Microplastic quantification in Sabellaria reefs: a validated protocol for extraction from biogenic agglutinated matrices.

Microplastic pollution affects all marine ecosystems, particularly coastal areas inhabited by sedentary reef-building organisms that rely on sand grains to build arenaceous reefs (e.g., Sabellariid polychaetes). These agglutinated reefs passively trap microplastics, thus increasing the potential risk to benthic organisms that live on and within the reef. An accurate quantitative assessment of microplastics accumulated within these arenaceous reefs is currently hindered by a lack of standardized methodologies. This study addresses this gap by developing and validating a reliable and reproducible protocol specifically designed to extract and quantify microplastics cemented within bioconstructed agglutinated matrices. The proposed protocol evaluated digestion procedures aimed at the release of microplastics from agglutinated matrices. The subsequent density extraction procedure was validated via a spiking experiment using both bioconstruction and sediment samples spiked with known quantities of polyethylene terephthalate, polypropylene, and polyvinyl chloride. Scanning electron microscopy and µ-Raman spectroscopy confirmed that the adopted digestion procedures did not alter the plastic polymers. Results also showed that the NaI solution yielded a significantly higher microplastic recovery than NaCl. Notably, microplastic recovery using NaCl was influenced by the initial sample weight, suggesting possible matrix interference at higher sample weights. Our multistep approach provides a validated, cost-effective, and reproducible protocol that improves microplastic quantification in agglutinated matrices. By employing common laboratory equipment and specific procedures, this methodology represents a significant step towards standardizing microplastic pollution monitoring in coastal bioengineered habitats. HIGHLIGHTS: • A step-by-step protocol for MP density extraction from biogenic agglutinated matrices was validated. • Agglutinated matrices require disaggregation to release MP, ensuring a correct density extraction. • Preliminary drying and disaggregation procedures do not alter the chemical integrity of MP. • NaI solution is significantly more efficient than NaCl for MP extraction. • Substrate type (sediment vs Sabellariid bioconstruction) had no influence on  density extraction efficiency.