Microplastic pollution in different environmental matrices of Tyrrhenian Sea’ marine caves

Microplastic (MP) pollution has been detected in many habitats of the marine ecosystem, but it was never studied in marine caves before a few years ago. Investigating the marine caves for the MP's presence in different matrices (water, sediment, and biota) could be useful to quantify the anthropogenic pressures in this habitat and provide useful indications for their protection and conservation. Marine caves have widely variable shapes and sizes due to changing geological and geomorphological characteristics and are affected by wide spatial and temporal environmental variability and/or extreme conditions. Due to this high environmental variability, they are biodiversity hot spots and refuge habitats. However, they are particularly vulnerable to pollution because, due to the enclosed conditions and scarce water exchange, pollutants may remain in the water, stored in the sediments, and incorporated by living organisms. To date, three caves in the Tyrrhenian Sea with very different characteristics have been studied for microplastic pollution: Bue Marino (Sardinia), Argentarola (Tuscany), and Ficarella (Sicily). MPs have been detected in water and sediments and also in the shells of sediment-dwelling single-celled organisms (benthic foraminifera). Results highlighted MPs in all the cave sediments: Argentarola (7.8 ± 3.6 items/kg), Bue Marino (17.2 ± 7.7 items/kg), and Ficarella (39.3 ± 40.8 items/kg). In all cases, MPs are mostly fragments and fibers <0.5 mm, while the composition is variable among caves, with Polyethylene and Polypropylene often abundant. Polyethylene and Polypropylene were detected in the shells of some species of agglutinated benthic foraminifera. This study demonstrates that MPs, even if at low concentrations, have contaminated cave habitats and that benthic foraminifera, at the beginning of the trophic chain, have used items to build their shells.