Morphological,compositional and ultrastructural changes in the Scrobicularia plana shell in response to environmental mercury – An indelible fingerprint of metal exposure?

The study aimed to assess morphological, structural and compositional alterations in Scrobicularia plana nacre environmentally exposed to mercury in order to seek out the possibility of the assessed alterations as a monitoring tool to handle complexity and interactions of metals in the environment involving a non-invasive methodology. Bivalves were collected from a mercury contaminated site (Laranjo basin – Ria de Aveiro, Portugal) and a reference site in the same aquatic system. The combination of scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) technique depicted a sheet like morphology of bivalve nacre collected from the reference site. Moreover, EDS plot exhibited the presence of potassium, oxygen, calcium, and carbon elements. Shells collected from the contaminated area depicted lamellar patches like structures with particle like morphology composition. SEM images corresponding to the elemental analysis by EDS plot clearly denoted the presence of mercury. SEM images from the other locations of the contaminated shells depicted large surface area, a broken or ruptured symmetry of organic matrix as well as crack-like gaps. The influence of environmental mercury affecting the surface morphology of S. plana nacre showed dimple like morphology (as proved by transmission electron microscopy, TEM). The possible explanation may be the replacement of calcium elements with other elements or alloys from the nacre composite collected from contaminated region. Therefore, the nacre fingerprint may be useful as innovative knowledge and applicable tool aiming at risk reduction from noxious mercury present in the environment. Overall results suggested the use of shell as an indelible fingerprint of metal exposure.