Rhizobacterial isolates from the metal hyperaccumulator noccaea caerulescens growing in a Ni enriched soil

Introduction: Ultramafic (serpentine) outcrops occupy about 1% of the planet and are constituted by igneous or metamorphic rocks with low content in silicon (< 45 % SiO2) more than 70 % of ferromagnesium minerals and usually with high contents of several toxic trace elements (TEs) (Brooks, 1987). The soils developed over these rocks are characterized by deficiency in organic matter and essential nutrients, unfavourable Ca/Mg ratio, high concentrations of potentially phytotoxic trace metals such as Ni, Co and Cr, etc. which make them a harsh environment for the plant growth (serpentine syndrome). The organisms inhabiting these environments developed numerous adaptation mechanisms, as a result, they host a highly valuable biodiversity. The ultramafic flora includes a particular group of plants called hyperaccumulators, which are able to accumulate in their aboveground tissues extremely high concentrations of Ni. The ultramafic outcrops in the NW of Spain host two endemic TE hyperaccumulators: Noccaea caerulescens (J.Presl & C.Presl) and Odontarrhena serpyllifolia (Desf.) Jord. & Fourr. The project Phy2SUDOE (SOE4/P5/E1021), funded by Interreg SUDOE, works on the establishment and maintenance of phytomanagement options, as well as, on the conservation and valorisation of endemic plant and microbial biodiversity in several degraded areas, among them a serpentine quarry in Bandeira (NW Spain). In the frame of this project, this study presents data relative to the hyperaccumulating characteristics of the population of N. caerulescens growing in the Bandeira quarry, as well as, the first results on the diversity of isolates cultivated from the rhizosphere of individuals of this species. Material and methods: An ultramafic area in Bandeira (42o 45´N; 8o 17´W, NW of Spain) was selected for the study (Fig.1). Several individuals of Noccaea caerulescens were collected in the surroundings of a serpentine quarry (Fig. 2). The rhizosphere soil tightly attached to the roots was carefully separated and extracted with 1% sodium hexametaphosphate solution. Dilutions of the soil extract were cultivated in mineral medium amended with 3 mM Ni and with glucose fructose, gluconate, lactate and pyruvate as C sources. 48 bacterial strains were isolated after repeated subculturing. Partial sequencing of the gene 16S rRNA was used for the identification of isolates. Properties related to plant growth promotion (PGP) and mobilization of TE of the rhizobacterial isolates are being studied (e.g. P solubilization, production of organic acids, siderophores and indoleacetic acid (IAA)). Conclusions: The N. caerulescens growing in the serpentine quarry hyperaccumulates Ni and shows capacity of uptake and translocation of Co and Zn. The N. caerulescens studied seems unsuitable as monocrop in phytomining because of its reduced biomass; however, it is interesting to explore its usefulness in intercropping with other hyperaccumulators. Most of the rhizobacterial isolated obtained from N. caerulescens where classified as Pseudarthrobacter, Streptomyces or Mesorhizobium, genera which include known PGP strains. The identified isolates with PGP and TE mobilization capacities may have potential application in phytomining or in other biotechnologies aiming at the mobilization of valuables elements from soils and other TE enriched substrates.