“Plasticagglutinated” reef by the genus Sabellaria Lamarck 1818 (Polychaeta, Annelida) in Mediterranean and Atlantic: microplastic pollution and associated risk

Microplastic pollution in the marine environment is a global threat, particularly in coastal systems, where this anthropogenic pollutant arrives from lands and can temporarily accumulates, before its subsequent transport offshore. Despite microplastics are considered an ubiquitous pollutant, little is known about their effects on benthic organisms that live in this transitional environment, particularly those that utilize the sandy grains to build their external protections (e.g. some marine sedentary polychaetes belonging to the Family Sabellaridae). This PhD thesis aims to fill this knowledge gap by investigating the possible impact of microplastic pollution on reef-building polychaetes of the genus Sabellaria, specifically S. spinulosa and S. alveolate from both Mediterranean and Atlantic sites. These marine organisms are important ecosystem engineers in temperate coastal areas, where they build large and complex, agglutinated reefs, that in turn, provide essential habitat, foster biodiversity, and also significantly contribute to coastal protection. Given their significant ecological role in the littoral environment, elucidating the effects of microplastics when entered these ecosystems is paramount for effective conservation and management strategies. In this thesis, the study of microplastics pollution in sabellariid bioconstructions based on a pioneering, multidisciplinary approach that combines specific techniques and methods from both Geological and Biological Sciences. This PhD research successfully integrates traditionally separate aspects by examining both the “geological aspect” of the bioconstruction (the living agglutinated rock) and the “biological aspect” of the reef-building organisms (ecology and organism physiological response). The comparative analysis of microplastics quantified in both bioconstruction and surrounding sediment, offered new insights into the accumulation mechanism, while laboratory manipulation experiments shed light on adverse effects induced by MPs pollution. The first step of the Phd research was addressed to fill a substantial methodological gap: to effectively extract microplastics from a complex biogenic matrix where microplastics are cemented with the other sedimentary grains. Consequently, a standardized, highly reproducible, and scientifically validated protocol for microplastic extraction, identification and quantification was developed. Subsequently, comparative field investigations were conducted across two very different environments: the largest known Mediterranean S. spinulosa reef and S. alveolata reefs developed along the Atlantic French coast—among the largest bio-engineered structures in Europe. Findings from both Mediterranean and Atlantic consistently highlighted a passive "trapping mechanism," revealing that these biogenic structures exhibited similar or higher microplastic abundance patterns compared to adjacent shoreface sediments. These observations confirm that Sabellariid reefs act as sedimentary traps for microplastics in the littoral environment. During the final step of the PhD the potential effect of microplastic on the physiological status of Sabellaria spcimens was assessed. Initially, a comprehensive physiological baseline characterization was established for S. spinulosa, detailing antioxidant defense mechanisms and glycolytic metabolism strategies, including size-related variations in enzyme activity. Subsequently, a controlled laboratory experiment assessed the effects of microplastic exposure on S. alveolata. Employing a multi-proxy approach that monitored feeding behavior alongside stress biomarkers, the experiment yielded robust evidence of stress responses induced by presence of microplastics.