Benthic Ecosystems as Reservoirs of Plastic-Associated Bacteria: A Culture-Based Study from the Maldives

Plastic pollution represents a major threat to marine ecosystems, particularly in biodiversity hotspots such as the Maldives, where coral reefs and seagrass meadows sustain essential ecological functions. Filter-feeding and sessile organisms, including sponges, ascidians and soft corals, are particularly exposed to suspended particles and may act as biological concentrators of microplastics, promoting the establishment of specialized microbial communities associated with plastic particles within host tissues. This study explores benthic invertebrates (i.e. ascidians, sponges and soft corals) and surrounding sediments from Faafu Atoll (Maldives) as potential sources of bacteria with plastic-related functional traits. Sampling was conducted at two sites characterized by contrasting levels of anthropogenic influence to assess how environmental pressure may shape the occurrence of plastic-associated microbiota. A culture-dependent approach was employed to isolate bacteria from both environmental matrices and host-associated microbiomes through direct isolation and enrichment cultures. Samples (sediment elutriates and animal homogenates) were directly seeded on Marine agar and on a minimal medium supplemented with 1% (w/v) polyethylene (PE) powder as the sole carbon source, and incubated at 30 °C for 10 days. In parallel, enrichment cultures were prepared by incubating sediment and biological materials in minimal medium amended with 1% (w/v) PE powder at 30 °C for 30 days under constant agitation, and subsequently plated on Marine Agar and minimal medium containing PE, under the same condition as direct isolation. Bacterial isolates were screened for traits relevant to marine bioremediation, including tolerance to heavy metals and ability to grow in the presence of hydrocarbon-like substances, as well as potential involvement in plastic degradation processes, particularly relevant in marine environments where plastics co-occur with other pollutants. Notably, growth observed on minimal media supplemented with plastic suggests the presence of microbial communities adapted to colonise or potentially degrade plastic materials. Our findings support the hypothesis that benthic ecosystems can host microbial communities involved in plastic-related processes, serving as ecological hotspots and potential sources of novel microbial resources for sustainable bioremediation strategies.