The combined effects of polystyrene microplastics and temperature stress on Mytilus galloprovincialis, Lamarck, 1819

Microplastic pollution and rising seawater temperature are widely recognized threats to marine biota, but their combined impacts remain poorly explored. In this context, the present study investigated whether the exposure of Mytilus galloprovincialis to environmentally relevant polystyrene microplastics (100 MPs/L) is amplified by 5 days of exposure to rising temperatures (22 °C, 25 °C, 28 °C), by evaluating the mussels' mortality, oxidative damage, and histological responses. Results showed a progressive increase in oxidative damage, with a noticeable rise in Malondialdehyde (MDA) content. Mortality occurred only at the highest temperature (28 °C), with clear signs of histological damage reported for digestive glands and gills, highlighted by lipofuscin accumulation, haemocyte infiltrations, epithelial alterations, inflammatory elements, melanin aggregates, and vacuolation. Results provided clear evidence of the combined effects of microplastic ingestion and elevated temperature on marine mussels, improving the knowledge base on the synergic impact that climate change and plastic pollution can have on marine biota. Further studies investigating their interacting effects are essential to better understand the fundamental mechanisms of chronic exposure. • Environmentally relevant MPs and elevated temperature induced histological and oxidative damage in Mytilus galloprovincialis . • Mortality occurred only at 28 °C, mainly in microplastic-exposed mussels. • MDA levels significantly increased in gills and digestive glands under higher temperatures and microplastic exposure. • Histological analyses showed clear signs of inflammation in both gills and digestive gland tissues. • A total mortality of 7% was recorded across all groups after 5 days of exposure.