RESEARCH ARTICLE Physiological response of fish under variable acidic conditions: a molecular approach through the assessment of an eco-physiological marker in the brain Amrita Mukherjee 1 & Amiya Ranjan Bhowmick 2 & Joyita Mukherjee 3 & Mahammed Moniruzzaman 4 Received: 13 April 2018 /Accepted: 27 May 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract The current study demonstrates oxidative damage and associated neurotoxicity following pH stress in two freshwater carp Labeo rohita and Cirrhinus cirrhosus. Carp (n = 6, 3 replicates) were exposed to four different pH (5.5, 6, 7.5, and 8) against control (pH 6.8 ± 0.05) for 7 days. After completion of treatment, levels of enzymatic (superoxide dismutase [SOD], catalase [CAT], glutathione reductase [GRd]) and non-enzymatic antioxidants (malondialdehyde [MDA], glutathione [GSH]), brain neurological parameters (Na + -K + ATPase, acetylcholinesterase [AcHE], monoamine oxidase [MAO], and nitric oxide [NO]), xanthine oxidase (XO), heat shock proteins (HSP70 and HSP90), and transcription factor NFkB were measured in carp brain. Variation in the pH caused a significant alteration in the glutathione system (glutathione and glutathione reductase), SOD-CAT system, and stress marker malondialdehyde (MDA). Xanthine oxidase was also induced significantly after pH exposure. Brain neurological parameters (MAO, NO, AChE, and Na + -K + ATPase) were significantly reduced at each pH-treated carp group though inhibition was highest at lower acidic pH (5.5). Cirrhinus cirrhosus was more affected than that of Labeo rohita. Molecular chaperon HSP70 expression was induced in all pH-treated groups though such induction was more in acid-stressed fish. HSP90 was found to increase only in acid-stressed carp brain. Expression of NFkB was elevated significantly at each treatment group except for pH 7.5. Finally, both acidic and alkaline pH in the aquatic system was found to disturb oxidative balance in carp brain which ultimately affects the neurological activity in carp. However, acidic environment in the aquatic system was more detrimental than the alkaline system regarding oxidative damage and subsequent neurotoxicity in carp brain. Keywords Labeo rohita . Cirrhinus cirrhosus . pH . Antioxidant . Brain . Heat shock protein . Oxidative stress Introduction Vertebrate brain is probably the most vulnerable organ to dif- ferent types of environmental fluctuations, as most of the physiological responses are initiated and synchronized by the central nervous system (Tseng et al. 2011). Aquatic sys- tems are extremely vulnerable due to their inclination to high concentrations of chemical substances that regularly enter into water bodies (Pal et al. 2014). Acidification or increase in alkalinity or contamination by other hazardous chemicals in the water bodies is a common phenomenon due to the release of pollutants from industries and household activities in the present day life. Detergents and some other pollutants present in urban or rural sewage reduce dissolved oxygen level, alter pH of receiving freshwater bodies, and thereby affect oxygen Responsible editor: Thomas Braunbeck Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11356-019-05602-3) contains supplementary material, which is available to authorized users. * Mahammed Moniruzzaman moni.vbu@gmail.com 1 Department of Zoology, Hiralal Mazumdar Memorial College for Women, Dakshineswar, Kolkata 700035, India 2 Department of Mathematics, Institute of Chemical Technology, Mumbai 400019, India 3 Department of Zoology, Krishna Chandra College, University of Burdwan, Birbhum, West Bengal 731124, India 4 Department of Zoology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata, West Bengal 700019, India Environmental Science and Pollution Research https://doi.org/10.1007/s11356-019-05602-3