Contamination of Bivalve Mytilus galloprovincialis, the Case of Radiocobalt in a Context of Environmental Release

Since the end of nuclear weapon testing, anthropogenic metallic radionuclides have originated from nuclear accidents such as Chernobyl and Fukushima and controlled releases from the nuclear industry. ⁶⁰Co is an activation product found in the effluents of nuclear power plants, mobile nuclear reactors, and fuel reprocessing facilities. In this paper, we are addressing the question of (radio)cobalt speciation upon bioaccumulation in the sentinel organism Mytilus galloprovincialis after in vivo contamination in a pseudo-natural system. For this study, inductively coupled plasma mass spectrometry and gamma spectroscopy were used to quantify the cobalt distribution in the various organs: hepatopancreas, gills, visceral mass, mantle, foot, and byssus, as well as in subcellular compartments. Two X-ray spectroscopic techniques were used to decipher cobalt speciation and localization, bulk X-ray absorption spectroscopy (XAS with EXAFS and XANES regimes), and micro X-ray fluorescence imaging (μ-XRF). Lastly, secondary ion mass spectrometry images provided information on cobalt distribution at a subcellular scale. The accumulation of cobalt exhibits significant differences depending on the origin of the individuals, with higher concentration factor values for mussels from the Toulon Naval Base (considered as polluted) compared to Villefranche sur Mer, France (considered as unpolluted). However, concentration in organs always follows the same order: byssus ≫ hepatopancreas ≫ other organs. In terms of spatial distribution, cobalt has been visualized in the hepatopancreas, revealing the presence of preferred zones within some digestive cells and this could be linked to detoxification mechanisms. Finally, the determination of speciation data using XAS suggested the presence of a Co(II)-metallothionein complex in the hepatopancreas and a potential Co(II)-mfp-1 complex in the byssus. While they can be challenging, accumulation and speciation studies in radioecology are essential steps for a comprehensive approach to the impact of trace metallic radionuclides on the marine biota.