Natural Bioremediation of Heavy Metals Through Nematode Parasite of Fish

INTRODUCTION

Pollutants might promote increased parasitism in aquatic animals, especially fish by impairing the host‘s immune response or favoring the survival and reproduction of the intermediate hosts (Khan and Thulin, 1991). Parasites are naturally occurring organism (Barus et al., 1999), attracting increasing interest as potential indicator of environmental quality due to variety of ways in which they respond to anthropogenic pollution (Barak and Mason, 1990; Sures, 2001). Deteriorating state of the environment in the aquatic resources is reflected by the increased degree of fish nematode parasite infection and elevated metallic pollutants levels. Some trace toxic metals have been found to accumulate to higher extent in parasite than in tissues of their host organism and to demonstrate a good suitability as a biomonitor of environmental pollution (Schludermann et al., 2003). The parasitic infection rate in fish is often concern as indication of the degree of immunity of organism, or community or system subjected to exogenous, potentially recurring stimulus (Szefer et al., 1998).

Most reports of pollution effects on endoparasites suggest increased parasitism in fish hosts. This also implies to fish living in the areas which receive industrial/thermal effluents. Parasite might in turn enhance their hosts, susceptibly to pollutants (Chibani et al., 2001).

Several types of pollutants, including domestic sewage, pesticides, polychlorinated biphenyls, heavy metals, pulp and paper effluents, petroleum aromatic hydrocarbons, acid rain and others are known to effect marine life (Ucman et al., 2001). Many of the later are parasitized and under natural environmental condition most fish parasites are believed to cause little or no harm. However, chronic exposure to pollutants over a period of time cause physiological, behavioral and biochemical host changes that ultimately can influence the prevalence and intensity of parasitism (Khan and Thulin, 1991). Brotheridge et al. (1998) reported that Trout were collected from two different sampling stations, one site adjacent to the river Otra South of Evje where Nickel mined and trout likely to be infected with parasites nematode, Eustrogylidies sp. due to the presence of Ni and Cu in fish and other from down stream was non infected.

The objective of this study to asses the degree of infection in marine fish and to determine the concentration of heavy metals in water, parasites and tissues of fish.

MATERIALS AND METHODS

Sixty-seven specimens of Liza vaigiensis, were targeted to study the concentration of heavy metals and parasitic infection. Fishes were collected from fish harbor Karachi in July 2006 in early morning through trawler with water samples. Samples were collected in sterile flasks and keep air tight to avoid any contamination in laboratory. Each fish specimen was washed with deionized water and then surface dried with filter paper.

The examination was aimed to study the infection in fishes by nematode parasites and heavy metal found in edible fish. Total Length (TL), Total Weight (TW) of fishes and infection intensity by the parasites were measured in individual fish sample. The total length of fishes were ranged from 7-10 inches where as weight were ranged from 100-150 g.

Investigation of nematode parasite: All fishes were dissected by ordinary method and gastrointestinal tract were removed, placed in a saline solution. Guts were dissected longitudinally and examined for nematode parasites under the binocular microscope. Nematodes collected from the gut were fixed in 70% alcohol, preserved in glycerin and 70% alcoholic mixture and cleared in lactophenol. Temporary mounts of nematodes were made in pure glycerin and examined under stereomicroscope. Photomicrographs were made with an automatic camera attached to a compound microscope using Nomarski`s interference contrast system.

Detection of toxic metals: Water samples were digested in specific volume of nitric acid (conc.) and then sequentially evaporated, after filtration each sample were made up to 100 cm³ with deionized water (conductivity below 1 μS cm^-1). Detection of heavy metals were made by Hitachi (H-28750-10) Atomic Absorption Spectrophotometer having band width of 0.2 nm and adjustable wavelength range 190-900 nm with 250 mm Elbert mount diffraction grating monochromator, burner was titanium head for air acetylene.

Nematode parasite of fish, tissue samples of different organs of fishes (wet sample) were digested using HNO₃and H₂SO₄ for detection of heavy toxic metals like Pb, Hg, Cd, As, Zn and Fe in a closed system by atomic absorption Spectrophotometry.

RESULTS AND DISCUSSION

Heavy metals analysis of seawater, fish and fish nematode parasites showed that Pb, As, Cd, Hg, Zn and Fe were detected. Table 1 showed the concentration of heavy metals in seawater collected from different coastal beaches of Karachi Coast. Table 2 showed the bioaccumulation potential of heavy metals in muscles and guts of Liza vaigiensis. Table 2 also indicate that infected fish contain low concentration of heavy metal as compared to non-infected fish whereas Table 3 showed the heavy metals burden in parasite, Echinocephalus sp. and Ascaris sp.

Fishes were dissected for studying infection intensity by parasite. It was found that out of sixty-seven specimens twenty-four were infected by nematode parasite. Numerous nematode parasites were collected from the intestine of fish, which were identified as Echinocepalus Molin (1858) and Ascaris Linnaeus (1758). Echinocephalus sp. and Ascaris sp. are common parasites of fish of Karachi Coast for its pathogenic effects and reported here for the first time as a natural bioremediator of heavy metals within fish which shows more toxin burden in their body and help in the survival of the host in polluted environment.

In environmental impact studies certain organisms provide valuable information about chemical state of their environment not through their presence or absence but through ability to concentrate environmental toxins within their tissues (Tenora et al., 2000; Ucman et al., 2001; Sures et al., 2001). Table 1 showed that heavy metals are more prevalent and in higher concentration in the different beaches of Karachi Coast which may be attributed to the continuous discharge of industrial/domestic effluent in the aquatic resources (Trucekova et al., 2002; Tariq et al., 1993). Having high density, metals have more tendencies to accumulate in the tissue organs of animals as compared to essential micro and macronutrient ions as shown in Table 2 (Azmat et al., 2006a). According to EPA (Environmental Protection Agency) the residual tolerance level of Pb, Hg, Cd and As is 0.05 mg kg^-1, 0.1 μg g^-1, 0.05 mg kg^-1 and 0.04-0.15 μg g^-1, respectively which is comparable with the results of Table 1-2. This indicates the toxic effect of heavy metals for both human and fish, which was the provider of quality meat for human beings (Khansari et al., 2005).

Results in Table 3 showed that heavy metals concentration in non infected fish was higher as compared to infected fish by Echinocephalus sp. and Ascaris sp. It signify that parasites accumulate more concentration of toxic metals in the soft tissues of their body and provide natural remediation for pollutants to survive its host in contaminated environments. Heavy metals concentration differed significantly between organs of fish and nematode parasites Echinocephalus sp. and Ascaris sp.(Table 2, 3 and Fig. 1) with level to several fold higher in the parasite (Szefer et al., 1998).

Present results were supported by results of earlier investigators like Tenora et al. (1999) who reported more heavy metals burden like Pb, Cd, Cr and Ni in the parasites-host system in cyprinid fish. Similarly certain parasites, particularly intestinal Acanthocephalus of fish can accumulate heavy metals to concentration orders of magnitude higher than those in the host tissues (Sures et al., 1997). The amount of heavy metals was generally higher in parasites and lower in liver of perch infected with parasites than in the non-infected fish (Trucekova et al., 2002; Chibani et al., 2001).


Fig. 1:	Echinocephalus sp. (A): Anterior region, (B): Posterior region Ascaris sp., (C): Anterior region, (D): Posterior region

Table 1:	Concentration of heavy metals (μg g^-1) found in sea water from different sampling stations

Table 2:	Heavy metal concentration (μg g^-1) in infected and non infected Liza vaigiensis fish

Table 3:	Bioaccumulation of heavy metal potential (μg g^-1) in nematode parasite of fish

Continuous discharge of industrial toxic material in water reservoir and presence of nematodes and heavy metals in fish can alter the metabolic and physiological function of the fish (Voegborlo et al., 1999). This also affects the quality of meat of fish. Because parasites (Barus et al., 2001) by definition require digestion of host tissue for their own nutrition and they rely on protrinases and oxidative damage for the tissue penetration and to supply their nutrients (Navratil et al., 1999). Table 3 shows trace elements like Pb, Cd, As, Hg, Fe and Zn that have been found to accumulate to higher extent in nematode parasites as compared to tissues of host (Table 2) and demonstrate its suitability as a bioindicators of environmental pollution and suggest that they are sharing more burden in their soft tissues as well as persistent in contaminated environment (Azmat et al., 2006b) therefore act as bioremediator for fish (removing heavy metal)and help in the survival of fish with toxins. The parasitic infection rate in fish is often considered as an indicator of the degree of immunity of organism (Schludermann et al., 2003; Rolbiecki et al., 2000), Very high percentage of Fe in parasite is reported in Table 3 which may be attributed with lowering of iron (Table 2) in fish which in turns produced growth retardation, stunting and poor cognitive development in human and in animal.

CONCLUSION

Investigation revealed that parasites reducing environmental stress in fishes through bioremediation by concentrating the metals in their soft body tissues and minimizing the site disturbance within fish body compared with conventional clean up technologies.

ACKNOWLEDGMENT

Authors are thankful to Dr. Rafia Rehana Ghazi, Ex-Director, VPCI, PARC, University of Karachi, for the identification of fish nematodes and also for her helpful comments and suggestions.