Unveiling the Hidden Threat : Tracking Uranium Accumulation in Marine Mussels with Dynamic SIMS

The Silent Intruder in Our Oceans

Uranium, a naturally occurring radioelement of the actinide family, is found in our marine ecosystems, primarily as a resource but also through human nuclear activities. Understanding its environmental impact is not just a scientific curiosity but uranium may also serve as a chemical model for other heavier actinidaes. The distribution, speciation, and bioaccumulation of uranium in marine organisms can reveal much about the accumulation and transfer of actinide elements within oceans trophic system and the potential risks to human health. This blog delves into a recent study that uses Dynamic Secondary Ion Mass Spectrometry (D-SIMS) to track uranium accumulation in marine mussels, offering a window into the unseen world of radioactive contamination. The research, conducted by researchers from the Institute of Chemistry of the Côte d’Azur University, with analytical support from PATERSON platform at the French Institute of Radioprotection and Nuclear Safety. 

The Mussel: Nature’s Sentinel

Marine mussels, particularly Mytilus galloprovincialis, serve as excellent bioindicators due to their stationary nature and ability to filter large volumes of seawater during their respiration and feeding phase. By examining these organisms, researchers can gain valuable insights into the levels and effects of uranium in marine environments. 



This study focuses on mussels from two distinct sites on the French Mediterranean coast: Toulon Naval Base (TNB) and Villefranche-sur-Mer (VFM), providing a comparative analysis of uranium accumulation. While overall uranium concentrations were found to be similar between the two sites, a deeper dive revealed that mussels from TNB had significantly higher levels of uranium in their hepatopancreas. This organ, crucial for digestion and detoxification, became the focal point for further analysis, alongside the byssus—the threads mussels use to anchor themselves.

Key findings from Dynamic SIMS analysis 

Dynamic Secondary Ion Mass Spectrometry (D-SIMS) emerged as a key analytical tool for this study. Its high spatial resolution and ability to detect trace elements provided detailed insights into the distribution and accumulation of uranium in marine mussels. It allowed researchers to obtain high-resolution ion maps of uranium isotope 238, as well as of ²³Na⁺ and ⁴⁰Ca⁺ for histological context.

Two contamination procedures were implemented to obtain information on uranium accumulation. In vivo contamination is characterized by the mussel producing a byssus thread during uranium exposition, while ex vivo contamination involves exposing byssus threads to a seawater solution doped with uranium.

Study of in vivo contamination using D-SIMS showed that uranium was present in the core of the byssus threads (Figure 1). No visible occurrence of U-precipitates (uranium condensed inorganic phases) appears in the byssus images, suggesting homogeneous storage rather than precipitation. This homogeneous distribution indicates that uranium is stored in a soluble form, likely as uranyl ions, within the thread matrix.


Figure 1: Byssus contaminated in vivo in doped sea water with [U] = 5.10^-5 M during 12 days. Histological image (HI) and D-SIMS ion images of ²³Na⁺, ²³⁸U⁺ and overlay of  ²³⁸U⁺ (in red) and ²³Na⁺ (in blue). Green arrows help to identify byssus.

Byssal thread samples were also analyzed by D-SIMS imaging (Figure 2) to obtain information on the distribution of uranium after ex vivo contamination. D-SIMS images reveal a thin uranium layer around the byssus threads, localized at the level of the protective cuticle. Data also suggest the diffusion of uranium from the cuticle into the thread matrix, indicating the migration of a soluble form of uranyl.


Figure 2: Byssus contaminated ex vivo in doped seawater with [U] = 10^-3 M during 12 days. Histological image (HI) and D-SIMS ion images of  ²³Na⁺, ⁴⁰Ca⁺, ²³⁸U⁺, overlay of ²³⁸U⁺ (in red) and ²³Na⁺ (in green), and overlay of  ²³⁸U⁺ (in red) and ⁴⁰Ca⁺ (in green). Red and blue arrows help identify and differentiate byssus. Yellow squares (1) and (2) indicate the areas with uranium in the cuticle and diffused uranium in the matrix.

Several analytical techniques were used for this study prior to D-SIMS: ICP-MS quantified uranium distribution in various mussel organs, X-ray absorption deciphered uranium speciation, and TEM provided information on uranium distribution at different scales, from organ to sub-cellular levels. However, due to its high lateral resolution, D-SIMS offered a more precise localization of uranium within the sub-structure of the aquatic mollusc, complementing the data from these other techniques. In fact, only D-SIMS was able to accurately highlight the diffuse localization of uranium, suggesting the migration of a soluble form of uranyl within the biological matrix.

Romain Stefanelli, PhD Student at the time of the study, and David Suhard, Manager of the SIMS (Secondary Ion Mass Spectrometry) imaging platform at IRSN.

Comparative Findings and conclusions

D-SIMS images showed that after in vivo contamination most of the uranium is contained inside the thread core, implying an active elimination mechanism by the mussels. In contrast, images obtained after ex vivo contamination showed uranium mainly on the cuticle surface. This localization was not detected after in vivo contaminations, which could be due to the lower uranium concentration compared to ex vivo experiments. This study revealed that uranium accumulates in a diffuse pattern, likely linked to protein complexing functions, without forming a condensed inorganic phase. 

Such studies on uranium accumulation and localization in marine organisms are crucial for understanding the behavior and impact of metallic radionuclides (actinides) in marine environments, particularly following accidental releases.

Dynamic SIMS: The Analytical Powerhouse

The ability to detect trace elements and capability to provide imaging of elemental distribution at high spatial resolution within biological tissues make D-SIMS a powerful tool for studying the bioaccumulation and speciation of elements like uranium in marine organisms. This study underscores the importance of using advanced analytical techniques to monitor and understand the environmental impact of radioactive contaminants in marine ecosystems.


For more information:
https://www.sciencedirect.com/science/article/abs/pii/S0013935124007813

Authors: Laura Créon (SIMS Product Manager), Paula Peres (Applications Lab Manager)
Other participants / Contributors: David Suhard (SIMS Imaging Platform Manager at IRSN), Romain Stefanelli (PHD Graduate Student, Côte d’Azur University)