Assimilation Of Se Research Articles

Overlapping Transcriptome Alterations Reveal the Mechanism of Interaction between Selenium and Zinc and Their Common Effect on Essential Nutrient Metabolism in Mung Bean (Vigna radiata L.).

Selenium (Se) and zinc (Zn) deficiencies have become serious global food security and public health problems. Biofortification through foliar fertilizer is a nonspecific, low-tech, and cost-effective strategy. Se and Zn have overlapping physiological roles and interacting relationships in plants. Mung bean is superior for Se enrichment and an excellent Zn carrier. However, the molecular mechanism underlying the interaction between Se and Zn in the mung bean remains unclear. Herein, Se and Zn accumulation, antioxidant activities, physiological determination, and transcriptomic analysis were performed under both Se and Zn treatments. Common essential roles of Se and Zn in mung bean were reflected by the comprehensively altered ten physiological indexes under both Se2 (24 g·ha-1) and Zn1 (1.2 kg·ha-1) treatments. Overlapping transcriptome changes and common DEGs in two compared groups revealed that the upregulated expression of sulfate transporters (SULTRs), phosphate transporters (PHTs), and Zinc-regulated/Iron-regulated-like protein (ZIP) family genes under Se and Zn treatments directly promoted both Se and Zn intakes. Furthermore, the altered Se/Sulfur, nitrogen, and carbohydrate metabolisms are closely interlinked with the uptake and assimilation of Se and Zn via the 20 key genes that we filtered through the protein-protein interaction (PPI) network analysis. Further analysis indicated that l-methionine γ-lyase (E 4.4.1.11) genes may play an important role in the transamination of selenomethionine and its derivatives; glutamine synthetase (GS), nitrate reductase (NR), and starch synthase (SS) genes may regulate the nitrogen assimilation and carbohydrate metabolism, which provide more carriers for Se and Zn; glutathione peroxidase (GPx), glutamate-cysteine ligase catalytic subunit (GCLC), and serine acetyltransferase (SAT) genes may accelerate the GSH-GSSH cycle and promote Se and Zn storages. This study provides new molecular insights into the comprehensive improvement of the nutritional quality of mung beans in Se and Zn biofortification productions.

Interaction between sulfur and selenium in agronomic biofortification of cowpea plants under field conditions

AimsSelenium (Se) as selenate shares similarities with sulfate in transport and assimilation by plants. Uptake and assimilation of Se might be affected by S and vice-versa, which could affect Se and S concentration in plant tissues, and metabolic pathways such as biosynthesis of sugars, amino acids, and storage proteins. This study aimed to evaluate Se and S combination on cowpea plants under field conditions.MethodsThe experimental design was a 4 × 4 interaction between four rates of Se (0, 10, 25, and 50 g ha− 1) and four rates of S (0, 15, 30, and 60 kg ha− 1) in two consecutive years of cowpea cultivation. Concentrations of Se, S, total sugars, sucrose, total free amino acids, and storage proteins in plant tissue were measured.ResultsThe Se x S interaction did not affect cowpea yield or growth. Antagonistic effects of S on Se concentrations in leaves and seeds were observed mainly for the second crop season. Selenium did not decrease S concentrations in leaves and seeds of cowpea plants. The combination of 25 g Se ha− 1 and 30 kg S ha− 1 provided the greater concentrations of total sugars. Interaction between Se and S was associated with greater sucrose, amino acids, and storage proteins concentrations in cowpea seeds.ConclusionsThe Se and S interaction did not impair plant growth but application of S decreased Se content in cowpea. Further studies are needed to better understand the physiological roles of Se and S combination in producing primary metabolic compounds.

Transcriptome analysis revealed accumulation-assimilation of selenium and physio-biochemical changes in alfalfa (Medicago sativa L.) leaves.

Selenium (Se) is an increasing concern for investigators predominantly because of its consumption in the human body mainly from crops. As the fourth largest plant crop globally, alfalfa is one of the most important forages. Alfalfa was fertilized with selenium(IV) (Se(IV)) under field conditions to study the accumulation and assimilation of Se(IV) and to assess the impact of Se fertilization. It was analyzed that the physio-biochemistry, Se species, combined with transcriptome after spraying Se(IV) at different times (0, 12, and 48 h). 9402 and 12 607 differentially expressed genes (DEGs) were identified at 12 h (versus 0h) and 48 h (versus 12 h). DEG functional enrichments proposed two time-specific biological processes: Se(IV) accumulation was the primary process at 0-12 h, and its assimilation mainly occurred during 12-48 h. This was further proved by the separation of various Se speciation at different times. It showed that Se-supplementation also affected the soluble protein, soluble sugar, pigment contents and antioxidant capacity. Selenium-biofortification could improve the stress resistance of alfalfa by enhancing antioxidant system to scavenge reactive oxygen species (e.g. hydrogen peroxide) and boosting carbohydrate metabolism. By integrating physio-biochemistry, Se-related metabolites, and transcriptome under Se(IV) treatment, this study provides data to guide further work on Se-fortification in alfalfa. © 2022 Society of Chemical Industry.

Uptake, transport, and metabolism of selenium and its protective effects against toxic metals in plants: a review.

Selenium (Se) is an essential trace element of fundamental importance to humans, animals, and plants. However, the uptake, transport, and metabolic processes of Se and its underlying mechanisms in plants have not been well characterized. Here, we review our current understanding of the adsorption and assimilation of Se in plants. First, we discussed the conversion of Se from inorganic Se into organic forms, the mechanisms underlying the formation of seleno-amino acids, and the detoxification of Se. We then discussed the ways in which Se protects plants against toxic metal ions in the environment, such as by alleviating oxidative stress, regulating the activity of antioxidant enzymes, sequestering metal ions, and preventing metal ion uptake and accumulation. Generally, this review will aid future research examining the molecular mechanisms underlying the antagonistic relationships between Se and toxic metals in plants.

Bioavailability Comparison of Nine Bioselenocompounds In Vitro and In Vivo.

Selenium (Se) shows biologically ambivalent characteristics in animals. It is an essential element but becomes severely toxic when the amount ingested exceeds the adequate intake level. Its biological, nutritional, and toxicological effects are strongly dependent on its chemical form. In this study, we evaluated the toxicity and bioavailability of nine naturally occurring Se compounds, or the so-called bioselenocompounds, in vivo and in vitro. Selenite and selenocystine showed higher toxicity than the other bioselenocompounds in vitro. In an in vitro membrane permeability study using Caco-2 cells, selenomethionine and Se-methylselenocysteine were more efficiently transported than the other bioselenocompounds. The effect of bioselenocompounds on nutritional availability was quantitatively determined from the recovery of serum selenoproteins in Se-deficient rats by speciation analysis. In contrast to the in vitro study, there were no significant differences in the assimilation of Se into serum selenoproteins among the bioselenocompounds, including selenoamino acids, selenosugar, and inorganic Se species, such as selenite, selenate, and selenocyanate, except trimethylselenonium ion. These results indicate that animals can equally assimilate both inorganic and organic naturally occurring selenocompounds except trimethylselenonium ion, which is the urinary metabolite of excess Se. We confirmed that the bioselenocompounds except trimethylselenonium ion had equivalent nutritional availabilities.

The interactive effects of selenomethionine and methylmercury on their absorption, disposition, and elimination in juvenile white sturgeon

Selenium (Se) and mercury (Hg) are prevalent pollutants of industrialized watersheds. However, when co-administered, Se has protective effects on organisms from Hg. The mechanism is not fully understood, but it is thought that Se reduces Hg availability, either by forming biologically inert complexes and/or associating with selenoproteins. Despite concerns with aquatic contaminations, relatively little information is available on the interaction in aquatic organisms. In the present study, the interactive effects of Se and Hg on their absorption, disposition, and elimination were examined in juvenile white sturgeon, a benthic fish species at high risk to exposures of both contaminants. Selenium and Hg were provided as l-selenomethionine (SeMet) and methylmercury (MeHg), respectively. Groups of 10 sturgeon were orally intubated with a single dose of either 0 (control), SeMet (500μg Se/kg body weight; BW), MeHg (850μg Hg/kg BW), or their combination (Se/Hg; 500μg Se/kg and 850μg Hg/kg BW). The blood was repeatedly sampled and urine collected from the fish, over a 48h post intubation period. At 48h, the fish were sacrificed for Se and Hg tissue concentration and distribution. The co-administration of SeMet and MeHg significantly (p<0.05) lowered blood concentrations of both Se and Hg and tissue Se concentrations. Similarly, assimilation of Se and Hg was also reduced significantly. The interaction has a more quantitative effect on Se metabolism because the reduction in the overall tissue Se is a consequence of reduced Se absorption at the gut and not from the metabolic effects after absorption. In contrast, given the pulse increase in blood Hg concentration, tissue redistribution, and increased urinary elimination, the interactive effect on tissue Hg concentration is likely to be post-absorption. Even in the absence of exogenous SeMet, Se and Hg co-accumulated in tissue at a Se:Hg molar ratio greater than 1. Thus, similar to mammals, maintaining at least a 1:1 molar ratio of Se and Hg is of great physiological importance in the white sturgeon. Interestingly, SeMet did not divert Hg from the brain. Allocation of Se from the kidneys may have occurred in order to maintain the high Se:Hg molar ratios in the brain of white sturgeon. In the current study, the combined use of kinetic analysis and that of the conventional approach of measuring tissue concentration changes provided a comprehensive understanding of the interactive effect of SeMet and MeHg on their respective metabolic processes in juvenile white sturgeon.

Differences in uptake and translocation of selenate and selenite by the weeping willow and hybrid willow

Due to its essentiality, deficiency, and toxicity to living organisms and the extensive use in industrial activities, selenium (Se) has become an element of global environmental and health concern. Se removal from contaminated sites using physical, chemical, and engineering techniques is quite complicated and expensive. The goal of this study was to investigate uptake and translocation of Se in willows and to provide quantitative information for field application whether Se phytoremediation is feasible and ecologically safe. Intact pre-rooted plants of hybrid willows (Salix matsudana Koidz x alba L.) and weeping willows (Salix babylonica L.) were grown hydroponically and treated with selenite or selenate at 24.0 +/- 1 degrees C for 144 h. Removal of leaves was also performed as a treatment to quantify the effect of transpiration on translocation and volatilization of Se. At the end of the study, total Se in the hydroponic solution and in different parts of plant tissues was analyzed quantitatively by hydride generation-atomic fluorescence spectrometry. The capacity of willows to assimilate both chemical forms of Se was also evaluated using detached leaves and roots in sealed glass vessels in vivo. Translocation efficiency of Se in both plants was estimated. Significant amounts of the applied selenite and selenate were eliminated from plant growth media by willows during the period of incubation. Both willows showed a significantly higher removal rate for selenate than for selenite (p < 0.05). Substantial differences existed in the distribution of both chemical forms of Se in plant materials: lower stems and roots were the major sites for accumulation of selenite and selenate, respectively. Translocation efficiency for selenite was significantly higher than that for selenate in both willow species (p < 0.01). Compared to the intact trees, remarkable decrease in the removal rate of both chemical forms of Se was found for willows without any leaves (p < 0.01). Volatilization of Se by plant leaves was estimated to be approximately 10% of the total applied selenite or selenate. Significant reduction (>20%) of selenate was observed in the sealed vessel with excised roots of willows, whereas trace amounts of selenite were eliminated from the hydroponic solution in the presence of roots. Detached leaves from neither of them reduced the concentration of selenite or selenate in the solution. Due to the significant difference in the removal rate and the distribution of the two chemical forms of Se in plant materials, the conversion of selenate to selenite in hydroponic solution prior to uptake and within plant tissues is unlikely. An independent uptake and translocation mechanisms are likely to exist for each Se chemical species. Uptake of selenate is mediated possibly through an active transport mechanism, whereas that of selenite may possibly depend on plant transpiration. Uptake velocities of selenite are linear (zero-order kinetics), while selenate removal processes obey first-order kinetics. In experiments with detached leaves in closed bottles, the cuticle of leaves was the major obstacle to extract both chemical forms of Se from the hydroponic solution. Phytovolatilization is a biological process playing an important role in Se removal. Although faster removal rates of selenate than selenite from plant growth media were observed by both willow species, selenite in plant materials was more mobile than selenate. Significant decrease in removal rates of both chemical forms of Se was detected for willows without any leaves. Significant differences in extraction, assimilation and transport pathways for selenite and selenate exist in willow trees. Phytoremediation of Se is an attractive approach of cleaning up Se contaminated environmental sites. More detailed investigation on the assimilation of Se in plant roots and transport in tissues will provide further biochemical evidence to explain the differences in uptake and translocation mechanisms between selenite and selenate in willows. A relevant phytoremediation scheme can then be designed to clean up Se contaminated sites. Willows show a great potential for uptake, assimilation and translocation of both selenite and selenate. Phytotreatment of Se is potentially an efficient and practical technology for cleaning up contaminated environmental sites.

Influences of salinity on the biokinetics of Cd, Se, and Zn in the intertidal mudskipper Periophthalmus cantonensis

The biokinetics (aqueous uptake, dietary assimilation, and elimination) of Cd, Se, and Zn in the intertidal mudskipper, Periophthalmus cantonensis, were examined at different acclimated salinities using the radiotracer technique. The dietary assimilation efficiency from ingested radiolabeled polychaetes was the highest for Se (32–40%), followed by Zn (5–7%) and Cd (2–3%), and was not influenced by salinity within a range of 10–30 psu. Uptake from the dissolved phase typically exhibited a linear pattern within the first 12 h of exposure, followed by a second slower uptake. The highest concentration factor (CF) was found for Zn, followed by Cd and Se. Differences in salinity did not significantly affect the CF of the three metals within the first 12 h, but the CFs were significantly higher at lower salinities (10–20 psu) than at the highest salinity (30 psu) by the end of 48 h exposure. Because the degrees to which the uptake of Se (a metalloid) and Cd/Zn were affected by salinity were comparable, we concluded that metal speciation as a result of salinity change was not important in leading to a change in metal CF. Physiological changes may be responsible for the increasing uptake at lowered salinity. The elimination rates of the three metals (0.01–0.06 d −1) were not significantly affected by salinity, but Se was eliminated at a faster rate following aqueous uptake than following dietary ingestion. There was no consistent influence of exposure routes on Cd and Zn elimination. The accumulated Cd was mainly associated with the gut, whereas the muscle was the dominant target site for Se and Zn accumulation.

Dietary assimilation and elimination of Cd, Se, and Zn by Daphnia magna at different metal concentrations

This study examines the influences of Cd, Se, and Zn concentrations in ambient water on the assimilation, elimination, and release budget of metals, and the clearance rate of Daphnia magna through ingestion of phytoplankton diets. With increasing ambient metal concentration over two to three orders of magnitude, the assimilation efficiencies (AEs) of Cd and Se from two green algal species, Scenedesmus obliquus and Chlamydomonas reinhardtii, decreased significantly. In contrast, the Zn AE from ingested food, which typically was low (<7%), was not impacted by the difference in Zn or Cd concentrations in ingested cells. The Cd AE also was reduced significantly by a high Zn concentration in ingested algae, perhaps due to the competitive assimilation between the two metals. Metal elimination was determined after the daphnids were exposed to the radiolabeled diets for 3 d. The elimination rate constant was not dependent on the metal concentration in ingested food. Similarly, the routes of metal elimination (excretion, egestion, molting, and neonate production) were less influenced by the metal concentration. The rapid elimination of Se and Zn may be attributed to the transfer of these metals from mother to offspring. Except at the highest Cd (917 nM) and Se (1,333 nM) concentrations, there was no significant effect of metal concentration on the clearance rate of the animals. Consequently, the metal assimilation in D. magna may play an important role in the regulation of Cd and Se influx into the animals, whereas the regulation of the metal tissue burden in the animals is unlikely to be achieved through the metal elimination. The trophic transfer factors (TTF) of Cd and Se were related inversely to the metal concentration in the water, whereas the TTF of Zn was less variable at different Zn concentrations. The change of TTF with metal concentration will have important implication for ecotoxicity testing using D. magna, especially when the dietary exposure is incorporated.

A review of mass-dependent fractionation of selenium isotopes and implications for other heavy stable isotopes

Measurements of selenium (Se) isotope fractionation began about 1960, with work by Roy Krouse and Harry Thode on Se reduction. Se isotope research to date has confirmed these measurements, developed new measurement techniques, and investigated Se isotope fractionation during several other critical processes. As the geochemistry of Se is complex, current understanding of Se isotope systematics involves studies of many reactions, and synthesis of these results. Fractionation during dissimilatory Se reduction by pure bacterial cultures varies according to experimental conditions, but bacterial reduction in sediment slurries from three wetlands induces consistent 80Se/76Se fractionations of about 2.8‰ and 5.6‰ for Se(VI) reduction and Se(IV) reduction, respectively. Assimilation by higher plants, Se(IV) oxidation, sorption, and biological volatilization induce little or no isotopic fractionation. Preliminary experiments suggest that fractionation of about 1.0‰ accompanies assimilation of Se by algae. Thus, significant fractionation occurs in only a few reactions and if fractionation is observed in nature, it provides evidence that those reactions occur.Understanding of the systematics of other “heavy stable isotopes”, including those of calcium, iron, copper, molybdenum, chromium, and zinc, is currently being developed. Mass-dependent fractionation of these and other elements should provide a new set of tools for low-temperature biogeochemical applications, such as detecting redox reactions or distinguishing between potential contaminant sources. Some general conclusions from the Se isotopes work can be extended to the other heavy stable isotopes to aid in developing them and assessing their likely geochemical applicability.

Assimilation and retention of selenium and other trace elements from crustacean food by juvenile striped bass (Morone saxatilis)

Estimates of the assimilation and retention of trace elements from food by fish are useful for linking toxicity with the biogeochemical cycling of these elements through aquatic food webs. Here we use pulse‐chase radiotracer techniques to estimate the assimilation and retention of Se and four trace metals, Ag, Am, Zn, and Cd, by 43‐ and 88‐d‐old juvenile striped bass, Morone saxatilis, from crustacean food. Brine shrimp nauplii, Artemia franciscana, or adult copepods, Acartia tonsa, were fed radiolabeled diatoms and then fed to juvenile striped bass. Assimilation efficiencies (AEs ± SD) for 43‐d‐old fish were 18 ± 2%, 6 ± 1%, 23 ± 4%, 33 ± 3%, and 23 ± 2% for Ag, Am, Cd, Se, and Zn, respectively. For 88‐d‐old fish, the AEs were 28 ± 1%, 42 ± 5%, and 40 ± 5% for Cd, Se, and Zn, respectively. The higher AEs in the older fish may result from longer gut passage times for larger fish. The 44‐d‐old fish excreted 5 ± 0.8%, 4 ± 2.0%, 7 ± 0.3%, 9 ± 0.4%, and 1.3 ± 0.9% of the Ag, Am, Cd, Se, and Zn, respectively, they ingested from food per day, whereas the 88‐d‐old fish excreted 3 ± 1.0%, 8 ± 0.5%, and 3 ± 0.5% of the assimilated Cd, Se, and Zn per day, respectively. Predictions of steady state Se concentrations in juvenile striped bass tissues made using a biokinetic model and the measured AE and efflux rates ranged from 1.8 to 3.0 mg Se g−1 dry wt for muscle tissue and 6.8 to 11.6 mg Se g−1 dry wt for gut tissue. These predictions agreed well with average values of 2.1 and 13 mg Se g−1 dry wt measured independently in North San Francisco Bay, where elevated Se concentrations are of concern. The model results imply that the planktonic food web, including juvenile striped bass, does not transfer Se as efficiently to top consumers as does the benthic food web.

Trace metal ingestion and assimilation by the green mussel Perna viridis in a phytoplankton and sediment mixture

We examined trace metal ingestion and assimilation (Cd, Se, Zn) by the green mussel Perna viridis in a mixture of diatoms and sediment at concentrations below and above the pseudofeces production levels. Dual gamma radiotracers (109Cd, 65Zn) were used to investigate particle selection on marine diatoms and sediments. The diatom (Thalassiosira weissflogii) was radiolabeled with 109Cd, and the natural sediment was radiolabeled with 65Zn. By comparing the ratios of 109Cd:65Zn in the particle mixture, mussel tissues and pseudofeces within a short-term exposure period (35 min), the results demonstrated that the green mussels were able to selectively ingest the diatom particles at a high particle load. Efficiency of selection for nutritious particles (e.g. diatoms) increased with increasing ratio of sediments in the particle mixture. Pseudofeces contained a higher ratio of sediments relative to that in the feeding suspension. No major particle selection was observed at concentrations below the level for pseudofeces production. The assimilation of Cd, Se and Zn by the green mussels was quantified using a pulse-chase feeding technique. The assimilation of Se and Zn by the green mussels from ingested diatoms was reduced with the presence of sediment within the mussel gut, presumably due to the resorption of metals onto the sediment, leading to a quicker passage of metals through the digestive tract and a lower proportion of metals subjected to intensive digestion. In contrast, the presence of diatoms did not significantly affect metal assimilation from ingested sediment. A significant correlation between metal assimilation efficiency and metal gut passage time was also observed. Metal assimilation by the green mussels appeared to be little dependent on the particle concentration in seawater. Our study suggests that particle selection may potentially alter metal influx from ingested food sources, particularly at high particle concentrations. Selective feeding of nutritious particles, coupled with a high assimilation efficiency from these ingested particles, may increase metal influx into mussels from the dietary phase.

Comparative assimilation of Cd, Cr, Se, and Zn by the barnacle Elminius modestus from phytoplankton and zooplankton diets

Assimilation from the diet is established as a key factor in the accumulation of very high trace metal concentrations by the barnacle Elminius modestus. Assimilation efficiencies (AE) of Cd, Cr, Se and Zn were measured from a diet of different phytoplankton (2 diatoms, a dinoflagellate, a prasinophyte and a chlorophyte) and of zooplankton (the copepod Acartia spinicauda with metal accumulated from solution or diet). AEs of Cd, Se and Zn (but not Cr) varied greatly with phyto- plankton type, and for Cd and Se the AE was correlated with the percentage of metal burden held in the cytoplasm of the phytoplankton diet. AE was generally higher from the zooplankton diet than from a phytoplankton diet. Variation in AEs of Cd, Se and Zn was not explained by any correlation with the percentage of copepod metal burden held in the soft tissues, nor was there variation for either Cd or Zn according to whether the copepod prey had accumulated metal from dissolved or food sources. Comparisons of the assimilation and efflux of accumulated metals by the archaeo- balanid barnacle E. modestus and literature data for (phylogenetically younger) balanid species of the genus Balanus indicate some differences in digestive physiology of barnacles from the 2 families, tending towards higher AEs in the balanids. Modeling of the accumulation of Cd and Zn by E. mod- estus predicts that for each metal >97% of accumulated metal has been derived from dietary inges- tion. The dominance of dietary ingestion in trace metal accumulation is a function of the conspicu- ously high assimilation efficiencies and high ingestion activity of barnacles. This study adds to the small but growing list of examples highlighting the significance of trophic transfer in metal accumu- lation by aquatic invertebrates.

Tagging crustacean larvae: Assimilation and retention of trace elements

The uptake, assimilation, and retention of Se, Co, Ag, Ce, Eu, and Gd by crustacean larvae were measured to assess the suitability of these elements as tags for determining larval dispersal patterns and mortality rates. Gamma‐emitting radioisotopes were used to determine the uptake and retention of each element by larvae of three crabs, Dyspanopeus sayi, Uca pugnax, and Sesarma reticulatum, and one barnacle, Semibalanus bolunoides. Selenium was readily taken up by larvae from their food, was assimilated at efficiencies of 61–93%, was retained at concentrations detectable above background levels for weeks, and did not consistently affect larval survival or development in the laboratory. The other elements were not absorbed sufficiently from food or were not retained long enough to serve as appropriate tags for monitoring larval dispersal. Thus, only Se seemed to be transferred efficiently between trophic levels and is suitable as a larval tag.

Selenium-Induced Growth Reduction inBrassicaLand Races Considered for Phytoremediation

Brassicaspecies considered for use in selenium (Se) phytoremediation need to accumulate large amounts of Se to be successful. Retarded plant growth and impaired protein synthesis are common symptoms for plants grown under seleniferous soils. Selenium accumulation by different land races ofBrassica juncea(L.) Czern and Coss and one land race ofBrassica carinatawas investigated in Se-enriched water and soil cultures containing 2 mg Se kg−1. Effects of Se concentration in the root environment on the assimilation of Se, leaf surface area, dry matter yield, total leaf protein concentration, and free seleno-amino acid concentrations were analyzed for plants grown in Se-laden media. In water culture, shoot Se concentrations among the land races ranged from 501 to 1017 mg Se kg−1dry matter (DM), and in plants grown in Se-laden soil, concentrations ranged from 407 to 769 mg Se kg−1DM. Land races grown with Se exhibited decreases in dry matter yields from 12 to 23% and in leaf surface area from 5 to 26% compared to the same land races grown without Se. Protein content was significantly correlated both with shoot Se concentrations (r= 0.746,P< 0.001) and with leaf surface area (r= 0.446,P< 0.01) for all land races grown in Se-enriched water culture. There was also a significant correlation (r= 0.767,P< 0.001) between total Se assimilation and shoot protein for all land races. Free selenomethionine was detected for plants grown with Se and ranged from 92 to 958 ng g−1DM. Other seleno-amino acids, Se-methyl-selenocysteine and selenocysteine, were not consistently detected as free amino acids in the different land races. Although visual symptoms of Se toxicity were not observed in theBrassicaspecies, dry matter yield, leaf surface, and total shoot protein decreased, depending on the land races tested.

Assimilation of trace elements and carbon by the mussel Mytilus edulis: Effects of food composition

Assimilation efficiency (AE) is an important physiological parameter in determining trace element influx from food sources into aquatic animals. We used radiotracer techniques to examine the influence of diet [seven species of algae (two diatoms, two chlorophytes, a prasinophyte, and two dinoflagellates) and glass beads] on the assimilation of seven trace elements (Ag, Am, Cd, Co, Cr, Se, Zn) and C in the mussel Mytilus edulis. Trace element assimilation was related to C assimilation and cytoplasmic distribution in the algae and to gut passage time in the mussels. Mussels displayed different C AEs for the different algal diets; the chlorophytes, which had highly refractory cell walls, were the least digestible food. Assimilation of Cd, Se, and Zn was directly correlated with C assimilation; for Am, Ag, and Cr, no relationship with C assimilation was apparent. For each species except the chlorophytes, AEs of all elements significantly correlated with their cytoplasmic distribution within each algal cell. Among all species, AEs of Am, Co, and Se also increased with elemental penetration into the cytoplasm; however, this relationship was not statistically significant for Ag, Cd, or Zn. With the exception of Cr, AEs of elements increased with gut passage time, implying more efficient digestion‐absorption when the element was retained longer in the digestive tract. In waters containing large mussel populations, unassimilated particle‐reactive elements should be removed from suspension and enriched in biodeposits in sediments, whereas assimilated metals should be enriched in mussel tissues.

Assimilation and regeneration of trace elements by marine copepods

Assimilation efficiencies (AE) of five trace elements (Am, Cd, Co, Se, and Zn) and carbon by neritic copepods (Acartia tonsa and Temora longicornis) feeding at different food concentrations and on different food types (diatoms, green algae, flagellates, dinoflagellates, and Fe oxides) were measured with radiotracer techniques. Food concentration had little influence on AEs of C, Cd, Co, and Se within a range of 16‐800 µg C liter‐1. AEs of Am and Zn were highest at low food concentrations (16‐56 µg C liter‐1) but remained relatively constant when food levels exceeded 160 µg C liter‐1. Different algal diets had no major influence on AEs, which generally were in the order Cd &gt; Se &gt; Zn &gt; Co &gt; Am. Metals (Cd, Co, and Zn) were assimilated from Fe oxides with 50% less efficiency than from algal cells. Element regeneration into the dissolved phase was a significant route for the release of ingested elements by copepods and increased with increased food concentration. Element regeneration rates for Cd, Se, and Zn were comparable to the regeneration rates of major nutrients such as P (30–70% daily). Retention half‐times of elements in decomposing fecal pellets ranged from &lt; 1d (Cd) to &gt;10 d (Am). The efficient assimilation and regeneration of Cd, Se, and Zn can significantly lengthen the residence time of these elements in ocean surface waters.

Kinetic determinations of trace element bioaccumulation in the mussel Mytilus edulis

Laboratory experiments employing radiotracer methodology were conducted to determine the assimilation efficiencies from ingested natural seston, the influx rates from the dissolved phase and the efflux rates of 6 trace elements (Ag, Am, Cd, Co. Se and Zn) in the mussel Mytilus edulis. A kinetic model was then employed to predict trace element concentration in mussel tissues in 2 locations for which mussel and environmental data are well described: South San Francisco Bay (California, USA) and Long Island Sound [New York, USA). Assimilation efficiencies from natural seston ranged from 5 to 18: for Ag, 0.6 to 1% for Am, 8 to 20% for Cd, 12 to 16% for CO, 28 to 34% for Se, and 32 to 41 ':, for Zn. Differences in chlorophyll a concentration in ingested natural seston did not have significant impact on the assimilation of Am, CO, Se and Zn. The influx rate of elements from the dissolved phase increased with the dissolved concentration, conforming to Freundlich adsorption isotherms. The calculated dissolved uptake rate constant was greatest for Ag, followed by Zn > Am -; Cd > CO > Se. The estimated absorption efficiency from the d~ssolved phase was 1.53 % for Ag, 0.34 % for Am, 0.31 X for Cd, 0.11 % for CO, 0.03' for Se and 0.89% for Zn. Salinity had an inverse effect on the influx rate from the dissolved phase and dissolved organic carbon concentration had no significant effect on trace element uptake. The calculated efflux rate constants for all elements ranged from 1.0 to 3.0% d-' The route of trace element uptake (food vs dissolved) and the duration of exposure to dissolved trace elements (12 h vs 6 d) did not significantly influence trace element efflux rates. A model which used the experimentally determined influx and efflux rates for each of the trace elements, following exposure from ingested food and from water, predicted concentrations of Ag. Cd, Se and Zn in mussels that were directly comparable to actual tissue concentrations independently measured in the 2 reference sites in national monitoring programs. Sensitivity analysis indicated that the total suspended solids load, which can affect mussel feeding activity, assimilation, and trace element concentration in the dissolved and particulate phases, can significantly influence metal bioaccumulation for particlereactive elements such as Ag and Am. For all metals, concentrations in mussels are proportionately related to total metal load in the water column and their assimilation efficiency from ingested particles. Further, the model predicted that over 96% of Se in mussels is obtained from ingested food, under conditions typical of coastal waters. For Ag, Am, Cd, CO and Zn, the relative contribution from the dissolved phase decreases significantly with increasing trace element parhtion coefficients for suspended particles and the assimilation efficiency in mussels of ingested trace elements, values range between 33 and 67% for Ag, 5 and 1 7 % for Am, 47 and 82% for Cd, 4 and 30% for CO, and 17 and 51 ?h for Zn.

EFFECTS OF SULFATE SALINITY AND SELENIUM TOLERANCE OF PLANTS ON SELENIUM ASSIMILATION IN WHITE CLOVER AND TALL FESCUE.

Substantial difference of selenium tolerance was found between the tall fescue (Festuca arundinacea Schreb.) and white clover (Trifolium rapens L.) An inverse relationship between Se accumulation and Se tolerance suggests an exclusion mechanism that restricts Se uptake by the plant with greater Se tolerance. A positive relationship between the increase of protein Se concentration and growth inhibition in the plants suggests that assimilation of Se into protein is responsible for the reducing Se toxicity at the protein level. No evidence of a Se exclusion mechanism which exclude Se from incorporating into protein plays any major role of Se tolerance in this two species.

Selenium Assimilation and Nutrient Element Uptake in White Clover and Tall Fescue under the Influence of Sulphate Concentration and Selenium Tolerance of the Plants

Tall fescue (Festuca arundinacea Schreb.) and white clover (Trifolium repens L.) were examined for Se assimilation and nutrient element uptake. Substantial Se tolerance difference was found between the two species. An inverse relationship between Se accumulation and Se tolerance suggests an exclusion mechanism that restricts Se uptake by the plant with greater Se tolerance. A positive relationship between the increase of protein Se concentration and growth inhibition in the plants suggests that assimilation of Se into proteins is responsible for the Se toxic effect and this study indicates that the antagonistic effect of sulphate is responsible for reducing Se toxicity at the protein level. No evidence that a Se exclusion mechanism which excludes Se from incorporating into protein, such as that found in Astragalus species, plays any major role of Se tolerance in these two species. Under Se treatment, plant tissue Ca concentration was increased, but P concentration was decreased. A higher Fe concentration was found in white clover which increased with increasing tissue Se concentration. Copper, Mn, and Zn concentrations only increased in the white clover under conditions of severe growth inhibition.