THE EFFECTS OF PHOSPHITE ON PHOSPHATE STARVATION RESPONSES OF ULVA LACTUCA (ULVALES,CHLOROPHYTA)1

Effects of phosphite (Phi) on phosphate (Pi) starvation responses were determined in Ulva lactuca L. by incubation in Pi‐limited (1 μM NaH₂PO₄) or Pi‐sufficient (100 μM NaH₂PO₄) seawater containing 0–3 mM Phi. Exposure to 1 μM NaH₂PO₄ decreased the growth rate and the content of free Pi and esterified‐P but increased the activities of extracellular alkaline phosphatase (EC 3.1.2.1) and intracellular acid phosphatase (ACP; EC 3.1.2.2); two ACP isozymes observed by activity staining on isoelectric focussing (IEF) gel were induced. The K_m value of Pi uptake rate was decreased by incubation with 1 μM NaH₂PO₄ and the decrease in K_m value was inhibited by 2 mM Phi, reflecting the operation of a high‐affinity Pi uptake system at low Pi concentrations. In the presence of Phi, the growth rate of Pi‐sufficient and Pi‐starved thalli decreased as Phi concentrations were increased from 0 to 2 mM. As Phi concentrations were increased from 0 to 2 mM, the free Pi contents in both Pi‐sufficient and Pi‐starved thalli decreased, but the esterified‐P contents in Pi‐starved thalli increased, whereas those in Pi‐sufficient thalli increased at 1 mM Phi and decreased at 2 mM Phi. Cell wall localized AP activity in both Pi‐sufficient and Pi‐starved thalli decreased as Phi concentrations were increased from 0 to 2 mM. Intracellular ACP activity in Pi‐starved thalli decreased as Phi concentrations were increased from 0 to 2 mM but was not affected in Pi‐sufficient thalli. The induction of ACP isozyme activity and high‐affinity Pi uptake system in Pi‐starved thalli was inhibited by Phi. The present investigation shows that Phi interrupts the sensing mechanisms of U. lactuca to Pi‐limiting conditions.     

Quaternary ammonium compounds in plants in relation to salt resistance

Fourteen plant species exhibiting a wide range of salt resistance as halophytes, semi-resistant glycophytes and sensitive glycophytes, have been grown in nutrient solution culture under low and high salt conditions. Inorganic analyses and shoot sap osmotic pressure values of these plants confirm that osmotic compensation at high salt levels is largely achieved by the accumulation of Na salts. Choline was found in shoots and roots in the range 1.0-0.2 μmol g fr. wt^?1 and varied little following salt stress. Trigonelline was found in some of the sensitive glycophytes and did not increase significantly in stressed plants. Betaine levels were high (10 μmol g fr. wt^?1) in the shoot of the halophytes at low salt conditions, lower values (1–10 μmol g fr. wt^?1) were found in the semi-resistant glycophytes and none detected in the sensitive glycophytes. In the two resistant groups betaine accumulated to higher levels following NaCl stress. Shoot betaine levels always exceeded root levels. Proline occurred in all plants and in all cases was accumulated following NaCl stress.     

Relationships between the kinetics of NH4+ and NO3− absorption and growth in the cultivated tomato (Lycopersicon esculentum Mill, cv T-5)

Tomato growth was examined in solution culture under constant pH and low levels of NH₄⁺ or NO₃^?. There were five nitrogen treatments: 20 mmoles m^?3 NH₄⁺, 50 mmoles m^?3 NO₃^?, 100 mmoles m^?3 NH₄⁺ 200 mmoles m^?3 NO₃^?, and 20 mmoles m^?3 NH₄₊+ 50 mmoles m^?3 NO₃^?. The lower concentrations (20 mmoles m^?3 NH₄⁺ and 50 mmoles m^?3 NO₃^?) were near the apparent K_m for net NH₄⁺ and NO₃^? uptake; the higher concentrations (100 mmoles m^?3 NH₄⁺ and 200 mmoles m^?3 NO₃^?) were near levels at which the net uptake of NH₄⁺ or NO₃^? saturate. Although organic nitrogen contents for the higher NO₃^? and the NH₄⁺+ NO₃^? treatments were 22.2–30.3% greater than those for the lower NO₃^? treatment, relative growth rates were initially only 10–15% faster. After 24 d, relative growth rates were similar among those treatments. These results indicate that growth may be only slightly nitrogen limited when NH₄⁺ or NO₃^? concentrations are held constant over the root surface at near the apparent K_m concentration. Relative growth rates for the two NH₄⁺ treatments were much higher than have been previously reported for tomatoes growing with NH₄⁺ as the sole nitrogen source. Initial growth rates under NH₄⁺ nutrition did not differ significantly (P≥ 0.05) from those under NO₃^? or under combined NH₄⁺+ NO₃^?. Growth rates slowed after 10–15 d for the NH₄⁺ treatments, whereas they remained more constant for the NO₃^? and mixed NH₄⁺+ NO₃^? treatments over the entire observation period of 24–33 d. The decline in growth rate under NH₄⁺ nutrition may have resulted from a reduction in Ca²⁺, K⁺, and/or Mg²⁺ absorption.     

Characterization of phosphate absorption in maize root cortex segments

Uptake of phosphate ions by 1 mm segments of isolated maize root cortex layers was studied. Cortex segments (from roots of 8 days old maize plants) absorb phosphate ions from 1 mM KH₂PO₄ in 0.2 mM CaSCO₄ at the average rate of 34.3 ±3.2 μg P_i g^?1 (fr. m.) h^?1,i.e. 0.35± 0.02 μmol P_i g^?1 (fr. m.) h^?1. Phosphate uptake considerably increases after a certain period of “augmentation”,i.e. washing in aerated 0.2 mM CaSO₄. This increase is completely blocked by the presence of 10 μg ml^?1 cycloheximide. The relation of uptake rate to phosphate concentration in the medium was shown to have 3 phases in the concentration range of 0.02 - 40 mM. Transition points were found between 0.8–1 mM and 10–20 mM. Following K_m and V_max values were found: K_m[mM] : 0.37 - 3.82 - 27.67 V_max[μg P_i g^?1 (fr. m.) h^?1] : 3.33 - 39.40 - 66.67 We have found no sharp pH optimum for phosphate uptake. It proceeds at almost constant rate till pH 6.0 and then the uptake rate drops with increasing pH. At low phosphate concentrations (1 mM) the lowest uptake rate was found at 5 and 13 °C, while the uptake is higher at 5 °C than at 13 °C at phosphate concentrations higher than 1 mM. At these concentrations uptake rate at 35 °C is lower than at 25 °C. Phosphate uptake considerably decreased in anaerobic conditions. DNP and iodoacetate (0.1 mM) completely blocked phosphate uptake from 1 mM KH₂PO₄, while uptake from 5 and 10 mM KH₂PO₄ was left unaffected by these substances. The inhibitors of active - SH groups NEM and PCMB inhibited phosphate uptake: 10^?3 M NEM by 81.6%, 10⁴ M NEM by 42% and 10^?4 M PCMB by 42%.     

COPPER‐UPTAKE KINETICS OF COASTAL AND OCEANIC DIATOMS1

We investigated copper (Cu) acquisition mechanisms and uptake kinetics of the marine diatoms Thalassiosira oceanica Hasle, an oceanic strain, and Thalassiosira pseudonana Hasle et Heimdal, a coastal strain, grown under replete and limiting iron (Fe) and Cu availabilities. The Cu‐uptake kinetics of these two diatoms followed classical Michaelis–Menten kinetics. Biphasic uptake kinetics as a function of Cu concentration were observed, suggesting the presence of both high‐ and low‐affinity Cu‐transport systems. The half‐saturation constants (K_m) and the maximum Cu‐uptake rates (V_max) of the high‐affinity Cu‐transport systems (～7–350 nM and 1.5–17 zmol · μm^?2 · h^?1, respectively) were significantly lower than those of the low‐affinity systems (>800 nM and 30–250 zmol · μm^?2 · h^?1, respectively). The two Cu‐transport systems were controlled differently by low Fe and/or Cu. The high‐affinity Cu‐transport system of both diatoms was down‐regulated under Fe limitation. Under optimal‐Fe and low‐Cu growth conditions, the K_m of the high‐affinity transport system of T. oceanica was lower (7.3 nM) than that of T. pseudonana (373 nM), indicating that T. oceanica had a better ability to acquire Cu at subsaturating concentrations. When Fe was sufficient, the low‐affinity Cu‐transport system of T. oceanica saturated at 2,000 nM Cu, while that of T. pseudonana did not saturate, indicating different Cu‐transport regulation by these two diatoms. Using CuEDTA as a model organic complex, our results also suggest that diatoms might be able to access Cu bound within organic Cu complexes.     

EFFECTS OF LIGHT AND TEMPERATURE ON NET PHOTOSYNTHESIS AND DARK RESPIRATION OF GAMETOPHYTES AND EMBRYONIC SPOROPHYTES OF MACROCYSTIS PYRIFERA1

The rates of net photosynthesis as a function of irradiance and temperature were determined for gametophytes and embryonic sporophytes of the kelp, Macrocystis pyrifera (L.) C. Ag. Gametophytes exhibited higher net photosynthetic rates based on oxygen and pH measurements than their derived embryonic sporophytes, but reached light saturation at comparable irradiance levels. The net photosynthesis of gametophytes reached a maximum of 66.4 mg O₂ g dry wt^?1 h^?1 (86.5 mg CO₂ g dry wt^?1 h^?1), a value approximately seven times the rate reported previously for the adult sporophyte blades. Gametophytes were light saturated at 70 μE m^?2 s^?1 and exhibited a significant decline in photosynthetic performance at irradiances 140 μE m^?1 s^?1. Embryonic sporophytes revealed a maximum photosynthetic capacity of 20.6 mg O₂ g dry wt^?1 h^?1 (25.3 mg CO₂ g dry wt^?1 h^?1), a rate about twice that reported for adult sporophyte blades. Embryonic sporophytes also became light saturated at 70 μE m^?2 s^?1, but unlike their parental gametophytes, failed to exhibit lesser photosynthetic rates at the highest irradiance levels studied; light compensation occurred at 2.8 μE m^?2 s^?1. Light-saturated net photosynthetic rates of gametophytes and embryonic sporophytes varied significantly with temperature. Gametophytes exhibited maximal photosynthesis at 15° to 20° C, whereas embryonic sporophytes maintained comparable rates between 10° and 20° C. Both gametophytes and embryonic sporophytes declined in photosynthetic capacity at 30° C. Dark respiration of gametophytes was uniform from 10° to 25° C, but increased six-fold at 30° C; the rates for embryonic sporophytes were comparable over the entire range of temperatures examined. The broader light and temperature tolerances of the embryonic sporophytes suggest that this stage in the life history of M. pyrifera is well suited for the subtidal benthic environment and for the conditions in the upper levels of the water column.     

Seasonal nitrate physiology of Macrocystis integrifolia Bory

Ambient sea-water nitrate and tissue nitrogen (ethanol soluble nitrate and amino acids, as well as total nitrogen) of Macrocystis integrifolia Bory were monitored over a 2-yr period in Bamfield, Vancouver Island, British Columbia. Sea-water nitrate varied from a high of 12 μmol · 1^?1 (individual values as high as 23 μmol · 1^?1 were recorded) in late winter to below detection limits for most of the summer. Tissue nitrate and total nitrogen paralleled the ambient nitrate levels and showed summer minima and winter maxima (from 0 to 70 μmol · g fresh wt^?1 for nitrate and from 0.8 to 2.9% of dry wt for total N). The nitrate uptake capacity was inversely proportional to tissue nitrate concentration and, furthermore, was much higher for subapical surface blades (60–70 nmol · cm^?2 · h^?1) than for older, deeper blades (5–10 nmol · cm^?2 · h^?1). Nitrate uptake by subapical blade disks in summer is apparently higher in dark (1.0–1.7 μmol · g fresh wt^?1 · h^?1) than in light (0.6–1.3 μmol · g fresh wt^?1 · h^?1) and the data obtained in 36–108 h experiments indicate nitrate pool sizes of 30–90 μmol · g fresh wt^?1. These pools are $\text{2}\text{3}$ to nearly full in winter. Ammonium does not inhibit nitrate uptake. It is taken up and apparently utilized much faster than nitrate and it may well be an important source of nitrogen for marine macrophytes.     

Phosphorus mining efficiency declines with decreasing soil P concentration and varies across crop species

High soil P concentrations hinder ecological restoration of biological communities typical for nutrient-poor soils. Phosphorus mining, i.e., growing crops with fertilization other than P, might reduce soil P concentrations. However, crop species have different P-uptake rates and can affect subsequent P removal in crop rotation, both of which may also vary with soil P concentration. In a pot experiment with three soil-P-levels (High-P: 125–155 mg P_Olsen/kg; Mid-P: 51–70 mg P_Olsen/kg; Low-P: 6–21 mg P_Olsen/kg), we measured how much P was removed by five crop species (buckwheat, maize, sunflower, flax, and triticale). Total P removal decreased with soil-P-level and depended upon crop identity. Buckwheat and maize removed most P from High-P and Mid-P soils and triticale removed less P than buckwheat, maize, and sunflower at every soil-P-level. The difference in P removal between crops was, however, almost absent in Low-P soils. Absolute and relative P removal with seeds depended upon crop species and, for maize and triticale, also upon soil-P-level. None of the previously grown crop species significantly affected P removal by the follow-up crop (perennial ryegrass). We can conclude that for maximizing P removal, buckwheat or maize could be grown.     

GROWTH,PHOTOSYNTHESIS AND MAINTENANCE METABOLIC COST IN THE DIATOM PHAEODACTYLUM TRICORNUTUM AT VERY LOW LIGHT LEVELS 1

The compensation point for growth of Phaeodactylum tricornutum Bohlin is less than 1 μmol. m^?2s^?1. Growth at low PFDs (<3.5 μmol. m^?2.s^?1) does not appear to reduce the maximum quantum efficiency of photosynthesis (ø_m) or to greatly inhibit the potential for light-saturated, carbon-specific photosynthesis (P_m^c). The value for ø_m in P. tricornutum is 0.10–0.12 mol O₂-mol photon^?1, independent of acclimation PFD between 0.75 and 200 μmol.m^?2.s^?1 in nutrient-sufficient cultures. P_m^c in cells of P. tricornutum acclimated to PFDs <3.5 μmol m^?2?s^?1 is approximately 50% of the highest value obtained in nutrient-sufficient cultures acclimated to growth-rate-saturating PFDs. In addition, growth at low PFDs does not severely restrict the ability of cells to respond to an increase in light level. Cultures acclimated to growth at lees than 1% of the light-saturated growth rate respond rapidly to a shift-up in PFD after a short initial lag period and achieve exponential growth rates of 1.0 d^?1 (65% of the light- and nutrient-saturated maximum growth rate) at both 40 and 200 μmol.m^?2.s^?1     

Nitrogen relations of the sorghum-Sfriga hermonthica hostparasite association: growth and photosynthesis

The extent to which the parasitic angiosperm Striga hermonthica reduces the growth of its sorghum host is dependent on the concentration of nitrogen (as NH₄NO₃ in 40% Long Ashton Solution) supplied to the plants. The biomass of 0.5,1 and 2 mol m^?3 N-grown infected plants was 22,30 and 66%, respectively, of uninfected plants after 140d growth. The biomass of 3 and 4 mol m^?3 N-grown infected plants differed little from uninfected plants. No grain was set in 0.5 and 1 mol m^?3 N-grown infected plants, grain yield reached 42 and 73% of controls in 2 and 3 mol m^?3 N-grown plants, and was unaffected in 4 mol m^?3 N-grown plants. Striga hermonthica also altered the allometry and architecture of the host, at all but the highest N concentration. Higher N concentration (3 and 4 mol m ^?3 N) reduced the growth of S. hermonthica. Foliar N concentrations in sorghum ranged from 11 mg g^?1 dwt. in 0.5 mol m^?3 N-grown plants, to 28 mg g^?1 dwt. in 4 mol m^?3 N-grown plants, and were not affected by S. hermonthica. Higher N concentrations were measured in S. hermonthica, and ranged from 18 to 45 mg g^?1 dwt. in 0.5 and 3 mol m^?3 N-grown plants, respectively. The relationship between photosynthesis (CO₂ flux) and N concentration differed between uninfected and infected sorghum. This was most apparent in 0.5 mol m^?3 N-grown plants, with rates of 16 and 11 μmol m^?2 s^?1 in uninfected and infected plants, respectively (at 1500–1800 μmol m^?2 s^?1 photosynthetic photon flux density). At higher N concentrations, this difference was smaller, with both sets of plants reaching 26 μmol m^?2 s^?1 at 4 mol m^?3 N. Varying the level of S. hermonthica infection showed that the effect of N on host photosynthesis cannot be explained by differences in the mass or number of parasites supported by the host. At low levels of infection in 1 mol m^?3 N-grown plants, the negative effect of the parasite was reversed, and photosynthesis in infected plants exceeded that in uninfected plants by 20%. Photosynthesis in S. hermonthica at 3 mol m^?3 N (8 μmol m^?2 s^?1) was double that in 0.5 mol m^?3 N-grown plants. Stable carbon isotope and gas exchange measurements data demonstrated that this higher level of autotrophic carbon fixation was accompanied by a lower dependency on hetero trophic carbon. The latter ranged from 27 to 6% in 0 5 mol m^?3 and 3 mol m^?3 N-grown plants, respectively.     

PHOTOSYNTHESIS AND RESPIRATION OF THE CONCHOCELIS STAGE OF ALASKAN PORPHYRA (BANGIALES,RHODOPHYTA) SPECIES IN RESPONSE TO ENVIRONMENTAL VARIABLES1

Photosynthesis and respiration of three Alaskan Porphyra species, P. abbottiae V. Krishnam., P. pseudolinearis Ueda species complex (identified as P. “pseudolinearis” below), and P. torta V. Krishnam., were investigated under a range of environmental parameters. Photosynthesis versus irradiance (P–I) curves revealed that maximal photosynthesis (P_max), irradiance at maximal photosynthesis (I_max), and compensation irradiance (I_c) varied with salinity, temperature, and species. The P_max of Porphyra abbottiae conchocelis varied between 83 and 240 μmol O₂ · g dwt^?1 · h^?1 (where dwt indicates dry weight) at 30–140 μmol photons · m^?2 · s^?1 (I_max) depending on temperature. Higher irradiances resulted in photoinhibition. Maximal photosynthesis of the conchocelis of P. abbottiae occurred at 11°C, 60 μmol photons · m^?2·s^?1, and 30 psu (practical salinity units). The conchocelis of P. “pseudolinearis” and P. torta had similar P_max values but higher I_max values than those of P. abbottiae. The P_max of P. “pseudolinearis” conchocelis was 200–240 μmol O₂ · g dwt^?1 · h^?1 and for P. torta was 90–240 μmol O₂ · g dwt^?1 · h^?1. Maximal photosynthesis for P. “pseudolinearis” occurred at 7°C and 250 μmol photons · m^?2 · s^?1 at 30 psu, but P_max did not change much with temperature. Maximal photosynthesis for P. torta occurred at 15°C, 200 μmol photons · m^?2 · s^?1, and 30 psu. Photosynthesis rates for all species declined at salinities <25 or >35 psu. Estimated compensation irradiances (I_c) were relatively low (3–5 μmol · photons · m^?2 · s^?1) for intertidal macrophytes. Porphyra conchocelis had lower respiration rates at 7°C than at 11°C or 15°C. All three species exhibited minimal respiration rates at salinities between 25 and 35 psu.     

Characterization and comparisons of five N2-fixing Azolla-Anabaena associations,I. Optimization of growth conditions for biomass increase and N content in a controlled environment*

Abstract Biomass increase, C and N content, C₂H₂ reduction, percentage dry weight and chlorophyll a/b ratios were determined for clones of Azolla caroliniana Willd., A. filiculoides Lam., A. mexicana Presl., and A. pinnata R. Br. as a function of nutrient solution, pH, temperature, photoperiod, and light intensity in controlled environment studies. These studies were supplemented by a glasshouse study. Under a 16 h, 26°C day at a light intensity of 200 μmol m^?2 s^?1 and an 8 h, 19° C dark period, there was no significant difference in the growth rates of the individual species on the five nutrient solutions employed. Growth was comparable from pH 5 to pH 8, but decreased at pH 9. Using the same photoperiod and light intensity but constant growth temperatures of 15–40°C, at 5°C intervals, the individual species exhibited maximum growth, nitro-genase (N²ase) activity and N content at either 25° or 30°C. There was no difference in the temperature optima at pH 6 and pH 8. The tolerance of the individual species to elevated temperature was indicated to be A. mexicana> A. pinnata> A. caroliniana> A.filiculoides. At the optimum temperature, growth rates increased with increasing photoperiod at both pH 6 and pH 8 but N₂ase activity was usually highest at a 16 h light period. At photon flux densities of 100, 200, 400 and 600 μmol m^?2 s^?1, during a 16 h light period and optimum growth temperature of the individual species, N₂ase activity was saturated at less than 200 μmol m^?2 s^?1 and growth at 400 μmol m^?2 s^?1.No interacting effects of light and pH were noted for any species, nor were light intensities up to 1700 μmol m^?2 s^?1 detrimental to the growth rate or N content of any species in a 5 week glasshouse study with a natural 14.5 h light period and a constant temperature of 27.5°C. Using the optimum growth temperature, a 16 h light period, and a photon flux density of at least 400 μmol m^?2 s^?1, the Azolla species all doubled their biomass in 2 days or less and contained 5–6% N on a dry weight basis.     

Phosphate transport in Ehrlich ascites tumor cells and the effect of arsenate

The effect of extracellular Pi and arsenate on Pi-transport in Ehrlich ascites tumor cells has been studied. Pi-transport can be described by Michaelis-Menten kinetics; the maximal flux equal to 44 mmoles (kg cell water)^?1 hour^?1 and K_m equal to 3.3 × 10^?4 M . Arsenate is a competitive inhibitor of Pi-transport with an inhibition constant (K_i) equal to 2.41 × 10^?3 M . The data support the hypothesis that cellular Pi is regulated by the cell membrane through the mediation of a carrier system.     

Taurine uptake in synaptosomal fractions of rat cerebral cortex.

Abstract— [³⁵S]Taurine was found to be accumulated in synaptosomal fractions of rat cerebral cortex. Kinetic analysis in the range of 1–800 μm -[³⁵S]taurine revealed at least two different uptake processes. A high affinity uptake with a K_m of 20 μM and a low affinity uptake with a K_m of about 450 μM. The high affinity component was dependent on temperature and energy, and virtually abolished in the absence of sodium. Examination of the influence of structural analogues and putative transmitter substances indicates that the high affinity uptake of taurine into synaptosomal fractions of rat cerebral cortex is unique and highly specific. No specific actions of several centrally acting drugs on taurine uptake could be observed.     

Biotic and abiotic controls on the ecosystem significance of consumer excretion in two contrasting tropical streams

1. Excretion of nitrogen (N) and phosphorus (P) is a direct and potentially important role for aquatic consumers in nutrient cycling that has recently garnered increased attention. The ecosystem‐level significance of excreted nutrients depends on a suite of abiotic and biotic factors, however, and few studies have coupled measurements of excretion with consideration of its likely importance for whole‐system nutrient fluxes. 2. We measured rates and ratios of N and P excretion by shrimps (Xiphocaris elongata and Atya spp.) in two tropical streams that differed strongly in shrimp biomass because a waterfall excluded predatory fish from one site. We also made measurements of shrimp and basal resource carbon (C), N and P content and estimated shrimp densities and ecosystem‐level N and P excretion and uptake. Finally, we used a 3‐year record of discharge and NH₄‐N concentration in the high‐biomass stream to estimate temporal variation in the distance required for excretion to turn over the ambient NH₄‐N pool. 3. Per cent C, N, and P body content of Xiphocaris was significantly higher than that of Atya. Only per cent P body content showed significant negative relationships with body mass. C:N of Atya increased significantly with body mass and was higher than that of Xiphocaris. N : P of Xiphocaris was significantly higher than that of Atya. 4. Excretion rates ranged from 0.16–3.80 μmol NH₄‐N shrimp^?1 h^?1, 0.23–5.76 μmol total dissolved nitrogen (TDN) shrimp^?1 h^?1 and 0.002–0.186 μmol total dissolved phosphorus (TDP) shrimp^?1 h^?1. Body size was generally a strong predictor of excretion rates in both taxa, differing between Xiphocaris and Atya for TDP but not NH₄‐N and TDN. Excretion rates showed statistically significant but weak relationships with body content stoichiometry. 5. Large between‐stream differences in shrimp biomass drove differences in total excretion by the two shrimp communities (22.3 versus 0.20 μmol NH₄‐N m^?2 h^?1, 37.5 versus 0.26 μmol TDN m^?2 h^?1 and 1.1 versus 0.015 μmol TDP m^?2 h^?1), equivalent to 21% and 0.5% of NH₄‐N uptake and 5% and <0.1% of P uptake measured in the high‐ and low‐biomass stream, respectively. Distances required for excretion to turn over the ambient NH₄‐N pool varied more than a hundredfold over the 3‐year record in the high‐shrimp stream, driven by variability in discharge and NH₄‐N concentration. 6. Our results underscore the importance of both biotic and abiotic factors in controlling consumer excretion and its significance for nutrient cycling in aquatic ecosystems. Differences in community‐level excretion rates were related to spatial patterns in shrimp biomass dictated by geomorphology and the presence of predators. Abiotic factors also had important effects through temporal patterns in discharge and nutrient concentrations. Future excretion studies that focus on nutrient cycling should consider both biotic and abiotic factors in assessing the significance of consumer excretion in aquatic ecosystems.     

PHOSPHATE-LIMITED GROWTH KINETICS OF SELENASTRUM CAPRICORNUTUM (CHLOROPHYCEAE)1

Most theoretical studies of phytoplankton growth in aquatic environments assume that relative nutrient utilization abilities regulate species composition. The steady-state phosphate-limited growth kinetics of Selenastrum capricornutum Printz were examined using continuous cultures to characterize the green alga's ability to compete for orthophosphate (Pi) when Pi limits growth. The maximal specific growth rate for Selenastrum at 20 C was 1.20 day^?1, and the concentration where half maximal growth rate occurs was 40 nM Pi. There was an apparent threshold of 10 nM Pi. Cell yields varied inversely with growth rate; thus ability to utilize Pi could not be characterized in terms of the Monod half-saturation constant and maximal growth rate. Instead, we computed the Pi affinity from steady-state flux vs. external Pi concentrations. This affinity was 2.8 l·mg dry wt^?1· day^?1 for Selenastrum. Kinetic evidence from this study suggests that Selenastrum will not be growth competitive with some other common aquatic heterotrophs and autotrophs when Pi limits microbial growth in lakes.     

EVALUATION OF COMPETITIVE ABILITY OF STAURASTRUM LUETKEMUELLERII (CHLOROPHYCEAE) AND MICROCYSTIS AERUGINOSA (CYANOPHYCEAE) UNDER P LIMITATION1

The desmid Staurastrum luetkemuellerii Donat et Ruttner and the cyanobacterium Microcystis aeruginosa Kütz. showed pronounced differences in chemical composition and ability to maintain P fluxes. The cellular P:C ratio (Q_p) and the surplus P:C ratio (Q_sp) were higher in M. aeruginosa, indicating a lower yield of biomass C per unit of P. The subsistence quota (Q_p) was 1.85 μg P·mg C^?1in S. luetkemuellerii and 6.09 μg P·mg C^?1in M. aeruginosa, whereas the respective Q_p of P saturnted organisms (Q_s) were 43 and 63 μg P·mg C^?1. These stores could support four divisions in S. luetkemuellerii and three divisions in M. aeruginosa, which suggests that the former exhibited highest storage capacity (Q_s/Q₀). M. aeruginosa showed a tenfold higher activity of alkaline phosphatase than S. luetkemuellerii when P starved. The optimum N:P ratio (by weight) was 5 in S. luetkemuellerii and 7 in M. aeruginosa. The initial uptake of P_i pulses in the organisms was not inhibited by rapid (<1 h) internal feedback mechanisms and the short term uptake rote could be expressed solely as a function of ambient P_i. The maximum cellular C-based uptake rate (V_m) in P starved M. aeruginosa was up to 50 times higher than that of S. luetkemuellerii. It decreased with increasing growth rate (P status) in the former species and remained fairly constant in the latter. The corresponding cellular P-based value (U_m= V_m/Q_p) decreased with growth rate in both species and was about 10 times higher in P started M. aeruginosa than in S. luetkemuellerii. The average half saturation constant for uptake (K_m) was equal for both species (22 μg P·L^?1) and varied with the P status. S. luetkemuellerii exhibited shifts in the uptake rate of P_i that were characterized by increased affinity (U^m/K^m) at low P_i, concentrations (<4 μg P·L^?1) compared to that at higher concentrations. The species thus was well adapted to uptake at low ambient P_i, but M. aeruginosa was superior in P_i uptake under steady state and transient conditions when the growth rate was lower than 0.75 d^?1. Moreover, M. aeruginosa was favored by pulsed addition of P_i. M. aeruginosa relpased P_i at a higher rate than S. luetkemuellerii. Leakage of P_i from the cells caused C-shaped μ vs. P_i curves. Therefore, no unique K_s for growth could be estimated. The maximum growth rate (μ_m) (23° C) was 0.94 d^?1for S. luetkemuellerii and 0.81 d^?1for M. aeruginosa. The steady state concentration of P_i (P*) was lower in M. aeruginosa than in S. luetkemuellerii at medium growth rates. The concentration of P_i at which the uptake and release of P_i was equal (P_c was, however, lower in S. luetkemuellerii.     

ACCLIMATION OF NITRATE UPTAKE BY PHYTOPLANKTON TO HIGH SUBSTRATE LEVELS1

A literature review of data on nitrate uptake by phytoplankton suggests that nitrate levels above 20 μmol N·L^?1 generally stimulated uptake rates in cultured unicellular algae and natural phytoplankton communities. This phenomenon indicates that phytoplankton cells acclimate to elevated nitrate levels by increasing their uptake capacity in a range of concentrations previously considered to be saturating. Cyanobacteria and flagellates were found to present a considerable capacity for acclimation, with low (0.1–2 μmol N·L^?1) half‐saturation values (K_s) at low (5–20 μmol N·L^?1) substrate levels and high (1–80 μmol N·L^?1) K_s values at high (30–100 μmol N·L^?1) substrate levels. However, some diatom genera (Rhizosolenia, Skeletonema, Thalassiosira) also appeared to possess a low affinity nitrate uptake system (K_s between 18 and 120 μmol N·L^?1), which can help resolve the paradox of their presence in enriched seas. It follows that present models of nitrate uptake can severely underestimate the effects of high nitrate concentrations on phytoplankton dynamics and development. A more adequate approach would be to consider the possibility of multiphasic uptake involving several phase transitions as nitrate concentrations increased. Because it is a nonlinear phenomenon featuring strong thresholds, this effect appears to override that of other variables, such as irradiance, temperature, and cell size. Within the present context of eutrophication and for a range of concentrations that is becoming more and more ecologically relevant, equations are tentatively presented as a first approach to estimate K_s from ambient nitrate concentrations.     

SHORT TERM UPTAKE OF NUTRIENTS BY ENTEROMORPHA PROLIFERA (CHLOROPHYCEAE)1

Rates of NH₄⁺ and NO₃^? uptake were determined by accumulation of ¹⁵N in plant tissue and by disappearance of nutrient from the medium. Agreement between rates calculated by the two methods was good, averaging 82.7% (SD = 15.8%) and 91.2% (SD = 13.7%) for NH₄⁺ and NO₃^? uptake, respectively. An average of 93.4 and 96.0% of added ¹⁵NH₄⁺ and ¹⁵NO₃^? was recovered from the medium and /or plant tissue at the end of the incubations. Both bacterial uptake and regeneration of NH₄⁺ may contribute to discrepancies between NH₄⁺ uptake rates calculated by ¹⁵N accumulation and disappearance of NH₄⁺ from the medium. The influence of tissue composition on uptake of NH₄⁺, NO₃^? and PO₄^3- by Enteromorpha prolifera (Müller) J. Agardh was examined. For NH₄⁺ uptake, V_max was 188 μmol NH₄^+. g dry wt^?1. h^?1 and K_s ranged from 9.3 to 13.4 μM, but there was no correlation between kinetic parameters and tissue nitrogen content. For NO₃^?, both kinetic parameters were higher for plants with low tissue nitrogen than for plants with high tissue nitrogen. Maximum rates were 169 and 75.4 μmol NO₃^?. g dry wt^?1. h^?1, and K_s was 13.3 and 2.31 μM for low and high tissue nitrogen plants, respectively. Estimates of uptake in the field suggested that NH₄⁺ accounted for 65% and NO₃^? for up to 35% of total nitrogen uptake during the summer. Nutrient uptake rates of field-collected plants also indicated that E. prolifera in Yaquina Bay, Oregon was not likely to have been nitrogen-limited, but may have been phosphorus-limited.     

Adaptation to chilling: photosynthetic characteristics of the cultivated tomato and a high altitude wild species

Abstract When tomato plants of the high-altitude species Lycopersicon hirsutum and of the cultivated Lycopersicon esculentum were grown at 24/18°C (day/night), the effects of temperature, photon flux density, and intercellular CO₂ concentration up to about 600 μl l^?1 on net CO₂ uptake were similar in the two species. Acclimation of these plants at 12/6°C (day/night) resulted, after 4 d or longer, in a similar downward shift of about 5°C in the optimum temperature for CO₂ uptake. However, in comparison with the cultivated species, the high-altitude plants achieved a higher rate of CO₂ uptake at saturating concentrations of intercellular CO₂, maintained a higher level of saturating-light CO₂ uptake rate at 10°C after exposure to chilling stress (10°C and photon flux density of 400 μmol m^?2s^?1 d and 5°C night) for 7–18 d, and displayed a better capacity for rapid recovery after prolonged stress. The greater capacity for CO₂ uptake observed in the high-altitude species during and after exposure to chilling stress was also reflected in its higher growth rate under those conditions compared with plants of L. esculentum. These advantages of the high-altitude species may partly explain its ability to survive and complete its life cycle under the environmental conditions prevailing in its natural habitat.