Delineation of heavy metal uptake pathways (seawater, food and sediment) in tropical marine bivalves using metal radiotracers

Full text: The SW lagoon of New Caledonia is subjected to intense heavy metal input from land-based mining industries exploiting metals such as Ni and, secondarily, Cr and Co. However, as is the case for most tropical environments, little is known about the contamination status of this lagoon. In order to allow a better ecotoxicological characterization of the New Caledonian lagoon through monitoring of its seawater quality status, three locally common and abundant species were selected for laboratory radiotracer experiments. The objective of the study was to characterize and possibly validate their usefulness as bioindicator species in programmes for local pollution surveys and monitoring of the coastal zone. The three species selected were the edible clam Gafrarium tumidum, and the oysters Isognomon isognomon and Malleus regula. Bioaccumulation and tissue distribution of six elements (Ni, Cd, Co, Cr, Zn, Ag) were determined following exposures via sea water, sediment and food using radiotracers (63Ni, 109Cd, 57Co, 5Cr, 65Zn, 110mAg) and highly sensitive nuclear detection techniques (beta and gamma spectrometry). Transfer from food was shown to be quite efficient; assimilation efficiencies (AE) of metals ingested with phytoplankton (Isochrysis galbana) ranged between 34 and 77%, and these ingested fractions of metals were retained for long periods of time (biological half- lives ranged between 20 and 400 days depending upon the element). In contrast, in terms of transfer efficiency, sediments were found to be a much less important pathway than sea water; for example, concentration factors from sea water estimated at steady-state, CFSS, were three to five orders of magnitude higher that steady-state transfer factors from sediments, TFSS. Nevertheless, metal concentrations in sediment are several orders of magnitude higher than those in sea water and, in terms of quantities of transferred elements, their impact on total metal accumulation may not be negligible. Therefore, it was necessary to use a mathematical model to estimate the relative contribution of each uptake pathway to the total bioaccumulation of metals in bivalves. The kinetic parameters measured in the different experiments investigating the exposure routes have been used to run the model. Computations suggest that sediment always accounts for only a minor part of the total bioaccumulation in the 3 bivalves (< 3%; except for Cd in the oysters: 20% in I. isognomon and 30% in M. regula). In the clams, the predominant uptake route for all metals was found to be the food (95-99%). In contrast, both oysters accumulated Co and Cd mainly from food, whereas sea water was the predominant route for Zn accumulation. Interestingly, the two oysters displayed the same behaviour with respect to all the metals except Ag. The latter element was far more efficiently taken up and retained in the soft parts of I. isognomon than in M. regula. For this metal the bioaccumulation model indicated that I. isognomon took up Ag mainly from sea water (88%) whereas M. regula accumulated it principally from its food (90%). Furthermore, our results suggest that differing mechanisms (or induction threshold level) of Ag detoxification/storage occur in the two oyster species, and that the mechanism in I. isognomon is particularly sensitive and efficient. In conclusion, food and, secondarily, sea water appear to be the predominant routes for the uptake of these metals in the three tropical bivalves investigated. (author)