Amelioration of municipal sludge by Pistia stratiotes L.: Role of antioxidant enzymes in detoxification of metals q Anamika Tewari, Ragini Singh, Naveen Kumar Singh, U.N. Rai * Ecotoxicology and Bioremediation, National Botanical Research Institute, Rana Pratap Marg, Lucknow 226 001, India article info Article history: Received 17 December 2007 Received in revised form 7 April 2008 Accepted 8 April 2008 Available online 21 May 2008 Keywords: Municipal sludge Leachate Pistia stratiotes Metals Antioxidant enzymes abstract Pistia stratiotes when exposed to mixture of municipal sludge and effluent accumulated substantial amount of metals in the fronds and roots. With the increase in the metal accumulation by the plants, a reduction in the concentration of metals was found in leachates. The treated plants showed reduced level in chlorophylls but enhanced level of carotenoids and protein. The plant showed a concomitant increase in the activities of antioxidant enzymes; superoxide dismutase, guiacol peroxidase and also an enhanced level of lipid peroxidation. The activities were more in the root tissues as compared to frond. Initially a reduced level of cysteine content in roots of sludge treated plant as compared to control was found, but with time duration it increased progressively. The level of non-protein thiols also increased gradually at all the durations in both fronds and roots. Thus, beside the reduction of metals from muni- cipal sludge, the ability of P. stratiotes to combat metal generated damages by induced synthesis of anti- oxidant enzymes and other metal binding ligands shows its suitability for the phytoremediation of the waste. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The fast industrialization and modernization has led to an unfortunate consequence of production of large amounts of solid waste. Lucknow, the capital of Uttar Pradesh (India) stands third in the per capita generation of municipal solid waste (MSW) amongst the 23 metropolitan cities (CPCB, 1999). These solid wastes are dumped in unsecured landfills or water bodies without any proper treatment leading to the emanation of toxic leachates in surface water. Our study carried on swiss albino male mice dem- onstrated that leachates of municipal sludge (MS) possess geno- toxic contaminants and may exert deleterious effects on populations residing near such unsecured waste dump sites (Te- wari et al., 2005). Sang et al. (2006) studied the cytogenetic dam- age in the root tips of Hordeum vulgare exposed to municipal landfill leachate and concluded that long term exposure to leach- ates even at low concentration may pose potential genotoxic risk to organisms. Leachate emanated from the water percolation is a heterogenous mixture of toxicants and is a potential source of con- tamination to the ground water posing numerous health conse- quences. Therefore, in the present scenario of increasing generation of MS, there is need to renovate such toxicant through some low cost technology. In this context, aquatic plants are well known for their potential to accumulate the heavy metals and they hold a prime position in food chain as primary producers, regulators of oxygen level and maintaining a homeostasis of elements via biogeochemical cycling (Sinha et al., 1996, 2002; Aslan et al., 2003; Rai et al., 2004; Mishra et al., 2006). Heavy metals (Fe, Cu, Ni, Cr, Pb) present in the MS ex- hibit the ability to produce reactive oxygen species (ROS), causing lipid peroxidation, DNA damage, altered calcium homeostasis and depletion of sulphydryls. These metals at supraoptimal condition become phytotoxic and result in the generation of superoxide rad- ical, hydrogen peroxide (H 2 O 2 ) and hydroxyl radical either by elec- tron transfer or by metal mediated inhibition of metabolic reaction (Stohs and Bagchi, 1995). The tissues damaged by oxidative stress cause increased production of carbonylated proteins and malondi- aldehyde (MDA) by the removal of hydrogen from unsaturated fatty acids causing the formation of free radicals and reactive alde- hydes thereby causing distortion of lipid layers and membrane proteins. Plants are surfeited with mechanisms to combat ROS. Plants possess both enzymatic (superoxide dismutase (SOD), cata- lase and ascorbate peroxidase) and non-enzymatic (glutathione, tocopherols, ascorbate and carotenoids) defense systems to protect cells from oxidative stress. Antioxidative enzymes play an impor- tant role in the interruption of uncontrolled oxidation cascades occurring in different cell compartments. SOD enzyme dismutates superoxide radical to H 2 O 2 which is utilized by guiacol peroxidase (GPX) in vacuoles. The other intracellular defense strategy includes production of organic ligands rich in cysteine and non-protein 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.04.018 q NBRI Research Publication No. 590. * Corresponding author. Tel.: +91 522 2205831/32x229; fax: +91 522 2205839. E-mail address: rai_nbrieb@rediffmail.com (U.N. Rai). Bioresource Technology 99 (2008) 8715–8721 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech