pCO2-driven seawater acidification affects aqueous-phase copper toxicity in juvenile flounder Paralichthys olivaceus: Metal accumulation, antioxidant defenses and detoxification in livers.

Ocean acidification potentially influences the biotoxicity of metals and the antioxidant defense systems of marine organisms. This study investigated how pCO2-driven seawater acidification (SA) affected aqueous-phase copper (Cu) toxicity in the juvenile flounder Paralichthys olivaceus from the perspective of hepatic oxidative stress and damage to better understand the mechanisms underlying the biological effects produced by the two stressors. Fish were exposed to aqueous-phase Cu at relevant ambient and polluted concentrations (0, 5, 10, 50, 100 and 200 µg L-1) at different pH levels (no SA: pH 8.10; moderate SA: pH 7.70, pCO2 ∼1353.89 µatm; extreme SA: pH 7.30, pCO2 ∼3471.27 µatm) for 28 days. A battery of biomarkers in the livers was examined to investigate their roles in antioxidant defense and detoxification in response to coexposure. Hepatic Cu accumulation (30.22-184.90 mg kg-1) was positively correlated with Cu concentrations. The biomarkers responded adaptively to different redox states following SA and Cu exposure. In unacidified seawater, increases in Cu concentrations significantly induced hepatic lipid peroxidation (LPO, by up to 27.03 %), although compensatory responses in antioxidant defenses and detoxification were activated. Moderate SA helped maintain hepatic redox homeostasis and alleviated LPO through different defense strategies, depending on Cu concentrations. Under extreme SA, antioxidant-based defenses were activated to cope with oxidative stress at ambient-low Cu concentrations but failed to defend against Cu toxicity at polluted Cu levels, and LPO (by up to 63.90 %) was significantly induced. Additionally, thiols (GSH and MT) responded actively to cope with Cu toxicity under SA. SOD, CAT, EROD, and GST were also sensitively involved in defending against hepatic oxidative stress during coexposure. These findings highlight the notable interactive effects of SA and Cu and provide a basis for understanding antioxidant-based defenses in marine fish confronting environmental challenges.

Identification of unique species in the marine ecosystem based on the weighted trophic field overlap.

Based on the trophic field overlap of species in the food webs, we propose using the weighted trophic field overlap (WTO) to determine the uniqueness of species in a topological network by considering the food web structure and the proportions of prey in the diets of predators. This proposed method measures uniqueness structurally and mathematically and considers cannibalism and mutual predation between species to overcome the deficiencies of the traditional method (the sum of trophic field overlap, STO), which only relies on the topological structure of the food web. Species with the lowest WTO values have high interaction strengths with other species in the food web weighted by the proportion of prey and play important roles as prey in the initial ecosystem, which are not recognized by the traditional method. The proposed index is sensitive to changes in the diets of predators since slight fluctuations may cause the index to vary considerably. The proposed methodology could be extended to other marine ecosystems to identify unique species from a practical and dynamic perspective and will contribute to the protection of unique species that maintain the trophic diversity of food webs and ecosystem robustness. •A WTO index was proposed for identifying unique species in food webs.•This index considers both the topological network structure and proportion of prey.•Cannibalism and mutual predation between species are also accounted for.