Short Communication Hydrolysis of fenamiphos and its toxic oxidation products by Microbacterium sp. in pure culture and groundwater Tanya. P. Cáceres, Mallavarapu Megharaj * , Seidu Malik, Michael Beer, Ravi Naidu CERAR, Centre for Environmental Risk Assessment and Remediation, University of South Australia, Mawson Lakes Boulevard SA 5095; Cooperative Research Centre for Contamination Assessment and Remediation of Environments (CRC CARE), University of South Australia, Mawson Lakes SA 5095, Australia article info Article history: Received 3 October 2008 Received in revised form 22 December 2008 Accepted 22 December 2008 Available online 4 February 2009 Keywords: Fenamiphos Fenamiphos sulfoxide Fenamiphos sulfone Microbacterium sp. Biodegradation abstract A bacterium with an exceptional ability to hydrolyse fenamiphos and its toxic oxidation products fenam- iphos sulfoxide and fenamiphos sulfone, all possessing PAOAC bond was isolated from soil. Based on 16S rRNA gene determination, this bacterium was putatively identified as Microbacterium esteraromaticum. The phenols (fenamiphos phenol, sulfoxide phenol and sulfone phenol) formed during bacterial hydroly- sis resisted further degradation in mineral salts medium and sterile groundwater, but were transitory in non-sterile groundwater due to the catabolism of native microorganisms. Also, the cell-free preparation of this bacterium was highly effective in hydrolysing fenamiphos and its oxides. These results demon- strate the potential of this bacterium to detoxify pesticide waste in the environment including the groundwater. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Organophosphorus compounds are the most widely used insec- ticides, accounting for an estimated 34% of world-wide insecticide sales (Singh and Walker, 2006). Fenamiphos (ethyl 4-methylthio- m-tolyl isopropylphosphoramidate), an organophosphorus pesti- cide is extensively used as a systemic and contact insecticide against soil nematodes in golf greens and horticultural crops all over the world. This pesticide is applied at various stages of plant growth such as pre-planting, at-planting, pre- and post-harvest on a variety of plants including tobacco, turf, bananas, pineapples, citrus and other fruit vines, vegetables, and grains (Kamrin, 1997). Generally, under environmental conditions, fenamiphos can be oxidized primarily to fenamiphos sulfoxide (FSO) followed by fur- ther oxidation to fenamiphos sulfone (FSO 2 )(Ou et al., 1994; Các- eres et al., 2008). These oxidation products have nematicidal activity and toxicity similar to the parent compound. According to the available data on sorption and leaching studies, fenamiphos can be classified as a chemical with low mobility (Tomlin, 2000). However, it has been observed that this compound as well as the degradation products, fenamiphos sulfoxide and fenamiphos sul- fone, may leach through the soil profile. Residues of these chemi- cals were found in groundwater in Florida, USA (Patrick et al., 2001) as well as in Perth, Australia (Franzmann et al., 2000). Con- tamination of groundwater with fenamiphos up to 2000 lgl 1 due to inappropriate disposal of pesticide containing material has also been reported (Franzmann et al., 2000). Fenamiphos has been re- ported to persist for several years in groundwater (Franzmann et al., 2000) and the calculated half-life for its mineralization in an anaerobic aquifer in Australia was as high as 1000 years (Franz- mann et al., 2000). Also, supplementation of oxygen or bioaugmen- tation could not help to decontaminate fenamiphos contaminated groundwater. In a study of 16 soils originating from 11 countries, its half-life at 22 °C ranged between 12 and 87 days (Simon et al., 1992). However, in soils with a previous history of fenami- phos use, transformations of fenamiphos to equally toxic fenami- phos sulfoxide and fenamiphos sulfone are further enhanced (Ou et al., 1994; Chung and Ou, 1996). Cáceres et al. (2008) reported that fenamiphos sulfoxide was the major degradation product in all the five soils originated from India, Australia and Ecuador stud- ied for fenamiphos degradation. It is important to note that oxida- tion of fenamiphos to its sulfoxide or sulfone is not a detoxification, while its hydrolysis can result in detoxification. Fenamiphos and its major degradation products are considered as highly toxic to fish, aquatic invertebrates and most terrestrial organisms. Several incidents of mortality caused by fenamiphos in birds and fish in USA have been reported (Patrick et al., 2001). In addition, the pes- ticide is classified as extremely toxic due to its potential risk to hu- man health (WHO, 2001). Due to the environmental concern associated with the accumulation of these compounds in soil and ground water, there is a great need to develop safe, convenient and feasible methods for their detoxification. Biodegradation is a reliable, cost effective technique for pesticide removal. Microbial 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.12.043 * Corresponding author. Tel.: +61 8 83025044; fax: +61 8 83023057. E-mail address: Megharaj.Mallavarapu@unisa.edu.au (M. Megharaj). Bioresource Technology 100 (2009) 2732–2736 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech