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Our question this month comes from Antarika Sen, who asks us:

“Do freshwater and seawater molluscs/bivalves absorb heavy metal? If they do, how can I test/ experiment for this in my school lab using a relatively simple and affordable technique?”

Our expert: Dr Chris Hauton

Chris Hauton is a marine biologist who works at the National Oceanography Centre, Southampton. Outside of the lab Chris spends as much time as possible underwater, SCUBA diving on wrecks and reefs throughout the UK.

It is quite true that molluscs absorb heavy metals from seawater. In fact some of these metals are essential for metabolism. For example, and depending on the definition used to describe heavy metals, copper may be regarded as a heavy metal. Copper is an essential metal required for the transport of oxygen in some molluscs. Copper atoms form part of a molecule called haemocyanin which carries oxygen around the tissues. Similarly other heavy metals, including manganese and cobalt, are essential in trace amounts as cofactors which activate enzymes or as components of vitamins, such as vitamin B12.
Molluscs are also well known for their ability to bioaccumulate other heavy metals, such as cadmium, mercury, lead and silver. These metals, which can accumulate to very high levels, are often bound to proteins called metallothioneins which help to control the toxic effect of the heavy metal in the mollusc.

Needless to say the accumulation of high levels of heavy metals in the tissues of molluscs can cause toxic problems for humans during consumption. One such notorious example occurred in Minamata Bay in Japan in the 1950s. Here the local community were heavily reliant on fish and shellfish in their diet. In 1952 a new factory was opened locally which produced vinyl chloride and acetaldehyde. In the following years 2000 cases of poisoning were diagnosed which resulted in 43 deaths. Subsequent studies showed that this was caused by the factory discharging mercury into the local waters which then accumulated in the fish and shellfish, including molluscs.

The conventional method for analysing heavy metals in molluscs, or any organism, is a lengthy process which is expensive and which requires a specialist instrument, called an Atomic Absorption Spectrometer (AAS). The first step requires the biological tissues to be digested using concentrated acids, such as nitric acid. The concentrated nitric acid dissolves the biological tissue and frees the heavy metal atoms into solution. Once the heavy metals are in solution a sample can be burnt in a graphite furnace, a process which transfers the metals atoms to a vapour phase. The atomic-absorption spectrometer (AAS) then uses the absorption of specific wavelengths of light to measure the concentration of gas-phase atoms which are compared to standard samples of known concentration.

Unfortunately there is no real affordable alternative to this method.

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