BIOAMPUFICATION OF SELECTED HEAVY METALS IN AN ECOSYSTEM

Abstract

Bioconcentration of toxic metals and the factors that govern the uptake by organisms in an aquatic system both qualitatively and quantitatively has been a subject of scientific research. Rapid industrialization and concomitant exploitation of nature has resulted in the prevalence of trace metals in natural waters. Aquatic organisms take up heavy metals from their aquatic environment and accumulate them. The incidence of Minimata and Itai-Itai diseases have focussed the attention of scientific community on the dangers of metal pollution, bioconcentration and bioamplification through a food chain. Bioconcentration of Heavy Metal ions (HMs) is affected by an array of physico-chemical and biological factors. Existing literature reveals that most of the bioconcentration studies have been carried out as monitoring of body burdens of hazardous trace metals. Casual references are also available wherein the factors affecting the uptake and the levels of biomagnification have been worked out. Scientific research is required to evaluate kinetics of uptake and to define site specific levels in tissues and cells, more so for environmentally sensitive metals. In the present investigation bioconcentration of'Cd, Pb, and Zn along with impact of physico-chemical conditions on bioconcentration have been studied in a natural riverine ecosystem, Hindon and Kalinadi. In the past forty years there have been rapid industrialization in the Hindon basin. Most of the industrial wastes generated unfortunately gain access into the river along with domestic wastes without any specific treatment. Though the river system has an important geographical location, there is a lack of data on spatial and temporal variations of

water quality parameters. Six representative stations were, therefore, selected to study the bioconcentration of metals in the biota. The physico-chemical characteristics of water in river Hindon and Kalinadi provide baseline information on the pollution of riversby various industrial and anthropogenic activities. Concentrations of Cd, Pb, and Zn in water, plankton and fish were highly elevated. Bioconcentration of HMs in plankton and fish was variable with abiotic environment of the stations. An attempt to correlate the bioconcentration ofHMs with physico-chemical parameters of water revealed that pH, dissolved organic carbon (DOC), water hardness, alkalinity, and metal content collectively influence the bioconcentration of Cd, Pb and Zn in the aquatic environment. The approach was extended further and a Water Quality Index (WQI) was formulated. Important determinants ofbioconcentration were employed in the formulation of the index. Weightages were assigned to pH, DOC, total hardness, alkalinity and metal in complexed form. Concomitant decrease in bioconcentration factors (BCFs) of Cd, Pb, and Zn in plankton and fish with an increase in index values corroborates the suitability of the index and define a relationship between pollution levels and BCFs of Cd, Pb, and Zn. Further, a conceptual model of persistent Cd, Pb, and Zn indifferent trophic status of the riverine ecosystem was developed. Levels of Cd, Pb, and Zn concentration in water, plankton and fish exemplify bioamplification (higher concentration of HMs at high trophic level) at selected stations for Cd (in winter season), Pb (in postmonsoon season), and in all the observations for Zn. Biodiminution was found to beprominent at selected stations in postmonsoon and summer seasons (for Cd) and in winter season (for Pb).

Laboratorystudies show thatCdandPbbioconcentration isa function oftime and sublethal metal concentration in the exposure medium. Measurements of Cd and Pb contained in individual organs expound that bioconcentration of HMs is site specific. High concentrations of Cd in gills and liver and of Pb in gills and muscles havebeen reported. Hourly and daily variations of uptake of Cd and Pb in fish tissue (gills, liver and muscles) have also been worked out. Bioconcentration kinetics of both Cd and Pb was found to comprise of an initial rapid phase and a later slow phase responsible for net accumulation. An attempt has also been made to scan Cd and Pb in gills, liver, and muscle tissues offish exposed to sublethal Cd and Pb by histochemical and micro-analytical techniques. Modified Silver Sulphide Technique (SST) was employed to localize the exogenous trace metals. Transmission Electron Microscopy (TEM) of gills, liver and muscle tissues show metal inclusions in gill head and gill lamellae regions. Metal inclusions were also cited in liver and muscle cells. Presence of Cd and Pb was corroborated by Electron Probe X-ray Microanalysis (EPMA) of thin sections of fish tissue. A gist of conclusions of the present investigation reveals compartmentation and bioamplification of Cd, Pb, and Zn in an aquatic ecosystem. Selected physicochemical parameters were found to moderate the bioconcentration of Cd, Pb, and Zn. Further, the accumulated metals have been localized, scanned and quantified in fish tissue (at cellular level) using TEM and EPMA.