Accumulation and distribution of trace metals within soils and the austral cordgrass Spartina densiflora in a Patagonian salt marsh

Arbuscular mycorrhizal fungi regulate amino acid metabolism, phytohormones and glycolysis pathway to promote the growth of Suaeda salsa under combined Cd and NaCl stresses

The use of halophytes in conjunction with arbuscular mycorrhizal (AM) fungi has been found to enhance the removal efficacy of heavy metals and salts in heavy metals contaminated saline soil. The mechanisms of AM fungi on promoting halophyte growth and regulating metabolism remain unclear. In this study, combinations of 0 g kg-1 NaCl and 3 mg kg-1 Cd (S0Cd3), 6 g kg-1 NaCl and 3 mg kg-1 Cd (S6Cd3), and 12 g kg-1 NaCl and 3 mg kg-1 Cd (S12Cd3) were employed to explore the impact of Funneliformis mosseae on the growth and metabolism of Suaeda salsa. The results showed that AM fungi increased the biomass and the P, K+, Ca2+, and Mg2+ accumulations, reduced the Cd and Na+ concentrations in S0Cd3 and S6Cd3, and increased the Cd concentrations in S12Cd3. AM fungi inoculation reduced the Cd and Na+ transfer factors and increased the Cd and Na+ accumulations in S6Cd3. The metabolomics of S6Cd3 showed that AM fungi upregulated the expression of 5-hydroxy-L-tryptophan and 3-indoleacid acid in tryptophan metabolism, potentially acting as crucial antioxidants enabling plants to actively cope with abiotic stresses. AM fungi upregulated the expression of arbutin in glycolysis process, enhancing the plants' osmoregulation capacity. AM fungi upregulated the expression of 2-hydroxycinnamic acid in phenylalanine metabolism and dopaquinone in tyrosine metabolism. These two metabolites help effectively remove reactive oxygen species. Correspondingly, AM fungi decreased MDA content and increased soluble sugar content. These results indicate that AM fungi improve the stress resistance of S. salsa by increasing nutrient uptake and regulating physiological and metabolic changes.

Holobiont dysbiosis or acclimatation? Shift in the microbial taxonomic diversity and functional composition of a cosmopolitan sponge subjected to chronic pollution in a Patagonian bay

Dysbiosis and acclimatization are two starkly opposing outcomes of altered holobiont associations in response to environmental pollution. This study assesses whether shifts in microbial taxonomic composition and functional profiles of the cosmopolitan sponge Hymeniacidon perlevis indicate dysbiotic or acclimatized responses to water pollution. To do so, sponge and water samples were collected in a semi-enclosed environment (San Antonio Bay, Patagonia, Argentina) from variably polluted sites (i.e., eutrophication, heavy metal contamination). We found significant differences in the microbiome of H. perlevis with respect to the pollution history of the sites. Several indicators suggested that acclimatization, rather than dysbiosis, explained the microbiome response to higher pollution: 1) the distinction of the sponge microbiome from the water microbiome; 2) low similarity between the sponge and water microbiomes at the most polluted site; 3) the change in microbiome composition between sponges from the different sites; 4) a high similarity in the microbiome among sponge individuals within sites; 5) a similar ratio of common sponge microbes to opportunistic microbes between sponges at the most and least polluted sites; and 6) a distinctive functional profile of the sponge microbiome at the most polluted site. This profile indicated a more expansive metabolic repertoire, including the degradation of pollutants and the biosynthesis of secondary metabolites, suggesting a relevant role of these microbial communities in the adaptation of the holobiont to organic pollution. Our results shed light on the rearrangement of the H. perlevis microbiome that could allow it to successfully colonize sites with high anthropogenic impact while resisting dysbiosis.

Evaluating metal phytorremediation and biondication potential of Spartina alterniflora in a South American estuary

Soil metal pollution has been widely studied in salt marshes but mainly regarding non-essential metals. The aim of this study was to assess the levels of two essential metals (Fe and Mn) and one non-essential one (Cd) in Spartina alterniflora salt marshes in a South American estuary in order to evaluate the potential of this species as a phytoremediator and/or bioindicator of Fe, Mn and Cd and to analyze the distribution of these metals according to the edaphic conditions. The metals present in the soils varied among the three sites studied according to the content of organic matter and fine sediments. In comparison with other Spartina-dominated salt marshes worldwide, in this study Fe and Mn were approximately in the same range, whereas Cd levels were always lower, with a high number of samples below the method detection limit (MDL). All metals were highly correlated with each other suggesting an association of Cd with Mn and Fe oxides/hydroxides or sulfides and/or a common anthropogenic source. Metals in plant tissues also varied from site to site and between the aboveground and belowground tissues. Compared to the metal levels in Spartina tissues in other salt marshes, our levels of Fe and Mn were in the same range, whereas the Cd levels were lower, among most samples, especially those from aboveground tissues that were below the MDL. The bioconcentration factor (metal in belowground tissues/metal in soil) was always lower than one for Fe and Mn meaning that there is no accumulation of these metals in belowground tissues, but this factor for Cd was sometimes higher than one, even as high as 3.45, implying that S. alterniflora can accumulate this metal in its tissues, pointing to a potential role of this species in Cd phytoremediation. Translocation factors (metal in aboveground tissues/metal in belowground tissues) were always lower than one for Fe and could not be calculated for Cd but were usually higher than one for Mn, showing the role of this element in photosynthetic tissues and a possible function of this species for phytoextraction of Mn. In most samples the Fe levels in plant tissues were higher than the permissible levels reported in the literature, suggesting a potential role of S. alterniflora in Fe phytoremediation. No correlation was observed between metal concentrations in soils and aboveground tissues; therefore, S.alterniflora is not a good bioindicator for the metals studied. Although our results are not conclusive, they reinforce the importance of local edaphic conditions on the behavior of metals in salt marshes and shed light on the potential role of S. alterniflora in the phytoremediation of highly toxic metals such as Cd or poorly studied metals such as Fe and Mn.

Assessment of halophyte plant phenotypic responses under heavy metals pollution. Implications for monitoring and phytoremediation

While phytoremediation is a highly valued practice to address local pollution problems, the use of early biomarkers of stress is useful for monitoring environments since they allow us to take measures before deleterious effects are irreversible. In this framework the goals are: to evaluate the pattern of leaf shape variation of Limonium brasiliense plants related to a metal soil gradient in the San Antonio salt marsh; to assess whether seeds from sites with different pollution levels show the same pattern of leaf shape variations under optimal growing conditions; and to compare the growth, the Pb accumulation pattern, and the leaf shape variation pattern of plants germinated from seeds originated in sites with different pollution levels in response to an experimental Pb rise. The results obtained from leaves collected in the field showed that the leaf shape changed depending on the soil metal levels. Plants germinated from seeds collected at the different sites expressed all the variation in leaf shape independently of the origin site, and the mean shape of each site was close to the consensus. Instead, when looking for the leaf shape components that maximize the differences between the sites from a growth experiment exposed to an increase in Pb in the irrigation solution, the pattern of variation found in the field disappeared. That is, only plants from the polluted site did not show variations in leaf shape in response to Pb additions. Finally, Pb accumulation in the roots was highest in plants germinated from seeds from the site where the soil pollution is greater. That suggests that seeds of L. brasiliense from polluted sites are better to use in phytoremediation practices, specifically to stabilize Pb in its roots whilst plants from the non-polluted site are better to detect pollutant soils using the leaf shape as an early biomarker.

Assessing the use of two halophytes species and seaweed composting in Cu-pollution remediation strategies

In order to evaluate suitable remediation strategies for Cu-polluted soils, the growth, tolerance, and Cu accumulation of Sarcocornia perennis and Limonium brasiliense were studied in hydroponic culture using different Cu concentrations, with and without Undaria pinnatifida compost. Most measured variables (e.g., water content, aboveground dry weight, malondialdehyde, pigments concentrations, tolerance index) showed a negative effect of high Cu levels in plants without compost but not in plants with compost. Plants accumulated high Cu levels in belowground tissues (bioaccumulation factor > 1) showing low translocation to aboveground parts. Based on the results, we suggest two remediation strategies: a short-term strategy: root absorption of Cu by halophytes, and a long-term strategy: using halophytes and U. pinnatifida compost, involving absorption of Cu by the plants together with metal immobilization in the substrate. This last strategy offers an additional advantage: it provides a use for seaweed waste, considered a problem for several coastal cities.

Crab carapace shape as a biomarker of salt marsh metals pollution

Coastal environmental pollution is a global problem that has been growing for decades. For this reason, different approaches have been sought to address and detect its environmental implications. Some organisms are considered bioindicators or biomonitors of contamination, which provide information about environmental quality. Previous studies used the crab Neoelice granulata (Brachyura, Varunidae) as bioindicator of the presence of metals by the analysis of soft tissues to evaluate physiological and molecular markers. However, the contaminant levels accumulated in these soft tissues have shown to be affected by seasonal variations, suggesting that environmental assessments based on soft tissue samples may be unreliable. Within this framework, we aimed to describe the crab body (carapace) variations related to a known soil metal gradient in a Patagonian salt marsh and to evaluate the use of the body shape as an alternative biomarker for monitoring the quality of salt marsh systems. We studied the carapace shape variations using geometric morphometrics (GM) based on a 2D structure with object symmetry. We observed symmetric and asymmetric components of carapace shape variation. While the latter was not found associated with a gradient of contamination by metals, the symmetric component responded to environmental changes; therefore, it could be considered a stress biomarker related to metal contamination. Consequently, we recommended using GM analysis because it is inexpensive, faster and non-seasonal and could be used on living organisms, avoiding destroying individuals to measure the environmental stress.

Potential mobility assessment of metals in salt marsh sediments from San Antonio Bay

The BCR method was applied on sediments from the salt marsh of San Antonio Bay (SAB). It presents several channels among which the Encerrado is the most important and is impacted by abandoned mining wastes. The pseudototal concentrations of metals measured within this channel were relatively higher than in outer sites, and according to the I_geo index, its contamination level was low. The metal distribution in the different phases of sediment particles showed that the residual component, considered the safest from the environmental point of view, accounted for most of the Fe, Cd, Cu, and Zn contents. Conversely, Pb was mainly in the non-residual component as part of the reducible fraction, thus constituting the main environmental hazard among the studied elements. The predominance of residual and reducible fractions indicated a historic contamination of metal such as Pb, Cu, and Zn from the mining wastes. The low exchangeable and oxidizable fractions would indicate no actual input of metals.

Leaf shape variation as a potential biomarker of soil pollution

Halophytic plants play a fundamental role in salt marshes, influencing their structure, dynamics, and cycling of nutrients and minerals. These plants have the ability to retain metals in the soil, or absorb and retain them in underground structures, or transport them to their aerial structures. Here we aim to study shape variation in the leaves of Cressa truxillensis inhabiting the salt marsh of San Antonio Oeste, according to their proximity to a source of metals in the soil. A gradient of bioavailability of metal was observed in the soil, decreasing from the site closest to the source to the most distant point, where Zn was the most abundant metal followed by Pb and Cu. We used landmark-based geometric morphometric tools to study leaf shape variation. We observed more oval leaf growth on the farthest point of the pollutant's source, and lanceolate shape close to it. No significant among-site size differences were found. Collectively, these results suggest that the stress conditions associated with the soil metals' concentration generate changes in the leaf shape of Cressa truxilensis. Considering that this species has not been extensively analyzed, this study establishes a baseline and supports the use of the leaf as an early biomarker of stress by contamination in plants associated with marshes.

Metal biomonitoring in a Patagonian salt marsh

Patagonian salt marshes are not affected by pollution, but historical mining wastes are a continuous source of metals to salt marsh in San Antonio Bay. The present study evaluated the concentration of metals in sediments and used the halophyte Spartina spp. and the crab N. granulata as biomonitors. The levels of metals in sediment and organisms in SAB remained at levels corresponding to a slight enrichment or contamination. The highest levels corresponded to innermost sites of the Encerrado channel and close to the mining wastes. Spartina is a phytostabilizer so its aboveground tissues do not reflect the concentrations in the sediment; although, it retains the metals in its belowground tissues and in the rhizosediment. N. granulata showed to be a useful biomonitor for Pb, but not for the other metals.

Iron plaque formation and heavy metal uptake in Spartina alterniflora at different tidal levels and waterlogging conditions

Tidal flat elevation in the estuarine wetland determines the tidal flooding time and flooding frequency, which will inevitably affect the formation of iron plaque and accumulations of heavy metals (HMs) in wetland plants. The present study investigated the formation of iron plaque and HM's (copper, zinc, lead, and chromium) accumulation in S. alterniflora, a typical estuarine wetland species, at different tidal flat elevations (low, middle and high) in filed and at different time (3, 6, 9, 12 h per day) of waterlogging treatment in greenhouse conditions. Results showed that the accumulation of copper, zinc, lead, and chromium in S. alterniflora was proportional to the exchangeable fraction of these metals in the sediments, which generally increased with the increase of waterlogging time, whereas the formations of iron plaque in roots decreased with the increase of waterlogging time. Under field conditions, the uptake of copper and zinc in the different parts of the plants generally increased with the tidal levels despite the decrease in the metals' exchangeable fraction with increasing tidal levels. The formation of iron plaque was found to be highest in the middle tidal positions and significantly lower in low and high tidal positions. Longer waterlogging time increased the metals' accumulation but decreased the formation of iron plaque in S. alterniflora. The binding of metal ions on iron plaque helped impede the uptake and accumulation of copper and chromium in S. alterniflora.

Modulation of Spartina densiflora plant growth and metal accumulation upon selective inoculation treatments: A comparison of gram negative and gram positive rhizobacteria

Metal contamination of estuaries is a severe environmental problem, for which phytoremediation is gaining momentum. In particular, the associations between halophytes-autochthonous rhizobacteria have proven useful for metal phytostabilization in salt marshes. In this work, three bacterial strains (gram-negative and gram-positive) were used for Spartina densiflora inoculation. All three bacteria, particularly Pantoea strains, promoted plant growth and mitigated metal stress on polluted sediments, as revealed from functionality of the photosynthetic apparatus (PSII) and maintenance of nutrient balance. Pantoea strains did not significantly affect metal accumulation in plant roots, whereas the Bacillus strain enhanced it. Metal loading to shoots depended on particular elements, although in all cases it fell below the threshold for animal consumption. Our results confirm the possibility of modulating plant growth and metal accumulation upon selective inoculation, and the suitability of halophyte-rhizobacteria interactions as biotechnological tools for metal phytostabilization in salt marshes, preventing metal transfer to the food chain.

Geochemical processes controlling the distribution and concentration of metals in soils from a Patagonian (Argentina) salt marsh affected by mining residues

Heavy metal pollution that affects salt marshes is a major environmental concern due to its toxic nature, persistence, and potential risk to organisms and to human health. Mining waste deposits originated four decades ago, by the metallurgical extraction of heavy metals, are found near to the San Antonio salt marsh in Patagonia. The aim of the work was to determine the geochemical processes that control the distribution and concentration of Cu, Fe, Pb and Zn in the soils of this Patagonian salt marsh. A survey of the mining waste deposits was carried out where three dumps were identified. Samples were collected to determine soil texture, Eh pH, organic matter and metal contents and the soil mineralogical composition. The results shows that the soils developed over the mining waste deposits are predominantly reddish constituted mainly by iron oxide, hydroxide and highly soluble minerals such as Zn and Cu sulphates. The drainage from these deposits tends to move towards the salt marsh. Within the salt marsh, the highest concentrations of Cu, Pb and Zn occur in the sectors closest to the mining wastes deposits. The sulphide oxidation and the dissolution of the Cu, Pb and Zn sulphates could be the mainly source of these metals in the drainage water. The metals in solution that reach the salt marsh, are adsorbed by the organic matter and the fine fraction of the soils. These adsorbed metals are then remobilized by tides in the lower sectors of the marsh by desorption from the cations present in the tidal flow. On the other hand, Fe tends to form non soluble oxides, hydroxides and sulphates which remain as altering material within the mining waste deposit. Finally, the heavy metal pollutants recorded in the San Antonio salt marsh shows that the mining waste deposits that were abandoned four decades ago are still a source metal contamination.

Mechanism of removal and retention of heavy metals from the acid mine drainage to coastal wetland in the Patagonian marsh

The attenuation of the acid mine drainage is one of the most important environmental challenges facing the mining industry worldwide. Mining waste deposits from an ancient metallurgical extraction of heavy metals were found near to the San Antonio marsh in Patagonia. The aim of this work was to determinate which mechanisms regulate the mobilization and retention of metals by acid drainage. A geological and geomorphological survey was carried out and samples from the mining waste deposits and the marsh were collected to determine soil texture, Eh pH, organic matter, Cu, Pb, Zn and Fe content, and soil mineralogical composition. Metals in marsh plants were determined in above- and below-ground structures. In the mining waste deposits polymetallic sulphides were recognized where the oxidation and formation of oxy-hydroxides and sulphates of Fe, Cu, Pb and Zn occurs. Then, by the alteration of those minerals, the metals enter in solution and are mobilized with the surface drainage towards the marsh where adsorption in the soils fine fraction and organic matter and/or by plants occurs. Locally, in the mining waste deposits, the precipitation/dissolution of Cu, Pb, and Zn sulphates take place in small centripetal drainage basins. In topographically lower portions of the marsh desorption and removal of metals by tidal flow could also be happen. The results allow to concluding that the marsh adjacent to the mining waste deposits is a geochemically active environment that naturally mitigates the contamination caused by acid drainage.

Possible developments for ex situ phytoremediation of contaminated sediments, in tropical and subtropical regions - Review

The growing problem of remediation of contaminated sediments dredged from harbor channels needs to be resolved by a cost effective and sustainable technology. Phytoremediation, by ex situ remediation plants, seems to have the potential to replace traditional methods in case of moderately contaminated sediments. On the other side, the need to mix sediments with soil and/or sand to allow an easier establishment of most employed species causes an increase of the volume of the processed substrate up to 30%. Moreover the majority of phytoremediating species are natives of temperate climate belt. Mangroves, with a special focus on the genus Avicennia - a salt secreting species - should represent an effective alternative in terms of adaptation to salty, anoxic sediments and an opportunity to develop ex situ phytoremediation plants in tropical and subtropical regions. The use of humic acid to increase root development, cell antioxidant activity and the potential attenuation of the "heavy metals exclusion strategy" to increase phytoextraction potentials of mangroves will be reviewed.

Phytochelatins and monothiols in salt marsh plants and their relation with metal tolerance

Phytochelatins (PCs) and monothiols and their relation with trace element concentrations were studied in three plant species from two Portuguese salt marshes. Belowground tissues showed always higher element concentrations, while enhanced values of monothiols were found in aboveground biomass. Glutathione was usually the most abundant monothiol. The concentration of total PCs was higher in leaves or stems than in roots of Halimione portulacoides and Sarcocornia perennis, while in Spartina maritima the highest concentrations were reported in large roots. PC₂ was synthesized by all tissues and species and was higher in large roots of S. maritima. PC₄ and PC₅ were in high levels in small roots of S. maritima. PC₂ was positively correlated with As, Zn and Pb. Although being the first evidence of PCs and monothiols in these species under natural conditions, our results do not point to a simple relationship with elements concentrations, suggesting a complex mechanism involved.

Comparison of phytoremediation potential capacity of Spartina densiflora and Sarcocornia perennis for metal polluted soils

Phytoremediation is considered the most appropriate technique to restore metal polluted soil, given its low cost, high efficiency and low environmental impact. Spartina densiflora and Sarcocornia perennis are perennial halophytes growing under similar environmental conditions in San Antonio marsh (Patagonia Argentina), therefore it is interesting to compare their phytoremediation potential capacity. To this end, we compared concentrations of Pb, Zn, Cu, and Fe in soils and in below- and above-ground structures of S. perennis and S. densiflora. It was concluded that both species are able to inhabit Pb, Zn, and Cu polluted soils. Although Sarcocornia translocated more metals to the aerial structures than Spartina, both species translocated only when they were growing in soils with low metal concentrations. It seems that the plants translocate only a certain proportion of the metal contained in the soil. These results suggest that both species could be considered candidates to phytostabilize these metals in polluted soils.

Hazardous metal pollution in a protected coastal area from Northern Patagonia (Argentina)

The San Antonio Bay is a protected natural coastal area of Argentina that has been exposed to mining wastes over the last three decades. Iron and trace metals of potential concern to biota and human health (Cd, Pb, Cu, and Zn) were investigated in the sediments from the bay and in the soils of the Pile (mining wastes). Concentrations of Cd (45 mg kg^-1), Pb (42,853 mg kg^-1), Cu (24,505 mg kg^-1), and Zn (28,686 mg kg^-1) in the soils Pile exceeded guidelines for agricultural, residential, and industrial land uses. Risk assessment due to exposure to contaminated soils (Pile) was performed. Hazard quotients were superior to non-risk (HQ >1) for all trace metals, while accumulative hazard quotient index indicated a high risk for children (HI = 93) and moderate for adults (HI = 9). In the bay, sediments closest to the Pile (mudflat and salt marsh) exceeded sediment quality guidelines for protection of biota. Results of different acid extraction methods suggest that most of the pseudototal content was potentially mobile. Principal component analysis indicated that the sites near the Pile (Encerrado channel) were more polluted than the distal ones. Tissues of Spartina spp. located within Encerrado channel showed the highest metal levels among all studied sites. These results show that the problem still persists and the mining wastes are the sources of the pollution. Furthermore, the Encerrado channel is a highly impacted area, as it is shown by their metal enriched sediments.