Are Marine Heatwaves Responsible for Mortalities of Farmed Mytilus galloprovincialis? A Pathophysiological Analysis of Marteilia Infected Mussels from Thermaikos Gulf, Greece.

Abstract

Simple SummaryClimate change effects strongly and negatively influence the health and productivity of marine bivalves and particularly bivalve aquaculture. It is therefore of critical importance to study the physiological response of bivalves against marine heatwaves under field conditions. Thus, in the present study, we evaluated the pathophysiological pathways of the Mediterranean mussel originating from mortality events attributed to extreme seawater temperatures in the context of Marteilia refrigens infection. Our study is focused on the Thermaikos Gulf, the most important mussel cultivation area in Greece, comparing the traditional bouchot-like mussel farming system with the modern long-line system. Heatwaves increased all examined molecular, biochemical and pathophysiological markers in M. galloprovincialis mussels, which were reinforced in Marteilia infected individuals. Therefore, these results enlighten us on the biological impacts of heatwaves, providing valuable information regarding the underlying mechanisms. Thus, insights for the future management of the marine aquatic sector could be provided and subsequently address measures for the decrease or elimination of this phenomenon and restoration of marine production.Marine heatwaves (excessive seawater temperature increases) pose high risk to bivalves’ health and farming. The seawater temperature increase is responsible for various pathogen population expansions causing intense stress to marine organisms. Since the majority of knowledge so far derives from laboratory experiments, it is crucial to investigate stress responses in field conditions in order to understand the mechanisms leading to bivalves’ mortality events after exposure to temperature extremes. Thus, we evaluated the pathophysiological response of the Mediterranean mussel Mytilus galloprovincialis originating from mortality events enhanced by intense heatwaves in Thermaikos Gulf, north Greece, along with Marteilia refrigens infection. Mussels that have been exposed to high environmental stressors such as high temperature were examined for various molecular and biochemical markers, such as hsp70, bax, bcl-2, irak4 and traf6 gene expression, as well as the enzymatic activity of the hsp70, hsp90, bax, bcl-2, cleaved caspases, TNFa and ll-6 proteins. Furthermore, histopathology and molecular positivity to Marteilia sp. were addressed and correlated with the gene expression results. Our findings elucidate the molecular and biochemical pathways leading to mortality in farmed mussels in the context of Marteilia infection, which according to the results is multiplied by heatwaves causing a significant increase in pathophysiological markers.