The sea urchin Paracentrotus lividus as a bioeroder of plastic

It is increasingly recognised that plastic pollution of the marine environment is highly dynamic in nature. Larger plastic items are fragmented or eroded into smaller and smaller pieces as its moves through marine ecosystems and small particles can be fouled or flocculate into larger aggregates. Whilst physical processes play a major part in photo- and oxidative degradation of plastic debris, biological process may also contribute to the breakdown of larger plastic items into smaller particulates, yet this has not been studied well to date. Here, we demonstrate the potential for the sea urchin Paracentrotus lividus to act as bioeroders of macroplastics. We found that urchins readily graze on a plastic surface, with this grazing activity generating microplastics, when held in experimental systems together. On average each urchin produced 91.7 (±33.8 pieces) smaller plastic pieces (118-15,797 μm) from one macroplastic item over a ten day period. This plastic fragmentation by the urchins grazing activity was strongly influenced by the additional availability of natural food and by the presence of fouling of the macroplastic surface. Fragmentation of macroplastic by urchins dropped by 97% when urchins were exposed to virgin plastic in the presence of natural food (kelp). However, when macroplastic was biofouled urchins acted to fragment this plastic irrespective of the presence of additional food. The majority of fragments produced were negatively buoyant due to both the biofouling process and indeed the fouling by faecal matter, sinking to the bottom of the exposure systems. This smaller size range of plastic would then bioavailable to a much wider suite of species than the original macroplastic item; hence this bioerosion process has the potential to contribute to the transfer plastic fragments through benthic food webs.