Efficient and sustainable management of rapidly mounting environmental issues has been the focus of current intensive research. The present study aimed to investigate the impact of plant phenological development stage variation on mercury (Hg) tolerance, accumulation, and allocation in two salt marsh macrophytes Triglochin maritima and Scirpus maritimus prevalent in historically Hg-contaminated Ria de Aveiro coastal lagoon (Portugal). Both plant samples and the sediments vegetated by monospecific stands of T. maritima and S. maritimus were collected from reference (R) and sites with moderate (M) and high (H) Hg contamination in Laranjo bay within Ria de Aveiro lagoon. Hg tolerance, uptake, and allocation in T. maritima and S. maritimus, physico-chemical traits (pH, redox potential, and organic matter content) and Hg concentrations in sediments vegetated by these species were impacted differentially by phenological development stages variation irrespective of the Hg contamination level. In T. maritima, Hg concentration increased with increase in Hg contamination gradient where root displayed significantly higher Hg followed by rhizome and leaf maximally at H. However, in S. maritimus, the highest Hg concentration was perceptible in rhizome followed by root maximally at M. Between the two studied plant species, S. maritimus displayed higher Hg tolerance index (depicted by higher plant dry mass allocated to reproductive stage) and higher available Hg at M (during all growth stages) and H (during senescent stage) when compared to T. maritimus. Both plant species proved to be Hg excluder (low root/rhizome-leaf Hg translocation). Additionally, T. maritima also acted as Hg stabilizer while, S. maritimus as Hg accumulator. It can be inferred from the study that (a) the plant phenological development stage variations significantly influenced plant Hg sensitivity by impacting sediment chemistry, plant growth (in terms of plant dry mass), Hg accumulation, and its subsequent allocation capacity, contingent to Hg contamination gradient; (b) S. maritimus accumulated higher Hg but restricted its translocation to above-ground part using exclusion process at both M and H due to its accelerated growth during Hg-tolerant reproductive/metabolically active phenological development stage greater than its counterpart T. maritima; and (c) the studied salt marsh plants although hailed from the same C3 and monocot group did not necessarily display similar phenotypic plasticity and behavior towards Hg-contaminated scenario during their life cycle.

Salt marshes are very important areas for biogeochemical cycling, sediment accretion, pollution filtration and retention and erosion and stabilization of the river margins. The high organic matter content in the salt marsh plant sediments along with the radial oxygen diffusion provided by these halophyte root systems gather the ideal conditions for the development of a microbial rhizosphere community. Due to the quick feedback of the microbial communities to an environmental change, these organisms become important monitors for environmental impact assessment. A Salt marsh Sediment Microbial Index (SSMI) that reflected physical-chemical and microbial parameters was applied to plant rhizosphere sediments of five salt marshes from three important water bodies from Portugal. The SSMI revealed to be plant-independent evaluating efficiently the different marshes according to their maturity degree and disturbance influence. Mature salt marshes SSMI classification grouped all the systems at this development stage, while the younger salt marshes are classified in different groups according to their evolution degree. Also the impact degree is reflected at this level discriminating also the more adversely impacted salt marshes. Being a multi-metric index, the SSMI sub-metrics are also susceptible of ecological interpretation, giving important backstage information about the underlying biogeochemical cycling processes.

This study assessed the oxidative stress status, antioxidant metabolism and polypeptide patterns in salt marsh macrophyte Juncus maritimus shoots exhibiting differential mercury burdens in Ria de Aveiro coastal lagoon at reference and the sites with highest, moderate and the lowest mercury contamination. In order to achieve these goals, shoot-mercury burden and the responses of representative oxidative stress indices, and the components of both non-glutathione- and glutathione-based H2O2-metabolizing systems were analyzed and cross-talked with shoot-polypeptide patterns. Compared to the reference site, significant elevations in J. maritimus shoot mercury and the oxidative stress indices such as H2O2, lipid peroxidation, electrolyte leakage and reactive carbonyls were maximum at the site with highest followed by moderate and the lowest mercury contamination. Significantly elevated activity of non-glutathione-based H2O2-metabolizing enzymes such as ascorbate peroxidase and catalase accompanied the studied damage-endpoint responses, whereas the activity of glutathione-based H2O2-scavenging enzymes glutathione peroxidase and glutathione sulfo-transferase was inhibited. Concomitantly, significantly enhanced glutathione reductase activity and the contents of both reduced and oxidized glutathione were perceptible in high mercury-exhibiting shoots. It is inferred that high mercury-accrued elevations in oxidative stress indices were obvious, where non-glutathione-based H2O2-decomposing enzyme system was dominant over the glutathione-based H2O2-scavenging enzyme system. In particular, the glutathione-based H2O2-scavenging system failed to coordinate with elevated glutathione reductase which in turn resulted into increased pool of oxidized glutathione and the ratio of oxidized glutathione-to-reduced glutathione. The substantiation of the studied oxidative stress indices and antioxidant metabolism with approximately 53-kDa polypeptide warrants further studies.

This study investigates largely unexplored physiological/biochemical strategies adopted by salt marsh macrophyte Halimione portulacoides (L.) Aellen for its adaptation/tolerance to environmental mercury (Hg)-exposure in a coastal lagoon prototype. To this end, a battery of damage (hydrogen peroxide, H2O2; thiobarbituric acid reactive substances, TBARS; electrolyte leakage, EL; reactive carbonyls; osmolyte, proline) and defense [ascorbate peroxidase, APX; catalase, CAT; glutathione peroxidase, GPX; glutathione sulfo-transferase, GST; glutathione reductase, GR; reduced and oxidized glutathione (GSH and GSSG, respectively), and GSH/GSSG ratio] biomarkers, and polypeptide patterns were assessed in H. portulacoides roots and leaves at reference (R) and the sites with highest (L1), moderate (L2) and the lowest (L3) Hg-contamination gradients. Corresponding to the Hg-burdens, roots and leaves exhibited a differential modulation of damage- and defense-endpoints and polypeptide-patterns. Roots exhibiting the highest Hg-burden (at L3) failed to maintain a coordination among enzymatic-defense endpoint responses which resulted into increased oxidation of reduced glutathione (GSH) pool, lowest GSH/GSSG (oxidized) ratio and partial H2O2-metabolism. In contrast, the highest Hg-burden exhibiting leaves (at L1) successfully maintained a coordination among enzymatic-defense endpoints responses which resulted into decreased GSH-oxidation, enhanced reduced GSH pool and GSH/GSSG ratio and lower extent of damage. Additionally, increased leaf-carotenoids content with increasing Hg-burden implies its protective function. H. portulacoides leaf-polypeptides did not respond as per its Hg-burden but the roots did. Overall, the physiological/biochemical characterization of below (roots)- and above (leaves)-ground organs (studied in terms of damage and defense endpoints, and polypeptides modulation) revealed the adaptive responses of H. portulacoides to environmental Hg at whole plant level which cumulatively helped this plant to sustain and execute its Hg-remediation potential.

Major endogenous biochemical properties can make plants ideal agents for metal/metalloid-contaminated site cleanup. This study investigates the biochemistry of Juncus maritimus (Lam) roots for its high mercury (Hg) stabilization potential in the sediments of the Ria de Aveiro coastal lagoon (Portugal), which received Hg-rich effluents from a chlor–alkali industry between 1950 and 1994. J. maritimus plants were collected at a reference (R) site and three sites with the highest (L1),moderate (L2), and the lowest (L3) Hg contamination levels. The highest Hg-harboring/stabilizing J. maritimus roots at L1, exhibited significantly elevated damage endpoints (H2O2; lipid peroxidation, LPO; electrolyte leakage, EL; protein oxidation, PO; proline) which were accompanied by differential changes in H2O2-metabolizing defense system components (ascorbate peroxidase, catalase, glutathione peroxidase, glutathione S-transferase), glutathione reductase and the contents of both reduced and oxidized glutathione. Trends in measured endpoints reached maximum levels at L1 followed by L2 and L3. Cross-talks on root–Hg status and the studied biochemical traits revealed (a) high Hg-accrued elevations in oxidative stress as an obvious response; (b) Hg-stabilization potential of J. maritimus roots as a result of a successfulmitigation of elevated highHginduced H2O2, and its anomalies such as LPO, EL, and PO; and (c) the induction of and a fine synchronization between non-glutathione and glutathione-based systems. Overall, the study unveiled biochemical mechanisms underlying root tolerance to Hg burden-accrued anomalies which, in turn, helped J. maritimus during Hg-stabilization.

A greenhouse experiment was performed, to verify if phytoremediation of cadmium can be done by using the macrophyte Scirpus maritimus from the Obidos Lagoon (Portugal), plus if water salinity can influence its phytoremediation effectiveness. Two concentrations of cadmium were tested (50 and 100 ƒÊg l-1) to evaluate the plant capability to accumulate cadmium, plus possible toxicity effects of this pollutant. The contamination levels were tested at different water salinity conditions (values equal to 0.0, 5.0, 10.0 and 20.0), in which the plants are usually submerged in, at their natural environment. S. maritimus mortality was mostly determined by the water salinity and not for the cadmium contamination, in which more plants died at higher water salinities. The surge of new shoots, plant length and biomass increments were not proved to be affected by either of those factors, however, plant length and reduction and biomass loss can be induced by increasing water salinities. There was a biofilm development in all trial vessels, independently of the type of treatment involved. These suspended microorganisms and senescent organic matter presented more cadmium, according to initial water level of cadmium. The amount of water dissolved cadmium was positively related to the initial contamination levels of the experiment, but it increased also at higher water salinities. This may have resulted from tissue degradation and disruption from dead plants, possibly potentiated as well by symbiotic organisms and those from the biofilm, some of which may have promoted tissue decomposition and bioavailability of dissolved cadmium. The plants presented more cadmium in the rhizomes, followed by the stems, and less in the leaves. More cadmium accumulated in the plant fs rhizomes, according to higher water contamination levels and lower water salinities. According to the results, salinity could influence the transport of cadmium in the plant, between roots and leaves. However, the phytoremediation of the Obidos Lagoon by S. maritimus, or other similar systems, may be not be possible, due to the salt stress of the plants, the salt interference with the cadmium accumulation, and the normal functions of a plant. But the optimization of phytoremediation processes by S. maritimus could turn possible the use of this plant in freshwater ecosystems.

The major aim of this study was to evaluate the capacity of Salicornia ramosissima, a halophyte present in Obidos lagoon (Portugal), on Cadmium (Cd) phytoremediation, when submitted to different salinities and Cd concentrations, for a month. Two Cd concentrations were used to evaluate this capacity and, consequently, the toxic effect of Cd on the plant development. The contamination levels were tested in different salinities (0, 5 and 10), taking into consideration the natural conditions that S. ramosissima is subject in its natural environment. The mortality registered in the end of the experiment, as the growth parameters measured (length and weight), did not showed significantly differences between the treatments, which means were not affected for the salinity or Cd concentrations, neither for the interaction of those two stress factors. However, length elongation and the variation of weight, observed in the end of the experiment, may have been induced by the increase of salinity and Cd concentration. A highest Cd accumulation was detected in the roots of S. ramosissima, compared with the aerial portion. The Cd accumulation in roots decreased with the increase of salinity and Cd concentration, even though statistically there were no significant differences between treatments. In all treatments, a biofilm was form, highlighting treatment salinity 0 x 50 μg Cd.l-1, where microorganisms and, eventually, organic matter (from the plant), suspended revealed a higher Cd amount relatively to the water initial Cd concentrations. The dissolved Cd concentration, detected in the final of the experiment, was in accordance with the initial contamination levels used in this study; however an increase was observed for some treatments, possible as a result of decomposition of organic matter and consequent release of Cd to the water. Salicornia ramosissima is a potential candidate for Cd phytoremediation at salinities close to 0, where development is maximum and, consequently, is Cd accumulation capacity. The optimization of phytoremediation processes by S. ramosissima could turn possible the use of this plant in freshwater ecosystems.

The present paper aims to discuss the functional and ecophysiological traits of two abundant succulent halophytes in Mediterranean salt marsh systems, considering two ecotypes differing on their tidal exposure. In Halimione portulacoides the higher root/shoot (R/S) biomass ratio verified in low inundation frequency (LF) marshes, as compared with that in high inundation frequency (HF), indicates an optimal partitioning strategy, inducing differential allocation patterns along with varying environmental factors. Conversely, Sarcocornia perennis appear to have an isometric allocation strategy, based on intrinsic biomass allocation mechanisms independent from the surrounding environmental conditions. Similarly, the photosynthetic traits appear to reflect the intrinsic ecology of the species. Sarcocornia perennis habitat adaptation to HF conditions is clearly revealed by its high photosynthetic efficiency under HF conditions. On the other hand, the cosmopolitan H. portulacoides shows no differences between ecotypes, in which concerns this trait, mostly due to the high carotenoid content of the HF ecotype, which allows this specie to counteract the stress generated during submersion. Additionally, anti-oxidant enzymatic activities revealed that S. perennis presents a high degree of stress tolerance under HF conditions, while H. portulacoides is highly adapted to LF conditions. These functional (biomass-related) and physiological traits determine the optimal habitats for H. portulacoides and S. perennis. While H. portulacoides optimal biomass partitioning, along with its efficient anti-oxidant defence system, makes this species fitter to LF conditions, S. perennis isometric allocation strategy, along with a higher photosynthetic efficiency under frequent flooding conditions, provides this species the mechanisms to efficiently colonize the intertidal habitat.

It has been shown that salt marshes may function as efficient sinks for contaminants, namely for mercury. At the rhizo-sediment Hg may be associated with Mn and Fe oxyhydroxides, precipitated as sulphides or incorporated into organic matter. However, to our knowledge, in situ studies have not focused on the related processes at a daily or tidal cycle scales. Thus, the present work aims to study the effect of a common salt marsh halophyte in temperate latitudes (Sarcocornia perennis) on dissolved Fe, Mn and Hg concentrations in the water column. The in situ approach was carried out at a mercury-contaminated salt marsh and at the adjacent non-vegetated area (distance ≤ 4 m), covering two consecutive tidal cycles in order to include the photosynthetic active period and the night processes. During high tide no daily or spatial effects were observed on the concentrations of Mn, Fe and Hg in the water column, due to the dilution effect of the incoming seawater. During low tide the concentrations of Mn, Fe and Hg were significantly higher in the overlaying water column of the salt marsh. At S. perennis mats the concentration of dissolved total Hg was significantly related with the concentration of Mn (r = 0.459, p = 0.028, n = 23), but not with that of Fe (r = 0.367, p = 0.085, n = 23) while no significant relations were found at the adjacent non-vegetated sediments. This study highlights the complexity of the biogeochemical processes that take place in salt marshes, due to the daily photosynthetic cycle of halophytes and to the tidal action in mesotidal systems.

The present study intends to increase the knowledge on the mobility of mercury in a salt marsh colonised by Halimione portulacoides. Mercury distribution in the sediment layers and its incorporation into the plant biomass were assessed, as well as the potential export of mercury from the contaminated area to the adjacent environment. Mercury pools in the sediments ranged from 560 to 943 mg m−2 and are largely associated with the solid fraction, with just a small amount being associated with the pore waters. Estimated diffusive fluxes of reactive mercury ranged from 1.3 to 103 ng m−2 d−1. Despite the above ground biomass values being comparatively higher than below ground biomass values, the mercury pools were much higher in the root system (0.06–0.16 mg m−2 and 29–102 mg m−2, respectively). The annual bioaccumulation of mercury in above ground tissues was estimated in 0.11 mg m−2 y−1, while in below ground biomass the values were higher (72 mg m−2 y−1). The turnover rates of H. portulacoides biomass suggest higher mercury mobility within the plant rhizosphere. Taking into account the pools of mercury in above ground biomass, the export of mercury by macro-detritus following the “outwelling hypothesis” is not significant for the mercury balance in the studied ecosystem. The mercury accumulated in the below ground part of the plant is quite mobile, being able to return to the sediment pool throughout the mineralisation process.