engleski

Acanthocephalans, fish intestinal parasites, as bioindicators of metal exposure in rivers impacted by mining waste

In the past decades, the interrelation between parasites and contaminants has gained increasing interest, especially in aquatic ecotoxicology. Fish intestinal parasites, acanthocephalans, were reported to effectively accumulate metals in concentrations that are orders of magnitude higher than those in the commonly used indicator organisms like fishes, bivalves or crustaceans. Acanthocephalans lack a gastrointestinal system and consequently, depend on host micronutrients and highly absorb essential metals from the fish intestine, as elements of physiological importance. Due to their competition for essential elements, non-essential elements are also absorbed by the parasites, even more effectively than essential metals. Accordingly, attempts have been made to utilize acanthocephalans as biological indicators of metal exposure in environmental risk assessment studies. Application of acanthocephalans as bioindicators in metal exposure assessment of mining impacted rivers in the north-eastern Macedonia involved parasites hosted by Vardar chub (Squalius vardarensis) dwelling in the Zletovska and Kriva River impacted by Pb/Zn active mines, and in the less contaminated Bregalnica River. Sample preparation involved acid digestion of acanthocephalans in a drying oven at 85°C for 3.5 hours using 1.5 ml of concentrated HNO3 and 0.5 ml H2O2, followed by measurement of 24 trace and 4 macroelements by high resolution inductively coupled plasma-mass spectrometer (HR ICP-MS). For the purposes of calculation of bioconcentration factors (BCFs), cytosolic gastrointestinal metal levels were multiplied with tissue dilution (6 fold) and BCFs were determined as the ratio of the element concentration in the parasites to that in host gastrointestinal tissue (BCF = C[parasite]/C[host intestine]). Two acanthocephalan species, Pomphorhynchus laevis and Acanthocephalus sp., were found in Vardar chub, but only from Bregalnica River and not from two mining affected rivers. Such occurrence of acanthocephalans only in moderately contaminated river disables their application as bioindicators of environmental pollution in mining impacted rivers. It follows that absence of acanthocephalans might be considered as a first sign of highly polluted aquatic environment. In the Bregalnica River, prevalence in both sampling periods, spring (May/June) and autumn (October), was comparable, i.e. around 70% of infected chub specimens. In spring, all measured metal concentrations were higher in P.laevis than Acanthocephalus sp., statistically significantly for 14 trace elements, while in autumn only for 7 trace elements. The data on BCF confirmed higher metal accumulation in P. laevis than Acanthocephalus sp., which was evident in both seasons for all measured metals, except for Sr, Na, Mg and Ca. Estimation of metal accumulation based on BCF indicated in both acanthocephalan species and in both seasons 10-45 times higher Al, Pb, Sr, Tl, Li, and Ba, 3-20 times higher Cr, Ti, Ca, Cu, and Mn, and 1-6 times higher Cd, Ni, V, U, Fe, Mg, As, and Na concentrations in acanthocephalans than in fish gastrointestinal tissue. In contrast, Se, Rb, Mo, Cs, Co, Zn and K were not accumulated by parasites. Although involvement of acanthocephalans in metal exposure assessment gives valuable data, which might serve as a sensitive bioindicator of bioavailable metal levels in the freshwater ecosystem, acanthocephalan-chub system unfortunately cannot be applied as an indicator of acute or chronic metal exposure in mining impacted rivers. Evaluation of possible protective role of acanthocephalans against metal accumulation in chub, regarding high metal uptake capacity of acanthocephalans, pointed to significantly higher Rb, Cs, Tl, Pb, Mn and Cu levels in uninfected compared to chub infected with acanthocephalans in spring, while in autumn only for Cd and Tl. It follows that acanthocephalans can reduce levels of some metals in the gastrointestinal tissue of their host, related to the uptake of bile-bound metals by parasites and reduction of metal levels available for reabsorption by the host. Our data suggest that in metal exposure assessment acanthocephalans should be considered as confounding factors, due to their ability to modify metal concentrations in fish gastrointestinal tissue.

chub ; intestinal parasites ; acanthocephalans ; metals ; bioaccumulation

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