ENVIRONMENTAL MICROBIOLOGY Arbuscular Mycorrhizal Fungi, Bacillus cereus, and Candida parapsilosis from a Multicontaminated Soil Alleviate Metal Toxicity in Plants Rosario Azcón & María del Carmen Perálvarez & Antonio Roldán & José-Miguel Barea Received: 31 July 2009 / Accepted: 8 November 2009 / Published online: 16 December 2009 # Springer Science+Business Media, LLC 2009 Abstract We investigated if the limited development of Trifolium repens growing in a heavy metal (HM) multi- contaminated soil was increased by selected native micro- organisms, bacteria (Bacillus cereus (Bc)), yeast (Candida parapsilosis (Cp)), or arbuscular mycorrhizal fungi (AMF), used either as single or dual inoculants. These microbial inoculants were assayed to ascertain whether the selection of HM-tolerant microorganisms can benefit plant growth and nutrient uptake and depress HM acquisition. The inoculated microorganisms, particularly in dual asso- ciations, increased plant biomass by 148% (Bc), 162%, (Cp), and 204% (AMF), concomitantly producing the highest symbiotic (AMF colonisation and nodulation) rates. The lack of AMF colonisation and nodulation in plants growing in this natural, polluted soil was compensated by adapted microbial inoculants. The metal bioaccumulation abilities of the inoculated microorganisms and particularly the microbial effect on decreasing metal concentrations in shoot biomass seem to be involved in such effects. Regarding microbial HM tolerance, the activities of antioxidant enzymes known to play an important role in cell protection by alleviating cellular oxidative damage, such as superoxide dismutase, catalase, glutathione reduc- tase, and ascorbate peroxidase, were here considered as an index of microbial metal tolerance. Enzymatic mechanisms slightly changed in the HM-adapted B. cereus or C. parapsilosis in the presence of metals. Antioxidants seem to be directly involved in the adaptative microbial response and survival in HM-polluted sites. Microbial inoculations showed a bioremediation potential and helped plants to develop in the multicontaminated soil. Thus, they could be used as a biotechnological tool to improve plant develop- ment in HM-contaminated environments. Introduction Heavy metals (HMs) in soil can cause detrimental effects on ecosystems [1]. High concentrations of metals such as zinc, nickel, mercury, cadmium, or copper cause environ- mental pollution because they have a strong persistence. The use of plants for bioremediation purpose is based on their ability to tolerate HMs. Thus, AMF symbiosis and/or saprophytic beneficial microorganisms may contribute to phytoremediation via strategies such as HM sequestration or accumulation, keeping metal concentrations in the plants below critical values and improving plant growth and nutrition [2–5]. Plant and microorganisms living in HM- polluted soils are often adapted to such detrimental conditions, and the use of such organisms to remediate HM-contaminated soils is an attractive possibility for the elimination or transformation of the contaminants [6]. At high concentrations, HMs may be toxic to plants and soil microorganisms, reducing growth. However, adapted and HM-tolerant soil microorganisms can enhance plant growth and nutrition, by acting as plant growth-promoting agents and also by interacting with metals through bioaccumula- tion or biotransformation processes [7]. Thus, plants have R. Azcón (*) : M. del Carmen Perálvarez : J.-M. Barea Departamento de Microbiología del Suelo y Sistemas Simbióticos, Estación Experimental del Zaidín (CSIC), C/Profesor Albareda no. 1, 18008 Granada, Spain e-mail: rosario.azcon@eez.csic.es A. Roldán Department of Soil and Water Conservation, Centro de Edafología y Biología Aplicada del Segura (CSIC), P.O. Box 164, Campus de Espinardo, 30100 Murcia, Spain Microb Ecol (2010) 59:668–677 DOI 10.1007/s00248-009-9618-5