diatom abundance cannot be explained on this basis. Both the high diatom concentrations and their seasonal variations probably are due mainly to factors other than growth rates; two factors considered important are diatom flotation and seasonal changes in wind-driven water transport. C. armatum usually dominates the phytoplankton biomass in the surf zone, and evidence suggests that this species is strongly dominant in terms of primary production.Contribution No. 1391 of the School of Oceanography, University of Washington, Seattle, Washington, USA     

Photoadaption in marine phytoplankton: Response of the photosynthetic unit

Some species of phytoplankton adapt to low light intensities by increasing the size of the photosynthetic unit (PSU), which is the ratio of light-harvesting pigments to P700 (reaction-center chlorophyll of Photosystem I). PSU size was determined for 7 species of marine phytoplankton grown at 2 light intensities: high (300 E m^-2 s^-1) and low (4 E m^-2 s^-1); PSU size was also determined for 3 species grown at only high light intensity. PSU size varied among species grown at high light from 380 for Dunaliella euchlora to 915 for Chaetoceros danicus. For most species grown at low light intensity, PSU size increased, while the percentage increase varied among species from 13 to 130%. No change in PSU size was observed for D. euchlora. Photosynthetic efficiency per chlorophyll a (determined from the initial slope of a curve relating photosynthetic rate to light intensity) varied inversely with PSU size. In contrast, photosynthetic efficiency per P700 was enhanced at larger PSU sizes. Therefore, phytoplankton species with intrinsically large PSU sizes probably respond more readily to the rapid fluctuations in light intensity that such organisms experience in the mixed layer.Contribution No. 1180 from the Department of Oceanography, University of Washington, Seattle, Washington, USA     

Heterotrophic nutrition of the marine pennate diatom Nitzschia angularis var. affinis

The nutritional pattern for heterotrophic growth of Nitzschia angularis var. affinis (Grun.) Perag. is more complex than for other diatom species studied previously. This species grew slowly in the dark in the presence of single amino acids, either glutamate or alanine; other amino acids when supplied singly were not used as substrates. Carbon from glutamate was converted to cell carbon with an efficiency of 43%. Glutamine was inhibitory both in the light and in the dark, and aspartate inhibited heterotrophic growth on glutamate. Glucose and tryptone supplied singly did not support heterotrophic growth, but when combined, together they allowed for rapid growth of N. angularis (generation time of 16 h). Glucose in combination with glutamate, alanine, aspartate, or asparagine (but not with any other amino acids) also supported growth in the dark, at a rate considerably more rapid than with glutamate alone. In the presence of excess glucose and limiting concentrations of glutamate, approximately 50% of the cell carbon for heterotrophic growth came from glucose, while in combination with tryptone about 25% of the cell carbon came from glucose. Amino acids were taken up by cells grown either photoautrophically or in the dark in the presence or absence of organic substrates; uptake rates were some-what higher for dark-grown than for light-grown cells. Glucose was taken up only by dark-grown cells; induction of a glucose uptake system in the dark required the presence of glutamate but not of glucose. The rates of uptake of glutamate and glucose by cells incubated in the dark with glutamate were sufficiently high to account for the observed rates of growth on these substrates in the dark. The uptake systems of N. angularis have relatively high affinities for glucose (K _s =0.03 mM) and glutamate (K _s =0.02 mM).Contribution No. 890 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Influence of relative water motion on the growth,ammonium uptake and carbon and nitrogen composition of Ulva lactuca (Chlorophyta)

In a series of multifactorial laboratory experiments, Ulva lactuca discs were grown in an apparatus in which they were exposed simultaneously to 3 simulated current speeds (7.5, 15, 22.5 cm s^-1) and a still control, and either 3 ammonium concentrations (0–10, 35–45 and 115–145 M) under ample uniform light (ca 200 E m^-2 s^-1) or 3 light intensities (approximately 35, 90 and 270 E m^-2 s^-1) with uniform surplus, ammonium. Disc growth rates were determined in each experiment as well as tissue nitrogen and carbon composition and fluxes of NH₄, NO₃/NO₂ and PO₄ in media. In a supplementary series of field experiments, U. lactuca discs were simultaneously exposed to 2 different water motion regimes in adjacent chambers at several sites characterized by widely different ammonium concentrations. In field experiments, growth rates were calculated and analyzed as a function of water motion at the various sites. The application of simulated current consistently enhanced disc growth rates in the laboratory, except at the lowest light intensity. In most cases this enhancement was fully realized at the lowest applied simulated current (7.5 cm s^-1). Simulated current slightly enhanced ammonium uptake rates by U. lactuca discs, relative to rates in still water, except at the highest ammonium concentration. C:N ratios of discs generally declined with increases in simulated current, except at the highest ammonium concentration. This decline was primarily attributable to increases in per cent N and was, again, mainly realized at 7.5 cm s^-1. The results suggested that simulated current compensated for N limitation, except when light was sufficiently low to become the overriding limiting factor, but that the enhancement of growth by simulated current could not be explained in terms of N metabolism alone. Field experiments showed that the higher level of water motion consistently enhanced growth at sites with comparatively low ammonium concentrations, but not at sites with moderate or high ammonium concentrations.     

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.     

Ecological growth strategies in the seaweeds Gracilaria foliifera (Rhodophyceae) and Ulva sp. (Chlorophyceae): Soluble nitrogen and reserve carbohydrates

The seaweeds Gracilaria foliifera (Rhodophyceae) and Ulva sp. (Chlorophyceae) were grown in an outdoor continuous-flow system at both ambient incident light (I₀) and 0.13 I₀. During the winter, both species accumulated substantial soluble nitrogen reserves (up to 1020 g-at N·g dry wt^-1 in G. foliifera and 630 g-at N·g dry wt^-1 in Ulva sp.). The rate at which these N reserves were depleted was proportional to the growth rate. Seaweeds grown at 0.13 I₀ had lower growth rates and higher levels of soluble tissue N than plants grown at I₀. During the spring-summer growing season, peaks in tissue N followed nutrient peaks in the ambient seawater. Ulva sp. had higher nutrient uptake and growth rates than G. foliifera and showed greater fluctuations in soluble tissue N. This may characterize opportunistic seaweed species with high biomass turnover rates. At I₀, the levels of starch (up to 340 mg·g dry wt^-1 in G. foliifera and 170 mg·g dry wt^-1 in Ulva sp.) were highest during the spring and summer. During this period, fluctuations in starch content were inversely related to growth rate and soluble tissue N. Seaweeds grown at 0.13 I₀ did not accumulate starch. Neither species was found to overwinter with starch reserves.     

Blooms of surf-zone diatoms along the coast of the Olympic Peninsula,Washington. XI. Regeneration of ammonium in the surf environment by the Pacific razor clam Siliqua patula

A 20 month field study was conducted on ammonium excretion rates of the Pacific razor clam Siliqua patula Dixon along the beaches of Washington State, USA. Excretion rates of all those nutrients likely to be regenerated in sufficient quantity to affect surf diatom growth were measured; ammonium appeared to be the most important metabolite. Excretion of ammonium by razor clams far exceeded that by other beach fauna. Ammonium excretion rates of razor clams were positively correlated with shell length, but no correlation between ammonium excretion rate and water temperature was evident. This may be an artifact or may represent some degree of seasonal acclimation of the species to temperature. Weight-specific ammonium excretion rates were negatively related to clam size, indicating a possible large (and unknown) contribution of regenerated ammonium by smaller clams in their first year of growth; smaller clams were rarely captured during this study.Contribution No. 1048 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Phosphate uptake by intertidal algae in relation to zonation and season

The removal of phosphate from ambient seawater by whole plants of five species of fucoid algae, collected from the east coast of N. Ireland in 1988 and 1989, was followed over 6-h periods. A transient uptake pattern was observed forPelvetia canaliculata (L.) Dcne. et Thuret,Fucus spiralis L.,F. vesiculosus L. andF. serratus L., consisting of an initial period of high uptake, followed by a phase of zero uptake and then a period at an intermediate rate.Ascophyllum nodosum (L.) Le Jolis had a constant slow rate of uptake over 6 h. The initial uptake rate ofF. spiralis was significantly greater than that of any other species. Phosphate uptake over a 2-h period was measured at concentrations ranging from that of ambient seawater to 25µg-at. l^–1 for whole plants ofF. spiralis andF. serratus, using a large scale batch method. A small scale batch method was used for whole plants ofP. canaliculata and sections of the other four species investigated. Uptake abilities of the algae at low concentrations of phosphate were compared using the parameterV ₁ (the uptake rate at 1µg-at. l^–1) and at high concentrations usingV _max, the maximum uptake rate. These kinetic parameters of uptake were calculated using a method that avoids bias and permits statistical evaluation of the results. The fucoid algae studied could be divided into two distinct groups on the basis of their abilities to take up phosphate from seawater.P. canaliculata andA. nodosum had low values ofV ₁ in winter, which were also correlated with their positions on the shore and did not vary between winter and summer. TheFucus species had higher values ofV ₁ in winter, which were also correlated with their positions on the shore. In summer, however,V ₁-values for these species decreased and no longer correlated with their shore heights. TheV _max-value forF. spiralis was higher in winter than in summer but was signifcantly greater than that of any other species at all times of year. The ecological significance ofV _max is discussed in relation to nutrient limitation and the possible occurrence of patches of high nutrient concentration in the intertidal environment.     

Utilization of glutamate and glucose for heterotrophic growth by the marine pennate diatom Nitzschia laevis

Nitzschia laevis Hustedt grew in the dark in the presence of either glutamate or glucose as substrate. Complex mixtures of yeast extract or tryptone plus lactate also supported good heterotrophic growth, while tryptone alone only supported very slow growth in the dark. The observed growth rates of N. laevis in the dark at different concentrations of glutamate or glucose could be accounted for by the measured uptake rates of these compounds. The affinity of the uptake systems for glutamate and glucose (K _s =0.03 mM for each) was quite high, and similar for dark- and light-grown cells. The lack of a lag-phase when cells were transferred from photoautotrophic to heterotrophic growth conditions can be explained by the presence of uptake systems for glutamate and glucose in ligh--grown cells, as well as in dark-grown cells. However, the uptake capacity was generally higher in the latter than the former. N. laevis also took up alanine and lactate according to Michaelis-Menten kinetics, with a K _s for alanine of 0.02 mM and for lactate of 0.4 mM. Malate and glycerol were not taken up to a significant extent by the cells. Cells grown in continous light had a doubling time of 18 h. The shortest doubling time observed in the dark on glutamate was 48 h and on glucose 24 h. Glutamate was used for heterotrophic growth with an efficiency of 43% and glucose with an efficiency of 48%.Contribution No., 945 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Nitrogen fixation by blue-green algae associated with the siphonous green seaweed Codium decorticatum: effects on ammonium uptake

Nitrogen fixation (acetylene reduction) at rates of up to 1.2 g N₂ g dry wt^-1 h^-1 was measured for the siphonous green seaweed Codium decorticatum. No nitrogenase activity was detected in C. isthmocladum. The nitrogenase activity was light sensitive and was inhibited by the addition of DCMU and triphenyl tetrazolium chloride. Additions of glucose did not stimulate nitrogen fixation. Blue-green algae (Calothrix sp., Anabaena sp., and Phormidium sp.) were implicated as the organisms responsible for the nitrogenase activity. They occurred in a reduced microzone within the C. decorticatum thallus where nitrogen fixation was optimized. Nitrogen fixation did not affect the kinetic constants for ammonium uptake in C. decorticatum (K_s=12.0 M, V_max=13.4 mol NH₃ g dry wt^-1 h^-1) determined using the perturbation method. Nevertheless, C. decorticatum thalli which fixed nitrogen had internal dissolved nitrogen concentrations which were over 1.4 times higher than in non-fixing thalli. This suggests that if C. decorticatum does derive part of its nitrogen requirement from the blue-green algae which it harbors, the transfer does not involve competition between this process and the uptake of ambient ammonium.     

Effects of simulated current on the growth rate and nitrogen metabolism of Gracilaria tikvahiae (Rhodophyta)

In a series of multifactorial laboratory experiments, Gracilaria tikvahiae apical segments were grown in an apparatus in which they were exposed simultaneously to 3 simulated current speeds (7.5, 15, 22.5 cm s^-1) and a still control, and either 3 ammonium concentrations (<6, 37–39, and 119–136 M) under ample uniform light (ca 200 E m^-2 s^-1) or 3 light intensities (ca 35, 90, and 270 E m^-2 s^-1) with uniform surplus ammonium. Growth rates of apical segments were determined in each experiment as well as nitrogen and carbon composition of tissues and fluxes of NH₄, NO₃/NO₂, and PO₄ in media. In a supplementary series of field experiments, apical segments of G. tikvahiae weresimultaneously exposed to 2 different regimes of water motion in adjacent chambers at several sites characterized by widely different ammonium regimes. The application of simulated current significantly enhanced growth rates in all experiments which utilized recently collected plants. Generally, this enhancement was fully realized at 7.5 cm s^-1, with growth rates tending to plateau above that speed. Growth enhancement by simulated current was independent of ammonium concentration and was considerably reduced at the lowest light intensity. In experiments conducted with plants that had been maintained for several months in aquaria, simulated current failed to enhance growth rates. This suggested that growth responses were at least partly a function of prior conditioning. Growth rates were a direct function of light intensity and an inverse function of ammonium concentration, indicating ammonium inhibition at the higher applied concentrations. Simulated current slightly enhanced rates of ammonium uptake but this did not consistently result in reduced C:N ratios, suggesting that the growth-stimulating effect of relative water motion was attributable to factors other than N uptake. There was evidence of luxury consumption of ammonium. In field experiments, growth rates were not significantly related to exposure to water motion.     

Uptake and release of phosphorus by phytoplankton in the Chesapeake Bay estuary,USA

The phytoplankton uptake and release rates for inorganic phosphate, dissolved organic phosphate and polyphosphate were estimated during 5 cruises on the Chesapeake Bay over a 9-month period. Phosphorus in all pools turned over in several minutes to 100 h, and each soluble pool appeared to contain fractions which were metabolically useful to the phytoplankton. Maximal uptake rates (V _m ) for orthophosphate ranged from 0.02 to 2.95 μg-at P (1.h)^-1 with half saturation constants (K _s ) between 0.09 and 1.72 μg-at P l^-1. At low soluble reactive phosphorus concentrations, the uptake rate of trace ³²P orthophosphate was initially rapid, but declined after 15 to 60 min incubation. The data suggest that the initial uptake phase was dominated by exchange of ³²PO₄ ^≡ for ³¹PO₄ ^≡ in the membrane transport systems whereas the subsequent phase represented the net incorporation of orthophosphate into phytoplankton cells.     

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.