Determination of LC50 and behavioural responses
Triclosan has been classified as a class III drug (compounds with low permeability and high solubility) by FDA due to their broad spectrum antimicrobial activity
(Dhillon et al., 2015). Toxicity studies conducted on various organisms proved that TCS is acutely and chronically toxic to all the organisms especially to the aquatic organism through wastewaters from different sources. Earlier studies observed the median lethal concentration (LC50) of TCS on different fishes as 4400 µg.l
-1 in
Onchorynchus mykiss (Adolfssonerici et al., 2002), 370 and 1700 µg.l
-1 in
Oryzias latipes (Orvos et al., 2002 and
Nassef et al., 2009), 340 µg.l
-1 in
Danio rerio (Oliviera et al., 2009), 1480 µg.l
-1 in
Xiphophorus helleri (Liang et al., 2013), 2810 µg.l
-1 in
Oreochromis niloticus (Vijitha et al., 2017), 1458 µg.l
-1 in
Pangasianodon hypophthalmus (Sahu et al., 2018), 1767 µg.l
-1 in
Anabas testudineus (Priyatha and Chitra, 2018), 360 µg.l
-1 in
Gibelion catla and 390 µg.l-1 in
Labeo rohita (Hemalatha et al., 2019a and 2019b). In the present study, the LC50 value of TCS on
O.
mossambicus was calculated based on the 96 h mortality data of fishes exposed to TCS. The data were plotted as graph and there was a positive correlation (P<0.05) between the mortality of fishes and concentration of TCS (Fig 1). The calculated 96 h LC50 value at 95% confidence limits for TCS for
O.
mossambicus was 740 µg.l
-1. During the exposure period the fishes showed behavioural irregularities such as mucous formation, surfacing, erratic movement, air gulping, hanging at the top and discoloration of skin.
Activity of acetylcholinesterase enzyme
Acetylcholinesterase (AChE) activity is widely being used as a biomarker to study the neurotoxic effects of wide range of toxicants in the aquatic environment. Inhibition of AChE activity results in build up of acetylcholine which causes prolonged excitatory post synaptic potential which negatively impacts the overall behaviour and nervous system of fishes
(Banaee et al., 2011). In the present study, the mean AChE activity of brain tissue of tilapia exposed to control medium was 0.345±0.01 μM.min
-1.mg.protein
-1. In tilapia, on exposure to 96 h acute toxicity of TCS the AChE activity of the brain got significantly reduced to 0.266±0.03, 0.209±0.02, 0.175±0.01 and 0.088±0.01 μM.min
-1.mg.protein
-1 at 131, 262, 523 and 1046 µg.l
-1 respectively (Fig 2). AChE activity in the brain tissue of tilapia was significantly (p<0.05) reduced on exposure to different concentrations of TCS. There was no significant difference in enzyme activity between the control and solvent control group of tilapia in the acute toxicity test during the study period. The degree of reduction of enzyme activity was found to be increase with increase in concentration of TCS during the exposure. The reduction of AChE activity in tilapia on exposure to acute concentrations of TCS is shown in Fig 2. Highest amount of inhibition of AChE was observed at 1046 µg.l-1 of TCS in tilapia. Similar observation of reduction in enzyme activity in brain tissue was observed in zebra fishes by
Oliviera et al., (2009) and
Pullaguri et al., (2020).
Activity of glutathione S transferase
Glutathione S transferase is involved in detoxification of xenobiotics at cellular level and their activity in different organs acts as a potential biomarker in toxicity tests
(Sturve et al., 2008). In the present study, the observed increase in concentration of GST enzyme in liver tissue can be attributed to the detoxification process of toxic metabolites and it is an indicator of defence mechanism
(Modesto and Martinez, 2010; Sahu et al., 2018). The mean GST activity in liver tissue of tilapia at control medium during acute toxicity test was 2.637±0.13 n mol. min
-1 mg.protein
-1. The GST activity was induced on exposure to TCS in fishes with significant increase in activity with increase in concentration of TCS. The GST activity on exposure to 96 h acute toxicity test significantly increased to 3.022±0.06, 3.346±0.10, 3.860±0.10 and 3.551±0.10 n mol. min-1 mg.protein
-1 at 131, 262, 523 and 1046 µg.l
-1 respectively (p<0.05) (Fig 3). Similarly, an elevation in GST activity due to TCS toxicity were reported in zebra fish by
Oliviera et al., (2009), in sword tail fish by
Liang et al., (2013) and also in mussels and zooplankton
(Canesi et al., 2007; Binelli et al., 2011; Peng et al., 2013; Han et al., 2016). GST inhibition at the early larval stages of
Solea senegalensis observed by
Araujo et al., (2019) proves that TCS inflicts more damage to fishes at their early life stages.
DNA damage in gill and liver of tilapia exposed to acute and sublethal toxicity of TCS
The integrity of DNA damage in blood cells and different tissues have been proved as potential biomarker for environmental toxicology. In the present investigation, significant dose dependent DNA damage intensity was recorded in gill and liver tissues of tilapia and rohu. The difference in extent of DNA damage in various tissues could be attributed to number of alkali – labile sites and variability in DNA of different tissues
(Paul et al., 2019) because different cells have different levels of DNA single strand breaks due to variation in antioxidant concentration, metabolic and repair activity
(Lee and Steinert, 2003). Effect of TCS on the index of DNA damage (DI) on gill and liver tissues of tilapia during acute toxicity test is presented in Fig 4. DNA damage was not observed in the control group of fishes in both gill and liver tissues of tilapia during 96 h acute toxicity test. In gill tissue, the mean DI values were observed as 37.564 ± 1.92, 49.275±2.34, 61.005±1.47 and 98.014±1.69 at 96 h acute toxic concentrations of 131, 262, 523 and 1046 µg.l
-1 respectively. In liver, the mean DI values at the concentrations of 131, 262, 523 and 1046 µg.l
-1 were 28.789±1.86, 38.877±1.39, 46.226±1.32 and 50.705±1.58 respectively. The DNA damage increased with increased in concentration of TCS in gill and liver tissue. The DI value was found to be higher in gill than the liver tissue. The TCS induced genotoxicity study on rainbow trout by
Capkin et al., (2017) in red blood cells showed significant DNA damage. In addition to fishes, studies by
Silva et al., (2015) and
Xu et al., (2015) revealed the TCS induced DNA damage in live food organisms such as
Daphnia magna and
Artemia salina. So, the present investigation proved that TCS can affect the target tissue of fishes and induces DNA strand breakage.