Impacts of microplastics on reef-building corals: Disentangling the contribution of the chain scission products released by weathering

Microplastics (MPs) have recently been shown to impact the health of corals negatively. The primary effects are linked to the physical interaction of the particles with coral tissues, such as abrasion that causes wounds. Additionally, MPs can leach contaminants into the seawater, not only the formulation additives but also molecular fragments resulting from the photo-oxidative degradation of the plastic polymer. These contaminants may have further detrimental effects. Currently, the relative contributions of these factors and their potential synergistic actions are not well understood. To address this, we conducted tests on nubbins of the soft coral Pinnigorgia flava and we evaluated the toxicity of reference additive-free MPs (LDPE and PP, sized 125–250 μm) before and after photo-aging, of MPs collected from beaches, and of the corresponding leachates. By FTIR and SEM analysis, we highlighted similarities between photo-aged reference MPs and the beached MPs, both in terms of surface oxidation and structural defects. GC–MS and dissolved organic carbon (DOC) analysis indicated the release of a variety of molecular species from the photo-aged MPs not detected in the pristine counterpart (accounting for 0.6–2.1 % of the original mass), including short-chain poly-oxidized compounds. The exposure of nubbins to the reference MPs highlighted a significant mucus production with PP and LDPE at 1 mg/L. Leachates from the photo-aged PP induced significant polyp retraction at 10 mg/L, while the leachates from photo-aged LDPE induced significant polyp retraction in P.flava at the concentration of 1 mg/L. No significant stress responses were highlighted with the photo-aged MPs and the leachates from pristine MPs. Beached MPs induced significant responses with the lowest observed effect concentration (LOEC) at 0.1 mg/L. This effect was related to the occurrence of plastic additives not detected in the reference MPs. Overall, the results highlighted the importance of considering the contribution of the photo-degradation products released by MPs in ecotoxicological assessments.