Contents lists available at ScienceDirect Aquatic Toxicology journal homepage: www.elsevier.com/locate/aqtox Biochemical parameters in skin and muscle of Pelophylax kl. esculentus frogs: Influence of a cyanobacterial bloom in situ Branka R. Gavrilović a, *, Marko D. Prokić a , Tamara G. Petrović a , Svetlana G. Despotović a , Tijana B. Radovanović a , Imre I. Krizmanić b , Miloš D. Ćirić c , Jelena P. Gavrić a a Department of Physiology, Institute for Biological Research “Siniša Stanković”– National Institute of Republic of Serbia, University of Belgrade, Bulevar despota Stefana 142, 11060 Belgrade, Serbia b Institute of Zoology, Faculty of Biology, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia c Scientific Institution Institute of Chemistry, Technology and Metallurgy – National Institute, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia ARTICLE INFO Keywords: Oxidative stress Biotransformation Neurotoxicity Frog Microcystin ABSTRACT There is little information in scientific literature as to how conditions created by a microcystin (MC) producing cyanobacterial bloom affect the oxidant/antioxidant, biotransformation and neurotoxicity parameters in adult frogs in situ. We investigated biochemical parameters in the skin and muscle of Pelophylax kl. esculentus from Lake Ludaš (Serbia) by comparing frogs that live on the northern bloom side (BS) of the lake with those that inhabit the southern no-bloom side (NBS). A higher protein carbonylation level and lower antioxidant defense system capability in the skin of frogs living in conditions of the cyanobacterial bloom were observed. Inhibition of glutathione-dependent machinery was the major mechanism responsible for the induction of cyanobacterial bloom-mediated oxidative stress in frog skin. On the other hand, the detected higher ability of muscle to overcome bloom prooxidant toxicity was linked to a higher efficiency of the biotransformation system through glutathione-S-transferase activity and/or was the consequence of indirect exposure of the tissue to the bloom. Our results have also revealed that the cyanobacterial bloom conditions induced the cholinergic neuro- transmitter system in both tissues. This study provides a better understanding of the ecotoxicological impact of the MC producing cyanobacterial bloom on frogs in situ. However, further investigations of the complex me- chanism involved in cyanobacterial bloom toxicity in real environmental conditions are required. 1. Introduction Decades of investigation into cyanotoxin-induced toxicity indicate that the occurrence of cyanobacterial blooms in freshwater eutrophic ecosystems presents a large-scale ecotoxicological problem with an increasing frequency worldwide (Merel et al., 2013). Based on their mode of action and according to the systems and organs they target, cyanotoxins can be classified as hepatotoxins, neurotoxins, cytotoxins, dermatotoxins and irritant toxins (Svrcek and Smith, 2004). Despite this classification, recent toxicological studies showed that all examined cyanotoxin groups exhibit multi-organ/system toxicity (Meriuloto et al., 2017). Specific mechanisms of toxicity for most cyanotoxins have been described in detail (Huang and Zimba, 2019; Wiegand and Pflugmacher, 2005). However, oxidative stress has been highlighted as one of the major cyanotoxin-induced adverse effects on aquatic or- ganisms (Cazenave et al., 2006; Lance et al., 2016; Qiu et al., 2007). Different secondary metabolites (e.g. microcystin – MCs, cylindrospermopsin, saxitoxin) produced by cyanobacteria trigger in- creased formation of reactive oxygen species (ROS), which results in lipid peroxidation (LPO), protein oxidation and DNA damage. The key role of the antioxidant system (AOS) in scavenging the excess ROS was demonstrated in previous studies on aquatic animals exposed to cya- notoxins (Amado and Monserrat, 2010). In addition to the AOS, bio- transformation processes have also been reported as an important de- fense mechanism against cyanobacterial toxicity in general (Pflugmacher et al., 1998). Recent studies have revealed a link between oxidative stress and some neurotoxicity parameters (e.g. acet- ylcholinesterase – AChE) (Rodríguez-Fuentes et al., 2015), which is very important considering that all major cyanotoxin groups have the potential to cause neurotoxicity (Calado et al., 2018; Hinojosa et al., 2019; Qian et al., 2018). Although considered as hepatotoxins, MCs can damage other organs (e.g. gills, intestine, kidneys, gonads, brain, muscle, heart) in different aquatic organisms (Amado et al., 2011; Cazenave et al., 2006; Gélinas https://doi.org/10.1016/j.aquatox.2019.105399 Received 30 October 2019; Received in revised form 26 December 2019; Accepted 26 December 2019 ⁎ Corresponding author. E-mail address: perendija@ibiss.bg.ac.rs (B.R. Gavrilović). Aquatic Toxicology 220 (2020) 105399 Available online 27 December 2019 0166-445X/ © 2019 Elsevier B.V. All rights reserved. T