Macroalgal habitat shapes physiological responses of associated invertebrates to legacy and emerging contaminants.

Pollution from legacy and emerging contaminants is a major threat to coastal ecosystems. Yet, the influence of habitat-forming species on the physiological responses of associated fauna to these stressors remains poorly understood. Macroalgal forests create complex environmental conditions that may alter contaminant sensitivity, but this possibility has received little experimental attention. To address this gap, the present study investigated the role of macroalgal forests, usingGongolaria barbata as a model habitat-forming species, in modulating the physiological responses and sensitivity of the invertebrates Carcinus aestuarii and Ophiothrix fragilis to mercury and glyphosate contamination. In O. fragilis, contaminant-specific responses were observed, with mercury reducing respiration rate and glyphosate increasing ammonia excretion and lipid peroxidation. However, prior acclimation to macroalgae did not consistently alter these responses, suggesting limited habitat-mediated modulation in this species. Conversely, C. aestuarii showed clear acclimation-dependent responses. Acclimation to macroalgal habitat conditions strongly reduced lipid peroxidation in the hepatopancreas independently of treatment and modulated antioxidant responses in a contaminant-specific manner. Haemocyte size also responded to the interaction between acclimation and mercury exposure, supporting the involvement of immune-related mechanisms in stress modulation. Results from crabs indicate a potential effect of the algal habitat on oxidative balance and detoxification pathways. Overall, our results support the existence of a macroalgal habitat effect on the sensitivity of associated fauna to contaminants, although this effect is strongly species-, tissue-, and compound-specific. These findings highlight the importance of incorporating ecologically relevant biotic interactions into ecotoxicological frameworks.