Direct laser infrared microscopy for the monitoring of microplastics in Holothuria poli and sediments of the Mar Menor coastal lagoon

Plastics represent a major threat to marine and terrestrial ecosystems if they are not recycled or disposed properly. Plastics undergo processes of physical erosion and chemical degradation, generating one of the most worrying emerging pollutants: microplastics (MPs), which may be incorporated into the food chain through ingestion by the different organisms that live in that habitat. This study investigates the possible accumulation of MPs in the gut content and integument of deposit feeder sea cucumbers, Holothuria poli Delle Chiaje, 1824 from the Mar Menor lagoon, as well as in the sediments of the areas they inhabit, using laser direct infrared microscopy (LDIR) technique. LDIR is a novel methodology based on illuminating samples with a mid-infrared quantum cascade laser, which was applied to analyse the chemical composition of MPs, as well as their size and shape. This technique offers high sensitivity and selectivity in the measurement of MPs. The most effective sample procedures involved the addition of 3 mL of HNO for 48 h at 65 °C for integument samples (1 g) and 4 mL of HO for 2 h at 65 °C for sediment and gut content samples (1 g), with subsequent density flotation process for these two last matrices. The presence of MPs in all analysed samples was confirmed through the proposed methodology, especially polyamide and polyethylene particles, confirming the accumulation of these materials in the marine organism and the sediments. These results are key to understand MPs pollution in marine ecosystems and its possible impact on marine fauna.