When we hear about organisms contaminated by heavy metals, we generally think of pollution from human activity. However, there are always exceptions to the rule and cadmium is one of them. This metal, which is used to manufacture rechargeable batteries among other things, also occurs naturally in the marine environment.
Cadmium is leached out of soil by rain water and then transported by rivers to the marine environment.
Left, Iceland scallop (Chlamys islandica); right, sea scallop (Placopecten magellanicus). Credit: Fisheries and Oceans Canada
Certain organisms, such as scallops and oysters, naturally accumulate cadmium and have very high concentrations in their meat even though they have been harvested in uncontaminated areas. Claude Rouleau, a research scientist at the Maurice Lamontagne Institute, is interested in all aspects of the bioaccumulation of chemical compounds from natural and human sources in marine organisms. He is presently focussing on the scallops of the lower North Shore. His objective is to better understand the mechanisms resulting in the accumulation of specific heavy metals in mollusks that have significant commercial value and are highly prized by consumers.
An in-depth study of this phenomenon was initiated out of necessity when scallop producers of the lower North Shore were refused access to European markets in 2001. The cadmium concentrations in their products exceeded the European standards in place at the time of 2 mg/kg in fresh tissues. As a result, Claude Rouleau participated in three sampling projects conducted in the lower North Shore, from Sept-Îles to Blanc-Sablon, off Anticosti Island and around the Magdalen Islands. The two main commercial species studied were the Iceland scallop and the sea scallop. The results of the analyses revealed that both scallop species sampled at sites visited contained high concentrations of cadmium.
Distribution of cadmium in sea scallop tissues, after exposure to cadmium-contaminated water. The highest concentrations are found in the kidney and digestive gland. The metal concentration in other tissues is low. Credit: Fisheries and Oceans Canada
Subsequent laboratory experiments provided an opportunity to closely study the two naturally occurring means of cadmium accumulation: direct contact with water and phytoplankton consumption. With techniques using radioactive materials, Claude Rouleau and his team were able to prove that cadmium is found mainly in certain parts of the scallop, such as the kidney and digestive gland, which generally are not eaten. The analyses also showed that the muscle, the part usually found on our plate, contains little cadmium.
However, commercial standards for scallops in most importing countries are set for the entire organism, including organs that are not eaten but accumulate high levels of metal! It should be noted that this phenomenon of hyperaccumulation is specific to certain species of mollusks, such as scallops and oysters.
Claude Rouleau is a member of an international IAEC (International Atomic Energy Agency) committee on the application of nuclear techniques, for example in experiments with radiotracers to expand knowledge about contaminants in marine organisms. He also contributes to scientific advisories for international regulatory bodies such as the Food and Agriculture Organization of the United Nations (FAO).
Research is being conducted not only in light of international commercial standards but also with a view to economic development. Given that cadmium occurs naturally, is non-biodegradable and present in all soils, controlling it in situ is not feasible. Studies being conducted will make it possible to better predict cadmium accumulation in scallops based on their life cycle and the quantity of the metal present in the marine environment. The idea is to select better sites for new scallop beds by choosing areas near watersheds where the soil is poor in cadmium.
In addition, the research will examine seasonal physiological factors, such as the reproduction period, which affect cadmium content in the tissue of these mollusks in order to determine whether a particular time of year is better for harvesting. This is promising research.
Source: Fisheries and Oceans Canada