Symbiotic efficiency of autochthonous arbuscular mycorrhizal fungus (G. mosseae) and Brevibacillus sp. isolated from cadmium polluted soil under increasing cadmium levels A. Vivas a , I. Vo¨ ro¨ s b , B. Biro´ b , E. Campos a , J.M. Barea a , R. Azco´ n a, * a Departamento de Microbiologı´a del Suelo y Sistemas Simbio ´ticos, Estacio ´n Experimental del Zaidı´n, CSIC, Profesor Albareda 1, 18008 Granada, Spain b Research Institute for Soil Science and Agricultural Chemistry of the Hungarian Academy of Sciences, Herman O. u ´t 15., Budapest, H-1022, Hungary Received 4 December 2002; accepted 26 May 2003 ‘‘Capsule’’: Selected ubiquitous microorganisms are important components of Cd tolerance in plants. Abstract The effect of inoculation with indigenous naturally occurring microorganisms [an arbuscular mycorrhizal (AM) fungus and rhi- zosphere bacteria] isolated from a Cd polluted soil was assayed on Trifolium repens growing in soil contaminated with a range of Cd. One of the bacterial isolate showed a marked PGPR effect and was identified as a Brevibacillus sp. Mycorrhizal colonization also enhanced Trifolium growth and N, P, Zn and Ni content and the dually inoculated (AM fungus plus Brevibacillus sp.) plants achieved further growth and nutrition and less Cd concentration, particularly at the highest Cd level. Increasing Cd level in the soil decreased Zn and Pb shoot accumulation. Coinoculation of Brevibacillus sp. and AM fungus increased shoot biomass over single mycorrhizal plants by 18% (at 13.6 mg Cd kg 1 ), 26% (at 33.0 mg Cd kg 1 ) and 35% (at 85.1 mg Cd kg 1 ). In contract, Cd transfer from soil to plants was substantially reduced and at the highest Cd level Brevibacillus sp. lowered this value by 37.5% in AM plants. Increasing Cd level highly reduced plant mycorrhization and nodulation. Strong positive effect of the bacterium on nodule formation was observed in all treatments. Results show that selected ubiquitous microorganisms, applied as enriched inocula, are important in plant Cd tolerance and development in Cd polluted soils. # 2003 Elsevier Ltd. All rights reserved. Keywords: Arbuscular mycorrhizal fungus; Rhizobium sp; Brevibacillus sp; Cadmium levels 1. Introduction Arbuscular mycorrhizal (AM) fungi improve nutrient transfer from the soil to the roots of the colonised host plant since the mycorrhizal interface developed between roots and soil is very effective in assimilating nutrients (Barea et al., 1987). Soil contamination by heavy metals is of major importance in industrialized areas. The detrimental effects of heavy metals on soil biochemical and biologi- cal properties have been reported (Ba¨a¨th, 1989). High metal concentrations in soil are toxic to bacteria and fungi but roots of most plant growing in polluted soils are colonised by AM fungi (Shetty et al., 1994a). This is an indication of the ability of AM fungi to develop tol- erance to contaminants. When metals are at toxic concentrations in soil, mycorrhizal rather than non-mycorrhizal host plants are able to colonize these polluted sites (Shetty et al., 1994a, 1994b). Thus, mycorrhizal colonization may be the key to plant survival on contaminated environments by enhancing metal resistance in plants and also by improving essential nutrients uptake. Nevertheless, metal resistance in AM fungi have not been exten- sively investigated in relation to their host plant (Meharg and Cairney, 2000). The co-evolution of both symbionts and other associated microbial groups in metal contaminated soils indicates the existence of adaptation strategies that enable them to survive in soil with toxic concentrations of metals (Biro´ et al., 0269-7491/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0269-7491(03)00195-7 Environmental Pollution 126 (2003) 179–189 www.elsevier.com/locate/envpol * Corresponding author. Tel.: +34-958-18-16-00x298; fax: +34- 958-12-96-00. E-mail address: rosario.azcon@eez.csic.es (R.Azco´n).