Germination and establishment of the invasive cordgrass Spartina densiflora in acidic and metal polluted sediments of the Tinto River

Distribution of PAHs and trace elements in Spartina densiflora and associated sediments from low to highly contaminated South American estuarine saltmarshes

Estuarine saltmarshes from South America are exposed to several anthropogenic impacts due to diverse human activities that occur in both Atlantic/Pacific coastal environments. Primarily, chemical and petrochemical industries negatively impact saltmarshes generating inputs/deposition of non-essential trace elements (NTEs) and polycyclic aromatic hydrocarbons (PAHs) in sediments. The native cordgrass Spartina densiflora inhabits a wide range of environments, from non-impacted to highly impacted areas. It is important to know its performance towards pollution in different environmental settings in South America. The content of Cd, Hg, Pb, and PAHs was determined in the roots and leaves of S. densiflora, bulk sediments (Bs), and rhizosediments (Rs) of estuaries from Argentina, Brazil, and Chile. Differences in NTEs and PAHs levels were observed between Bs, Rs, and Spartina tissues from different saltmarsh areas. Differences in Rs/Bs (RHICF; rhizosediments concentration factors), roots/Bs (RCF; roots concentration factors) and leaves/roots (TF; translocation factors) factors were also found. In terms of NTEs, S. densiflora showed a high capability to increase levels in their Rs (RHICF>1) and bioconcentrate Cd in roots (RCF > 1), while no general translocation (TF < 1) was observed. Conversely, in cordgrass tissues, Bs and Rs, PAHs contents showed RCF and TF > 1, which was in line with lower levels in Rs related to Bs (RHICF<1) in most sites. These findings showed the S. densiflora capacity to retain, remove and/or translocate priority contaminants depending on intrinsic chemical characteristics and the level of contamination. The present study enables future considerations regarding the biomonitoring and phytoremediation/stabilization capabilities of Spartina in coastal environments.

Adaptation of the Invasive Plant (Sphagneticola trilobata L. Pruski) to a High Cadmium Environment by Hybridizing With Native Relatives

Invasive species can evolve rapidly in the invasion areas to adapt to new habitats. Sphagneticola trilobata L. Pruski, an invasive species, was studied for its tolerance to cadmium (Cd) in the soil and compared with its natural hybrid. From the perspective of photosynthetic physiology, antioxidant characteristics, and leaf hormone levels, the differences between the leaves of the two species before and after Cd treatment were compared. The results showed that the hybrid had stronger tolerance to Cd stress than invasive species. After Cd stress, the indexes of gas-exchange [net photosynthetic rate (Pn), intercellular CO₂ concentration (Ci), stomatal conductance (Gs), and transpiration rate (Tr)] of the hybrid was higher than invasive species, while the content of non-enzymatic antioxidants (flavonoids and total phenols) and antioxidant enzyme activities [peroxidase (POD) and superoxide dismutase (SOD)] was lower in hybrid than in invasive species. The changes in the content of plant hormones [auxin (IAA) and abscisic acid (ABA)] under Cd stress showed that hybrid can still maintain growth and prevent leaf senescence. Furthermore, the differences in gene expression between hybrid and invasive species in photosynthetic physiology, the antioxidant capacity of leaves, and endogenous hormone (IAA and ABA) synthesis pathway also showed that hybrid has stronger Cd tolerance than invasive species. This suggests that invasive species will realize the invasion through hybridization with the native relatives to overcome the stress from environmental factors. The study implied that hybridization between invasive species and native relatives is an important way for invasive species to spread in a wider and new environment that invasive species have not experienced in the area of origin.

Metal effects on germination and seedling development in closely-related halophyte species inhabiting different elevations along the intertidal gradient

Seed germination and seedling establishment are very sensitive plant stages to metal pollution. Many halophyte species colonizing salt marshes are able to germinate and establish in highly contaminated habitats and low marsh halophyte species seem to show higher tolerance to metals than high marsh species. We analyzed the effects of copper, zinc and nickel in concentrations up to 2000 μM on seed germination and seedling growth in two closely related species of Sarcocornia, S. perennis, a low marsh species, and S. fruticosa, a high marsh species. Germination of both halophytes was not affected by any metal concentration, and their seedling growth, mainly radicle length, was reduced by increasing metal concentrations. Seedlings of S. perennis showed higher tolerance to the three metals than those of S. fruticosa. Our results are useful for designing ecotoxicological bioassays and planning phytoremediation projects in salt marshes.

Spatial and seasonal metal variation, bioaccumulation and biomonitoring potential of halophytes from littoral zones of the Karachi Coast

Coastal wetlands primarily serve as natural sinks of trace metals and their importance for phytoremediation is well known at global level. There are some reports on trace metal availability in soil sediments of the Karachi coast but studies on accumulation and translocation to harvestable (Shoot) parts among halophytes of the littoral zones have not been conducted. Hence, phytoremediation potential of six naturally existing halophytes (Aeluropus lagopoides, Arthrocnemum macrostachyum, Atriplex stocksii, Avicennia marina, Cressa cretica and Suaeda fruticosa) was assessed for cleaning metal (Mn, Zn, Pb and Cr) polluted soils of the Karachi coast. Seasonal (winters, pre and post-monsoon summers) and spatial (three littoral zones: viz., site - I: Sandspit, site - II: Do-Dariya/Clifton and site - III: Korangi creek) variations in soil and plant metals of the Karachi coast were studied. Soil Zn, Pb and Cr were generally higher in winters, Mn and organic matter in summers (7-11%) while pH values ranged between 7.15 and 7.5 in all seasons at site - III. All tested species had potential for cleaning Pb through their harvestable part (shoots) with A. stocksii as prominent candidate (16 mg kg^-1) at site - I. Cressa cretica emerged as exclusive candidate for Zn phytoremediation (96 mg kg^-1) at site - I, while S. fruticosa, A. macrostachyum and A. lagopoides showed bioaccumulation in pre-monsoon summers at site II. Aeluropus lagopoides with higher Mn in post monsoon summers (62 at site - III and 53 mg kg^-1 at site - II) and Cr (7.1 mg kg^-1 at site - II and 14 mg kg^-1 at site - III) appeared exclusive bioindicator with potential of for cleaning all metals (Mn, Zn, Pb and Cr) at different sites. Metal bioaccumulation at study sites appeared species specific and varied seasonally among tested halophytes.

Effects of heavy metal pollution on germination and early seedling growth in native and invasive Spartina cordgrasses

Seed germination and seedling establishment are the stages most sensitive to abiotic stress in the plant life cycle. We analyzed the effects of copper, zinc and nickel on seed germination and early seedling growth of native Spartina maritima and invasive S. densiflora from polluted and non-polluted estuaries. Germination percentages for either species were not affected by any metal at any tested concentration (up to 2000 μM). However, the increase in metal concentration had negative effects on S. densiflora seedlings. The primary effect was on radicle development, representing initial seedling emergence. Spartina densiflora seedlings emerging from seeds from Tinto Estuary, characterized by high bioavailable metal loads, showed higher tolerance to metals than those from less polluted Odiel and Piedras Estuaries. Comparing our results to metal concentrations in the field, we expect S. densiflora seedling development would be negatively impacted in the most metal-polluted areas in Odiel and Tinto Estuaries.

Bacterial inoculants for enhanced seed germination of Spartina densiflora: Implications for restoration of metal polluted areas

The design of effective phytoremediation programs is severely hindered by poor seed germination on metal polluted soils. The possibility that inoculation with plant growth promoting rhizobacteria (PGPR) could help overcoming this problem is hypothesized. Our aim was investigating the role of PGPR in Spartina densiflora seed germination on sediments with different physicochemical characteristics and metal pollution degrees. Gram negative Pantoea agglomerans RSO6 and RSO7, and gram positive Bacillus aryabhattai RSO25, together with the consortium of the three strains, were used for independent inoculation experiments. The presence of metals (As, Cu, Pb and Zn) in sediments reduced seed germination by 80%. Inoculation with Bacillus aryabhattai RSO25 or Pantoea agglomerans RSO6 and RSO7 enhanced up to 2.5 fold the germination rate of S. densiflora in polluted sediments regarding non-inoculated controls. Moreover, the germination process was accelerated and the germination period was extended. The consortium did not achieve further improvements in seed germination.

Competition from native hydrophytes reduces establishment and growth of invasive dense-flowered cordgrass (Spartina densiflora)

Experimental studies to determine the nature of ecological interactions between invasive and native species are necessary for conserving and restoring native species in impacted habitats. Theory predicts that species boundaries along environmental gradients are determined by physical factors in stressful environments and by competitive ability in benign environments, but little is known about the mechanisms by which hydrophytes exclude halophytes and the life history stage at which these mechanisms are able to operate. The ongoing invasion of the South American Spartina densiflora in European marshes is causing concern about potential impacts to native plants along the marsh salinity gradient, offering an opportunity to evaluate the mechanisms by which native hydrophytes may limit, or even prevent, the expansion of invasive halophytes. Our study compared S. densiflora seedling establishment with and without competition with Phragmites australis and Typha domingensis, two hydrophytes differing in clonal architecture. We hypothesized that seedlings of the stress tolerant S. densiflora would be out-competed by stands of P. australis and T. domingensis. Growth, survivorship, biomass patterns and foliar nutrient content were recorded in a common garden experiment to determine the effect of mature P. australis and T. domingensis on the growth and colonization of S. densiflora under fresh water conditions where invasion events are likely to occur. Mature P. australis stands prevented establishment of S. densiflora seedlings and T. domingensis reduced S. densiflora establishment by 38%. Seedlings grown with P. australis produced fewer than five short shoots and all plants died after ca. 2 yrs. Our results showed that direct competition, most likely for subterranean resources, was responsible for decreased growth rate and survivorship of S. densiflora. The presence of healthy stands of P. australis, and to some extent T. domingensis, along river channels and in brackish marshes may prevent the invasion of S. densiflora by stopping the establishment of its seedlings.

Scouting contaminated estuaries: heavy metal resistant and plant growth promoting rhizobacteria in the native metal rhizoaccumulator Spartina maritima

Spartina maritima is a native endangered heavy metal rhizoaccumulator cordgrass naturally growing in southwest coasts of Spain, where is used as a biotool to rehabilitate degraded salt marshes. Fifteen bacterial strains were isolated from the rhizosphere of S. maritima growing in the estuary of the Tinto River, one of the most polluted areas in the world. A high proportion of bacteria were resistant towards several heavy metals. They also exhibited multiple plant growth promoting (PGP) properties, in the absence and the presence of Cu. Bacillus methylotrophicus SMT38, Bacillusaryabhattai SMT48, B. aryabhattai SMT50 and Bacilluslicheniformis SMT51 were selected as the best performing strains. In a gnobiotic assay, inoculation of Medicago sativa seeds with the selected isolates induced higher root elongation. The inoculation of S. maritima with these indigenous metal-resistant PGP rhizobacteria could be an efficient method to increase plant adaptation and growth in contaminated estuaries during restoration programs.

Potential of Spartina maritima in restored salt marshes for phytoremediation of metals in a highly polluted estuary

Sedimentary abiotic environment, and concentration and stock of nine metals were analyzed in vegetation and sediments to evaluate the phytoremediation capacity of restored Spartina maritima prairies in the highly polluted Odiel Marshes (SW Iberian Peninsula). Samples were collected in two 10 -m long rows parallel to the tidal line at two sediments depths (0-2 cm and 2-20 cm). Metal concentrations were measured by inductively coupled plasma spectroscopy. Iron, aluminum, copper, and zinc were the most concentrated metals. Every metal, except nickel, showed higher concentration in the root zone than at the sediment surface, with values as high as ca. 70 g Fe kg(-1). The highest metal concentrations in S. maritima tissues were recorded in its roots (maximum for iron in Spartina roots: 4160.2 +/- 945.3 mg kg(-1)). Concentrations of aluminum and iron in leaves and roots were higher than in superficial sediments. Rhizosediments showed higher concentrations of every metal than plant tissues, except for nickel. Sediment metal stock in the first 20 cm deep was ca. 170.89 t ha(-1). Restored S. maritima prairies, with relative cover of 62 +/- 6%, accumulated ca. 22 kg metals ha(-1). Our results show S. maritima to be an useful biotool for phytoremediation projects in European salt marshes.

The effects of heavy metals on germination and seedling characteristics in two halophyte species in Mediterranean marshes

The influence of different concentrations (10-2000 μM) of heavy metals (Cu, Mn, Ni, Zn) was analysed in Atriplex halimus and Salicornia ramosissima germination pattern and seedling size. They are two halophyte species that grow in the Estuary of Huelva (Southwest Iberian Peninsula, Spain), one of the most heavy metal-polluted environments in the world. All of the metals tested affected the final germination percentage in A. halimus and only Ni reduced germination in S. ramosissima. The germination rate was unaffected in both species. The study of seedling development shows that S. ramosissima, an intertidal annual species, has a higher tolerance of metals than A. halimus, a bush that inhabits the upper part of the marshes. Taking into account the metal concentrations in the estuary and the effects of these on the seedling development of the species analysed, we conclude that metals might limit plant colonisation in some parts of the marshes.

Comparison of germination, growth, photosynthetic responses and metal uptake between three populations of Spartina densiflora under different soil pollution conditions

Spartina densiflora has demonstrated a high tolerance to heavy metal contamination and a high capacity for accumulating metal in its tissues. In the Gulf of Cadiz this species has colonized habitats with different degrees of metal pollution. The aim of this study is to analyse the responses of populations of Spartina densiflora to this pollution. Germination, growth, photosynthesis and metal uptake of two populations of Spartina densiflora collected from contaminated sites (Odiel and Tinto marshes) and of one population from a clean site (Piedras marshes) were examined through two reciprocal experiments, in which seeds and adult plants were exposed to metal-contaminated and uncontaminated soil under greenhouse conditions. The seeds of Spartina densiflora were able to germinate in all sediments with little differences between populations, even in more contaminated soils. However, these conditions decreased the growth and survival of the seedlings to a similar degree for all populations. Likewise, no differences were recorded in relation to physiological and metal uptake. Contrarily, in the adult experiment, we found that the Odiel population differed from the other populations in growth and metal uptake, with overall greater values. These differences in growth were strongly supported by lower photosynthetic rates and stomatal conductance in the Piedras and Tinto populations. The reduction in photosynthetic performance was largely due to the reduction in photosynthetic pigment concentration in both populations. Despite these differences, there was insufficient evidence to support that Spartina has evolved to heavy-tolerant ecotypes, since all Spartina densiflora populations proved to have a great capacity for accumulating heavy metals in its roots. Nonetheless, this finding suggests that the Odiel population could have a greater phytoremediation potential.