Marine Angiosperm Zostera Marina Research Articles

Insights into the regulation of energy metabolism during the seed-to-seedling transition in marine angiosperm Zostera marina L.: Integrated metabolomic and transcriptomic analysis.

Seed development is a crucial phase in the life cycle of seed-propagated plants. As the only group of angiosperms that evolved from terrestrial plants to complete their life cycle submerged in marine environments, the mechanisms underlying seed development in seagrasses are still largely unknown. In the present study, we attempted to combine transcriptomic, metabolomic, and physiological data to comprehensively analyze the molecular mechanism that regulates energy metabolism in Zostera marina seeds at the four major developmental stages. Our results demonstrated that seed metabolism was reprogrammed with significant alteration of starch and sucrose metabolism, glycolysis, the tricarboxylic acid cycle (TCA cycle), and the pentose phosphate pathway during the transition from seed formation to seedling establishment. The interconversion of starch and sugar provided energy storage substances in mature seeds and further acted as energy sources to support seed germination and seedling growth. The glycolysis pathway was active during Z. marina germination and seedling establishment, which provided pyruvate for TCA cycle by decomposing soluble sugar. Notably, the biological processes of glycolysis were severely inhibited during Z. marina seed maturation may have a positive effect on seed germination, maintaining a low level of metabolic activity during seed maturation to preserve seed viability. Increased acetyl-CoA and ATP contents were accompanied with the higher TCA cycle activity during seed germination and seedling establishment, indicating that the accumulations of precursor and intermediates metabolite that can strengthen the TCA cycle and facilitate energy supply for Z. marina seed germination and seedling growth. The large amount of oxidatively generated sugar phosphate promotes fructose 1,6-bisphosphate synthesis to feed back to glycolysis during seed germination, indicating that the pentose phosphate pathway not only provides energy for germination, but also complements the glycolytic pathway. Collectively, our findings suggest these energy metabolism pathways cooperate with each other in the process of seed transformation from maturity to seedling establishment, transforming seed from storage tissue to highly active metabolic tissue to meet the energy requirement seed development. These findings provide insights into the roles of the energy metabolism pathway in the complete developmental process of Z. marina seeds from different perspectives, which could facilitate habitat restoration of Z. marina meadows via seeds.

Retraction Note: The highly efficient NDH-dependent photosystem I cyclic electron flow pathway in the marine angiosperm Zostera marina.

Retraction Note: The highly efficient NDH-dependent photosystem I cyclic electron flow pathway in the marine angiosperm Zostera marina.

Disturbance decreases genotypic diversity by reducing colonization: Implications for disturbance-diversity feedbacks.

One objective of eco-evolutionary dynamics is to understand how the interplay between ecology and evolution on contemporary timescales contributes to the maintenance of biodiversity. Disturbance is an ecological process that can alter species diversity through both ecological and evolutionary effects on colonization and extinction dynamics. While analogous mechanisms likely operate among genotypes within a population, empirical evidence demonstrating the relationship between disturbance and genotypic diversity remains limited. We experimentally tested how disturbance altered the colonization (gain) and extinction (loss) of genets within a population of the marine angiosperm Zostera marina (eelgrass). In a 2-year field experiment conducted in northern California, we mimicked grazing disturbance by migratory geese by clipping leaves at varying frequencies during the winter months. Surprisingly, we found the greatest rates of new colonization in the absence of disturbance and that clipping had negligible effects on extinction. We hypothesize that genet extinction was not driven by selective mortality from clipping or from any stochastic loss resulting from the reduced shoot densities in clipped plots. We also hypothesize that increased flowering effort and facilitation within and among clones drove the increased colonization of new genets in the undisturbed treatment. This balance between colonization and extinction resulted in a negative relationship between clipping frequency and net changes in genotypic richness. We interpret our results in light of prior work showing that genotypic diversity increased resistance to grazing disturbance. We suggest that both directions of a feedback between disturbance and diversity occur in this system with consequences for the maintenance of eelgrass genotypic diversity.

An underlying mechanism of qE deficiency in marine angiosperm Zostera marina.

Non-photochemical quenching (NPQ) of photosystem II (PSII) fluorescence is one of the most important protective mechanisms enabling the survival of phototropic organisms under high-light conditions. A low-efficiency NPQ, characterized by weak NPQ induction capacity and a low level of protective NPQ, was observed in the marine angiosperm Zostera marina, which inhabits the shallow water regions. Furthermore, chlorophyll fluorescence and Western blot analysis verified that the fast-inducted component of NPQ, i.e., the energy-dependent quenching (qE), was not present in this species. In contrast with the lack of PSII antenna quenching sites for qE induction in brown algae and the lack of functional XC in Ulvophyceae belonging to green algae, all the antenna proteins and the functional XC are present in Z. marina. A novel underlying mechanism was observed that the limited construction of the trans-thylakoid proton gradient (ΔpH) caused by photoinactivation of the oxygen evolving complex (OEC) did not induce protonation of PsbS, thus explaining the inability to form quenching sites for qE induction. Although the ΔpH established under light exposure activated violaxanthin (V) de-epoxidase enzyme to catalyze conversion of V via antheraxanthin (A) and then to zeaxanthin (Z), the quenching capacity of de-epoxidized pigment was weak in Z. marina. We suggest that the low-efficiency NPQ was conducive to efficiently utilize the limited electrons to perform photosynthesis, resisting the adverse effect of OEC photoinactivation on the photosynthetic rate.

Chlororespiration Serves as Photoprotection for the Photo-Inactivated Oxygen-Evolving Complex in Zostera marina, a Marine Angiosperm.

As an alternative electron sink, chlororespiration, comprising the NAD(P)H dehydrogenase complex and plastid terminal plastoquinone oxidase, may play a significant role in sustaining the redox equilibrium between stroma and thylakoid membrane. This study identified a distinct role for chlororespiration in the marine angiosperm Zostera marina, whose oxygen-evolving complex (OEC) is prone to photo-inactivation as a result of its inherent susceptibility to excess irradiation. The strong connectivity between OEC peripheral proteins and key chlororespiratory enzymes, as demonstrated in the interaction network of differentially expressed genes, suggested that the recovery of photo-inactivated OEC was connected with chlororespiration. Chlorophyll fluorescence, transcriptome and Western blot data verified a new physiological role for chlororespiration to function as photoprotection and generate a proton gradient across the thylakoid membrane for the recovery of photo-inactivated OEC. Chlororespiration was only activated in darkness following excess irradiation exposure, which might be related to electron deficiency in the electron transport chain because of the continuous impairment of the OEC. The activation of chlororespiration in Z. marina was prone to proactivity, which was also supported by the further activation of the oxidative pentose-phosphate pathway synthesizing NADPH to meet the demand of chlororespiration during darkness. This phenomenon is distinct from the common assumption that chlororespiration is prone to consuming redundant reducing power during the short transition phase from light to dark.

The highly efficient NDH-dependent photosystem I cyclic electron flow pathway in the marine angiosperm Zostera marina.

Zostera marina, a fully submerged marine angiosperm with a unique evolutionary history associated with its terrestrial origin, has distinct photochemical characteristics caused by its oxygen-evolving complex (OEC) being prone to deactivation in visible light. Based on the present phylogenetic analysis, the chloroplast NADPH dehydrogenase-like (NDH) complex was found to be completed in of Z. marina, unlike other marine plants, suggesting its crucial role. Thus, the responses of electron transport to irradiation were investigated through multiple chlorophyll fluorescence techniques and Western blot analysis. Moreover, the respective contribution of the two photosystem I cyclic electron flow (PSI-CEF) pathways to the generation of trans-thylakoid proton gradient (∆pH) was also examined using inhibitors. The contributions of the two PSI-CEF pathways to ∆pH were similar; furthermore, there was a trade-off between the two pathways under excess irradiation: the PGR5/L1-dependent PSI-CEF decreased gradually following its activation during the initial illumination, while NDH-dependent PSI-CEF was activated gradually with exposure duration. OEC inactivation was continuously under excess irradiation, which exhibits a positive linear correlation with the activation of NDH-dependent PSI-CEF. We suggest that PGR5/L1-dependent PSI-CEF was preferentially activated to handle the excess electron caused by the operation of OEC during the initial illumination. Subsequently, the increasing OEC inactivation with exposure duration resulted in a deficit of electrons. Limited electrons from PSI might preferentially synthesize NADPH, which could support the function of NDH-dependent PSI-CEF to generate ∆pH and ATP via reducing ferredoxin, thereby maintaining OEC stability.

The role of O2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L.

This study investigates the role of O2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L. Electron transport rates (ETRs) and non-photochemical quenching (NPQ) of Z. marina were measured under saturating irradiance in synthetic seawater containing 2.2mM DIC and no DIC with different O2 levels (air-equilibrated levels, 3% of air equilibrium and restored air-equilibrated levels). Lowering O2 did not affect ETR when DIC was provided, while it caused a decrease in ETR and an increase in NPQ in DIC-free media, indicating that O2 acted as an alternative electron acceptor under low DIC. The ETR and NPQ as a function of irradiance were subsequently assessed in synthetic seawater containing (1) 2.2mM DIC, air-equilibrated O2; (2) saturating CO2, no O2; and (3) no DIC, air-equilibrated O2. These treatments were combined with glycolaldehyde pre-incubation. Glycolaldehyde caused a marked decrease in ETR in DIC-free medium, indicating significant electron flow supported by photorespiration. Combining glycolaldehyde with O2 depletion completely suppressed ETR suggesting the operation of the Mehler reaction, a possibility supported by the photosynthesis-dependent superoxide production. However, no notable effect of suppressing the Mehler reaction on NPQ was observed. It is concluded that during DIC-limiting conditions, such as those frequently occurring in the habitats of Z. marina, captured light energy exceeds what is utilised for the assimilation of available carbon, and photorespiration is a major alternative electron acceptor, while the contribution of the Mehler reaction is minor.

Influence of model selection on the predicted distribution of the seagrass Zostera marina

There is an increasing need to model the distribution of species and habitats for effective conservation planning, but there is a paucity of models for the marine environment. We used presence (131) and absence (219) records of the marine angiosperm Zostera marina L. from the archipelago of SW Finland, northern Baltic Sea, to model its distribution in a 5400 km2 area. We used depth, slope, turbidity, wave exposure and distance to sandy shores as environmental predictors, and compared a presence-absence method: generalised additive model (GAM), with a presence only method: maximum entropy (Maxent). Models were validated using semi-independent data sets. Both models performed well and described the niche of Z. marina fairly consistently, although there were differences in the way the models weighted the environmental variables, and consequently the spatial predictions differed somewhat. A notable outcome from the process was that with relatively equal model performance, the area actually predicted in geographical space can vary by twofold. The area predicted as suitable for Z. marina by the ensemble was almost half of that predicted by the GAM model by itself. The ensemble of model predictions increased the model predictive capability marginally and clearly shifted the model towards a more conservative prediction, increasing specificity, but at the same time sacrificing sensitivity. The environmental predictors selected into the final models described the potential distribution of Z. marina well and showed that in the northern Baltic the species occupies a narrow niche, typically thriving in shallow and moderately exposed to exposed locations near sandy shores. We conclude that a prediction based on a combination of model results provides a more realistic estimate of the core area suitable for Z. marina and should be the modelling approach implemented in conservation planning and management.

Widespread genetic mosaicism in the marine angiosperm Zostera marina is correlated with clonal reproduction

Somatic mutations are an underappreciated source of genetic variation within multi-cellular organisms. The resulting genetic mosaicism should be particularly abundant in large clones of vegetatively propagating angiosperms. Little is known on the abundance and ecological correlates of genetic mosaicism in field populations, despite its potential evolutionary significance. Because sexual reproduction restores genetic homogeneity, we predicted that in facultatively clonally reproducing organisms, the prevalence of genetic mosaicism increases with increasing clonality. This was tested among 33 coastal locations colonized by the ecologically important marine angiosperm Zostera marina, ranging from Portugal to Finland. Genetic mosaics were detectable as complex microsatellite genotypes at two hypervariable loci that revealed additional mosaic alleles, suggesting the presence of one or more divergent cell lineages within the same ramet. The proportions of non-mosaic genotypes in a population sharply decreased below a clonal richness of 0.2. Accordingly, more genetic mosaics were found at the southern and northern limit of the distribution of Z. marina in Europe where sexual reproduction is rare or absent. The genetic mosaics observed at neutral microsatellite markers suggest the possibility of within-clone variation at selectively relevant loci and supports the notion that members of clones are seldom genetically identical.

Genetic sub-structure and intermediate optimal outcrossing distance in the marine angiosperm Zostera marina

The spatial distribution of genetic variability depends on the spatial patterns of clonal and sexual reproduction, gene flow, genetic drift and natural selection. Species with restricted dispersal may exhibit genetic structuring within populations with immediate neighbours being close relatives, and may show differentiation among populations. Genetic structuring of a species may have important genetic, evolutionary and ecological consequences including distance-dependent mating success. In this study we used microsatellite markers to show that clones of Zostera marina in a population in the Ria Formosa, Portugal, were aggregated and covered distances of up to 3–4 m. Clones within 4 m of each other exhibited significant and positive coancestry values, reflecting the limited seed dispersal of this species. Hand-pollinations between near (0–10.9 m), intermediate (11–32 m) and far (15 km) individuals resulted in similar levels of seed set, although the near pollinations had higher, although not statistically significant, levels of seed abortion during maturation. Seeds from intermediate-distance pollinations had a significantly higher proportion of seeds germinate and shorter germination time than both the near and far seeds. Similarly, the average number of seedlings produced per pollination, used as an overall estimate of fitness, was significantly greater for the intermediate distance when compared to both near and far pollinations. These results suggest that the genetic structuring observed may result in both inbreeding and outbreeding depression, which gives rise to an intermediate optimal outcrossing distance.

Effects of edge/interior and patch structure on reproduction in Zostera marina L. in Chesapeake Bay, USA

We examined the effects of location (patch edge vs. interior) and shoot density (individual, patchy, continuous) on reproduction in three natural and two transplanted Chesapeake Bay (USA) stands of the submerged marine angiosperm Zostera marina L. (eelgrass; Zosteraceae). There were no edge effects on demographic-based reproductive effort or reproductive output (propagule production), and patch structure (individual, patchy, continuous) alone never accounted for the majority of variability in any metric. Transplant site was the most important predictor of eelgrass reproduction response, and relationships among metrics were predictable within sites. Our results suggest that, in Chesapeake Bay eelgrass, environmental factors acting at a regional scale (km) have a stronger impact on reproductive investment than those at a patch scale (1–10 m). Since tradeoffs between clonal and sexual production are mediated primarily by exogenous environmental factors, site selection may be more critical than transplant configuration for producing self sustaining stands, and achieving long-term restoration success.

Integrin-like protein at the invaginated plasma membrane of epidermal cells in mature leaves of the marine angiosperm Zostera marina L.

By immunoblotting with anti-human integrin beta polyclonal antibodies (beta1, beta3 or beta5), a single distinct band of about 60 kDa was detected in total protein extracts from mature leaves of the seagrass Zostera marina L., but no band was detected in total protein extracts from immature seagrass leaves, freshwater grass leaves or Arabidopsis thaliana (L.) Heynh. leaves. This integrin-like protein was detected by indirect immunofluorescence microscopy on the surface of non-spherical protoplasts of epidermal cells isolated from mature seagrass leaves using an anti-integrin beta3 polyclonal antibody. Electron-microscopic analyses with the same antibody indicated that this integrin-like protein was localized specifically in the invaginated plasma membrane of epidermal cells in mature seagrass leaves. Therefore, this integrin-like protein of about 60 kDa may be involved in the developmentally regulated invagination of the plasma membrane in epidermal cells of the seagrass Z. marina.

Flexible mating: cross-pollination affects sex-expression in a marine clonal plant.

Many functionally hermaphroditic plants have evolved mechanisms to reduce interference between the sex functions and to optimize reproductive output. In addition to physical mechanisms such as the spatial (herkogamy) and temporal (dichogamy) separation of male and female functions, plasticity in sex expression by means of mate-recognition (flexible mating) could be important in plants with variable access to cross-pollen. This applies particularly to clonal plants because of their modular growth form. We experimentally tested for the effects of pollen source and vegetative neighbourhood on instantaneous sex ratio and seed production in the self-compatible clonal marine angiosperm Zostera marina L. To this end, we exposed the (monoecious) flowering shoots to self and cross-pollen and to neighbourhoods of their own and a mix of foreign vegetative shoots. Flowering shoots that had been exposed to cross-pollen showed (1) a significantly lower female/male ratio at peak flowering, evidence for mate-recognition, and (2) a significantly higher seed set by the end of the season. Both effects were independent of the genetic composition of their vegetative neighbourhood. The results suggest that Z. marina maintains a cryptic self-incompatibility system not previously described for angiosperms with sub-aqueous pollination. In Z. marina, and possibly other self-compatible clonal plant species, mate-recognition could be a means of increasing the out-crossing probability for flowering shoots with central positions within their clone.

Seasonal Variations in Carbon and Nitrogen Constituents in Eelgrass (Zostera marina L.) as Influenced by Increased Temperature and Water-Column Nitrate

Fluctuations in nitrogen and carbon compounds were examined over an autumn growing season in the submersed marine angiosperm Zostera marina L. (eelgrass). The experimental design included replicated controls (ambient NO 3 − , typically < 2 μM), increased water-column nitrate (8 μMNO 3 − above ambient, pulsed daily), increased environmental temperature (3 to 4 °C above 20-year weekly means), and combined increased water-column nitrate and temperature. Above- and belowground tissues were collected weekly to biweekly and assayed for total soluble carbohydrates, non-reducing carbohydrates, starch, α-cellulose, lipids, free amino acids, total protein, tissue nitrate, tissue nitrite, and tissue ammonium. Tissue nitrate declined, and amino acids, proteins, lipids, and cellulose increased as the growing season progressed in both control and treated plants. In addition, there were seasonal quadratic responses for tissue ammonium, soluble carbohydrates, and non-reducing sugars, with maxima during periods of optimal plant growth (mid- to late September). Increased temperature promoted periodic increases in amino acids and soluble carbohydrates, but decreased accumulation of α-cellulose by the end of the experiment. Moreover, increases in water-column nitrate led to periodic increases in tissue ammonium and amino acids, as well as decreases in non-reducing sugars. Toward the end of the experiment, increases in soluble carbohydrates for plants grown under higher temperatures may have been associated with an extension of the growing season. In contrast, decreased non-reducing sugars in nitrate-enriched plants may have resulted from an increased carbon demand during nitrate assimilation/reduction, as well as a reallocation of carbon to enhance amino acid synthesis.

Sodium-dependent nitrate transport at the plasma membrane of leaf cells of the marine higher plant Zostera marina L.

NO(3)(-) is present at micromolar concentrations in seawater and must be absorbed by marine plants against a steep electrochemical potential difference across the plasma membrane. We studied NO(3)(-) transport in the marine angiosperm Zostera marina L. to address the question of how NO(3)(-) uptake is energized. Electrophysiological studies demonstrated that micromolar concentrations of NO(3)(-) induced depolarizations of the plasma membrane of leaf cells. Depolarizations showed saturation kinetics (K(m) = 2.31 +/- 0.78 microM NO(3)(-)) and were enhanced in alkaline conditions. The addition of NO(3)(-) did not affect the membrane potential in the absence of Na(+), but depolarizations were restored when Na(+) was resupplied. NO(3)(-)-induced depolarizations at increasing Na(+) concentrations showed saturation kinetics (K(m) = 0.72 +/- 0.18 mM Na(+)). Monensin, an ionophore that dissipates the Na(+) electrochemical potential, inhibited NO(3)(-)-evoked depolarizations by 85%, and NO(3)(-) uptake (measured by depletion from the external medium) was stimulated by Na(+) ions and by light. Our results strongly suggest that NO(3)(-) uptake in Z. marina is mediated by a high-affinity Na(+)-symport system, which is described here (for the first time to our knowledge) in an angiosperm. Coupling the uptake of NO(3)(-) to that of Na(+) enables the steep inwardly-directed electrochemical potential for Na(+) to drive net accumulation of NO(3)(-) within leaf cells.

Physiological evidence for a proton pump and sodium exclusion mechanisms at the plasma membrane of the marine angiosperm Zostera marina L.

The basic electrical plasma membrane characteristics of leaf cells from the seagrass Zostera marina L. have been investigated with respect to its primary transport system and its Na + /K + selectivity. In natural seawater Z. marina exhibits a membrane potential of -156 ± 10 mV. The phytotoxin fusicoccin stimulates H + extrusion and hyperpolarizes the plasma membrane. Ouabain, an inhibitor of the mammalian Na + /K + -ATPase did not depolarize the plasma membrane of Z. marina. Both flushing the leaves with CO 2 and 'light off' acidified the cytoplasm and hyperpolarized the cells. It is suggested that a H + -ATPase rather than a Na + -ATPase is the primary pump in Z. marina. In the presence of cyanide plus salicylhydroxamic acid the membrane potential changed to -64 ± 11 mV. This so-called diffusion potential was sensitive to external [K + ] from 0.05 to 0.5 mM in the presence of 0.5 M Na + and revealed a relative permeability P K+ /P Na+ of 303. We suggest that this high ratio is the basic adaptation which permits Z. marina to grow in high [Na + ] conditions and to exhibit a rather negative resting potential. Since amiloride, an inhibitor of the nH + /Na + antiporter, hyperpolarized the plasma membrane, it is suggested that this transporter could be present in the plasma membrane of Z. marina acting as an overflow valve for Na + which leaks into the cell.

Physiological evidence for a proton pump and sodium exclusion mechanisms at the plasma membrane of the marine angiosperm Zostera marina L

Physiological evidence for a proton pump and sodium exclusion mechanisms at the plasma membrane of the marine angiosperm Zostera marina L

Risk posed by the antifouling agent Irgarol 1051 to the seagrass, Zostera marina

Irgarol 1051 (2-( tert-butylamino)-4-cyclopropylamino)-6-(methylthio)-1,3,5-triazine) is a triazine herbicide that is increasingly being used to boost the effectiveness of antifouling paints. Estuarine plants, such as the marine angiosperm Zostera marina L. (eelgrass) may accumulate, and be affected by, Irgarol 1051, in locations with high boat densities. Bioconcentration of Irgarol 1051 within Zostera tissue was determined in field plants and laboratory semi-static exposure experiments. Effects of Irgarol 1051 upon the growth rate and photosystem II photosynthetic efficiency of Zostera were examined over a concentration range of 0 to 25 μg dm −3. Growth rate was assessed by comparison of leaf specific biomass ratios, and was found to be reduced at and above an Irgarol 1051 concentration of 10 μg dm −3. Photosynthetic efficiency was assessed using fluorescence induction kinetics: efficiency was significantly reduced at 0.18 μg dm −3 (0.4 μg g −1 dry weight leaf tissue) and a 10-day EC 50 value of 2.5 μg dm −3 (1.1 μg g −1) calculated. Longer-term exposure revealed a 36-day EC 50 value of 0.2 μg dm −3. Uptake of Irgarol 1051 was rapid within the Zostera leaves: tissue concentrations (dry weight basis) in excess of 300 times the water concentration were found within 2 days of exposure. Leaf concentrations in excess of 14 times root tissue concentration were found. Estuaries sampled in S.W. England had low aqueous Irgarol 1051 contamination, typically <0.003 μg dm −3, but Zostera leaf tissue concentrations (dry weight basis) were up to 25 000 times the aqueous values; this was only 15 times below the 10-day EC 50 value. The reported results will enable the level of risk to isolated Zostera meadows from Irgarol 1051 to be assessed based on leaf tissue concentration and also have implications for the siting of marinas.

Developmental changes of sucrose synthase activity and abundance in root and leaf tissues of Zostera marina

The activity of sucrose synthase (EC 2.4.1.13) is correlated with tissue carbon (C) demand in vascular plants. The patterns of SS activity and abundance, and soluble protein abundance are described along age gradients within leaf bundles and in roots along the subterranean rhizome of the marine angiosperm Zostera marina L. (eelgrass). Soluble protein content decreased exponentially with increasing age in root tissue (concentrations in oldest roots were 20% of those of the youngest roots), but decreased in a linear fashion and overall to a lesser extent in leaves (to 62% of values in youngest shoots). Variation of SS activity with tissue age depended on the tissue and parameter to which measurements were normalized. Biomass-specific root SS activity decreased exponentially from 90–40 μmol g −1 fresh weight h −1 in root bundles numbers 1 (youngest) through 9, while protein-specific activity remained constant in the same tissue (approximately 16 μmol mg −1 protein h −1). Roots 10–12 had elevated, but variable SS activities. In leaves numbers 1–5, the decreases in both biomass- (200 to 50 μmol g −1 FW h −1) and protein-specific SS activity (23–12 μmol mg −1 protein h −1) were approximately linear. The abundance of SS protein in roots and leaves (determined by western analysis) also decreased with the increasing age, though not to the same extent (leaves), or with similar pattern (roots) as for SS activity. The age-dependent changes in SS activity are consistent with other indices of C metabolism (e.g., respiratory rates, carbohydrate content), but not with an index of N metabolism (glutamine synthetase activity) during development of Z marina tissues.

On the co-occurrence of two exotic, invasive marine organisms: the brown seaweed Sargassum muticum (Yendo) Fensholt and the Spirorbid tube worm Janua (Neodexiospira) brasiliensis (Grube), in association with the indigenous eelgrass, Zostera marina L. and Wrack, Fucus serratus L. in the south-west Netherlands and the Channel Islands, Europe

Janua (Neodexiospira) brasiliensis, an immigrant spirorbid, has extended its range in Europe Once restricted to Portsmouth Harbour, south coast of Britain, the tubeworm has subsequently established isolated, but well-developed populations in a small saline canal in the south-west Netherlands and in an intertidal outflow channel from the Jersey Power Station at St. Helier in the Channel Islands, in Portsmouth Harbour, where J . brasiliensis was first discovered as an European immigrant species, the tubeworm is associated with another introduced organism, the brown seaweed Sargassum muticum ; in the south-west Netherlands J . brasiliensis develops most prolifically upon the marine angiosperm Zostera marina , whilst at St. Helier the brown wrack Fucus serratus has become most heavily colonized. Both new sites of establishment are influenced by seasonally elevated seawater temperatures (25 °C), which may be responsible for the notable, localized population developments on Z. marina and F . serratus (mean settlement densities of 29.7 ± 2.5 and 55.1 ± 3.5 individuals per cm 2 respectively were recorded).