Mercury speciation in marine birds tissus

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  Mercury is a global contaminant which readily bioaccumulates in aquatic organisms and biomagnifies in marine food webs mainly as neurotoxic methylmercury (MeHg). Its volatile character is responsible for its ubiquity in all the natural compartments around the world whichever the geographical origin of the pollution source. In this context, we investigate the fate of mercury ingested by high predators such as the Giant Petrel sea birds, and evidenced by transmission electron microscopy (TEM) the formation of HgSe nanocristals as a mechanism to detoxify its contaminated food.

Keywords : Pollutant, mercury, transmission electron microsopy (TEM), EDX spectroscopy, crystallography

 

 

Speciation of mercury in marine bird tissues

  Mercury is a global contaminant which readily bioaccumulates in aquatic organisms and biomagnifies in marine food webs mainly as neurotoxic methylmercury (MeHg). Being high in the food webs, seabirds are particularly exposed to MeHg. Total mercury concentration in seabird commonly reaches several hundreds ppm in liver and several tens ppm in muscle and kidney. Due to the high affiity of mercury for selenium, the resistance to such high amounts of toxic mercury is thought to be achieved by detoxification in association with Se through biomineralization as HgSe, similarly to marine mammals.

   This hypothesis was tested on the southern giant petrel Macronectes giganteus using Hg L3-edge X-ray absorption spectroscopy (HR-XANES and EXAFS) and transmission electron microscopy in STEM-HAADF mode coupled to elemental mapping (EDS). HgSe precipitates have been identified in liver (that contains 1499 ppm Hg), kidneys (414 ppm), and pectoral muscle (89 ppm).

  The installation in 2018-19 of a new high performance TEM, the Themis-Z G3, allow to image for the first time in STEM-HAADF mode at 80kV the formation of these precipitates. Larges aggregates of HgSe  3-5 nm nanoparticles were observed in muscles and kidneys (Fig 2-3), up to 40 nm in the liver (Fig. 1). The chemical composition of these precipitates was confirmed by elemental mapping (STEM-EDS). Electron diffraction, high-resolution imaging HRTEM at 300kV (Fig. 1b and 3b) or HRSTEM at -170°C (Fig. 3d) demonstrated the crystalline cubic structure Fd-3m of the HgSe precipitates.

The coupling of XAFS spectroscopies and TEM characterization allowed proposition a detoxification mechanism of MeHg leading to a much higher resistance of these marine birds compared to terrestrial birds.

Figure 1 : (a) Low magnification STEM-HAADF image of the two large HgSe grains in petrel liver tissues. (b) HRTEM image of the two HgSe nanoparticles located in the boxed region of (a); (c) FFT patterns of large grain in (b), corresponding to the [112] zone axis of HgSe, in the cubic Fd-3m space group with a = 6.08 Å;  (d) EDS maps of Hg (red) and Se (green)

 

 

Figure 2 : (a) Low magnification STEM-HAADF image of a small (arrow) and larger (box) aggregate of HgSe nanoparticles in the muscle tissues of petrel; (b) zoom of the larger aggregate boxed in (a); (c) corresponding EDX map of Hg (red) and Se (green); (d) post-acquisition EDX line profile in the box area in (b) of the atomic fraction (%) of Hg (red) and Se (green), with HAADF contrast (black)

 

 

Figure 3 : (a) TEM image of a micron-size aggregate of HgSe nanoparticles in a piece of dried kidney tissue of petrel; (b) HRTEM image of 3 to 8 nm isolated HgSe nanocrystals crystals and FFT of the boxed area showing an hexagonal pattern compatible with the common {111} orientation of the cubic structure HgSe; (c) HRSTEM image obtained from 25 drift-corrected individual frames of two HgSe nanocrystals in the petrel muscle tissues (from low magnification area in the inset), and corresponding FFT image from the lower grain. This STEM image was recorded at −170°C and 60 pA electron current to minimize radiation damage

 

 

References

Article INEE-CNRS : Les oiseaux marins detoxifient le mercure ingéré dans leur alimentation

Vidéo de l'ESRF : The birds that detoxify mercury

In Vivo Formation of HgSe Nanoparticles and Hg–Tetraselenolate Complex from Methylmercury in Seabirds—Implications for the Hg–Se Antagonism, Manceau et al, 2021, Environmental Sciences & Technology, 55(3), 1515–1526

 

Collaborations

A. Manceau, ISTERRE Univ. Grenoble

P. Bustamante, LiENSs Univ. La Rochelle