Effects of CO 2 enrichment on photosynthesis,growth, and nitrogen metabolism of the seagrass Zostera noltii

Seagrass ecosystems are expected to benefit from the global increase in CO ₂ in the ocean because the photosynthetic rate of these plants may be C_i‐limited at the current CO ₂ level. As well, it is expected that lower external pH will facilitate the nitrate uptake of seagrasses if nitrate is cotransported with H⁺ across the membrane as in terrestrial plants. Here, we investigate the effects of CO ₂ enrichment on both carbon and nitrogen metabolism of the seagrass Zostera noltii in a mesocosm experiment where plants were exposed for 5 months to two experimental CO ₂ concentrations (360 and 700 ppm). Both the maximum photosynthetic rate (P_m) and photosynthetic efficiency (α) were higher (1.3‐ and 4.1‐fold, respectively) in plants exposed to CO ₂‐enriched conditions. On the other hand, no significant effects of CO ₂ enrichment on leaf growth rates were observed, probably due to nitrogen limitation as revealed by the low nitrogen content of leaves. The leaf ammonium uptake rate and glutamine synthetase activity were not significantly affected by increased CO ₂ concentrations. On the other hand, the leaf nitrate uptake rate of plants exposed to CO ₂‐enriched conditions was fourfold lower than the uptake of plants exposed to current CO ₂ level, suggesting that in the seagrass Z. noltii nitrate is not cotransported with H⁺ as in terrestrial plants. In contrast, the activity of nitrate reductase was threefold higher in plant leaves grown at high‐CO ₂ concentrations. Our results suggest that the global effects of CO ₂ on seagrass production may be spatially heterogeneous and depend on the specific nitrogen availability of each system. Under a CO ₂ increase scenario, the natural levels of nutrients will probably become limiting for Z. noltii. This potential limitation becomes more relevant because the expected positive effect of CO ₂ increase on nitrate uptake rate was not confirmed.     

Are epiphytes a significant component of intertidal Zostera noltii beds?

The role of epiphytes in an intertidal Zostera noltii seagrass bed in Marennes-Oléron Bay was assessed in comparison with the other main benthic primary producers (Z. noltii, microphytobenthos) at two bathymetric levels and on a seasonal basis. Assemblage and biomass of epiphytes were studied using scanning electron microscopy (SEM). Z. noltii and its detrital matter followed a typical seasonal pattern: microphytobenthos was present in large quantities throughout the year representing 21% of the total biomass while detrital matter, above-ground parts and below-ground parts accounted for 65, 9 and 5%, respectively. Only two species of epiphytic diatoms, Cocconeis scutellum and Cocconeis placentula, were observed on seagrass leaves. Epiphyte biomass was very low, representing on average less than 0.001% of that of microphytobenthos or leaves. This low epiphyte biomass is linked with the absence of macroalgae and also with the low biovolume of Cocconeis, which formed a monolayer of cells on leaves. This can be explained by the severe conditions of the intertidal and the high leaf turn-over of Z. noltii leaves.     

Evaluation of Zostera detritus as a potential new source of zosteric acid

Detritus of the seagrasses Zostera noltii and Z. marina collected on the beaches of Arcachon Lagoon (France) over a 3-year period was screened as a new source of zosteric acid (ZA). This natural sulphated phenolic acid is a high value-added product capable of preventing settlement of marine organisms and protecting crops from fungal diseases. The seasonal variation of the ZA content was quantified in methanolic and aqueous crude extracts using high-performance liquid chromatography. The concentration found ranged from 65 to 456 μg g⁻¹ dry wt for Z. noltii and 51–692 μg g⁻¹ dry wt for Z. marina, respectively. This is the first report of ZA in Zostera noltii. Detrital leaves of Zostera have never before been screened for their bioactive substances. These results show that this low cost, very abundant and renewable, but heretofore unused, marine resource has potential as a source of a rare and naturally occurring bioactive product.     

Changes in the kinetics of phosphate and potassium absorption in nutrient-deficient barley roots measured by a solution-depletion technique

From measurements of the rates of depletion of labelled ions from solution in the low concentration range, we described the phosphate and potassium uptake characteristics of the roots of intact barley plants in terms of the kinetic parameters, K _m and I _max (the maximum rate of uptake). In relatively young (13 d) and older (42 d) plants, cessation of phosphate supply for 4 d or more caused a marked increase in I _max (up to four times), without concomitant change in K _m, which remained between 5 and 7 M. By contrast, 1 d of potassium starvation with 14-d plants caused a decline in the K _m (i.e. an increased apparent affinity for potassium) from 53 M to 11 M, without alteration to I _max. After longer periods of potassium starvation, I _max increased (about two times) while the K _m remained at the same low value. Growth of shoots and roots were unaffected by these treatments, so that concentrations of ions in the tissues declined after 1 d or more of nutrient starvation, but we could not identify a characteristic endogenous concentration for either nutrient at which changes in kinetic parameters were invariably induced. The possible mechanisms regulating carriermediated transport, and the importance of changes induced in kinetic parameters in ion uptake from solution and soil are discussed.Symbol I _max the maximum rate of absorption at saturating concentrations     

Differential anaerobic decomposition of seagrass (Zostera noltii) and macroalgal (Monostroma obscurum) biomass from Arcachon Bay (France)

Arcachon Bay is characterized by extensive meadows of the seagrass Zostera noltii. Moreover, as a consequence of eutrophication, massive proliferations of the macroalga (Monostroma obscurum) have occurred since the beginning of 1990s.This paper describes the anaerobic decomposition of biomass of both species under experimental conditions by two methods. Firstly, the dynamics of decomposition were studied in situ using litter bags. The remaining biomass and the elemental composition of the decomposing macrophytes were monitored. Secondly, degradation was studied in experimental containers under anoxic conditions in which the release of inorganic nutrients and the development of fermentative and sulfate-reducing bacterial populations were followed.The decomposition rate of total biomass was faster for macroalgae than for the vascular plants, thus corroborating previous observations. However, both in situ and laboratory experiments showed that the anaerobic decomposition of the seagrass Z. noltii resulted in rapid release of inorganic N and P, and increasing C/N and C/P ratios of the residual biomass. As a result, the recycling of inorganic nitrogen and phosphorus compounds was slightly more efficient for Z. noltii than for M. obscurum. Recycling of inorganic nutrients appears to be of a great importance to the whole ecosystem, because of the extensive spreading of Z. noltii in the bay.     

Submerged versus air-exposed intertidal macrophyte productivity: from physiological to community-level assessments

The photosynthetic productivity of the intertidal communities dominated by the seagrass Zostera noltii and the cordgrass Spartina maritima was assessed in two contrasting situations during a tidal cycle, i.e., air exposure and water immersion. Two complementary methods were used: infra red gas analysis of CO₂ flux measurements in whole communities and chlorophyll a fluorescence measurements of individual plants photosynthetic activity. Higher photosynthetic rates of Z. noltii in air were observed both at the individual plants response level determined by chlorophyll fluorescence and at the community level measured as gas exchange (CO₂ uptake). S. maritima plants consistently showed low photosynthetic response when immersed. Gross community production (GCP) measured as carbon dioxide uptake was always higher in air than in water for both communities. When immersed, the GCP of both communities was similar. However, when exposed to the air, the GCP of the S. maritima community was higher than the one of Z. noltii's. The key factor in CO₂ assimilation by air-exposed Z. noltii was the retention of water in sediment microdepressions. During low tide, depressions in the sediment retain a considerable amount of water, enough to maintain leaf hydration. In these conditions, rapid air-water CO₂ diffusion occurs, making it readily available to plants. The community gas exchange measurements compared well with the fluorescence indications. Both Z. noltii and S. maritima were shown to be responsible for the overall pattern of photosynthetic carbon fixation within their respective communities, both during submersion and emersion periods. The short-term incubations method described in this report proved to be a valuable tool for field measurements of intertidal lagoon productivity. It provides fast and precise values of carbon dioxide fixation, both in submerged and air-exposed communities.     

Leaf dynamics and standing stocks of intertidal Zostera noltii Hornem. and Cymodocea nodosa (Ucria) Ascherson on the Banc d'Arguin (Mauritania)

Leaf dynamics and standing stocks of intertidal seagrasses were studied in the Baie d'Aouatif (Parc National du Banc d'Arguin, Mauritania) in April and September 1988. Standing stocks of Zostera noltii Hornem. suggest a unimodal seasonal curve similar to what is found for populations at higher latitudes. Also, leaf growth rates (0.03 cm² cm^–2 day^–1 on average) were similar to those found at higher latitudes in these months. Variation in leaf loss over tidal depth, time and different locations in the Baie d'Aouatif was larger and more often significant than variation in leaf growth. In general, Z. noltii beds in the Baie d'Aouatif had comparable leaf growth rates and standing stocks. In both months losses were almost always higher than or equal to growth.Variation in leaf loss over time was much higher in the plots that were situated high in the intertidal than in lower plots. This is explained by differences in susceptibility to sloughing, which is presumably higher in periods with low tide around noon for shallow depths.In an experiment using artificial shading nets, in situ leaf growth was affected negatively from 94% shading onwards. This shading was observed to reduce the light intensity reaching the seagrass bed to a level below the reported range of light compensation points for Z. noltii. Cymodocea nodosa (Ucria) Ascherson on average had higher leaf area and relative growth rates than Z. noltii and much lower loss rates, resulting in a positive net increase in September. Standing stocks were also higher than for Z. noltii. A mixed seagrass bed containing the above two species and Halodule wrightii Ascherson had the highest observed total biomass: 335 g m^–2 ash-free dry weight.     

Larval abundance and recruitment of Carcinus maenas L. close to its southern geographic limit: a case of match and mismatch

In the Ria Formosa lagoon (S. Portugal), Carcinus maenas larvae have been observed only during the cold part of the year (October–May), when the water temperature stays below 23 °C. Larvae suffer high mortality during day time (about 80% during 6 h). Hence, they will only develop outside the lagoon, as proved by the observation of zoea I stages exclusively. Main recruitment starts in April, i.e. much later than the first larvae appear. Small crabs prefer Zostera noltii patches and colonize other sites when the carapace width exceeds about 5 mm. A reserve of some small crabs is found in Zostera noltii patches nearly all year round. It is postulated that one of the key factors for successful propagation of the species is a match between physiological reactions (here in particular temperature effect on reproductive cycle and larval release), and the ecological conditions for larval survival and recruitment, such as predator impact, food availability or currents.     

Seasonal dynamics of <Emphasis Type="Italic">Zostera noltii</Emphasis> Hornem. in two Mediterranean lagoons

The density, biomass and shoot morphology of two populations of Zostera noltii were monitored from January 1998 to July 1999 at two shallow Mediterranean lagoons of Biguglia and Urbino, which differ in hydro-morphological conditions and nutrient loading. Monitoring included the principal biological and foliar parameters (shoot density, aboveground and belowground biomass, length, width and number of leaves, LAI and coefficient A: percentage of leaves having lost their apex), the organic matter contents of the sediment and the environmental conditions (salinity, turbidity, temperature, nutrient concentrations and dissolved oxygen levels). The two populations of Z. noltii displayed seasonal changes in density (1600–19600 m²), aboveground biomass (11–153 g. DW. m⁻²), leaf length (33–255 mm), and leaf width (0.9–1.8 mm). Temperature and turbidity were significant environmental factors influencing the temporal changes observed in the Z. noltii meadows studied. Conversely, the belowground biomass, the number of leaves per shoot and the LAI did not undergo any seasonal changes. In the Biguglia lagoon, the functioning dynamics of the Z. noltii seagrass beds are determined by the catchment area and the inputs of nutrients derived from it, whereas in the Urbino lagoon the dynamics of the Z. noltiibeds depend on low levels of water turbidity.     

Long-distance signals positively regulate the expression of iron uptake genes in tobacco roots

Long-distance signals generated in shoots are thought to be associated with the regulation of iron uptake from roots; however, the signaling mechanism is still unknown. To elucidate whether the signal regulates iron uptake genes in roots positively or negatively, we analyzed the expressions of two representative iron uptake genes: NtIRT1 and NtFRO1 in tobacco (Nicotiana tabacum L.) roots, after shoots were manipulated in vitro. When iron-deficient leaves were treated with Fe(II)-EDTA, the expressions of both genes were significantly reduced; nevertheless iron concentration in the roots maintained a similar level to that in roots grown under iron-deficient conditions. Next, all leaves from tobacco plants grown under the iron-deficient condition were excised. The expression of two genes were quickly reduced below half within 2 h after the leaf excision and gradually disappeared by the end of a 24-h period. The NtIRT1 expression was compared among the plants whose leaves were cut off in various patterns. The expression increased in proportion to the dry weight of iron-deficient leaves, although no relation was observed between the gene expression and the position of excised leaves. Interestingly, the NtIRT1 expression in hairy roots increased under the iron-deficient condition, suggesting that roots also have the signaling mechanism of iron status as well as shoots. Taken together, these results indicate that the long-distance signal generated in iron-deficient tissues including roots is a major factor in positive regulation of the expression of NtIRT1 and NtFRO1 in roots, and that the strength of the signal depends on the size of plants.     

Temperature tolerance and survival of intertidal populations of the seagrass <Emphasis Type="Italic">Zostera noltii</Emphasis> (Hornemann) in Southern Europe (Ria Formosa,Portugal)

The dwarf seagrass Zostera noltii is an important primary producer in Atlantic coastal ecosystems from Mauritania to southern Norway and the Mediterranean Sea. Sessile intertidal organisms existing at the interface between marine and terrestrial environments may be particularly vulnerable to environmental change. In this study, we asked how near to thermal tolerance limits natural populations of Z. noltii are in the Ria Formosa coastal lagoon system in southern Portugal. We recorded the maximum temperatures in the Ria Formosa during the 2007 summer, and conducted experiments to determine the sub-lethal temperature of Z. noltii shoots sampled at two sites located at different tidal heights. Mortality rates and photosynthetic performance were recorded within a range of heat shock temperatures between 35 and 41°C. Survival was recorded ≤37°C, while higher temperatures led to a sudden drop in photosynthetic capacity followed by mortality (shoot loss) that occurred more rapidly with increasing temperatures. At 39°C and above, the rate of shoot mortality in both sites was close to 100%, occurring between 5 and 13 days after the heat shock. Survival was ca. 95 and 90% at 35 and 37°C, respectively. From these results for Z. noltii populations in the Ria Formosa we estimated sub-lethal temperature to be approximately 38°C for Z. noltii, close to the maximum of 36°C recorded in the summer 2007. Considering predicted trajectories in the coming decades, these results raise concern as to the future viability of intertidal Z. noltii populations near the southernmost edge of their distribution. Handling editor: S. M. Thomaz     

IN SITU UPTAKE KINETICS OF AMMONIUM AND PHOSPHATE AND CHEMICAL COMPOSITION OF THE RED SEAWEED GRACILARIA TIKVAHIAE1

The uptake kinetics of ammonium and phosphate by Gracilaria tikvahiae McLachlan were studied under field conditions. Seaweeds, pulse fed once a week for 6 h over a 4-week period, had maximum uptake rates of 19 μmol·g fwt^?1·h^?1 for ammonium and 0.28 μmol·g fwt^?1·h^?1 for phosphate. For both nutrients there was a positive linear correlation between uptake rate (v) and concentration (S) over the entire range of concentration tested. In a nutrient depletion experiment, the phosphate uptake curve determined over a wide range of concentrations consisted of two stages of saturation at low concentrations, and a linear phase at high concentrations. Ash free dry weight, chlorophyll a, phycoerythrin, and protein content were higher in pulse fed plants than in control plants receiving no nutrient additions, while the reverse held true for carbohydrate contents and the C/N ratios. The C/N ratio inversely correlated with ammonium and phosphate uptake rate as well as protein and phycoerythrin content, and positively with carbohydrate content.     

NUTRITIONAL STUDIES OF TWO RED ALGAE. II. KINETICS OF AMMONIUM AND NITRATE UPTAKE1, 2

Similar NH₄₊ and NO₃^?.uptake kinetic patterns were observed in Neoagardhiella baileyi (Harvey ex Kiitzing) Wyinne & Taylor and Gracilaria foliifera (Forssk?l) Borgesen. NO₃^? was taken up in a rate-sturating fashion described by the Michaelis-Menten equation. NH₄₊ uptake was multicomponent: a saturable component was accompanied by a diffusive or a high K component showing no evidence of saturation (at ≤50 μM [NH₄₊]). Nitrogen starved plantsi(C/N atom ratios > ca. 10) showed higher transient rates of NH₄₊ uptake at a given concentration than plants not N-Iimited. Only plants with high N content exhibited diel changes inNH₄₊ uptake rates, and showed transient rates of NH₄₊ accumulation which did not greatly exceed the capacity to incorporate N in steady-state growth. NH₄₊ was preferred over NO_3?even in plants preconditioned on NO_3?as the sole N. source, NO_3? uptake was suppressed at 5μM [NH₄₊], but simultaneous uptake occurred at unsurpressed rates at lower concentrations. Potential for N accumulation was greater via NH₄₊uptake than via NO_3?uptake. Changing capacity for NH₄₊ uptake with N content appears to be a mechanism whereby excessive accumulation of N was avoided by N-.satiated plants but a large accumulation was possible for N-depleted plants.     

Interactions of light and organic matter under contrasting resource simulated environments: the importance of clonal traits in the seagrass <Emphasis Type="Italic">Zostera noltii</Emphasis>

Light reduction in the water column and enhanced organic matter (OM) load into the sediments are two main consequences of eutrophication in marine coastal areas. This study addresses the combined effects of light, OM, and clonal traits in the seagrass Zostera noltii. Large Z. noltii plants were grown in sand with or without the addition of OM and under two light levels (high light and low light). Whereas some complete plant replicates were grown under homogeneous light and/or OM conditions, other replicates were grown under contrasting light and/or OM levels between the apical and the distal parts of the same plant. The three-way factorial design (light, OM load, and apex position) allowed us to determine the harmful effect of light reduction and OM enrichment on the growth, photosynthetic performance, and biochemical composition of Z. noltii. The addition of OM to the sediment promoted a decrease, or even an inhibition, in net plant growth regardless of the light level when the whole plants were grown under homogeneous light conditions. However, the results differed when plants were grown under contrasting light and/or OM conditions between apical and distal parts. In this case, the harmful effect of OM load was alleviated when apical parts were grown under high light conditions. OM loads also negatively affected the photosynthetic performance, evaluated as leaf fluorescence. The results indicate the importance of clonal traits in the response of Z. noltii growth to light conditions and OM enrichment. Guest editors: J. H. Andersen & D. J. Conley Eutrophication in Coastal Ecosystems: Selected papers from the Second International Symposium on Research and Management of Eutrophication in Coastal Ecosystems, 20–23 June 2006, Nyborg, Denmark     

Interactive effects of mycorrhization and elevated atmospheric CO2 on sulphur nutrition of young pedunculate oak (Quercus robur L.) trees

Pedunculate oak (Quercus robur L.) was germinated and grown at ambient CO₂ concentration and 650 μmol mol^?1 CO₂ in the presence and absence of the ectomycorrhizal fungus Laccaria laccata for a total of 22 weeks under nonlimiting nutrient conditions. Sulphate uptake, xylem loading and exudation were analysed in excised roots. Despite a relatively high affinity for sulphate (K_M= 1.6 mmol m^?3), the rates of sulphate uptake by excised lateral roots of mycorrhizal oak trees were low as compared to herbaceous plants. Rates of sulphate uptake were similar in mycorrhizal and non-mycorrhizal roots and were not affected by growth of the trees at elevated CO₂. However, the total uptake of sulphate per plant was enhanced by elevated CO₂ and further enhanced by elevated CO₂ and mycorrhization. Sulphate uptake seemed to be closely correlated with biomass accumulation under the conditions applied. The percentage of the sulphate taken up by mycorrhizal oak roots that was loaded into the xylem was an order of magnitude lower than previously observed for herbaceous plants. The rate of xylem loading was enhanced by mycorrhization and, in roots of mycorrhizal trees only, by growth at elevated CO₂. On a whole-plant basis this increase in xylem loading could only partially be explained by the increased growth of the trees. Elevated CO₂ and mycorrhization appeared to increase greatly the sulphate supply of the shoot at the level of xylem loading. For all treatments, calculated rates of sulphate exudation were significantly lower than the corresponding rates of xylem loading of sulphate. Radiolabelled sulphate loaded into the xylem therefore seems to be readily diluted by unlabelled sulphate during xylem transport. Allocation of reduced sulphur from oak leaves was studied by flap-feeding radiolabelled GSH to mature oak leaves. The rate of export of radioactivity from the fed leaves was 4–5 times higher in mycorrhizal oak trees grown at elevated CO₂ than in those grown at ambient CO₂. Export of radiolabel proceeded almost exclusively in a basipetal direction to the roots. From these experiments it can be concluded that, in mycorrhizal oak trees grown at elevated CO₂, the transport of sulphate to the shoot is increased at the level of xylem loading to enable increased sulphate reduction in the leaves. Increased sulphate reduction seems to be required for the enhanced allocation of reduced sulphur to the roots which is observed in trees grown at elevated CO₂. These changes in sulphate and reduced sulphur allocation may be a prerequisite for the positive effect of elevated CO₂ on growth of oak trees previously observed.     

Ammonium uptake kinetics in root and leaf cells of Zostera marina L.

Ammonium uptake rates and the mechanism for ammonium transport into the cells have been analysed in Zostera marina L. In the cells of this species, a proton pump is present in the plasmalemma, which maintains the membrane potential. However, this seagrass shows a high-affinity transport mechanism both for nitrate and phosphate which is dependent on sodium and is unique among angiosperms. We have then analysed if the transport of another N form, ammonium, is also dependent of sodium. First, we have studied ammonium transport at the cellular level by measurements of membrane potentials, both in epidermal root cells and mesophyll cells. And second, we have monitored uptake rates in whole leaves and roots by depletion experiments. The results showed that ammonium is taken up by a high-affinity transport system both in root and leaf cells, although two different of kinetics could be discerned in mesophyll cells (with affinity constants of 2.2 ± 1.1 μM NH₄⁺, in the range 0.01-10 μM NH₄⁺, and 23.2 ± 7.1 μM NH₄⁺, at concentrations between 10 and 500 μM NH₄⁺). However, only one kinetic could be observed in epidermal root cells, which showed a K_m = 11.2 ± 1.0 μM NH₄⁺, considering the whole ammonium concentration range assayed (0.01-500 μM NH₄⁺). The higher affinity of leaf cells for ammonium was consistent with the higher uptake rates observed in leaves, with respect to roots, in depletion experiments at 10 μM NH₄⁺ initial concentration. However, when an initial concentration of 100 μM was assayed, the difference between uptake rates was reduced, but still being higher in leaves. Variations in proton or sodium-electrochemical gradient did not affect ammonium uptake, suggesting that the transport of this nutrient is not driven by these ions and that the ammonium transport mechanism could be different to the transport of nitrate and phosphate in this species.     

Phosphorus uptake and translocation in salt-stressed melon plants

Melon seedlings (Cucumis melo L. cv.Galia) were grown hydroponically to study the effect of salinity (80 mmol/LNaCl) on phosphate (Pi) uptake and translocation at two levels of Pi (25 μmol/L and 1 mmol/L). Net uptake rates of Pi were determined by depletionof the medium and by plant content. Salinity decreased Pi uptake at low Pi (high affinity uptake mechanism), 25 μmol/L, although no specific competitive inhibition of Pi uptake by Cl⁻ was observed. When plants were grown with high Pi (1 mmol/L), the uptake of Pi through the low affinity system was increased by 80 mmol/L NaCl. Salinity also reduced the phosphorus flux, as Pi, through the xylem. It is hypothesised that high levels of NaCl decrease the mobility of Pi stored in vacuoles, and as a result, inhibit export from this storage compartment to other parts of the plant.     

PRODUCTION OF HYALINE HAIRS BY INTERTIDAL SPECIES OF FUCUS (FUCALES) AND THEIR ROLE IN PHOSPHATE UPTAKE1

A field study to determine the precise times of year at which three intertidal species of Fucus start to produce hyaline hairs and cease producing such hairs was conducted on the Isle of Man, U.K. Hairs were first observed during February, and within 6 days of their initial appearance, all tagged plants of all species at all tidal heights on the shore possessed hairs. Hair production continued until the beginning of October, at which time Fucus plants growing at the lowest stations (+ 3.0 m) had glabrous apical growth. Hair production continued later into the year for plants growing higher on the shore, and it was not until mid-November that glabrous apical growth was observed in all plants. Phosphate uptake rates of pilose (hairy) and glabrous (hairless) apical sections were measured in November 1988 for F. spiralis L. and in January 1989 for F. spiralis and F. serratus L., at phosphate concentrations ranging from 0.8 μM (ambient seawater) to 9.0 μM. In ambient seawater, pilose plants of F. spiralis removed phosphate 2–3 times faster than glabrous plants, whereas the uptake rates of pilose plants of F. serratus were about 50% greater than those of glabrous plants. The differences between uptake rates of pilose and glabrous plants of both species were smaller or nonsignificant at higher phosphate concentrations. The field and laboratory data are consistent with the hypothesis that hairs are formed in Fucus as a response to increased nutrient demand and that hairs facilitate the uptake of nutrients from seawater at concentrations typical of natural situations.     

Information about previous phosphate fluctuations is stored via an adaptive response of the high-affinity phosphate uptake system of the cyanobacterium Anacystis nidulans

The high-affinity uptake system of phosphatelimited cyanobacterium Anacystis nidulans [Synechococcus leopoliensis (Raciborski) Komarek] is characterized by a threshold value below which uptake cannot occur. Here it is shown that, if phosphate-limited cyanobacteria are challenged with a short pulse of high phosphate concentration that appreciably exceeds this threshold value, the uptake system undergoes an adaptive response, leading to the attainment of new kinetic properties and a new threshold value. These new properties are maintained for several hours after the pulse. A notable characteristic of this new state is a wide linear dependence of the uptake rate on the external phosphate potential that is a function of the driving force of the uptake process. According to theoretical arguments it is shown that this “linear operation mode” can be explained by the simultaneous operation of several uptake systems with different, staggered threshold values and kinetic properties. Moreover, the new linear uptake properties, in turn, reflect the prehistory of phosphate supply experienced by the population. The consequences of this result with regard to environmental fluctuations of the phosphate concentration in lakes are discussed.