Summary of First Year Research Work:
Majority of research work has been conducted on the toxic effects of single metal species on fish. However, the aquatic organisms are typically exposed to the mixtures of metals. Therefore, in order to provide data supporting the usefulness of freshwater fish as indicators of heavy metal’s pollution, the acute toxicity of 19 mixtures of five metals viz. Fe, Zn, Pb, Ni and Mn has been determined for five commercially important fish species (Catla catla, Labeo rohita, Cirrhina mrigala, Ctenopharyngodon idella and Hypophthalmichthys molitrix). The present work also focuses on the extent of metal’s bio-accumulation in fish organs during acute exposures viz. 96-hr LC50 and lethal. The 96-hr LC50 and lethal concentrations of each metal mixture were determined for each fish species at constant water hardness (225 mgL-1), pH (7.25) and water temperature (30oC) by using static bioassay. During acute toxicity trails, each test concentration was evaluated with three replications for each fish species, separately, in glass aquaria.
Acute toxicity tests revealed significant differences among five fish species for their tolerance limits against 19 mixtures of five metals. Regarding overall sensitivity of five fish species, Hypophthalmichthys molitrix were highly sensitive while Labeo rohita showed significantly least sensitivity. Regarding overall response of fish towards 19 mixtures, mean sensitivity of fish to the mixture of five metals (Fe+Zn+Pb+Ni+Mn) was significantly high, followed by that of four metals (Fe+Zn+Pb+Mn) with statistically significant difference while fish showed significantly least sensitivity towards a mixture of lead and manganese.
The toxicity of heavy metal mixtures to the fish was dependent upon their concentrations and specific metal compositions. As far as lethal concentrations, Hypophthalmichthys molitrix were significantly more sensitive to metal mixtures, followed by Catla catla, Cirrhina mrigala, Ctenopharyngodon idella and Labeo rohita. Fish were significantly less sensitive to a mixture of Zn and Pb while they showed significantly higher sensitivity towards a mixture of five metals (Fe+Zn+Pb+Ni+Mn), followed by that of 4 metal mixture (Fe+Zn+Pb+Mn) with statistically significant differences. A significantly direct relationship existed between 96-hr LC50 and lethal concentrations of fish showing additive or synergistic effects of metals in various combinations.
The 96-hr LC50 exposure of Fe+Zn+Pb+Mn mixture to the fish caused significantly higher iron accumulation in their bodies while manganese and nickel accumulations were significantly higher in Cirrhina mrigala due to Fe+Zn+Pb+Ni+Mn and zn+Pb+Ni mixtures, respectively. Hypophthalmichthys molitrix showed significantly highest ability to accumulate both zinc and lead during exposures of Zn+Ni and Fe+Zn+Pb+Mn, respectively. Lethal exposures to the five fish species resulted in significantly variable accumulation of all metals in their bodies. Lethal exposure of Fe+Zn+Pb+Mn mixture caused significantly highest accumulation of iron in Labeo rohita while manganese and nickel accumulations were significantly maximum due to the exposures of Zn+Mn and Pb+Ni in Hypophthalmichthys molitrix and Catla catla, respectively. Zinc accumulations were higher in both Ctenopharyngodon idella and Hypophthalmichthys molitrix. Hypophthalmichthys molitrix exhibited significantly highest ability to concentrate lead in its body when exposed to Fe+Zn+Pb+Ni+Mn mixture.
During both 96-hr LC50 and lethal exposures, fish liver showed significantly higher ability to concentrate manganese, nickel, zinc and lead while iron accumulation was significantly high in fish kidney. Generally, fish muscle and bones showed significantly least tendencies to accumulate various metals. Besides liver, fish kidney appeared as a target organ for the accumulation of significantly higher iron during acute exposure of metal mixtures, which implies that it is also the "critical" organ for toxic symptoms. Although fish were significantly more sensitive to the metal mixture of iron+zinc and zinc+lead but accumulation of these metals were not so escalated in body organs. Hence, indicating antagonistic, additive or synergistic interactions among various metals in a particular mixture to concentrate in fish body organs. The present research demonstrated the complexity of the uptake processes occurring in the medium containing a mixture of metals at chronic toxicity (LC50 and lethal) levels. These interactions are of key significance in understanding and predicting the metal uptake, accumulation and toxicity in multi-metal exposure scenarios.