Heavy metal content and mutagenic activity, evaluated by Vicia faba micronucleus test, of Tiber river sediments

Abstract

Tiber river sediment samples, collected in October 1995, were tested for mutagenicity by the micronucleus test in Vicia faba root tips. Four stations were studied within the urban area of Rome: (1) Castel Giubileo, at the entry of the urban area; (2) Ponte Tor di Quinto, immediately after the confluence of the tributary river Aniene; (3) Ponte Sublicio, in the middle of the city; and (4) Ponte della Magliana, immediately outside Rome, 20 km from the sea. Since no significant increase in micronucleus frequency was observed in any of the tested stations compared to control (while in previous campaigns mutagenic activity was observed in some of the same stations), it can be assumed an interesting recovery from mutagenic pollution in the 2 last years. The samples were analysed for pH value, nitrogen, organic matter and carbonate content, and the concentration of some potentially mutagenic heavy metal ions (Zn, Cd, Ni, V, Cu) was assessed. In all samples, a concentration of heavy metals higher than unpolluted areas was observed. However, the alkaline pH measured should keep them as non-bioavailable elements.

Introduction

Contaminated surface water has been shown to induce genetic damage in mammals 1, 2and aquatic organisms 3, 4, 5, 6. This fact has led the attention towards the development of short-term bioassays for environmental monitoring of mutagenic pollution.

Higher plants provide valuable genetic assay systems for screening and monitoring environmental pollutants [7]. It has been shown that for a specific chemical agent, comparable results, in terms of genetic abnormalities, are obtained in plant and animal systems [8]. In a survey of the cytogenetic effects of pesticides, Grant et al. [7]demonstrated that an excellent correlation existed between the frequency of both chromosomal aberrations and c-mitosis in plant and animal systems. Furthermore, results from mutagenic tests on plant systems show a good predictive value for human beings [9].

Among the different cytogenetic assays in plants, the micronucleus test in Vicia faba root tips is of particular interest because it is sensitive to both mitoclastic and clastogenic agents, it is simple, reliable, inexpensive, and the minimal laboratory facilities are required 10, 11, 12. Furthermore, the Vicia faba micronucleus test can also detect very weak mutagenic effects. A significant increase in micronucleus frequency was observed in root tips irradiated with 2 cGy of X-rays [13].

Since 1989, we have been studying the Tiber river, one of the main Italian rivers, and its tributaries' water quality within the urban area of Rome, testing the mutagenic activity of water and sediment samples by means of cytogenetic tests.

In the present work, the results from the last campaign (1995) are reported. The principal aim was to evaluate the evolution of mutagenicity of the Tiber river sediment samples in comparison to those obtained in our previous campaigns. Sediment was preferred to water sample because it is more stable in chemical composition so that it maintains a `longer term memory' of contamination. Moreover, it is known that some genotoxic metals and idrophobic chemicals are preferentially sequestrated from the environment as insoluble precipitates.

Samples collected in autumn (October, 1995), (in this period the pollution level gets worse 14, 15), were tested for mutagenicity under laboratory conditions by mean of the Vicia faba root tips micronucleus test. The same samples were analysed for pH value, nitrogen, organic matter and CaCO₃ content, the concentration of some potentially mutagenic heavy metal ions (Zn, Cd, Ni, V, Cu) was assessed, too. Heavy metals are a potentially mutagenic class of environmental pollutants and some of them are implicated in the induction of tumours in experimental organisms and exposed humans 16, 17, 18. Generally speaking, the main mutagenic effects induced by heavy metals, either alone or by enhancing the effect of other agents, are c-mitosis and chromosome fragmentation [19].

It is known that Tiber river pollution is mainly due to agriculture and municipal effluents rather than to industrial activities [20]and that the global quality is badly affected in the last 50 km by the effluents of the urban area of Rome and the confluence of the Aniene river [21]. In order to demonstrate the impact of the town on the mutagenic activity of the samples, four sampling stations were chosen in an upstream–downstream sequence within the urban area of Rome: the first station, Castel Giubileo, at the entry of the urban area; the second, Ponte Tor di Quinto, immediately after the confluence of the tributary Aniene river, a very polluted river [22]; the third, Ponte Sublicio, in the city centre and, the last station, Ponte della Magliana, immediately outside Rome, 20 km from the sea (Fig. 1).