Ecotoxicology and Environmental Safety 69 (2008) 541–545 Decomposing leaf litter: The effect of allochthonous degradation products on the antioxidant fitness and photosynthesis of Vesicularia dubyana Jorge Nimptsch, Stephan Pflugmacher à Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Mu ¨ ggelseedamm 301, D-12587 Berlin, Germany Received 22 February 2007; received in revised form 23 August 2007; accepted 16 September 2007 Available online 31 October 2007 Abstract Leaf litter is one of the major input sources of organic carbon and nutrients in freshwater ecosystems. Throughout the degradation and leaching of leaf litter in freshwater bodies, ‘‘new born’’ substances are continuously generated and may aggregate to form humic substances (HS). Although the effect of HS on the stress physiology of aquatic macrophytes has been case of several investigations, the effect of these ‘‘new born’’ compounds (leaf litter breakdown products) on the stress physiology of aquatic plants has not been studied yet. Our results show that leaf litter degradation extracts (LLDEs) from oak, beech, and mixed oak and beech leaves have deleterious effects on the physiology of the aquatic bryophyte Vesicularia dubyana, decreasing photosynthetic activity and enhancing oxidative stress response. These findings suggest that leaf litter degradation extracts may be an important environmental factor influencing community structure within freshwater ecosystems. r 2007 Elsevier Inc. All rights reserved. Keywords: Leaf litter; Aquatic plants; Degradation; Phenolics; Oxidative stress 1. Introduction Within the aquatic ecosystem, the decomposition of organic material from terrestrial origins (allochthonous sources) is an essential process within both terrestrial and aquatic ecosystems (Wetzel, 1995). In streams and lakes, a wide range of organic materials are degraded stepwise to particulate matter and then to dissolved organic matter by a network of organisms ranging from insects to bacteria (Ribas et al., 2006). The degradation rate of plant litter, i.e., breakdown and decomposition depends on several factors, comprising the chemical and structural composi- tion of the litter, as well as temperature, nutrient offer, and bacterial and invertebrate community structure, among others (Hanlon, 1982). Leaf litter is one of the major input sources of organic carbon and nutrients in freshwater ecosystems. As described by Meyer et al. (1998), leaf litter alone can contribute around 30% of the total dissolved organic carbon in streams, being humic substances one of the major components. About 60% of the carbon dissolved in the aquatic system consists of humic substances (HS). HS are organic compounds and are formed during degradation of organic matter coming from microbes, plants, and animals. HS can be divided according to their solubility into humic acids, fulvic acids, and humin (Pena-Mendez et al., 2005). It has been shown that HS have the ability to act as xenobiotics (Pflugmacher et al., 1999). It is known that xenobiotics are able to generate enhanced production of reactive oxygen species (ROS) in biological systems (Cross et al., 1998). ROS play a central role in stress defense of photosynthetic organisms. Since during photosynthesis ROS are generated continuously, plants have developed effective biochemical mechanisms against these free radicals, which have the ability to disrupt cell membranes, denature proteins, and even damage DNA (Blokhina et al., 2003; Dat et al., 2000), to avoid oxidative stress. These biochemical defense mechanisms (e.g., antioxidative defense) are based on a complex network of antioxidants and antioxidative enzymes that ARTICLE IN PRESS www.elsevier.com/locate/ecoenv 0147-6513/$ - see front matter r 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.ecoenv.2007.09.003 à Corresponding author. Fax: +49 30 64181682. E-mail address: pflugmacher@igb-berlin.de (S. Pflugmacher).