The way of contaminants from oceans into our dish

Ocean pollution mostly comes from land, including agriculture, heavy industry, untreated sewage and litter like plastics. Contaminants that enter the oceans do not disappear, they can be assimilated by edible marine organisms and ultimately end up in people’s dishes. In order to limit human exposure, it is important to understand the transfer of contaminants through the food web.
This requires evaluation on how contaminants are taken up by marine organisms, their path through the marine environment and how they are assimilated by the humans ⁽¹⁾.
The understanding of all the processes involved utilises nuclear techniques.

The use of radiotracers and the high sensitivity of their detection allow to work at very low concentrations, reflecting realistic conditions and significantly reducing biological variation during experiments. Moreover, nuclear techniques can be used also on living organisms, thus allowing measurements of the contaminants transfer mechanism and the biokinetics.
It is possible to analyze how contaminants move through the food chain, from marine algae to seafood, and to assess their impact on humans.

Radiotracers are used to track different types of contaminants: metals (e.g. mercury, cadmium); artificial radionuclides (e.g. cesium-137, americium); organic pollutants (e.g. pesticides, polychlorinated biphenyls).
The possibility to replicate the human digestive process in the laboratory by creating a mix of enzymes enables the determination of which contaminants are not broken down during digestion and thus remain in the system. For example, crabs make up 20% of all marine crustaceans consumed worldwide.
They can accumulate contaminants such as heavy metals and radionuclides: one of them being silver. Despite it being a rare naturally occurring metal, silver is released into the atmosphere by different anthropogenic activities including: emissions from smelting operations, manufacture and disposal of certain photographic and electrical supplies, coal combustion, and cloud seeding⁽²⁾.
The IAEA Marine Environment Laboratory studied the mechanism of assimilation of silver in crabs ⁽³⁾.
Using γ spectrometric techniques (i.e. Ge detectors), they were able to follow Ag behaviour in a small number of crabs that were allowed to ingest labelled shrimp during a short period of time (pulse-chase feeding technique).
Shrimp were labelled while alive with the ^110mAg radiotracer: this is produced from silver by the ¹⁰⁹Ag(n,γ)^110mAg nuclear reaction. Its half-life is 253 days and it decays by emitting a gamma ray of 660 keV (Figure 1). 

Figure 1 - Decay scheme of ^110mAg

Shrimps were labelled by exposing them to 0.5 Bq ml^-1 110mAg for 15 days. The crabs then fed on the labelled shrimp for 9 hours. In studies such as these it is important to introduce the biological half-life (T_b1/2): it corresponds to the time required for a biological system to eliminate, by natural processes, half of the amount of a substance, in this case a radioactive material, that has entered it. Loss kinetics of ingested ^110mAg were then followed individually in each crab for 4 months, using a high-resolution γ-spectrometry system. In parallel, the distribution of ^110mAg in their tissues was investigated using whole-body autoradiography (WBARG) on liquid nitrogen frozen crabs at different times.
Whole-body loss kinetics of ingested ^110mAg were best fitted by a two-component exponential model; a short-lived component due to the loss by excretion (T_b1/2 = 8 h) and a long lived component for the radiotracer assimilated by the organisms. The retention capacity of this metal was found to be extremely strong (T_b1/2 = 7.3 y); crabs are excellent long-term recorders of silver contamination in coastal environments. These analyses allow correlation of the assimilation parameters to the sex and the dimension of the crab.
Since the autoradiography evidenced that incorporated ^110mAg was exclusively localized in the hepatopancreas of the crabs, the mechanisms responsible for the very efficient sequestration of Ag in these crustaceans are most probably related to hepatic metabolism.
It should be noted that crabs were growing during the experiment and it was possible to determine the long-term retention of Ag only because of the use of a radiotracer that could be counted as whole-body activity against a background virtually equal to zero. Indeed, any observation using classical metal measurements (Atomic absorption spectroscopy, Inductively coupled plasma mass spectrometry) would have resulted in Ag concentrations decreasing with time due to growth dilution.