Microplastic dynamics in the littoral reefs created by Sabellariid Polychaetes in the southern Adriatic Sea

The Mediterranean Sea stands out as a semi-enclosed basin with high density of population who lives along the coastlines: this implies a high concentration of microplastics (MPs) originating from land sources. For this reason it can be considered a “Plastic Soup” (Suaria et al., 2016), with marine sediments playing the role of primary MPs sink. The impact of MPs extends beyond their presence in the sedimentary record and their effects on marine life are still largely unknown: for example, the recent record of MPs incorporated into the arenaceous bioconstructions of sedentary marine worms (Sabellariid polychaetes) has arisen interest(Mancin et al., 2022). Sabellaria spinulosa (Leuckart, 1849) reefs, thriving in subtidal littoral environments, are formed bya myriad of packed arenaceous tubes, meticulously built using agglutinated sand particles. This engineered substrate not only serves as a vital habitat for the polychaete and associated biota, but also plays a crucial role in the coastal ecosystem dynamics as a nutrient and sediment storage (Lisco et al., 2017). In this context, this works aims at understanding the dynamics of MPs interactions within the seafloor environment associated with Sabellariid reefs, and specifically to assess the accumulation of MPs within the largest known Mediterranean S. spinulosa reef in the South Adriatic Sea. Samples of sea-floor sediment and bioconstruction (36 replicates in total) were collected along six transects from the Torre Mileto reef (41 ◦ 55045.3” N; 15 ◦ 37012.37” E) to compare the abundance of MPs in both substrates. MPs were extracted by density difference and submitted to census count using a stereomicroscope equipped with a UV light. Suspected MPs and other anthropogenic particles sized 2μm to 5000μm were identified based on morphology, colour, fluorescence, and physical response (for example to a hot needle). Quantitative data were reported as number of particles per gram of dry sediment; MPs were classified into size classes and morphological categories. Furthermore, μRaman spectroscopy was performed on twelve samples (33% of the total samples) to obtain validated data on MPs composition. Results document MPs presence in all the collected samples, with fibers more abundant than fragments. The absolute abundance of MPs resulted comparable between sediment and reef substrates. Data suggest a mutual influence between the reef and the neighbouring sediment: the peculiar dynamic status of the larger Sabellariid reefs, characterized by an alternation between growth and erosion phases during the year, can control the abundance of MPs in the associated sediment. In particular, while the ancient, more stable portion of the reef are acting as sinks, the younger unstable one can become source of sediment and MPs. Lisco, S. et al., 2017. Sedimentological features of Sabellaria spinulosa biocontructions. Marine and Petroleum Geology 87, 203–212. https://doi.org/10.1016/j.marpetgeo.2017.06.013 Lo Bue, G. et al., 2023. First attempt to quantify microplastics in Mediterranean Sabellaria spinulosa(Annelida, Polychaeta) bioconstructions. Marine Pollution Bulletin 196, 115659. https://doi.org/10.1016/j.marpolbul.2023.115659 Mancin, N. et al., 2022. Fractal analysis highlights analogies in arenaceous tubes of Sabellaria alveolata (Metazoa, Polychaeta) and agglutinated tests of foraminifera (Protista). PLoS ONE 17, e0273096. https://doi.org/10.1371/journal.pone.0273096 Suaria, G. et al., 2016. The Mediterranean Plastic Soup: synthetic polymers in Mediterranean surface waters. Sci Rep 6, 37551. https://doi.org/10.1038/srep37551