Microplastic Bioaccumulation and Oxidative Stress in Key Species of the Bulgarian Black Sea: Ecosystem Risk Early Warning

Plastic pollution in marine environments poses a new global threat. Microplastics (MPs) can bioaccumulate in marine organisms, leading to oxidative stress (OS). This study investigates MP accumulation and associated OS responses in six invertebrate species (Bivalvia, Gastropoda, and Malacostraca) and three key fish species of the Bulgarian Black Sea ecosystems. The target hydrobionts were collected from nine representative coastal habitats of the northern and southern aquatory. MPs were quantified microscopically, and OS biomarkers (lipid peroxidation, glutathione, and antioxidant enzymes) were analyzed spectrometrically in fish liver and gills and invertebrate soft tissues (STs). The specific OS (SOS) index was calculated as a composite indicator of the ecological impact, incl. MP effects. The results revealed species-specific MP bioaccumulation, with the highest concentrations in Palaemon adspersus, Rathke (1837) (0.99 ± 1.09 particles/g ST) and the least abundance in Bittium reticulatum (da Costa, 1778) (0.0033 ± 0.0025 particles/g ST). In Sprattus sprattus (Linnaeus, 1758), the highest accumulation of MPs was present (2.01 ± 2.56 particles/g muscle). The correlation analyses demonstrated a significant association between MP counts and catalase activity in all examined species. The SOS index varied among species, reflecting different stress responses, and this indicated that OS levels were linked to ecological conditions of the habitat and the species-specific antioxidant defense potential to overcome multiple stressors. These findings confirmed the importance of environmental conditions, including MP pollution and the evolutionarily developed capacity of marine organisms to tolerate and adapt to environmental stress. This study emphasizes the need for novel approaches in monitoring MPs and OS to better assess potential ecological risks.