July 25, 2010

Assessing arsenic and cadmium susceptibility

BANGLADESH is a developing country where fisheries and aquaculture remain very important as a source to supplement animal origin nutrition, employment generation, improving environmental condition and earning revenue in foreign exchange. Environmental pollution now-a-days poses a great threat to human beings as well as to animal kingdom over the world. Water resource, being the prominent component of the environment, is getting polluted over the decades. Contamination of water environment with various pollutants has increased considerably in resent years in many parts of the world. Various heavy metal ions such as As, Cd, Zn, Cu, Cr, Ni, Pb, Hg etc. are entering into environment through various processes.

Arsenic is a well-known chemical element that has the symbol As and atomic number 33. Arsenic was first documented by Albertus Magnus in 1250 (Emsley and John, 2001). Its atomic mass is 74.92. Arsenic is a metalloid that is prevalent in the environment, occurring both naturally and as a consequence of pollution. Arsenic is ubiquitous in the biosphere and occurs naturally in both organic and inorganic forms in water, food, soil, dust, wood and other materials. The most important inorganic arsenic compounds are arsenic trioxide, sodium arsenite, arsenic trichloride, arsenic acid and arsenites (trivalent forms) and lead and calcium arsenates (pentavalent forms).
Inorganic arsenic is more toxic than organic and the trivalent forms are more toxic than pentavalents. Common organic arsenic compounds are arsanilic acid, methylarsonic acid (MMA), dimethylarsinic acid (DMA), and arsenobetaine (Friberg et al. 1986; Lau et al. 1987). In the nature, arsenic can also be found to a small extent in elemental form. Arsenic is one of the toxic environmental pollutants which has recently attracted attention because of its chronic and epidemic effects on human health.
Cadmium(Cd) is a silver-white, blue-tinged, lustrous metal that melts at 321°C and boils at 765°C. This divalent element has an atomic weight of 112.4 and an atomic number of 48. It is insoluble in water, although its chloride and sulphate salts are freely soluble (Windholz, 1976). The availability of Cd to living organisms from their immediate physical and chemical environs depends on numerous factors, including adsorption and desorption rates of cadmium from terrigenous materials, pH, Eh, chemical speciation, and many other modifiers. The few selected examples that follow demonstrate the complex behaviour of Cd in freshwater systems. A substantial toxicological data base for cadmium and freshwater biota demonstrates that ambient cadmium water concentrations exceeding 10 ppb are associated with high mortality, reduced growth, inhibited reproduction, and other adverse effects. In case of fish Cd accumulation is higher in liver, kidneys and other organs but low in flesh. Cadmium encourages kidney disease, high blood pressure to man and also may lead to kidney and lung damage. Resistance to cadmium is higher in marine than in freshwater organisms; survival usually is higher at lower temperatures and higher salinities for any given level of cadmium in water.
Cadmium contamination of the environment is especially severe in the vicinity of smelters and urban industrialized areas. There is no evidence that cadmium, a relatively rare heavy metal, is biologically essential or beneficial; on the contrary, cadmium is a known teratogen and carcinogen, a probable mutagen, and has been implicated as the cause of severe deleterious effects on fish and wildlife. The concentrations of 0.80 to 9.9 µg/L in water is lethal to several species of aquatic insects, crustaceans, and teleosts and concentrations of 0.70 to 570 µg/L is associated with sublethal effects such as decreased growth, inhibited reproduction and population alterations.
Fish are ideal indicator of heavy metal contamination in aquatic systems because they abound different trophic levels in aquatic ecosystem. Some heavy metals have accumulation effect in fish and poses potential risks of heavy metal contamination. Culture of fish and other aquatic organisms that is known as aquaculture, depends completely on the respective qualities of water, i.e. qualities of the aquatic environment. Suitable water quality parameters are prerequisite for a congenial aquatic environment and for the growth of adequate fish food organisms. In Bangladesh, elaborate data is available for arsenic only on tube-well water, very little work has been done on the presence of arsenic, cadmium and others heavy metals in freshwater fish, livestock and livestock products, as well as in human and animal food chains.
Many scientists suggest that arsenic is not a problem for human alone but it may accumulate in animal tissues and animal products (Calvert and Smith, 1980; Awal, 2007), thus human exposure may occur through their food chain. Many scientists from different countries are working on the arsenic problem in Bangladesh, especially on ground water. But effects of heavy metals and nanoparticles on aquatic and terrestrial organisms are yet to acquire research imperative for the human health concern.
The bioaccumulation of arsenic and cadmium compounds were analysed in gill, muscle, intestine and eye in three major carps viz. grass carp, Ctenopharyngodon idella (herbivore), catla , Catla catla (planktivore); and mrigal; Cirrhinus cirrhosus (debris feeder) collected from10 different ponds at Chaugachha Upazila in Jessore district. The experiment emphasized on arsenic and cadmium concentrations in pond water, depth of groundwater which was used for fish culture and trophic position of species. The digestion of samples and chemical analysis were carried out by using Atomic Absorption Spectrophotometer at Arsenic Detection and Mitigation Laboratory (ADM Lab), Department of Pharmacology and Professor Mohammad Hossain Central Laboratory Bhaban, Bangladesh Agricultural University (BAU).
The arsenic and cadmium concentrations in pond the water ranged from 0.004±0.001 to 0.013±0.002 and 0.012±0.002 to 0.015±0.002 ppm respectively. The mean concentrations of arsenic and cadmium were observed 0.006±0.001 and 0.013±0.002 ppm. The highest concentrations of arsenic and cadmium were found in 61.0 m and 46.0 m depth of underground water respectively. Water quality parameters were monitored and found suitable except PH and total alkalinity. The mean concentrations of cadmium accumulated on fish tissues were much higher than that for human acceptable range (WHO, 2001).
In fisheries sector, ground water is readily used in various stages such as in hatchery operation and in brackish water aquaculture. Furthermore surface water reserves are also getting polluted due to discharges of unmanaged industrial effluents and urban waste water. So this sector is in a potential risk of being severely affected by various pollutants' toxicity but no proper attention has yet been paid to understand the toxic effects of arsenic and cadmium especially at the molecular level on fish and other aquatic organisms in Bangladesh. Therefore, further research is imperative to develop biomarkers based analysis by applying modern genomic techniques and to investigate incorporation and intracellular localization of heavy metals and nanoparticles in aquatic living organisms.
Source: www.thedailystar.net

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