Underwater meadows of the seagrass Posidonia oceanica in the Mediterranean Sea are proving to be a significant natural defense against microplastic pollution. These vital marine plants form structures known as 'Neptune balls,' which effectively capture tiny plastic particles. As these Neptune balls are naturally expelled from the marine environment and wash ashore, they carry the trapped microplastics with them, showcasing nature's own clean-up mechanisms.
A 2024–2025 study highlighted the substantial role of Posidonia oceanica in microplastic capture, indicating that the plant's rhizomes accumulate these pollutants at a higher rate than its leaves. This natural filtration is crucial for mitigating microplastic contamination. However, the research also suggests potential impacts on benthic food webs due to this accumulation. Researchers from the University of Barcelona, led by Anna Sanchez-Vidal, found that Neptune balls can contain a high density of microplastics, with some samples yielding nearly 1,500 pieces per kilogram. It is estimated that these seagrass meadows collectively trap close to 900 million plastic items in the Mediterranean annually.
Neptune balls, also called aegagropilae, are formed from the fibrous remains of Posidonia oceanica leaves that tangle together. While their plastic-trapping capability is a recent discovery, the seagrass itself has long been recognized for its ecological importance. Posidonia oceanica meadows are essential for improving water quality, sequestering carbon dioxide, producing oxygen, and serving as critical nurseries and refuges for marine biodiversity. They also contribute to coastal protection by preventing erosion and dampening storm surges.
Despite their environmental benefits, Posidonia oceanica meadows face considerable threats, including climate change, invasive species, pollution, coastal development, and destructive fishing practices like bottom trawling. Conservation efforts are ongoing, with notable initiatives such as the EU LIFE+ Posidonia Andalusia project aiming to restore and protect these vital seagrass ecosystems. These projects involve mapping meadow extents, implementing measures against degradation sources, and raising public awareness. The decline of these meadows, with estimates suggesting a loss of up to 50% of their potential area since 1960 in the Mediterranean, underscores the urgency of these conservation efforts.