Exploring Microplastics Contamination in Mediterranean Octocorals

Marine litter (ML), and particularly microplastics (MPs), has emerged as a pervasive form of pollution affecting marine ecosystems from coastal areas to the deep sea, with increasing evidence of their occurrence across environmental compartments and taxonomic groups. Despite growing attention, deep-sea ecosystems and habitat-forming invertebrates remain poorly explored, especially in the Mediterranean Sea. Octocorals are among the most emblematic and ecologically relevant components of Mediterranean benthic communities, from shallow circalittoral assemblages to deep-sea coral forests, where they act as ecosystem engineers by increasing habitat complexity, supporting associated biodiversity, and contributing to benthic–pelagic coupling. Their functional role, longevity, and capacity to interact with particulate matter make them suitable indicators of environmental contamination; however, information on MP occurrence and biological effects in Mediterranean octocorals is still extremely limited. This PhD thesis aimed to characterize key aspects of MP pollution in Mediterranean octocorals through an integrated approach combining methodological development, field-based assessment, and experimental exposure studies. The first objective was to develop and validate a reliable digestion protocol for the isolation and identification of MPs from octocoral tissues, addressing the lack of standardized methodologies for anthozoans. Using the deep-sea bamboo coral Isidella elongata as a model species, a wide range of chemical digestion approaches was systematically tested in order to identify a protocol that ensured efficient organic matter removal while preserving MP integrity. This methodological framework provided the baseline for all subsequent investigations presented in this thesis. Following protocol validation, MP occurrence was assessed under natural conditions in I. elongata collected from a poorly explored deep-sea area of the Ionian Sea (Western Mediterranean). MPs were quantified in coral colonies and in deep-sea fish species living in close association with coral forests, using these organisms as biological indicators of contamination. In addition, seafloor ML deposited around coral collection sites was characterized to provide environmental context. By integrating biological and environmental compartments, this study represents one of the first ecosystem-based assessments of ML pollution in Mediterranean deep-sea coral forest systems. In addition, the first evidence of MP occurrence in Mediterranean octocorals was reported, with concerning implications for cold-water coral habitats. To further investigate biological interactions with MP, experimental exposure studies were conducted under controlled laboratory conditions. Due to practical constraints associated with the rarity and maintenance of I. elongata, experiments were performed on two Mediterranean mesophotic gorgonians with established laboratory protocols, Paramuricea clavata and Eunicella singularis, which share key functional traits with deep-sea octocorals, including suspension feeding and habitat-forming roles. Two complementary experiments were carried out, targeting different life history stages. The first focused on adult colonies of P. clavata, assessing MP ingestion and egestion dynamics during exposure. The second addressed early life history stages of E. singularis, evaluating larval survival, settlement, and MP uptake. By covering both adult and early developmental phases, this work provides insight into potential MP impacts across multiple biological levels. Overall, this thesis contributes novel methodological tools, field-based evidence, and experimental data to the understanding of MP contamination in Mediterranean octocorals. By integrating different spatial scales, ecological compartments, and life stages, it highlights the relevance of habitat-forming corals as indicators of ML pollution and underscores the need for further research on MP impacts in mesophotic and deep-sea ecosystems.