Nickel Hyperaccumulator Plants Research Articles

Reflectance spectroscopy as a promising tool for ‘sensing’ metals in hyperaccumulator plants

Main conclusionThe VNIR reflectance spectra of nickel hyperaccumulator plant leaves have spectral variations due to high nickel concentrations and this property could potentially be used for discovery of these plants. Hyperaccumulator plants accumulate high concentrations of certain metals, including manganese, cobalt, or nickel. Of these metals, the divalent ions of nickel have three absorption bands in the visible to near-infrared region which may cause variations in the spectral reflectance of nickel hyperaccumulator plant leaves, but this has not been investigated previously. In this shortproof-of-concept study, the spectral reflectance of eight different nickel hyperaccumulator plant species leaves were subjected to visible and near-infrared and shortwave infrared (VNIR-SWIR) reflectance spectrum measurements in dehydrated state, and for one species, it was also assessed in hydrated state. Nickel concentrations in the plant leaves were determined with other methods and then correlated to the spectral reflectance data. Spectral variations centred at 1000 ± 150 nm were observed and had R-values varying from 0.46 to 0.96 with nickel concentrations. The extremely high nickel concentrations in nickel hyperaccumulator leaves reshape their spectral reflectance features, and the electronic transition of nickel-ions directly contributes to absorption at ~ 1000 nm. Given that spectral variations are correlated with nickel concentrations it make VNIR-SWIR reflectance spectrometry a potential promising technique for discovery of hyperaccumulator plants, not only in the laboratory or herbarium, but also in the field using drone-based platforms. This is a preliminary study which we hope will instigate further detailed research on this topic to validate the findings and to explore possible applications.

The parameters determining hyperaccumulator rhizobacteria diversity depend on the study scale

Soils harbor some of the most diverse microbiomes on Earth and are essential for both nutrient cycling and carbon storage. Numerous parameters, intrinsic to plant physiology, life history and the soil itself, can influence the structure of rhizomicrobial communities. While our knowledge of rhizosphere microbial diversity is increasing, opinion is divided as to whether the factors that most impact this diversity are abiotic, climatic or plant selection. Here we focused on the rhizosphere bacterial diversity of nickel hyperaccumulator plants (28 species from Mediterranean or tropical climates). We showed, by leveraging 16S Illumina sequencing of 153 ultramafic rhizosphere soils, that bacterial genetic diversity was highest in Mediterranean habitats where plant diversity was the lowest. Concerning those parameters driving this diversity, we demonstrated that climate drives bacterial diversity, in particular with the annual temperature variation. Focusing on each region, we underlined the substantial role of soil physicochemical parameters. Our results highlight the importance of considering spatial scale when explaining bacterial community diversity.

Nickel distribution in

Context Hyperaccumulator plants are of considerable interest for their extreme physiology. Stackhousia tryonii is a nickel (Ni) hyperaccumulator plant endemic to ultramafic outcrops in Queensland (Australia) capable of attaining up to 41 300 μg g−1 foliar Ni. Aims This study sought to elucidate the distribution of Ni in S. tryonii by using synchrotron X-ray fluorescence micro-computed tomography (XFM-CT), complemented with elemental maps acquired from physically sectioned plant organs. Its Ni-enriched cylindrical photosynthetic stems make them particularly well suited samples for synchrotron XFM-CT. Methods XFM-CT enables ‘virtual sectioning’ of a sample, avoiding artefacts arising from physical sample preparation. The method can be used on fresh samples that are frozen during the analysis, which preserves ‘life-like’ conditions by limiting radiation damage. It also prevents/minimises other artefacts. Key results The results showed that Ni is mainly concentrated in the apoplastic space surrounding epidermal cells, and in some epidermal cell vacuoles. This finding is significant because this ‘free’ solute Ni is likely to be lost during physical sectioning. Conclusions and implications This case study has highlighted the utility of the XFM-CT approach for visualising metals within intact plant organs, which may be used across the plant sciences.

ARPA–E explores paths to emissions-free metal making

The agency is seeking advice from industry and academia on how to make mining, smelting, and remediation less polluting and more energy efficient.

Variation in the ionome of tropical ‘metal crops’ in response to soil potassium availability

In tropical ultramafic soils, potassium (K) is typically the most growth limiting nutrient. However, tropical nickel (Ni) hyperaccumulator plants, including Phyllanthus rufuschaneyi and Rinorea cf. bengalensis (which are ‘metal crops’ used in agromining) from Malaysia, have unusually high K shoot accumulation compared to other species, despite naturally growing on severely K-impoverished ultramafic soils. This study aimed to establish the response to soil K availability in relation to uptake of K and other elements in the roots and shoots of P. rufuschaneyi and R. cf. bengalensis. We undertook an experiment in which soluble K was dosed to ultramafic soil in pots with P. rufuschaneyi and R. cf. bengalensis in Sabah (Malaysia). The results show that root K concentrations increased markedly as the soil K availability increased by 35-fold, whilst the corresponding effect on K accumulation in the shoots of P. rufuschaneyi and R. cf. bengalensis was not significantly different in relation to soil K dosing. Observed divergent responses between root and shoot K accumulation in these species suggests a separate genetic control of K uptake and xylem loading in P. rufuschaneyi and R. cf. bengalensis. The tight control of root-to-shoot K translocation and constrained K accumulation in shoots under a soil K gradient is likely an adaptive mechanism to the evolution of these species to grow in highly nutrient-impoverished ultramafic soils. This study provides information that will be useful for better nutrient management of tropical Ni metal farms that use K-efficient Ni ‘metal crops’.

Blepharidium guatemalense, an obligate nickel hyperaccumulator plant from non-ultramafic soils in Mexico

Nickel hyperaccumulation in Blepharidium guatemalense Standl. (Rubiaceae) was found in the tropical forests of south-eastern Mexico. This study aimed to document the geographic extent of nickel hyperaccumulation in this species, to understand its process of hyperaccumulation and to explore nickel distribution within the tissues of this plant. To accomplish these objectives, a complete non-destructive elemental screening of herbarium specimens was performed with a hand-held X-ray fluorescence spectrometer. Besides, rhizosphere soils and plant tissues were collected in Mexico and analyzed for physical–chemical parameters. Finally, elemental distribution maps of nickel and other elements in plant tissues were obtained by X-ray fluorescence spectroscopy and microscopy. This study revealed that Blepharidium guatemalense is distributed throughout Chiapas, Tabasco and Campeche, reaching the maximum nickel concentration in leaves (4.3 wt%) followed by roots and seeds (2.0 wt%) and bark (1.8 wt%). Simultaneous hyperaccumulation of cobalt and nickel was found in 15% of the herbarium specimens. Blepharidium guatemalense has uncommon re-distribution mechanisms via phloem since this tissue is the highest nickel-enriched from all parts of the plant (from roots to leaves). A high total nickel (mean of 610 µg g−1) was found in rhizosphere soils even though no evidence of ophiolite emplacement in that area has been reported. Blepharidium guatemalense represents the first hypernickelophore (> 1 wt% Ni) to be reported as growing in soils that are neither ultramafic nor enriched by anthropogenic pollutants.

Bacterial community diversity and functional roles in the rhizosphere of Rinorea cf. bengalensis and Phyllanthus rufuschaneyi under a nickel concentration gradient

The tropical nickel (Ni) hyperaccumulator plants, Rinorea cf. bengalensis and Phyllanthus rufuschaneyi, are locally common on ultramafic soils in the Malaysian state of Sabah on the island of Borneo. The aim of this study was to determine whether the structure and diversity of R. cf. bengalensis rhizosphere microbial communities were dependant on the Ni concentrations in the different rhizosphere soils studied. Rinorea cf. bengalensis and Phyllanthus rufuschaneyi were subjected to a randomized block design experiment in which four Ni dose levels were applied (0, 60, 240, 600 mg Ni kg−1) × 5 replicates per level for 12 months cultivation. At the end of the trial foliar elemental concentrations were measured. In addition, rhizosphere soils were collected for elemental and microbial analyses. The microbial analysis consisted of measuring microbial enzymatic activities and a diversity analysis based on Illumina sequencing. The Illumina sequencing analysis yielded 2,649,138 sequences grouped under 2430 different Operational Taxonomic Units (OTUs) belonging to 25 phyla. No impact of the Ni amendments was shown for the four microbial enzymatic activities tested, even at the highest Ni dose level (600 mg Ni kg−1). Higher microbial enzymatic activities were found in the rhizosphere of R. cf. bengalensis relative to P. rufuschaneyi. Nickel concentrations appeared to have a high impact on the P. rufuschaneyi rhizosphere soils, whereas no effect was found for the rhizosphere bacterial community diversity of R. cf. bengalensis. There may be a significant role of rhizodeposits associated with R. cf. bengalensis that protect and preserve the microbial communities in the rhizosphere, regardless of the prevailing nickel concentrations.

Convergent patterns of tissue-level distribution of elements in different tropical woody nickel hyperaccumulator species from Borneo Island.

The Malaysian state of Sabah on the Island of Borneo has recently emerged as a global hotspot of nickel hyperaccumulator plants. This study focuses on the tissue-level distribution of nickel and other physiologically relevant elements in hyperaccumulator plants with distinct phylogenetical affinities. The roots, old stems, young stems and leaves of Flacourtia kinabaluensis (Salicaceae), Actephila alanbakeri (Phyllanthaceae), Psychotria sarmentosa (Rubiaceae) and young stems and leaves of Glochidion brunneum (Phyllanthaceae) were studied using nuclear microprobe (micro-PIXE and micro-BS) analysis. The tissue-level distribution of nickel found in these species has the same overall pattern as in most other hyperaccumulator plants studied previously, with substantial enrichment in the epidermal cells and in the phloem. This study also revealed enrichment of potassium in the spongy and palisade mesophyll of the studied species. Calcium, chlorine, manganese and cobalt were found to be enriched in the phloem and also concentrated in the epidermis and cortex of the studied species. Although hyperaccumulation ostensibly evolved numerous times independently, the basic mechanisms inferred from tissue elemental localization are convergent in these tropical woody species from Borneo Island.

Evaluation of the performance of nickel hyperaccumulator plants as combustion fuel

Agromining is an emerging technology aiming at recovering metals from soils using hyperaccumulator (HA) plants. Plant cultivation and pyro- and hydrometallurgical processes have been designed for nickel (Ni) recovery. In this chain of processes, dry plants are burnt to obtain Ni-rich ashes that are subsequently treated. To ensure the sustainability and the profitability of the process, energy recovered at the combustion stage would add some value. The combustion of three Ni HA plants (Odontarrhena chalcidica, Leptoplax emarginata and Berkheya coddii) was investigated. Namely the higher and lower heating values (HHV and LHV) were measured with a calorimeter bomb and calculated with correlations, thermo-gravimetric analysis was performed, dry plants and ashes were characterized, and gas emissions were monitored during combustion at two temperatures. In a complementary approach, the self-heating behaviour of plants and ashes were compared, and the activation energy of such phenomenon was determined. These results allowed the identification of the step limiting the self-inflammation of the plants, which gives relevant information for their safe storage.

Synchrotron µXRF imaging of live seedlings of Berkheya coddii and Odontarrhena muralis during germination and seedling growth

Nickel hyperaccumulator plants require highly evolved mechanisms to avoid cellular-level toxicity to cope with the high prevailing concentrations of nickel in their seeds and seedlings. This study aimed to investigate tissue-level distribution and redistribution of Ni and other physiologically relevant elements during the germination of hyperaccumulator plants. Berkheya coddii Roessler (Asteraceae) and Odontarrhena muralis sensu latu (Waldst. & Kit.) Endl. (O. chalcidica), formerly Alyssum murale (Waldst. & Kit.) (Brassicaceae), were germinated for several days at room temperature before using synchrotron micro X-ray Fluorescence Spectroscopy (µXRF) to obtain high-sensitivity and high-resolution elemental images of live/hydrated plants at various stages of seed germination and seedling growth. The results show that fruits and seeds of both species were highly enriched in nickel. In Berkheya coddii, nickel was located in the cotyledons, the micropylar area, the seed coat, and the point of attachment of the pappus with the cypsela body. In Odontarrhena muralis seeds, nickel occurred in the cotyledons and hypocotyl. The emergence of true leaves initiates nickel and calcium redistribution within seedlings in both species. The diversity in physiological responses to nickel in Berkheya coddii and Odontarrhena muralis does not only occur at the mature stage, but is inherent to both species as seed elemental storage and tolerance mechanisms during seedling development differ.

Combustion of nickel hyperaccumulator plants investigated by experimental and thermodynamic approaches

This work was undertaken to determine whether thermodynamic calculations can predict the amount and composition of ash produced by combustion of nickel hyperaccumulator (HA) plants in the context of agromining. To this end, three HA plants containing about 1wt% of nickel were submitted to isothermal combustion at 550, 815, 900°C and the solid residues were characterized by complementary techniques (TGA, XRD, ICP-OES, ion chromatography, SEM-EDX), while thermodynamic calculations were performed with the FactSage software using the dry biomass composition as input. We present an in-depth evaluation of inorganics behavior based on the combination and comparison of experimental data and calculations: effect of temperature on ash yield, nature of crystallized phases, ash elemental composition and inorganics volatilization. Our work confirms that equilibrium calculation is a powerful method to evaluate the behavior of inorganic elements during thermal treatment of biomass, despite some lacks in the databases. For the nickel HA plants, an innovative result is that, after combustion, nickel is always in the form of NiO particles, that can mix with MgO to form (Ni,Mg)O solid solutions. These results have direct implications for understanding the combustion process and for better design of the following leaching step aiming at Ni recovery.

Ecophysiology of nickel hyperaccumulating plants from South Africa - from ultramafic soil and mycorrhiza to plants and insects.

An overview of 30 years of studies related to South African nickel hyperaccumulators is presented. Only five species have so far been identified as Ni hyperaccumulator plants among very rich and diversified South African flora. All of them occur on soils derived from ultramafic (serpentine) rocks and belong to the family Asteraceae: Berkheya coddii Roessler, Berkheya zeyheri subsp. rehmannii var. rogersiana, Berkheya nivea, Senecio coronatus, Senecio anomalochrous. Several techniques and methods were used to investigate ecophysiological aspects of the Ni hyperaccumulation phenomenon, from basic field and laboratory studies, to advanced instrumental methods. Analysis of elemental distribution in plant parts showed that in most cases the hyperaccumulated metal was stored in physiologically inactive tissues such as the foliar epidermis. However, an exception is Berkheya coddii, which has a distinctly different pattern of Ni distribution in leaves, with the highest concentration in the mesophyll. Such a distribution suggests that different physiological mechanisms are involved in the Ni transport, storage location and detoxification, compared to other hyperaccumulator species. Berkheya coddii is a plant with high potential for phytoremediation and phytomining due to its large biomass and potentially high Ni yield, that can reach 7.6% of Ni in dry mass of leaves. Senecio coronatus is the only known hyperaccumulator with two genotypes, hyperaccumulating and non-hyperaccumulating, growing on Ni-enriched/metalliferous soil. Detailed ultrastructural studies were undertaken to characterize specialized groups of cells in the root cortex of Ni-hyperaccumulating genotype, that are not known from any other hyperaccumulator. The occurrence of arbuscular mycorrhiza (AM) in Ni-hyperaccumulating plants was found for the first time in South African hyperaccumulator plants, and this type of symbiosis has been proved obligatory in all of them. There is a significant influence of mycorrhiza on the concentration and distribution of several elements. Three highly specialized herbivore insects feeding only on Ni hyperaccumulator plants were identified: Chrysolina clathrata (formerly Chrysolina pardalina), Epilachna nylanderi and Stenoscepa sp. The Ni-elimination strategies of these specialised insects have been established. Microbiological studies have revealed several genera of fungi and bacteria isolated from B. coddii leaves as well as presence of specialised, Ni-resistant yeasts in the C. clathrata gut. Understanding ecophysiological response to harsh environment broadens our knowledge and can have practical applications in cleaning polluted environments through phytomining/agromining. Finally, conservation aspects are also discussed and lines for future research are proposed.

Bacterial community diversity in the rhizosphere of nickel hyperaccumulator plant species from Borneo Island (Malaysia).

The Island of Borneo is a major biodiversity hotspot, and in the Malaysian state of Sabah, ultramafic soils are extensive and home to more than 31 endemic nickel hyperaccumulator plants. The aim of this study was to characterize the structure and the diversity of the rhizosphere bacterial communities of several of these nickel hyperaccumulator plants and factors that affect these bacterial communities in Sabah. The most abundant phyla were Proteobacteria, Acidobacteria and Actinobacteria. At family level, Burkholderiaceae and Xanthobacteraceae (Proteobacteria phylum) were the most abundant families in the hyperaccumulator rhizospheres. Redundancy analysis based on soil chemical analyses and relative abundances of the major bacterial phyla showed that abiotic factors of the studied sites drove the bacterial diversity. For all R. aff. bengalensis rhizosphere soil samples, irrespective of studied site, the bacterial diversity was similar. Moreover, the Saprospiraceae family showed a high representativeness in the R. aff. bengalensis rhizosphere soils and was linked with the nickel availability in soils. The ability of R. aff. bengalensis to concentrate nickel in its rhizosphere appears to be the major factor driving the rhizobacterial community diversity unlike for other hyperaccumulator species.

X-ray fluorescence elemental mapping of roots, stems and leaves of the nickel hyperaccumulators Rinorea cf. bengalensis and Rinorea cf. javanica (Violaceae) from Sabah (Malaysia), Borneo

There are major knowledge gaps in understanding the translocation leading from nickel uptake in the root to accumulation in other tissues in tropical nickel hyperaccumulator plant species. This study focuses on two species, Rinorea cf. bengalensis and Rinorea cf. javanica and aims to elucidate the similarities and differences in the distribution of nickel and physiologically relevant elements (potassium, calcium, manganese and zinc) in various organs and tissues. High-resolution X-ray fluorescence microscopy (XFM) of frozen-hydrated and fresh-hydrated tissue samples and nuclear microprobe (micro-PIXE) analysis of freeze-dried samples were used to provide insights into the in situ elemental distribution in these plant species. This study has shown that the distribution pattern of nickel hyperaccumulation is typified by very high levels of accumulation in the phloem bundles of roots and stems. In the leaves, nickel is preferentially located in epidermal cell region, whereas manganese is located mainly in the lower epidermis and zinc in the upper epidermis and palisade mesophyll. The abundant formation of calcium-oxalate crystals, lining the collenchyma, is a prominent feature of both Rinorea cf. bengalensis and Rinorea cf. javanica. Future investigations on Rinorea cf. bengalensis and Rinorea cf. javanica should focus on unravelling the mechanisms of nickel uptake in the root, specifically targeting the identification of nickelspecific membrane transporters.

Uptake, translocation and accumulation of nickel and cobalt in Berkheya coddii, a 'metal crop' from South Africa.

Hyperaccumulator plants have the ability to efficiently concentrate metallic elements, e.g. nickel, from low-grade sources into their living biomass. Although the majority of nickel hyperaccumulator plant species restrict cobalt uptake, some species are able to co-accumulate cobalt when growing in ultramafic soils. The asteraceous perennial herb Berkheya coddii from South Africa is one of the most promising agromining crops known globally. It may accumulate nickel in excess of 30 000 μg g-1 in dry leaves, while co-accumulating up to 600 μg g-1 cobalt. This study aimed to elucidate the interactions between nickel and cobalt for uptake by and translocation into B. coddii through a pot experiment including various cobalt/nickel treatment combinations in soil, after which uptake and localisation were recorded. Cobalt in the substrate limits nickel uptake by B. coddii plants and is mainly retained in the basal leaves in contrast to Ni that is rapidly transferred to the top of the plant. B. coddii was more tolerant to high Ni concentration, whether in the substrate or internally but remains a promising crop which could be used, with suitable agronomic measures and practices, for cobalt agromining in areas with high soil cobalt but low soil nickel. A yield of 77 kg ha-1 nickel and 16.5 kg ha-1 cobalt may be attainable under optimum conditions.

Localization of nickel in the hyperaccumulator plant Breynia cernua (Poir.) Mull.Arg. discovered in the nickeliferous laterites of Zambales, the Philippines

A newly discovered nickel (Ni) hyperaccumulator plant, Breynia cernua (Phyllanthaceae) from the nickeliferous laterites of Acoje, Zambales, has elevated Ni concentrations. Its computed bioaccumulation factor is 1.7 and the translocation factor is 4.6. Leaf tissues which contain the highest Ni concentrations were analyzed using optical microscopy and Scanning Electron Microscopy with Energy-Dispersive X-ray (SEM/EDX). The results indicated that most of the Ni are stored in the epidermal cells of the leaves, followed by the mesophyll cells then the vascular bundles. The accumulation of Ni found in the subcellular locations, probably in the vacuoles, may prevent phytotoxicity that could affect the normal function of other cellular structures. A possible resistance mechanism through strain avoidance towards Ni accumulation makes B. cernua capable of tolerating elevated Ni concentrations in its tissues. The tolerance of B. cernua to Ni phytotoxicity suggests possible applications of this hyperaccumulator plant in phytoextraction technology.

Biogeochemistry of the flora of Weda Bay, Halmahera Island (Indonesia) focusing on nickel hyperaccumulation

Indonesia has one of the largest surface expressions of ultramafic rocks on Earth and in parallel hosts one of the most species-rich floras. Despite the extensive knowledge of the botanical diversity and the chemistry of these substrates, until recently the records for nickel hyperaccumulator plant species in the region have been scant. Identification of native local hyperaccumulator species is the critical initial step for developing local phytomining as these species naturally match ambient bioclimatic, geochemical and physiological conditions. Prior to this research just 11 nickel hyperaccumulator plant species were known from the whole of Indonesia. This field-based investigation at Weda Bay on the Island of Halmahera in the North Maluku Province revealed the existence of 13 nickel, two cobalt, one manganese and 10 aluminium hyperaccumulator plant species. Phylogenetic affinity for nickel hyperaccumulation is diverse and spans several orders, but was most frequent in the Malpighiales, as in other ultramafic regions of Southeast Asia. In contrast to global patterns, hyperaccumulation was infrequent in the Phyllanthaceae.

Rhizosphere chemistry and above-ground elemental fractionation of nickel hyperaccumulator species from Weda Bay (Indonesia)

The identification and use of hyperaccumulator plants in mining projects has been recognized as an important component of mine planning at several sites around the world. The objective of this research was to provide information on relevant plant tissue chemistry and an indicative assessment of the potential for phytomining at Weda Bay Nickel (WBN), Halmahera. The first stage was the identification of native nickel hyperaccumulator plants. In total, 280 plant tissue samples from 10 nickel accumulator species and 46 matching rhizosphere soil samples were collected. Chemical analyses of plant tissue samples were performed and physico-chemical parameters of the rhizosphere soils were also measured. A total of three species were considered as metal crops: Rinorea aff. bengalensis (up to 22,200 mg kg−1 dry weight at 2 m above ground level), Ficus trachypison (1060 mg kg−1) and Trichospermum morotaiense (5180 mg kg−1), but only R. aff. bengalensis has sufficiently high Ni concentrations in biomass to warrant field trials. Utilising a successional planting strategy, F. trachypison and T. morotaiense could be used to facilitate site conditions, followed by the metal crop R. aff. bengalensis. Using this design, a nickel yield of 330 kg per hectare would be possible every 4 years. In addition to allowing the recovery of nickel, this approach could be an integrated mine site rehabilitation strategy to mitigate environmental impacts, improve soil quality and facilitate transition to other land-uses such as native forest.

Community diversity and potential functions of rhizosphere-associated bacteria of nickel hyperaccumulators found in Albania

Ultramafic (i.e. serpentine) soils are widespread in the Balkans and particularly in Albania. They account for a large part of plant endemism in that region and host several hyperaccumulator species, which are characterized by leaf nickel concentrations frequently above 1%. This rich nickel hyperaccumulating flora could serve as candidate to be used in phytoextraction and agromining. Despite recent interest in metal hyperaccumulating plants and agromining, very few studies have investigated the bacterial diversity and the influence of environmental factors on microbial gene profiles in the rhizosphere of hyperaccumulator plants growing on ultramafic soils. Because rhizospheric bacteria could be crucial to the success of phytoremediation, we studied a total of 48 nickel-hyperaccumulating plants which were sampled from four species that are widespread in Albania: Noccaea ochroleuca, Odontarrhena smolikana, O. rigida and O. chalcidica. All samples were taken from the ultramafic regions of Librazhd and Pogradec in eastern Albania in October 2015. Our study shows that Proteobacteria, Actinobacteria and Acidobacteria dominated the soil bacterial communities. Of these three phyla, only Proteobacteria was relatively abundant. This study underlines the influence of soil Cation Exchange Capacity on the bacterial community's diversity and structure. Based on the predicted metagenomes, the genes belonging to amino acid, lipid and carbohydrate metabolisms were identified as major gene families. Our study sheds some light on our understanding of how bacterial communities are structured within and affect the rhizosphere of hyperaccumulator plants from ultramafic soils in Albania.

Bacterial community diversity in the rhizosphere of nickel hyperaccumulator species of Halmahera Island (Indonesia)

Ultramafic soils cover large areas in Indonesia, with over 8000 km2 on Halmahera Island. Nickel hyperaccumulator plants have evolved on these soils which are suspected to host highly nickel tolerant bacteria in their rhizosphere. To date, research has been limited on the characterization of the genetic diversity of such bacterial communities. The objective of this study was to investigate the genetic diversity of bacterial communities in the rhizosphere of nickel hyperaccumulator plants from Halmahera Island to improve knowledge on the factors that drive bacterial community diversity. We collected 45 rhizosphere soils from 10 woody nickel hyperaccumulator species at 16 sites to highlight the influence of plants, soil chemical parameters and site effects on bacterial diversity, richness and composition. A total of 2,508,874 bacterial 16S rRNA gene sequences were obtained and clustered into 6645 OTUs. In total, 40 phyla were identified with the most dominant phyla being Proteobacteria, Acidobacteria, Actinobacteria and Chloroflexi. Redundancy analysis between soil chemical characteristics and bacterial phyla relative abundances for the three main species (Rinorea aff. bengalensis, Ficus trachypison and Trichospermum morotaiense) showed that the main factor driving the bacterial diversity in the rhizosphere of Rinorea aff. bengalensis appears to be the plant itself (regardless of the characteristics of the soil), while the impact of soil conditions was important for the two other hyperaccumulator species. This may be explained by the fact that Rinorea aff. bengalensis (at least in Halmahera Island) is an obligate nickel hyperaccumulator.