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During the last decade, the world faced record-breaking giant fires as observed in Australia and California, a trend that is expected to increase in the forthcoming years due to climate change (Palinkas, 2020; Sharples et al., 2016; van Oldenborgh et al., 2021). In addition to their large ecological impacts, wildfires are more and more regarded for their potential threat to human health through air pollution (Xu et al., 2020). However, water pollution resulting from wildfires represents an underestimated pathway for wildfires-induced health risk (Abraham et al., 2017). This latter impact is related to the heat generated by wildfires that can propagate towards several centimeters in the soil and transform/destroy soil components. In addition to weakening soil physical stability, such transformation/destruction can change the speciation of potentially toxic elements (PTEs) that are associated with these soil components, leading to enhanced mobility towards waterways (Abraham et al., 2017; Terzano et al., 2021). One notable PTE is chromium, which is naturally present in soils mostly as trivalent Cr(III), but can represent an environmental and health issue when occurring as hexavalent Cr(VI). Recent studies reported Cr(III) oxidation to Cr(VI) upon laboratory-heating of Cr(III)-doped Fe-oxyhydroxides (Burton et al., 2019a; 2019b). Besides, Cr(III) oxidation to Cr(VI) upon controlled heating was also demonstrated for different types of soils (Burton et al., 2019b; Rascio et al., 2022; Thery et al., 2023). All these considerations suggest a significant effect of wildfires on Cr(III) oxidation to Cr(VI) in soils, with a possible influence on Cr mobility that could further impact freshwater quality. This risk of freshwater Cr(VI) pollution is expected to particularly concern ultramafic catchments because of the related occurrence of Cr-rich soils.We have tried to address this question by performing laboratory-heating of several soils types (Ferralsols, Cambisols and Vertisols) developed on various geological settings (ultramafic, mafic and volcano-sedimentary) in New Caledonia, a French overseas territory which is a good representative of wildfires-threatened tropical ultramafic catchments (Toussaint, 2020). The results obtained revealed a significant influence of soil heating on Cr(III) oxidation to Cr(VI), followed by an enhanced Cr(VI) mobility, in all soil types. However, the magnitude of Cr(III) to Cr(VI) oxidation and Cr mobility depended on the actual nature of the soil, Ferralsols showing the highest Cr(VI) release compared to Cambisols and Vertisols. These differences were further interpreted on the basis of the changes in Cr speciation (including redox) induced by laboratory-heating of the investigated soils, as revealed by synchrotron-based X-ray absorption spectroscopy analyses. Finally, a simple risk assessment relying on the hypothesized concentration of suspended particulate matter (SPM) issued from burned soils in the related waterways allowed to emphasize a risk of wildfires-induced freshwater Cr(VI) pollution for ultramafic catchments composed of Ferralsols (Thery et al., 2023). Beyond the single case of New Caledonia, the results of this study point to the need to foster collaborative studies in order to further evaluate this risk of wildfires-induced freshwater Cr(VI) pollution at tropical ultramafic catchments on a global scale.

In the context of global warming, environmental forcing of wildfires on continental ecosystems is on a growing trend. Among other environmental impacts, wildfires can enhance trace metals concentration in freshwater systems. In this regard, chromium is of particular environmental and health concern because the temperature-induced oxidation of the less mobile and toxic Cr(III) form to the most mobile and toxic Cr(VI) one might represent a risk of freshwater pollution. The aim of the present study was to make a first assessment of this risk in New Caledonia by quantifying the influence of soil heating on Cr(VI) mobility in Cr-rich Ferralsols that cover about 1/3 of this French overseas territory. The results obtained indicated an enhanced Cr(VI) mobility upon soil heating from 200 °C, with a marked turnaround from 400 °C. Based on these results, a solid/solution mass balance approach allowed to confirm the risk of freshwater Cr(VI) pollution as a function of suspended particulate matter (SPM) concentration issued from burned Cr-rich Ferralsols. In addition, the results obtained revealed an apparent control of the Fe-oxides/silicates ratio on Cr(VI) mobility upon soil heating. Since this ratio decreases from the top to the bottom of ultramafic catchments, this latter result suggests that the risk of freshwater Cr(VI) pollution could depend on the actual location of the wildfires at the top or slope/base of these catchments. Since freshwaters represent the major water resource for the inhabitants of New Caledonia, these results point to the need for a larger evaluation of the risk of wildfires-induced freshwater Cr(VI) pollution at other catchments composed of Cr-bearing soils like Cambisols and Vertisols. At the larger scale, they also point to the need to foster studies aimed at better evaluating the risk of wildfires-induced freshwater Cr(VI) pollution at Cr-bearing pedological settings worldwide.

International audience; Nowadays, ecological restoration is considered the best solution for the rehabilitation of mining-degraded areas, particularly when it concerns valuable ecosystems. This is the case in New Caledonia, a hot spot of biodiversity, with 82% of endemic plant species in ultramafic soils. The use of topsoil to restore mined areas is an important practice. However, topsoil stockpiling can reduce soil fertility. We studied the evolution of different properties of two topsoils stored in 10 m height piles on mined areas, in order to characterize possible degradations. The effects of storage duration, depth, and topsoil origin were analyzed, focusing mainly on bacterial and fungal diversity assessed after bar-coded pyrosequencing, and expressed by operational taxonomic units (OTU). Microbial activity (carbon dioxyde production), arbuscular mycorrhizal fungi (AMF) spore numbers, bulk density and different other physicochemical characteristics were also determined. The studied ultramafic soils were characterized by a relatively high microbial diversity with 45 bacterial phyla and 7 fungal phyla. Bacterial and fungal OTU numbers did not vary significantly after 12 months of storage, but the community structures of these groups were changed. AMF diversity was significantly reduced by the storage. Microbial activity, AMF spore numbers, soil organic carbon, total nitrogen, cation exchange capacity, and carbon to nitrogen ratio (C/N) decreased significantly starting from 3 to 9 months of topsoil storage. In contrast, soil bulk density and pH increased, and extractable concentrations of potentially toxic metals varied only slightly. All the changes induced by topsoil stockpiling in microbial population structure, AMF diversity, and other biotic and abiotic traits may negatively affect soil functions and create perturbations of the reconstructed ecosystems after revegetation. It is then necessary to reduce the storage of ultramafic topsoils as far as possible. In all cases, the stockpiling should not exceed six months before its use in ecosystem restoration. It is also recommended to reduce the height of the stockpiles, when possible, to 1 m or less to minimize the compaction and the anaerobiosis.

The OCBIL theory comprises a set of hypotheses to comprehend the biota of old, climatically buffered, infertile landscapes (OCBILs). Here, we review evidence from the literature to evaluate the extent to which this theory could apply to the biodiversity hotspot of New Caledonia. We present geological, pedological and climatic evidence suggesting how the island might qualify as an OCBIL. The predictions of OCBIL theory are then reviewed in the context of New Caledonia. There is evidence for a high rate of micro-endemism, accumulation of relict lineages, a high incidence of dioecy, myrmecochory and nutritional specializations in plants. New Caledonian vegetation also exhibits several types of monodominant formations that reveal the importance of disturbances on the island. Fires and tropical storms are likely to be important factors that contribute to the dynamic of New Caledonian ecosystems. Although naturally infertile, there is archaeological evidence that humans developed specific horticultural practices in the ultramafic landscapes of New Caledonia. Further comparisons between New Caledonia and other areas of the world, such as South Africa and Southwest Australia, are desirable, to develop the OCBIL theory into a more robust and generalized, testable framework and to determine the most efficient strategies to preserve their outstanding biodiversity.

Little is known regarding how the increased diversity of nitrogen-fixing bacteria contributes to the productivity and diversity of plants in complex communities. However, some authors have shown that the presence of a diverse group of nodulating bacteria is required for different plant species to coexist. A better understanding of the plant symbiotic organism diversity role in natural ecosystems can be extremely useful to define recovery strategies of environments that were degraded by human activities. This study used ARDRA, BOX-PCR fingerprinting and sequencing of the 16S rDNA gene to assess the diversity of root nodule nitrogen-fixing bacteria in former bauxite mining areas that were replanted in 1981, 1985, 1993, 1998, 2004 and 2006 and in a native forest. Among the 12 isolates for which the 16S rDNA gene was partially sequenced, eight, three and one isolate(s) presented similarity with sequences of the genera Bradyrhizobium, Rhizobium and Mesorhizobium, respectively. The richness, Shannon and evenness indices were the highest in the area that was replanted the earliest (1981) and the lowest in the area that was replanted most recently (2006).

Acacia spirorbis subsp. spirorbis Labill. is a widespread tree legume endemic to New Caledonia that grows in ultramafic (UF) and volcano-sedimentary (VS) soils. The aim of this study was to assess the symbiotic promiscuity of A. spirorbis with nodulating and nitrogen-fixing rhizobia in harsh edaphic conditions. Forty bacterial strains were isolated from root nodules and characterized through (i) multilocus sequence analyses, (ii) symbiotic efficiency and (iii) tolerance to metals. Notably, 32.5% of the rhizobia belonged to the Paraburkholderia genus and were only found in UF soils. The remaining 67.5%, isolated from both UF and VS soils, belonged to the Bradyrhizobium genus. Strains of the Paraburkholderia genus showed significantly higher nitrogen-fixing capacities than those of Bradyrhizobium genus. Strains of the two genera isolated from UF soils showed high metal tolerance and the respective genes occurred in 50% of strains. This is the first report of both alpha- and beta-rhizobia strains associated to an Acacia species adapted to UF and VS soils. Our findings suggest that A. spirorbis is an adaptive plant that establishes symbioses with whatever rhizobia is present in the soil, thus enabling the colonization of contrasted ecosystems.

Island biogeography theory is one of the most influential paradigms in ecology. That island characteristics, including remoteness, can profoundly modulate biological diversity has been borne out by studies of animals and plants. By contrast, the processes influencing microbial diversity in island systems remain largely undetermined. We sequenced arbuscular mycorrhizal (AM) fungal DNA from plant roots collected on 13 islands worldwide and compared AM fungal diversity on islands with existing data from mainland sites. AM fungal communities on islands (even those >6000 km from the closest mainland) comprised few endemic taxa and were as diverse as mainland communities. Thus, in contrast to patterns recorded among macro-organisms, efficient dispersal appears to outweigh the effects of taxogenesis and extinction in regulating AM fungal diversity on islands. Nonetheless, AM fungal communities on more distant islands comprised a higher proportion of previously cultured and large-spored taxa, indicating that dispersal may be human-mediated or require tolerance of significant environmental stress, such as exposure to sunlight or high salinity. The processes driving large-scale patterns of microbial diversity are a key consideration for attempts to conserve and restore functioning ecosystems in this era of rapid global change.

This study aims to characterize the ectomycorrhizal (ECM) communities associated with Acacia spirorbis, a legume tree widely spread in New Caledonia that spontaneously grows on contrasted edaphic constraints, i.e. calcareous, ferralitic and volcano-sedimentary soils. Soil geochemical parameters and diversity of ECM communities were assessed in 12 sites representative of the three mains categories of soils. The ectomycorrhizal status of Acacia spirorbis was confirmed in all studied soils, with a fungal community dominated at 92% by Basidiomycota, mostly represented by/tomentella-thelephora (27.6%), /boletus (15.8%), /sebacina (10.5%), /russula-lactarius (10.5%) and /pisolithus-scleroderma (7.9%) lineages. The diversity and the proportion of the ECM lineages were similar for the ferralitic and volcano-sedimentary soils but significantly different for the calcareous soils. These differences in the distribution of the ECM communities were statistically correlated with pH, Ca, P and Al in the calcareous soils and with Co in the ferralitic soils. Altogether, these data suggest a high capacity of A. spirorbis to form ECM symbioses with a large spectrum of fungi regardless the soil categories with contrasted edaphic parameters.

Acacia spirorbis subsp. spirorbis Labill. is a legume tree that grows on calcareous, metalliferous and siliceous soils and is endemic to New Caledonia. The present study assesses the nitrogen-fixing potential of A. spirorbis in its contrasted natural environments. Soil samples, the leaves of A. spirorbis and of co-occurring non-N2-fixing reference plant species were collected from nine study sites across New Caledonia. Soil properties were analyzed by ICP-AES. Nitrogen (N) content and natural 15N abundance (δ15N) were measured in A. spirorbis and in reference plants. The percentage of N derived from the atmosphere (%Ndfa) fixed by A. spirorbis was also assessed and correlated with soil parameters. Remarkably, mean N contents in A. spirorbis showed no significant differences whatever the soil categories (20.2 ± 3.5 g kg−1) whereas major differences were found in N contents in the reference plants. The average δ15N value of A. spirorbis was close to zero (+0.29‰), while that of the reference species ranged from −4.83 to +7.05‰. In eight of the nine sites studied, A. spirorbis %Ndfa was above 70%, regardless of the soil category. Unlike in the reference species, no statistical correlations were found between A. spirorbis N content and δ15N abundance and soil parameters. Despite the difficulty of using the δ15N abundance method in such contrasted environments, the resulting data show that A. spirorbis has the highest N2-fixing potential yet recorded in the genus Acacia and its nitrogen fixation is effective almost throughout its natural range.

Arbuscular mycorrhizal fungi (AMF) are obligatory biotrophs that have a symbiotic evolutionary relationship with about 80% of all terrestrial plant species. The fungus mainly supplies water and nutrients to the plant and receives photoassimilates. The AMF diversity affects both the competition among species and floristic composition of an area. Fluctuations in the population of this group of microorganisms can cause fluctuations in plant populations above ground. In this work, the AMF community profile in areas with different ages of revegetation was evaluated by morphological identification of spores and denaturing gradient gel electrophoresis (DGGE) techniques. We found 12 AMF species and dominance of the species Glomus macrocarpum determined by spore density. Since the richness level observed in each plot was low and there was predominance of one species of AMF, it can be concluded that these areas still present a high degree of disturbance. It was possible to detect complex band profiles by DGGE analyses for the two plant species studied, Visnia latifolia and Cecropia hololeuca. No relationship between AMF diversity and revegetation time was observed in these areas. Key words: Denaturing gradient gel electrophoresis (DGGE), symbiosis, succession, survey.

Nickel (Ni)-tolerant ectomycorrhizal Pisolithus albus was isolated from extreme ultramafic soils that are naturally rich in heavy metals. This study aimed to identify the specific molecular mechanisms associated with the response of P. albus to nickel. In presence of high concentration of nickel, P. albus Ni-tolerant isolate showed a low basal accumulation of nickel in its fungal tissues and was able to perform a metal efflux mechanism. Three genes putatively involved in metal efflux were identified from the P. albus transcriptome, and their overexpression was confirmed in the mycelium that was cultivated in vitro in the presence of nickel and in fungal tissues that were sampled in situ. Cloning these genes in yeast provided significant advantages in terms of nickel tolerance (+ 31% Ni EC50) and growth (+ 83% u) compared with controls. Furthermore, nickel efflux was also detected in the transformed yeast cells. Protein sequence analysis indicated that the genes encoded a P-type-ATPase, an ABC transporter and a major facilitator superfamily permease (MFS). This study sheds light on a global mechanism of metal efflux by P. albus cells that supports nickel tolerance. These specific responses to nickel might contribute to the fungal adaptation in ultramafic soil. (Resume d'auteur)

International audience; Ectomycorrhizal fungi (ECM) isolates of Pisolithus albus (Cooke and Massee) from nickel-rich ultramafic topsoils in New Caledonia were inoculated onto Acacia spirorbis Labill. (an endemic Fabaceae) and Eucalyptus globulus Labill. (used as a Myrtaceae plant host model). The aim of the study was to analyze the growth of symbiotic ECM plants growing on the ultramafic substrate that is characterized by high and toxic metal concentrations i.e. Co, Cr, Fe, Mn and Ni, deficient concentrations of plant essential nutrients such as N, P, K, and that presents an unbalanced Ca/Mg ratio (1/19). ECM inoculation was successful with a plant level of root mycorrhization up to 6.7%. ECM symbiosis enhanced plant growth as indicated by significant increases in shoot and root biomass. Presence of ECM enhanced uptake of major elements that are deficient in ultramafic substrates; in particular P, K and Ca. On the contrary, the ECM symbioses strongly reduced transfer to plants of element in excess in soils; in particular all metals. ECM-inoculated plants released metal complexing molecules as free thiols and oxalic acid mostly at lower concentrations than in controls. Data showed that ECM symbiosis helped plant growth by supplying uptake of deficient elements while acting as a protective barrier to toxic metals, in particular for plants growing on ultramafic substrate with extreme soil conditions. Isolation of indigenous and stress-adapted beneficial ECM fungi could serve as a potential tool for inoculation of ECM endemic plants for the successful restoration of ultramafic ecosystems degraded by mining activities.

New Caledonian serpentine (ultramafic) soils contain high levels of toxic heavy metals, in particular nickel, (up to 20 g kg−1) and are deficient in essential elements like carbon, nitrogen and phosphorus while having a high magnesium/calcium ratio. Although previous studies showed that ectomycorrhizal symbioses could play an important role in the adaptation of the endemic plants to ultramafic soils (FEMS Microbiol Ecol 72:238–49, 2010), none of them have compared the diversity of microbial communities from ultramafic vs non-ultramafic soils in New Caledonia. We explored the impact of edaphic characteristics on the diversity of ectomycorrhizal (ECM) fungi associated with different endemic species of Tristaniopsis (Myrtaceae) growing under contrasting soil conditions in the natural ecosystems of New Caledonia. ECM root tips were thus sampled from two different ultramafic sites (Koniambo massif and Desmazures forest) vs two volcano-sedimentary ones (Arama and Mont Ninndo). The molecular characterization of the ECM fungi through partial sequencing of the ITS rRNA gene revealed the presence of different dominant fungal genera including, both soil types combined, Cortinarius (36.1%), Pisolithus (18.5%), Russula (13.4%), Heliotales (8.2%) and Boletellus (7.2%). A high diversity of ECM taxa associated with Tristaniopsis species was found in both ultramafic and volcano-sedimentary soils but no significant differences in ECM genera distribution were observed between both soil types. No link could be established between the phylogenetic clustering of ECM taxa and their soil type origin, thus suggesting a possible functional—rather than taxonomical—adaptation of ECM fungal communities to ultramafic soils.

International audience; Mycorrhizal symbiosis is extremely important for tree growth, survival and resistance after transplantation particularly in Madagascar where deforestation is a major concern. The importance of mycorrhizal symbiosis is further increased when soil conditions at the planting site are limiting. To identify technical itineraries capable of improving ecological restoration in Madagascar, we needed to obtain native ectomycorrhizal (ECM) saplings with a wide diversity of ECM fungi. To this end, we transplanted ECM seedlings from the wild (wildlings) to a nursery. Using molecular characterisation of internal transcribed spacer (ITS) rDNA, we tested the effect of transplanting Asteropeia mcphersonii wildlings on ECM communities after 8 months of growth in the nursery. With or without the addition of soil from the site where the seedlings were sampled to the nursery substrate, we observed a dramatic change in the composition of fungal communities with a decrease in the ECM infection rate, a tremendous increase in the abundance of an operational taxonomic unit (OTU) taxonomically close to the order Trechisporales and the disappearance of all OTUs of Boletales. Transplanting to the nursery and/or to nursery conditions was shown to be incompatible with the survival and even less with the development in the nursery of most ECM fungi naturally associated with A. mcphersonii wildings.

Podoserpula miranda sp. nov. (Amylocorticiales) is described from an old Arillastrum gummifera stand in New Caledonia.

The main goal of this work was to describe the mycorrhizal status, AM or ECM or dual AM and ECM or nonmycorrhizal (NM), of different trees species in five coastal humid forests located along the eastern coast of Madagascar. Identification of the plants was carried out by carefully tracing the root system in the field. Several field collection surveys were under-taken and the biological material was examined in laboratory conditions to check the mycorrhizal status of roots and to identify sporophores. (Resume d'auteur)

The taxonomic status of eleven rhizospheric bacterial strains belonging to the genus Burkholderia and isolated from roots of Costularia (Cyperaceae), tropical herbaceous pioneer plants growing on ultramafic soils in New Caledonia, was investigated using a polyphasic taxonomic approach. The genetic analyses (16S rRNA genes, gyrB, recA, nreB and cnr) confirmed that all strains are Burkholderia and cluster into two separated groups. The DNA hybridization results showed low relatedness values to the closest relatives Burkholderia species. The phenotypic analyses confirmed that the two groups of strains could be differentiated from each other and from other known Burkholderia species. This polyphasic study revealed that these two groups of strains represent each a novel species of Burkholderia, for which the names Burkholderia novacaledonica sp. nov. (type strain STM10272(T)=LMG28615(T)=CIP110887(T)) and B. ultramafica sp. nov. (type strain STM10279(T)=LMG28614(T)=CIP110886(T)) are proposed, respectively. These strains of Burkholderia presented specific ecological traits such as the tolerance to the extreme edaphic constraints of ultramafic soils: they grew at pH between 4 and 8 and tolerate the strong unbalanced Ca/Mg ratio (1/19) and the high concentrations of heavy metals i.e. Co, Cr, Mn and Ni. Noteworthy B. ultramafica tolerated nickel until 10mM and B. novacaledonica up to 5mM. The presence of the nickel (nreB) and cobalt/nickel (cnr) resistance determinants encoding for protein involved in metal tolerance was found in all strains of both groups. Moreover, most of the strains were able to produce plant growth promoting molecules (ACC, IAA, NH3 and siderophores). Such ecological traits suggest that these new species of Burkholderia might be environmentally adaptable plant-associated bacteria and beneficial to plants.

Microbial species richness and assemblages across ultramafic ecosystems were investigated to assess the relationship between their distributional patterns and environmental traits. The structure of microorganism communities in the Koniambo massif, New Caledonia, was investigated using a metagenetic approach correlated with edaphic and floristic factors. Vegetation cover and soil properties significantly shaped the large phylogenetic distribution of operational taxonomic unit within microbial populations, with a mean per habitat of 3,477 (± 317) for bacteria and 712 (± 43) for fungi. Using variance partitioning, we showed that the effect of aboveground vegetation was the most significant descriptor for both bacterial and fungal communities. The floristic significant predictors explained 43% of the variation both for the bacterial and fungal community structures, while the edaphic significant predictors explained only 32% and 31% of these variations, respectively. These results confirm the previous hypothesis, that the distribution of microorganisms was more structured by the vegetation cover rather than the edaphic characteristics and that microbial diversity is not limited in ultramafic ecosystems.

Forest ecosystem disturbances generate changes including soil changes, generally with loss of topsoil and loss of much of the diversity of microorganisms, which are usually replaced by others that are more adapted to the new conditions. We describe the effect of rainforest clear-cutting in Madagascar, with stripping of surface soil horizons among ectomycorrhizal fungal communities associated with natural Asteropeia mcphersonii seedlings. An experimental device was placed on four sites, each including an undisturbed plot with litter and a plot in clear-cut areas on a soil composed of a homogeneous mineral substrate. On each of these eight plots the ectomycorrhizal infection rate was determined and root tips were collected to sequence the internal transcribed spacer. 219 sequences grouped into 59 operational taxonomic units (OTUs) were obtained. The main taxonomic groups were Russulaceae, Boletales and Thelephoraceae. Marked spatial aggregation was observed for OTUs. The disturbance significantly affected the ectomycorrhizal infection rate of A. mcphersonii seedlings and the OTU composition of mycorrhizal communities. However, some OTUs were found in both types of environments. We discuss the effect of the disturbance on mycorrhizal communities associated with natural ectomycorrhizal A. mcphersonii seedlings, particularly in view of its application to ecological restoration of disturbed forest environments.

Aims: In order to depict the fine interactions that lead to nodulation, absolute microbiological control of the symbiotic partners is required, i.e. the ability to obtain in vitro axenic nodulation, a condition that has never been fulfilled with the Casuarina–Frankia symbiosis. The effects of culture conditions on plant growth and nodule formation by Casuarina cunninghamiana were investigated. Methods and Results: Axenic (capped tubes with different substrates), and nonaxenic cultures (Gibson tubes, pot cultures) were tested. In axenic conditions, C. cunninghamiana, inoculated with Frankia, had poor growth and did not form nodules at 6 weeks. Plants cultivated in Gibson tubes reached the four axillary shoots stage within 6 weeks and formed nodules 4 weeks after inoculation. Sand-pot cultures allowed us to relate the plant development stage at inoculation with nodulation. Conclusions: The sterile replacement of the cap by a plastic bag increased plant growth and enabled nodule formation 6 weeks after inoculation. The new system of plant culture allows the axenic nodule formation 6 weeks after inoculation. Nodulation behaviour is related to plant development and confinement. Significance and Impact of the Study: This axenic plant nodulation system is of major interest in analysing the roles of Frankia genes in nodulation pathways.

Nickel (Ni)-tolerant ectomycorrhizal Pisolithus albus was isolated from extreme ultramafic soils that are naturally rich in heavy metals. This study aimed to identify the specific molecular mechanisms associated with the response of P. albus to nickel. In presence of high concentration of nickel, P. albus Ni-tolerant isolate showed a low basal accumulation of nickel in its fungal tissues and was able to perform a metal efflux mechanism. Three genes putatively involved in metal efflux were identified from the P. albus transcriptome, and their overexpression was confirmed in the mycelium that was cultivated in vitro in the presence of nickel and in fungal tissues that were sampled in situ. Cloning these genes in yeast provided significant advantages in terms of nickel tolerance (+ 31% Ni EC50) and growth (+ 83% μ) compared with controls. Furthermore, nickel efflux was also detected in the transformed yeast cells. Protein sequence analysis indicated that the genes encoded a P-type-ATPase, an ABC transporter and a major facilitator superfamily permease (MFS). This study sheds light on a global mechanism of metal efflux by P. albus cells that supports nickel tolerance. These specific responses to nickel might contribute to the fungal adaptation in ultramafic soil.

Acacia mangium grown in aeroponic culture was co-inoculated with selected strains of Bradyrhizobium sp. and Glomus intraradices. A single-step technique using alginate as an embedding and sticking agent for an inoculum composed of arbuscular mycorrhiza (AM)-infected sheared roots was used to infect plants. This method resulted in the successful establishment of AM in 100% of the inoculated plants after 7 weeks. The results indicated that dual microbial inoculation with Glomus intraradices strain S-043 and Bradyrhizobium strain AUST 13C stimulated the growth of A. mangium in aeroponic culture. The effects of single and dual microbial inoculations were also evaluated at two levels of P in the nutrient medium. A concentration of 5 mg P kg-1 stimulated the development of AM without affecting plant development or establishment of Bradyrhizobium symbiosis. In contrast, saplings supplemented with a higher concentration of P (25 mg kg-1) alone or co-inoculated with Bradyrhizobium had lower AM frequencies.

Phylogenetic studies comparing the Dipterocarpaceae and the Sarcolaenaceae, a tree family endemic to Madagascar, have shown that the Sarcolaenaceae share a common ancestor with Asian dipterocarps. This suggests that Asian dipterocarps drifted away from Madagascar with the India-Seychelles landmass and then dispersed through Asia. Although all dipterocarps examined so far have been found to be ectomycorrhizal, the ectomycorrhizal status of Sarcolaenaceae had not been investigated. Here we establish the ectomycorrhizal status of Sarcolaenaceae using histological and molecular methods. This indicates that the common ancestor of the Sarcolaenaceae and Asian dipterocarps was ectomycorrhizal, at least before the separation of the Madagascar-India landmass, 88 million years ago.

Nous avons effectue des inoculations ponctuelles sur le systeme racinaire de Medicago laciniata, cultivee in vitro La bacterie symbiotique Sinorhizobium meliloti et le champignon mycorhizien arbusculaire Glomus sp. S043 ont ete utilises. Nous avons etudie la reponse chimique du tissu racinaire en presence et en l'absence de la bacterie ou du champignon par observation en microscopie visible et a epi-fluorescence. L'inoculation ponctuelle de Sinorhizobium meliloti ou Glomus sp. S043 induit une accumulation de flavonoides localisee autour du point d'inoculation. Les autres zones de la racine ne presentent aucune accumulation de flavonoides. Il n'a ete observe d'accumulation de tannins ni chez le controle ni dans les racines inoculees. Le role de ces flavonoides est discute. (Resume d'auteur)

International audience; Ecological restoration in severely disturbed environments can fail because of lack of knowledge of the functioning of the original ecosystem. Nevertheless, facilitating establishment between plant species can help accelerate ecological succession, especially in stressful environments. Mycorrhizal symbiosis plays a key role in plant growth, particularly in harsh environments, and could also play a role in facilitation between plants, as mycorrhizal fungi can form a mycelial network that simultaneously interacts with the root systems of several plant species. In a high-elevation Malagasy tropical rainforest on acidic and iron-rich soil surrounding an active mining site, four genera of ectomycorrhizal plants are locally abundant: Leptolaena, Sarcolaena, Uapaca and Asteropeia. A floristic survey showed that only Asteropeia seedlings can grow on bare soil. Molecular analysis of ectomycorrhizal fungi ITS rDNA enabled us to describe ectomycorrhizal communities and their distribution among these four plant genera. Russulaceae, Boletales, Cortinariaceae and Thelephoraceae are abundant in these forests. There is extensive sharing between ectomycorrhizal communities associated with Asteropeia mcphersonii and other ectomycorrhizal plants. There are also many mycorrhizal fungi species which are common to ectomycorrhizal communities of seedlings and adult trees. This abundance of generalist fungi allows us to envisage the use of A. mcphersonii in the ecological restoration of the mine site. Planting ectomycorrhizal fungi in the bare soil at the beginning of ecological restoration could allow them to grow, thereby establishing a source of inoculum to colonize other ectomycorrhizal plants and consequently facilitate their establishment.

Le projet ECOMINE BIOTOP est centré sur le fonctionnement des topsoils et leur gestion lors de l’exploitation minière. Il propose à la fois des petit Poum - Verse Fredoexpérimentations sur le terrain, des travaux de recherche in situ et des expérimentations en serre et en laboratoire sur les topsoils, la caractérisation et le suivi de la banque de graines, l’activité microbienne, le potentiel mycorhizogène et son amélioration par inoculation. Les résultats obtenus permettent d’améliorer la gestion et l’utilisation des topsoils miniers néocalédoniens. Une étude sur la maîtrise de la diversité génétique des espèces végétales complète le projet. Ce programme aboutira à des recommandations pratiques sur les méthodes de gestion du topsoil, de mycorhizogène des plantes, de l’activité microbienne et de gestion de l’activité microbienne et de la dynamique des métaux en relation avec l’activité microbienne pour la restauration écologique après exploitation minière. Ce programme aboutit à des recommandations pratiques sur l’utilisation et la gestion du topsoil dans une optique de restauration écologique.

Metal-rich soils, especially heavy metal-polluted soils and ultramafic soils, are generally toxic to non-adapted plants and microorganisms. The role of mycorrhizal fungi in the metal tolerance of adapted plant species has become clear in the last decade. This review aims to synthesize the findings of representative studies of the effects of mycorrhizas on the alleviation of heavy metal toxicity on plants and on the absorption/accumulation of heavy metals in their roots and shoots. The adaptation to heavy metals by mycorrhizal symbionts is associated with their efficiency in metal-rich soils. More than 80 % of the studies have indicated a positive role of mycorrhizal fungi in the adaptation of plants to heavy metals in these soils, but the relationships between plant tolerance to heavy metals and the absorption of metals are complex and depend on a range of biological, physical and chemical factors.

Rhizosphere bacteria were isolated from Costularia spp., pioneer sedges from ultramafic soils in New Caledonia, which is a hotspot of biodiversity in the South Pacific. Genus identification, ability to tolerate edaphic constraints, and plant-growth-promoting (PGP) properties were analysed. We found that 105colony-forming units per gram of root were dominated by Proteobacteria (69%) and comprised 21 genera, including Burkholderia (28%), Curtobacterium (15%), Bradyrhizobium (9%), Sphingomonas (8%), Rhizobium (7%), and Bacillus (5%). High proportions of bacteria tolerated many elements of the extreme edaphic conditions: 82% tolerated 100 μmol·L–1chromium, 70% 1 mmol·L–1nickel, 63% 10 mmol·L–1manganese, 24% 1 mmol·L–1cobalt, and 42% an unbalanced calcium/magnesium ratio (1/16). These strains also exhibited multiple PGP properties, including the ability to produce ammonia (65%), indole-3-acetic acid (60%), siderophores (52%), and 1-aminocyclopropane-1-carboxylate (ACC) deaminase (39%); as well as the capacity to solubilize phosphates (19%). The best-performing strains were inoculated with Sorghum sp. grown on ultramafic substrate. Three strains significantly enhanced the shoot biomass by up to 33%. The most successful strains influenced plant nutrition through the mobilization of metals in roots and a reduction of metal transfer to shoots. These results suggest a key role of these bacteria in plant growth, nutrition, and adaptation to the ultramafic constraints.

The fungus Pisolithus albus forms ectomycorrhizal (ECM) associations with plants growing on extreme ultramafic soils, which are naturally rich in heavy metals such as nickel. Both nickel-tolerant and nickel-sensitive isolates of P. albus are found in ultramafic soils in New Caledonia, a biodiversity hotspot in the Southwest Pacific. The aim of this work was to monitor the expression of genes involved in the specific molecular response to nickel in a nickel-tolerant P. albus isolate. We used pyrosequencing and quantitative polymerase chain reaction (qPCR) approaches to investigate and compare the transcriptomes of the nickel-tolerant isolate MD06-337 in the presence and absence of nickel. A total of 1,071,375 sequencing reads were assembled to infer expression patterns of 19,518 putative genes. Comparison of expression levels revealed that 30% of the identified genes were modulated by nickel treatment. The genes, for which expression was induced most markedly by nickel, encoded products that were putatively involved in a variety of biological functions, such as the modification of cellular components (53%), regulation of biological processes (27%) and molecular functions (20%). The 10 genes that pyrosequencing analysis indicated were induced the most by nickel were characterized further by qPCR analysis of both nickel-tolerant and nickel-sensitive P. albus isolates. Five of these genes were expressed exclusively in nickel-tolerant isolates as well as in ECM samples in situ, which identified them as potential biomarkers for nickel tolerance in this species. These results clearly suggest a positive transcriptomic response of the fungus to nickel-rich environments. The presence of both nickel-tolerant and nickel-sensitive fungal phenotypes in ultramafic soils might reflect environment-dependent phenotypic responses to variations in the effective concentrations of nickel in heterogeneous ultramafic habitats.

Rhizobia are soil bacteria able to develop a nitrogen-fixing symbiosis with legumes. They are taxonomically spread among the alpha and beta subclasses of the Proteobacteria. Mimosa pudica, a tropical invasive weed, has been found to have an affinity for beta-rhizobia, including species within the Burkholderia and Cupriavidus genera. In this study, we describe the diversity of M. pudica symbionts in the island of New Caledonia, which is characterized by soils with high heavy metal content, especially of Ni. By using a plant-trapping approach on four soils, we isolated 96 strains, the great majority of which belonged to the species Cupriavidus taiwanensis (16S rRNA and recA gene phylogenies). A few Rhizobium strains in the newly described species Rhizobium mesoamericanum were also isolated. The housekeeping and nod gene phylogenies supported the hypothesis of the arrival of the C. taiwanensis and R. mesoamericanum strains together with their host at the time of the introduction of M. pudica in New Caledonia (NC) for its use as a fodder. The C. taiwanensis strains exhibited various tolerances to Ni, Zn and Cr, suggesting their adaptation to the specific environments in NC. Specific metal tolerance marker genes were found in the genomes of these symbionts, and their origin was investigated by phylogenetic analyses.

Use of the Australian genusEucalyptusin short rotation plantations in Africa and Madagascar has developed over the last century to such an extent that it is becoming the most frequently planted genus in Africa. In order to find ecologically well-adapted eucalypts, foresters have tested different species of various origins and the number of testedEucalyptusspecies now exceeds 150 in Africa. Due to the ability of eucalypts to naturally form ectomycorrhizae, even in the absence of any controlled introduction of compatible ectomycorrhizal fungal partners, their introduction in new ecosystems has direct consequences for ectomycorrhizal fungus communities. A bibliographical compilation, together with original field observations on putative ectomycorrhizal fungi associated with eucalypts in Africa and in Madagascar, has been drawn up in two lists: one for Africa and one for Madagascar where surprisingly high fungal diversity was observed. The level of diversity, the putative origin of the fungi, and their potential impact on native ectomycorrhizal fungi are discussed. The development of eucalypts plantations will inexorably lead to the increase of exotic fungal species being potentially invasive in the considered region.

International audience; In New Caledonia, a hot spot of biodiversity, plants from the Cyperaceae family are mostly endemic and considered pioneers of the nickel-rich natural serpentine ecosystem. The aim of the study was to highlight the mycorrhizal status of these Cyperaceae and to bring new insights into the role of this symbiosis in plant tolerance to ultramafic soils. Nine Cyperaceae species were studied and presented evidence of root colonization by arbuscular mycorrhizas (AMs), with frequencies ranging from 8% to 57%. The highest level of AM colonization was observed in plants from the endemic dominant genus Costularia. Molecular evidence demonstrated the presence of Glomus species inside the roots. In a controlled greenhouse assay, AM inoculation of Costularia comosa grown under ultramafic conditions significantly enhanced plant growth, with an increase in biomass by up to 2.4-fold for shoots and 1.2-fold for roots, and also reduced nickel content in roots by 2.5-fold, as compared with the controls. All these data support our hypotheses (i) that a relationship exists between the mycorrhizal status of Cyperaceae and their habitat, and (ii) that AM have a positive role in plant tolerance to ultramafic soils (mineral nutrition and metal tolerance), suggesting the use of these pioneer plants with AM management as potential tools for nickel mine site rehabilitation in New Caledonia.; En Nouvelle-Calédonie, un point chaud de la biodiversité, les plantes appartenant à la famille des Cypéracées sont principalement endémiques et sont considérées comme pionnières de l’écosystème naturel de la serpentine riche en nickel. Cette étude visait à éclaircir le statut mycorhizien de ces Cypéracées et d’apporter un nouvel aperçu de rôle de cette symbiose dans la tolérance des végétaux aux sols ultrabasiques. Neuf espèces de Cypéracées ont été étudiées et la colonisation des racines par des mycorhizes arbusculaires (MA) a été mise en évidence avec des fréquences variant de 8 % à 57 %. Le niveau de MA le plus élevé a été noté chez les plantes du genre endémique dominant Costularia. Les données moléculaires ont démontré la présence de Glomus sp. à l’intérieur des racines. Lors d’un essai en serre contrôlée, l’inoculation de MA chez Costularia comosa cultivée en conditions ultrabasiques a significativement accru la croissance des plants, avec une augmentation de la biomasse des pousses de 2,4 fois et des racines de 1,2 fois, réduisant aussi le contenu en nickel des racines de 2,5 fois, tout cela par rapport au contrôle. Toutes ces données plaident en faveur de l’hypothèse (i) d’une relation entre le statut en mycorhize des Cypéracées et leur habitat, et (ii) du rôle positif des MA dans la tolérance aux sols ultrabasiques (nutrition minérale et tolérance aux métaux), suggérant que ces plantes pionnières parallèlement à la gestion des MA puissent constituer des outils potentiels de réhabilitation des sites miniers de nickel en Nouvelle-Calédonie.

International audience; Ectomycorrhizal (ECM) Pisolithus albus (Cooke & Massee), belonging to the ultramafic ecotype isolated in nickel-rich serpentine soils from New Caledonia (a tropical hotspot of biodiversity) and showing in vitro adaptive nickel tolerance, were inoculated to Eucalyptus globulus Labill used as a Myrtaceae plant-host model to study ectomycorrhizal symbiosis. Plants were then exposed to a nickel (Ni) dose-response experiment with increased Ni treatments up to 60 mg kg (- 1) soil as extractable Ni content in serpentine soils. Results showed that plants inoculated with ultramafic ECM P. albus were able to tolerate high and toxic concentrations of Ni (up to 60 mu g g (- 1)) while uninoculated controls were not. At the highest Ni concentration tested, root growth was more than 20-fold higher and shoot growth more than 30-fold higher in ECM plants compared with control plants. The improved growth in ECM plants was associated with a 2.4-fold reduction in root Ni concentration but a massive 60-fold reduction in transfer of Ni from root to shoots. In vitro, P. albus strains could withstand high Ni concentrations but accumulated very little Ni in its tissue. The lower Ni uptake by mycorrhizal plants could not be explained by increased release of metal-complexing chelates since these were 5- to 12-fold lower in mycorrhizal plants at high Ni concentrations. It is proposed that the fungal sheath covering the plant roots acts as an effective barrier to limit transfer of Ni from soil into the root tissue. The degree of tolerance conferred by the ultramafic P. albus isolates to growth of the host tree species is considerably greater than previously reported for other ECM. The primary mechanisms underlying this improved growth were identified as reduced Ni uptake into the roots and markedly reduced transfer from root to shoot in mycorrhizal plants. The fact that these positive responses were observed at Ni concentrations commonly observed in serpentinic soils suggests that ultramafic ecotypes of P. albus could play an important role in the adaptation of tree species to soils containing high concentrations of heavy metals and aid in strategies for ecological restoration.

Contact: philippe.jourand@ird.fr; International audience; Isolates of ectomycorrhizal Pisolithus albus were sampled from both ultramafic and volcano-sedimentary soils in New Caledonia, a tropical hotspot of biodiversity, to investigate the relationships between genetic diversity and edaphic constraint through tolerance to nickel (Ni). Carpophore description, spore morphology and phylogenetic analysis based on internal transcribed spacer (ITS) rDNA sequences confirmed that all isolates belong to P. albus and are closely related to other Australasian specimens. Using molecular tools, ITS-restriction fragment length polymorphism and amplified fragment length polymorphism markers, we showed the existence of two distinct genetic clusters within P. albus: ultramafic and volcano-sedimentary. Mycelia response to Ni toxicity supports such a population structure. Pisolithus albus from ultramafic soils included isolates with a high diversity of in vitro Ni tolerance, with both Ni-tolerant isolates (average Ni EC(50) at 575 mu M) and Ni-sensitive isolates (average Ni EC(50) at 37 mu M). In contrast, all isolates from volcano-sedimentary soils were found to be Ni sensitive (average Ni EC(50) at 32 mu M). We highlight that (1) P. albus population from ultramafic soils of New Caledonia are genetically structured in ecotype, and that (2) Ni tolerance among ultramafic isolates suggests an adaptive physiological response to Ni toxicity.

International audience; The phylogenetic relationships among 200 Pisolithus basidiomata collected from pine, oak, and eucalypt forests and rockrose scrubs in Morocco were investigated. Using PCR-RFLP analysis of the internal transcribed spacer (ITS) of ribosomal DNA, this collection could be divided into 5 groups, by using PCR-RFLP analysis of the internal transcribed spacer (ITS) of ribosomal DNA. The ITS of a representative basidioma of each group was sequenced, and a phylogenetic analysis was performed. The dendrogram suggests that at least five Pisolithus species are present in Morocco. Pisolithus basidiomata collected in the Pinus and Quercus forests correspond to Pisolithus arrhizus and P. species 4 as previously described by Martin and colleagues in 2002. Those collected from the eucalyptus forests, under E. gomphocephala and E. camaldulensis, correspond to P. albus and P. microcarpus. Basidiomata collected from the rockrose scrubs, under Cistus crispus, C. monspeliensis or C. salviifolius, are all identified as Pisolithus species 3. Phylogenetic analyses showed that our different Pisolithus grouped well with Pisolithus specimens from other geographical origins.

La Nouvelle-Calédonie est une île du Pacifique sud reconnue comme un " hotspot " de biodiversité. Toutefois, l'inventaire des champignons de ce territoire de 17 000 km2 est encore très fragmentaire ; à peine 1 % des espèces auraient été décrites à ce jour ! Le récent développement d'un programme de recherche sur le rôle des microorganismes symbiotiques, notamment les champignons, dans l'adaptation des plantes aux sols extrêmes et la création de la Société mycologique de Nouvelle-Calédonie ont permis l'identification de plusieurs centaines d'espèces nouvelles pour ce territoire, mais aussi la découverte de nouvelles espèces, dont le spectaculaire Podoserpula miranda nom prov. (Résumé d'auteur)

International audience; Australian acacias like Acacia holosericea are excellent candidates for the revegetation of arid zones in Africa. Their high ability to develop multiple symbioses with soil microorganisms is crucial to their rapid development in adverse climatic and edaphic conditions. These symbioses include nitrogen fixation with rhizobia, vesicular arbuscular mycorrhization and ectomycorrhization. We set up an experiment to test the growth response of A. losericea to ectomycorrhizal inoculation in 13 different soils from Senegal. After autoclaving the soil, the experiment was conducted in a greenhouse for four weeks. Plants were inoculated with Pisolithus albus strain IR100. The following parameters were measured: plant biomass, N, P, K and Ca foliar composition, spontaneous nodulation rate, and ectomycorrhizal colonization. Data were analyzed in light of the physical, chemical and total microbial characteristics of the soil. The results demonstrated a global promoting effect of P. albus inoculation on plant (shoot and root) growth and on foliar P and K, together with a depressive effect on N, while Ca rates were barely affected. Interestingly, spontaneous nodulation with putative water- or airborne rhizobia was stimulated after P. albus inoculation. However, these nodules seemed poorly effective, asthey failed to cause any change in plant growth or in foliar N composition within the nodulated and nonnodulated control plants. These results show that plant growth response to ectomycorrhizal inoculation is greatly dependent on soil characteristics, and that root ectomycorrhizal colonization is influenced by biotic factors such as soil microbiota. From a practical point of view, data from the present study demonstrate that it is possible to optimize the effect of fungi on plant growth by screening soils under nursery conditions.