Effect of environmental and size related confounding factors in the responsiveness of cell and tissue level biomarkers in mussels Mytilus edulis against pollution along a latitudinal gradient in the north Atlantic sea.

Supervisors: Manu Soto, Denis Benito (PiE-UPV/EHU).
The biomarker approach has been widely used in mussel monitoring programs for several years. However, up to now it has not been commonly used in high latitude study areas. Different environmental and biological factors, such as pH, temperature, nutrients, photoperiod, as well as the metabolism, age, reproductive condition, and growth rates of the biota may vary with latitude. These can be considered confounding factors as they can induce a biomarker response not related to the presence of pollutants. Pathological conditions non related to chemical insult can affect gametogenesis as well as on the biomarker responses representing another confounding factor. Therefore, establishing a baseline biomarker variation along this latitudinal gradient could be a useful tool for the assessment of the overall pollution effects on this species. In order to establish these reference values for selected biomarkers in the Northern Atlantic Ocean, mussels of two sizes (small, 2-3 cm; large, 3.5-4.5 cm) from selected polluted (commercial harbors & ports, WWTP dumping area) and less impacted sites in Oslo (59 º 87´N), Trondheim (63º 26´N) and Tromsø (69º 40´N) were sampled in late summer 2017. Cellular level biomarkers such as neutral lipid accumulation, lysosomal membrane stability (LMS) and structural changes (LSC) were determined. In addition, tissue-level biomarkers including atrophy, basophilic cell volume density (VvBAS), relative proportion of digestive and connective tissue (CTD) and histopathological alterations in the digestive gland were measured. Moreover, PAH concentrations, gamete development and adipogranular cell levels were determined for each site and size of mussels. Finally, an IBR/n index was calculated using five biomarker responses representing different organization levels and/or general stress processes. Different biomarker responses were recorded indicating a difference among small mussels in the three study areas, possibly due to differences in growth rates and thus age and reproductive status. Variability in the response along the two sizes was significant only in some sites and endpoints, while variable responses of biomarkers to pollution were detected in all three study sites and most endpoints. There is a need to perform environmental variable measurements in order to elucidate the biomarker responses in these three study areas, and confidently define the baseline values of the given biomarkers. Overall, the most stressed situation was found in both sizes of mussels in the Oslo harbor and large mussels in the Tromsø harbor, while the least stressed were both sizes of mussels in the less impacted site and small mussels from the WWTP site in Trondheim. In conclusion, the selected battery of biomarkers was useful to discriminate mussel populations from non-polluted and chemically impacted sites, even with the simultaneous occurrence of confounding factors such as latitude-related environmental conditions and age of the animals.