Phenols,proline and low-molecular thiol levels in pea (<Emphasis Type="Italic">Pisum sativum</Emphasis>) plants respond differently toward prolonged exposure to ultraviolet-B and ultraviolet-C radiations

Pea (Pisum sativum L.) seedlings were exposed to low, moderate, and high regimes of ultraviolet-B (UV-B) (ld-B 4.4, md-B 13.3, and hd-B 26.5 kJ m⁻² day⁻¹), or ultraviolet-C (UV-C) (ld-C 0.1, md-C 0.3, and hd-C 0.6 kJ m⁻² day⁻¹) radiations. Concentrations of total phenols, free proline, and low-molecular thiol groups were determined in the last formed (young) and older leaves after irradiation for 7, 10 or 14 consecutive days. Shoot length and weight did not change markedly after 14 days of ld-B and ld-C, but reduced substantially after moderate and high regimes of both UV-B and UV-C. Proline decreased upon high doses of irradiation, while in ld-B treated plants, by contrast, an increase was observed. The reduction in total phenols and thiols was stronger after hd-B than after hd-C irradiations, although an induction was found in ld-B treated plants. In contrast to ld-B, ld-C regime led mainly to reductions or insignificant changes in proline, phenols, and thiols. Therefore, the stress-protection mechanisms are different between low UV-B and UV-C irradiation regimes in regard to proline, phenols, and thiols.     

Low doses of ultraviolet-B or ultraviolet-C radiation affect phytohormones in young pea plants

Pea (Pisum sativum L., cv. Scinado) seedlings were exposed to low doses of ultraviolet-B (UV-B; 4.4 and 13.3 kJ m⁻² d⁻¹) or UV-C (0.1 and 0.3 kJ m⁻² d⁻¹) radiation for 14 d. Aminocyclopropane carboxylic acid (ACC), indoleacetic acid (IAA) and abscisic acid (ABA) contents were quantified by gas chromatography coupled to mass spectrometry (GC-MS). The accumulation of ACC upon irradiation was dose-dependent. ABA content was reduced and IAA content increased upon UV-C treatment whereas the UV-B doses used did not cause significant changes in ABA and IAA contents.     

Understanding the effect of organic pollutant fluoranthene on pea in vitro using cytokinins,ethylene, ethane and carbon dioxide as indicators

Environmental contaminants like polycyclic aromatic hydrocarbons can influence many biochemical and physiological processes in plants. The effect of 0.1, 1 and 5 mg l⁻¹ of fluoranthene (FLT) in combination with indole-3-acetic acid (IAA, 0.1 mg l⁻¹) or a combination of IAA and N⁶-benzyladenine (BA, both 0.1 mg l⁻¹) on the growth and production of ethylene, ethane and CO₂ in Pisum sativum L. cultivated for 21 days in vitro was investigated. In 21 days old plants also net photosynthesis rate, content of FLT and cytokinins were evaluated. FLT 5 mg l⁻¹ significantly inhibited the growth of pea after 21 days in both IAA and IAA + BA treatments, increased production of ethylene (by 11% in IAA and 14% in IAA + BA treatments, respectively) and ethane (by 28 and 18%) and decreased production of CO₂ (by 23 and 29%). The net photosynthesis rate decreased in response to FLT concentration by up to 51% under saturating irradiation (600–1,200 μmol m⁻² s⁻¹), as found in IAA + BA + FLT 5 mg l⁻¹ treatment. The content of FLT in pea plant shoots well correlated with increasing FLT treatment in both IAA and IAA + BA medium. The content of cytokinins in pea shoots changed in response to FLT treatment. FLT 5 mg l⁻¹ caused a rise in level of trans-zeatin (by 16% in IAA and 9% in IAA + BA treatments, respectively), dihydrozeatin riboside (by 27 and 50%), benzyladenine (by 3 and 80%), benzyladenine riboside (by 44 and 17%) and meta-topolin riboside (by 139 and 214%), no change in isopentenyladenine level and a decrease in meta-topolin level (by 33% in IAA and 36% in IAA + BA treatments, respectively). Cultivation of plants in vitro allowed not only to assess their growth, photosynthetic activity and the level of cytokinins, but also to extend the knowledge about the effect of PAHs on production of gaseous stress indicators like ethylene, ethane and CO₂. Recorded changes in all studied parameters show, that persistent organic pollutants like PAHs can negatively influence plant growth and development.     

Fluoranthene-induced production of ethylene and formation of lysigenous intercellular spaces in pea plants cultivated in vitro

The effect of increasing concentration of polycyclic aromatic hydrocarbon (PAH) fluoranthene (FLT; 0.1, 1 and 5 mg l⁻¹) on the growth, ethylene production and anatomy of stems of 21-day-old pea plants cultivated in vitro in MS medium, with or without FLT, enriched with 0.1 mg l⁻¹ indole-3-acetic acid (IAA) or with combination of 0.1 mg l⁻¹ IAA + 0.1 mg l⁻¹ N⁶-benzyladenine (BA) were investigated. The low concentration of 0.1 mg l⁻¹ FLT, in both IAA- and IAA + BA-treated plants, significantly stimulated the growth of pea callus, while higher concentrations 1 mg l⁻¹ and especially 5 mg l⁻¹ FLT significantly inhibited it. Pea shoots were significantly influenced only after application of 5 mg l⁻¹ FLT in IAA treatment. Significantly increased production of ethylene was found in IAA + BA treatments in all concentrations of FLT, whereas in IAA treatments in 1 and 5 mg l⁻¹ FLT. The lysigenous aerenchyma formation in the cortex of pea stems significantly increased in all FLT treatments and its highest proportion was found in plants exposed to 1 mg l⁻¹ FLT.     

Polyamines increase carpospore output and growth during in vitro cultivation of Hydropuntia cornea

Carpospore output and development in the marine red alga Hydropuntia cornea J. Agardh. were increased by adding polyamines (PAs) (putrescine, spermidine and spermine) singly or in combinations at 10⁻⁹, 10⁻⁶ and 10⁻³ M. Cell divisions after spore release and development of apical axis between 17 and 21 days characterized carpospore development. PAs increased carpospore development by promoting cell divisions to form cell masses between day 2 and 3. Morphogenesis to develop apical axes occurred at day 7. Spermine at 10⁻⁶ M and a combination of putrescine 10⁻⁹ M + spermidine 10⁻⁹ M + spermine 10⁻⁹ M gave a higher number of carpospores and enhanced their further development to sporelings.     

Polyamines content and physiological and biochemical responses to ladder concentration of nickel stress in Hydrocharis dubia (Bl.) Backer leaves

Influence of ladder concentration of nickel (Ni) on the leaves of Hydrocharis dubia were studied after 3 days treatment. The accumulation of Ni, the content of polyamines, proline, malondialdehyde (MDA) and soluble protein, as well as the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) in the leaves were investigated. The result indicated that the toxicity of Ni manifested in respective aspect of physiological and biochemical characters. Significant increase of Ni concentration in the leaf tissue was observed, which was concentration dependent. Visible symptoms of Ni toxicity: chlorosis and necrosis occurred following the 3rd day. Meantime, treatment with Ni resulted in the increase in the generation rate of O₂^•− in the leaves. SOD and CAT activities decreased significantly in response to Ni treatment, it was possibly the reason of accumulation of O₂^•−. However, a several-fold decrease in POD activities was found. Our results indicated that because of prolonged increases in O₂^•− level, oxidative damage, measured as the level of lipid peroxidation, occured in the leaves of Ni treated fronds. The changes of the content of polyamines (PAs) were also investigated in the leaves of Hydrocharis dubia. Ni treatment significantly increased the putrescine (Put) level and lowered spermidine (Spd) and spermine (Spm) levels, thereby significantly reducing the ratio of free (Spd + Spm)/Put in leaves, which has been considered as the signal under stress. Although the trend that PS-conjugated PAs and PIS-bound PAs changed the same as free PAs, they changed in more less extent.     

The influence of ultraviolet-B radiation on the growth, pigment production and chlorophyll fluorescence of Norway spruce seedlings

Norway spruce (Picea abies (L.)Karst.) from seven seed sources was grown in a greenhouse with 8.3 and 14.7 kJ·m⁻²·d⁻¹ m UV-B_BE (biologically effective UV-B: 280–320 nm) irradiation, and with no supplemental irradiation as control. The seedlings total biomass (dry weight) and shoot growth decreased with high UV-B treatment but spruce from low elevation seed sources were more affected. The seedlings grown at the highest UV-B irradiance (14.7 kJ·m⁻²·d⁻¹) showed from 5 to 38% inhibition of total biomass and 15 to 70 % shoot growth inhibition. Norway spruce populations from higher altitude seed sources manifested greater tolerance to UV-B radiation compared to plants from low altitudes. Changes in phospholipids and protective pigments were also determined. The plants grown at the lower UV-B irradiance (8.3 kJ·m⁻²·d⁻¹) showed greater ability to concentrations UV-B-absorbing pigments then control plants. Chlorophyll a fluorescence parameter R_fd, (R_fd=(F_m-F_s)/F_s) showed a significant decrease in needles of UV-B treated plants and this correlated with the altitude of seed source. Exposure to UV-B affect levels of the ratio of variable to maximum fluorescence (F_v/F_m). Results from this study suggest that the response to increased levels of UV-B radiation is depended upon the ecotypic differentiation of Norway spruce and involved changes in metabolites in plant tissues.     

Winter as a nutritional bottleneck for North American porcupines (Erethizon dorsatum)

North American porcupines are distributed across a wide variety of habitats where they consume many different species of plants. Winter is a nutritional bottleneck for northern populations, because porcupines remain active when environmental demands are high and food quality is low. We used captive porcupines to examine physiological responses to low-quality diets at high energy demands during winter at ambient temperatures as low as −39°C. We did not observe an endogenous pattern of body mass gain or loss when porcupines were fed a low nitrogen diet (1.1% dry matter) ad libitum through winter. Dry matter intake declined from 43.6 to 14.6 g kg^−0.75 d⁻¹ even though ambient temperatures declined from −3 to −30°C, which indicates a seasonal decrease in metabolic rate. Porcupines consuming white spruce needles maintained digestive efficiency for energy (61%) and neutral detergent fiber (NDF) (50%). However, low requirements for energy (398 kJ kg^−0.75 d⁻¹) and nitrogen (209 mg kg^−0.75 d⁻¹) minimized the loss of body mass when intakes were low and plant toxins increased urinary losses of energy and nitrogen. Porcupines were also able to tolerate low intakes of sodium, even when dietary potassium loads were high. Porcupines use a flexible strategy to survive winter: low requirements are combined with a high tolerance for dietary imbalances that minimize the use of body stores when demands exceed supply. However, body stores are rapidly restored when conditions allow. Porcupines posses many physiological abilities similar to specialist herbivores, but retain the ability of a generalist to survive extreme conditions by using a variety of foods.     

Fluoranthene influences endogenous abscisic acid level and primary photosynthetic processes in pea (<Emphasis Type="Italic">Pisum sativum</Emphasis> L.) plants in vitro

The influence of increasing concentrations (0.1, 1.0 and 5.0 mg l⁻¹) of fluoranthene (FLT) on growth, endogenous abscisic acid (ABA) level and primary photosynthetic processes in 21-day-old pea plants (Pisum sativum L.) in vitro was investigated. Murashige and Skoog’s (MS) medium, with or without FLT, was enriched with indole-3-acetic acid (IAA; 0.1 mg l⁻¹) or a combination of IAA (0.1 mg l⁻¹) plus N⁶-benzyladenine (BA; 0.1 mg l⁻¹). The level of endogenous ABA significantly increased with increasing FLT concentrations in the presence of both IAA and IAA plus BA. An increased level of endogenous ABA was observed in plants treated with IAA alone. The growth of shoot, callus and the content of photosynthetic pigments (chlorophyll a and b, carotenoids), in both IAA- and IAA plus BA-treated plants, were significantly stimulated by FLT at its lowest concentration (0.1 mg l⁻¹) assayed in this study. However, FLT at higher concentrations (1.0 and 5.0 mg l⁻¹) significantly inhibited all these parameters. Chlorophyll fluorescence imaging showed that FLT only at the highest concentration (5.0 mg l⁻¹) in the presence of IAA (0.1 mg l⁻¹) significantly increased F₀, but decreased F_V/F_M and Φ_II.     

Comparison of responses of bean, pea and rape plants to UV-B radiation in darkness and in light

Effect of UV-B radiation on leaves of bean, pea and rape plants was studied. UV-B radiation (11.2 kJ·m⁻²) induced more distinct reduction of the primary photosynthesis activity when applied in darkness than the same UV-B dose, extended in time, and applied with photosynthetic active radiation (PAR). The pea plants were more susceptible to UV-B in darkness, but in the presence of PAR their tolerance was higher. The CO₂ fixation in the bean and rape plants, exposed to UV-B was decreased, but for the pea plants it remained unchanged. The UV-B irradiation caused an increase in the content of ultraviolet-absorbing pigments. Additionally, the bean plants grown at UV-B increased the thickness of leaves, described as SLW.     

Quasi steady state growth of <Emphasis Type="Italic">Lactococcus lactis</Emphasis> in glucose-limited acceleration stat (A-stat) cultures

Quasi steady state growth of Lactococcus lactis IL 1403 was studied in glucose-limited A-stat cultivation experiments with acceleration rates (a) from 0.003 to 0.06 h⁻² after initial stabilization of the cultures in chemostat at D = 0.2–0.3 h⁻¹. It was shown that the high limit of quasi steady state growth rate depended on the acceleration rate used—at an acceleration rate 0.003 h⁻² the quasi steady state growth was observed until μ _crit = 0.59 h⁻¹, which is also the μ _max value for the culture. Lower values of μ _crit were observed at higher acceleration rates. The steady state growth of bacteria stabilized at dilution rate 0.2 h⁻¹ was immediately disrupted after initiating acceleration at the highest acceleration rate studied—0.06 h⁻². Observation was made that differences [Δ(μ − D)] of the specific growth rates from pre-programmed dilution rates were the lowest using an acceleration rate of 0.003 h⁻² (< 4% of preset changing growth rate). The adaptability of cells to follow preprogrammed growth rate was found to decrease with increasing dilution rate—it was shown that lower acceleration rates should be applied at higher growth rates to maintain the culture in the quasi steady state. The critical specific growth rate and the biomass yields based on glucose consumption were higher if the medium contained S ₀ = 5 g L⁻¹ glucose instead of S ₀ = 10 g L⁻¹. It was assumed that this was due to the inhibitory effect of lactate accumulating at higher concentrations in the latter cultures. Parallel A-stat experiments at the same acceleration and dilution rates showed good reproducibility—Δ(μ − D) was less than 5%, standard deviations of biomass yields per ATP produced (Y _ATP), and biomass yields per glucose consumed (Y _XS) were less than 15%.     

Lead induced antioxidant defense system in pigeon pea and its impact on yield and quality of seeds

Toxic effects of lead (>0.2 mM Pb) were measured in pigeon pea (Cajanus cajan Mill) cv. UPAS grown in sand culture as reduction in growth, yield, and quality of seeds. Leaves containing >38 μg g⁻¹ Pb showed oxidative damage as decrease in chlorophyll content and induction of antioxidants such as carotenoids, proline, and non-protein thiol contents with enhanced activities of SOD and peroxidase. At excess (>0.2 mM) Pb, accumulation of >1,000 μg Pb g⁻¹ root tissue was associated with increase in non-protein thiol content. It is concluded that inhibition in root-to-shoot translocation of Pb and induction in the level of proline, chloroplast pigments, and non-protein thiols and activities of antioxidant enzymes SOD and peroxidase at <0.2 mM Pb could have protected the pigeon pea plants from the deleterious effects of Pb. However, excess Pb at >0.2 mM showed a decline in yield, boldness, and quality of seeds despite the expression of an additional band each of Cu–Zn SOD and peroxidase isoform. The threshold of toxicity and toxicity values in pigeon pea leaves of plants exhibiting 10 and 33% yield depression at 27 days after metal supply were 27 and 56 μg Pb g⁻¹, respectively.     

Characterization of an acid-labile, thermostable β-glycosidase from Thermoplasma acidophilum

A recombinant putative β-galactosidase from Thermoplasma acidophilum was purified as a single 57 kDa band of 82 U mg⁻¹. The molecular mass of the native enzyme was 114 kDa as a dimer. Maximum activity was observed at pH 6.0 and 90°C. The enzyme was unstable below pH 6.0: at pH 6 its half-life at 75°C was 28 days but at pH 4.5 was only 13 h. Catalytic efficiencies decreased as p-nitrophenyl(pNP)-β-d-fucopyranoside (1067) > pNP-β-d-glucopyranoside (381) > pNP-β-d-galactopyranoside (18) > pNP-β-d-mannopyranoside (11 s⁻¹ mM⁻¹), indicating that the enzyme was a β-glycosidase.     

Salinity effect on plant growth and leaf demography of the mangrove, Avicennia germinans L.

We assessed the effect of salinity on plant growth and leaf expansion rates, as well as the leaf life span and the dynamics of leaf production and mortality in seedlings of Avicennia germinans L. grown at 0, 170, 430, 680, and 940 mol m⁻³ NaCl. The relative growth rates (RGR) after 27 weeks reached a maximum (10.4 mg g⁻¹ d⁻¹) in 170 mol m⁻³ NaCl and decreased by 47 and 44% in plants grown at 680 and 940 mol m⁻³ NaCl. The relative leaf expansion rate (RLER) was maximal at 170 mol m⁻³ NaCl (120 cm m⁻² d⁻¹) and decreased by 57 and 52% in plants grown at 680 and 940 mol m⁻³ NaCl, respectively. In the same manner as RGR and RLER, the leaf production (P) and leaf death (D) decreased in 81 and 67% when salinity increased from 170 to 940 mol m⁻³ NaCl, respectively. Since the decrease in P with salinity was more pronounced than the decrease in D, the net accumulation of leaves per plant decreased with salinity. Additionally, an evident increase in annual mortality rates (λ) and death probability was observed with salinity. Leaf half-life (t _0.5) was 425 days in plants grown at 0 mol m⁻³ NaCl, and decreased to 75 days at 940 mol m⁻³ NaCl. Thus, increasing salinity caused an increase in mortality rate whereas production of new leaves and leaf longevity decreased and, finally, the leaf area was reduced.     

Influence of UV-light on carthamin accumulation and floret elongation in a cultivar of saffron thistle (Carthamus tinctorius L.)

Pre-matured florets of Benibana, a cultivar of saffron thistle (Carthamus tinctorius L.) was irradiated with UV-B (280–320 nm) or UV-C (254 nm) light for 48 h at 23±1 °C and the influence of UV-light on carthamin accumulation and floret elongation was investigated. UV-C light enhances carthamin accumulation most prominently, showing a specific value of 52.3 nmol carthamin·dm⁻³·h⁻¹·25μm⁻² (13.9 times of control), while it restricts floret elongation by a light-suppression manner (net elongation: 0.058 mm·h⁻¹, one ninth of control). UV-B light is also promotive for the red colour appearance (25.0 nmol carthamin·dm⁻³·h⁻¹·25 μm⁻², 6.7 times of control) with suppressing floret elongation (net elongation: 0.17 mm·h⁻¹, one third of control). Heterogeneous productivity of carthamin was seen in floret tissues after continuous treating under UV-C light. Carthamin accumulation, heterogeneous carthamin productivity and decrease of floret elongation restraint under UV-lights are discussed.     

Biodegradation of phenol by immobilized Aspergillus awamori NRRL 3112 on modified polyacrylonitrile membrane

Covalent immobilization of Aspergillus awamori NRRL 3112 was conducted onto modified polyacrylonitrile membrane with glutaraldehyde as a coupling agent. The polymer carrier was preliminarily modified in an aqueous solution of NaOH and 1,2-diaminoethane. The content of amino groups was determined to be 0.58 mgeq g⁻¹. Two ways of immobilization were used—in the presence of 0.2 g l⁻¹ phenol and without phenol. The capability of two immobilized system to degrade phenol (concentration—0.5 g l⁻¹) as a sole carbon and energy source was investigated in batch experiments. Seven cycles of phenol biodegradation were conducted. Better results were obtained with the immobilized system prepared in the presence of phenol, regarding degradation time and phenol biodegradation rate. Scanning electron micrographs of the polyacrylonitrile membrane/immobilized Aspergillus awamori NRRL at the beginning of repeated batch cultivation and after the 7th cycle were compared. After the 7th cycle of cultivation the observations showed large groups of cells. The results from the batch experiments with immobilized system were compared to the results produced by the free strain. Phenol biodegradation experiments were carried out also in a bioreactor with spirally wound membrane with bound Aspergillus awamori NRRL 3112 in a regime of recirculation. 10 cycles of 0.5 g l⁻¹ phenol biodegradation were run consecutively to determine the degradation time and rate for each cycle. The design of the bioreactor appeared to be quite effective, providing large membrane surface to bind the strain.     

Changes in Hg fractionation in soil induced by willow

This study investigated the effect of willow (Salix viminalis ×  S. schwerinii) on soil characteristics, including changes in Hg fractionation in the soil solid phase, and Hg accumulation and distribution in pot-grown plants cultivated for 32 and 76 days in aged Hg-contaminated soil (30 mg Hg kg⁻¹DW). Changes in soil pH and organic carbon content as well as in Hg fractionation were monitored in both rhizospheric soil and in soil without plants. Mercury fractionation was performed by a 5-step sequential soil extraction procedure. Organic carbon content increased while pH decreased in the rhizospheric soil. Both chemically defined exchangeable Hg (0.1) and Hg bound to humic and fulvic acids (1.1) decreased in the rhizospheric soil, whereas plant accumulation of Hg increased with cultivation time. The sum of the decrease of these two soil Hg fractions after 76 days of cultivation was approximately equal to the amount of Hg accumulated in plants. Furthermore, the major Hg fractions (Hg bound to residual organic matter (53), sulphides (43), and the residual fraction (2.5)) remained stable. Neither whole plant accumulation of Hg from the soil, approximately 0.2 of total Hg in soil after 76 days cultivation, nor the fraction of total plant Hg in the shoots, which accounted for about 3 of the total plant Hg pool regardless of the cultivation time, were high. The overall results suggest that plants might be suitable for phytostabilization of aged Hg-contaminated soil, where root systems trap bioavailable Hg and help to control both leaching of Hg and re-entrainment of Hg-containing particulates from a contaminated site.     

Alleviation of Ultraviolet-B Radiation-Induced Growth Inhibition of Green Gram by Triadimefon

Supplementary UV-B (12.2 kJ m⁻² d⁻¹ UV-B_BE) provided to Vigna radiata for 2 h d⁻¹ suppressed the length of root, shoot and whole plants, number of leaves, total leaf area, leaf area index, specific leaf mass, fresh and dry mass of leaves and shoot, relative growth rate and net productivity. In unstressed green gram plants (10 kJ m⁻² d⁻¹ UV-B_BE), triadimefon (TRIAD) (20 mg dm⁻³) enhanced growth in all parameters over control. The growth promoting effect of TRIAD enabled the UV-B impacted plants to overcome the growth inhibitions to varying degrees indicating its protective potential against UV-B stress. This revised version was published online in July 2006 with corrections to the Cover Date.     

High Frequency Plant Regeneration from Callus Culture of Pleione formosana Hayata

High frequency plant regeneration was induced from protocorm-derived callus cultured on half-strength of Murashige—Skoog medium with 2,4-dichlorophenoxyacetic acid (2,4-D, 0–5 mg l⁻¹) and 1-phenyl-3-(1,2,3-thiadiazol-5-yl, 0–1 mg l⁻¹) urea (TDZ) in the dark. Twelve totipotent callus lines were selected within 76 callus lines regenerated on half-strength of Murashige—Skoog (MS) medium with 0.5 mg l⁻¹ TDZ. The proliferation rate was 4–5-fold in fresh weight after 30 days of culture on half-strength MS medium containing 5 mg l⁻¹ 2,4-D and 0.5 mg l⁻¹ TDZ in the dark. The maximum number of shoot buds generated by 0.01 g callus explant was 134 after 4 months of culture. These calli were regenerated to plantlets via protocorm-like bodies (PLBs) after 75–150 days of culture. The shoots, with two true leaves, were transferred to hormone-free medium, rooting and eventually formed plantlets. Totipotent callus lines of Pleione formosana Hayata have been successfully established in this study.     

Phosphorus dynamics and bioavailability in sediments of the Penzé Estuary (NW France): in relation to annual P-fluxes and occurrences of Alexandrium Minutum

The macrotidal estuary of Penzé (Brittany, Western part of the Channel, France) has been subjected to recurrent annual toxic blooms of Alexandrium minutum since 1988. This study aims to specify the phosphorus dynamics and bioavailability in sediments in order to improve our understanding of Alexandrium occurrences. Sediment-P pools and diffusive phosphate fluxes were studied under similar hydrodynamic conditions, in the intermediate estuary in May, June and July 2003 and along the salinity gradient from August 2004 to June 2005. The results highlight a decrease in bioavailable phosphorus (iron and organic bound) from the inner part of the estuary seaward. The ratio of iron-bound phosphorus to iron-oxyhydroxides is lower in the inner and intermediate estuaries (5–8) than in the outer site (15), suggesting a saturation of sorption sites and greater phosphorus bioavailability in this area. Pools of bioavailable phosphorus in surficial sediments are about eight times higher than the annual net-export of P (7 ton year⁻¹). Phosphate releases from sediments are always lower than 5 μmol m⁻² d⁻¹ in March. The highest supplies occur in June and August in the intermediate area (up to 400 μmol m⁻² d⁻¹) where they represent up to 50% of river loadings. These results further suggest that phosphate pulses coincide with occurrences of Alexandrium reported in June.