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Arbuscular mycorrhizal fungi (AMF) are generally involved in the adaptation of native plants to ultramafic soils, especially in New Caledonia. These soils are deficient in major elements, particularly in phosphorus (P), and are rich in potentially toxic metals such as nickel, chromium, cobalt, and manganese. We aimed to test the effects of increasing P doses on mycorrhizal functions of plants grown on P-deficient ultramafic soil. We analyzed the effects of soil P fertilization on growth, mineral nutrition and potentially toxic metal absorption of plants inoculated or not with AMF native isolates. Three endemic plants frequently used in the ecological restoration were tested: Metrosideros laurifolia (Myrtaceae), Alphitonia neocaledonica (Rhamnaceae), and Tetraria comosa (Cyperaceae). They were grown in pots supplied with different doses of P, after being inoculated or not with AMF. P fertilization increased greatly the growth rate of all three species. In pots non-supplied with P, only M. laurifolia showed a higher growth rate when inoculated with AMF, but all plant species showed different positive effects of mycorrhizal symbiosis, such as better mineral nutrition, particularly for potassium (K) and calcium and a higher calcium/magnesium values (Ca/Mg). Mycorrhizal colonization was not reduced by P supply, but the specific positive effects of AMF on growth and mineral nutrition were reduced or suppressed. Negative effects of P fertilization on mycorrhizal functions were induced without reduction of mycorrhizal colonization. As the adaptive traits of the three plants to ultramafic soils were obtained by a reduction of their growth rate, we hypothesized that the high increase of this growth rate induced by P fertilization could have altered this adaptive structure.

The ecological restoration of nickel mining-degraded areas in New Caledonia is strongly limited by low availability of soil mineral nutrients, metal toxicity, and slow growth rates of native plant species. In order to improve plant growth for restoration programs, special attention was paid to interactions between plant and soil microorganisms. In this study, we evaluated the influence of inoculation with Curtobacterium citreum BE isolated from a New Caledonian ultramafic soil on arbuscular mycorrhizal symbiosis and growth of Tetraria comosa, an endemic sedge used in restoration programs. A greenhouse experiment on ultramafic substrate was conducted with an inoculum comprising two arbuscular mycorrhizal fungi (AMF) species isolated from New Caledonian ultramafic soils: Rhizophagus neocaledonicus and Claroideoglomus etunicatum. The effects on plant growth of the AMF and C. citreum BE inoculated separately were not significant, but their co-inoculation significantly enhanced the dry weight of T. comosa compared with the non-inoculated control. These differences were positively correlated with mycorrhizal colonization which was improved by C. citreum BE. Compared with the control, co-inoculated plants were characterized by better mineral nutrition, a higher Ca/Mg ratio, and lower metal translocation. However, for Ca/Mg ratio and metal translocation, there were no significant differences between the effects of AMF inoculation and co-inoculation.

In New Caledonia, one third of the main island is covered by ultramafic soils characterized by high levels of metals and low levels of plant nutrients. In these soils, metal tolerant bacteria may play a role in plant adaptation to the edaphic constraints. Recently, two new bacteria species belonging to the genus Burkholderia sensu lato were isolated from rhizospheric roots of pioneer plant growing in New Caledonian ultramafic soils and were able to tolerate high metal concentrations. The purpose of this study was to determine the major mechanisms by which these two bacteria were able to tolerate metals and if they were able to reduce metal absorption by plants from which they were isolated. Bioaccumulation, adsorption on cell surface, ability to form biofilm and production of exopolysaccharides were assessed. A focus was carried out on Ni, generally the most toxic metal in New Caledonian ultramafic soils. All experiments were performed in comparison with a reference strain isolated from a non-ultramafic habitat, Paraburkholderia caribensis MWAP64T. The two ultramafic bacterial species showed a high tolerance to Ni with a Ni–IC50 of 5 mM for Caballeronia novacaledonica STM10272T and 30 mM for Paraburkholderia ultramafica STM10279T, values significantly higher than those of P. caribensis MWAP64T (2 mM). The major mechanism involved in the bacterial Ni tolerance was related to biofilm formation for C. novacaledonica STM10272T and to acidic exopolysaccharide production for P. ultramafica STM10279T. Both species alleviated metal contents in roots of Tetraria comosa, an endemic pioneer herbaceous species growing on New Caledonian ultramafic soils.

International audience; Inoculation of arbuscular mycorrhizal fungi (AMF) as plant growth promoters has mostly been conducted using single-species inoculum. In this study, we investigated whether co-inoculation of different native AMF species induced an improvement of plant growth in an ultramafic soil. We analyzed the effects of six species of AMF from a New Caledonian ultramafic soil on plant growth and nutrition, using mono-inoculations and mixtures comprising different numbers of AMF species, in a greenhouse experiment. The endemic Metrosideros laurifolia was used as a host plant. Our results suggest that, when the plant faced multiple abiotic stress factors (nutrient deficiencies and high concentrations of different heavy metals), co-inoculation of AMF belonging to different families was more efficient than mono-inoculation in improving biomass, mineral nutrition, Ca/Mg ratio, and tolerance to heavy metals of plants in ultramafic soil. This performance suggested functional complementarity between distantly related AMF. Our findings will have important implications for restoration ecology and mycorrhizal biotechnology applied to ultramafic soils.

International audience; In this study, we examined interactive effects of elevated atmospheric CO2, concentrations, and increased tidal flooding on two mangroves species, Avicennia marina and Rhizophora stylosa. Leaf gas-exchange parameters (photosynthesis, transpiration rates, water-use efficiency, stomatal conductance, and dark respiration rates) were measured monthly on more than 1000 two-year-old seedlings grown in greenhouses for 1 year. In addition, stomatal density and light curve responses were determined at the end of the experiment. Under elevated CO2 concentrations (800 ppm), the net photosynthetic rates were enhanced by more than 37% for A. marina and 45% for R. stylosa. This effect was more pronounced during the warm season, suggesting that an increase in global temperatures would further enhance the photosynthetic response of the considered species. Transpiration rates decreased by more than 15 and 8% for A. marina and R. stylosa, respectively. Consequently, water-use efficiency increased by 76% and 98% for A. marina and R. stylosa, respectively, for both species, which will improve drought resistance. These responses to elevated CO2 were minimized (by 5%) with longer flooding duration. Consequently, future increases of atmospheric CO2 may have a strong and positive effect on juveniles of A. marina and R. stylosa during the next century, which may not be suppressed by the augmentation of tidal flooding duration induced by sea-level rise. It is possible that this effect will enhance seedling dynamic by increasing photosynthesis, and therefore will facilitate their settlements in new area, extending the role of mangrove ecosystems in carbon sequestration and climate change mitigation.

In this study, we examined interactive effects of elevated atmospheric CO

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.

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.

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.