Imaging of microplastics in bio-engineered marine substrates: a neutron tomography approach

Microplastics (MP) are an ubiquitous pollutant that can be considered a planetary boundary threat. The persistence and durability of this synthetic material in the environment allows MP to easily enter the food web and the sedimentary record. Several freshwater (e.g Trychopteran insects) and marine benthic species, such as Sabellariid polychaetes, rely on natural occurring materials as biogenic fragments and sandy grains to build their agglutinated cases. Particularly, Sabellariid polychaetes build arenaceous reefs in the intertidal and subtidal zone along the temperate Atlantic and Mediterranean European coasts. The reef consists of a myriad of arenaceous packed tubes made up of sandy particles mobilized by waves. Sabellariid reefs are dynamic engineered habitats that constitute nutrient and sediment storage (Lisco et al., 2017), thus they play a crucial role increasing local biodiversity and enhancing coastal protection. Recently MP abundance within Sabellaria spinulosa (Leuckart, 1849) reefs from Mediterranean Sea has been investigated (Lo Bue et al., 2023), but the applied method has required a destructive approach: the arenaceous tubes forming the bioconstruction have been digested in hydrogen peroxide and MP particles have been separated from the arenaceous matrix by density. As a result, MP quantification was provided as number of particles per gram of dry sediments. Unfortunately, information on MP distribution within three dimensional bioconstruction samples was irremediably lost. This work aims to perform neutron tomography, combining non-destructive X-ray and neutron imaging, on two portions of S. spinulosa reef. A similar approach had been previously utilized to identify microplastics (MP) in sediment samples (Tötzke et al., 2021). During our study, small reef portions, collected from the Torre Mileto bioconstruction (Gargano promontory, Southern Adriatic) were placed in two laboratory tanks: the first sample was manipulated with the addition of MPs at known concentration and size (up to 2mm); while the second one, utilized as a control, was not manipulated. After one month of exposition, both reef samples were analyzed at ICON, the cold neutron imaging facility at the neutron spallation source, Paul Scherrer Institut (Kaestner et al., 2011). Results showed that most of MP agglutinated within the samples were easily identifiable, but the real content was probably overestimated. Unfortunately, our bioconstruction samples showed additional complexity with respect to sediments previously observed with a similar technique. The agglutinated grains forming the bioconstruction are bounded by an organic glue used to aggregate the tubes. Furthermore, remains of the worm organic tissues could persist partially filling the tube cavities. As additional beamtime has been obtained at PSI facility, further analyses addressed to refine the analytical procedure will allow us to overcome this problem. Kaestner, A.P. et al., 2011. The ICON beamline – A facility for cold neutron imaging at SINQ. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 659, 387–393. https://doi.org/10.1016/j.nima.2011.08.022 Lisco, S. et al., 2017. Sedimentological features of Sabellaria spinulosa biocontructions. Marine and Petroleum Geology 87, 203–212. https://doi.org/10.1016/j.marpetgeo.2017.06.013 Lo Bue, G. et al., 2023. First attempt to quantify microplastics in Mediterranean Sabellaria spinulosa (Annelida, Polychaeta) bioconstructions. Marine Pollution Bulletin 196, 115659. https://doi.org/10.1016/j.marpolbul.2023.115659 Tötzke, C. et al., 2021. Non-invasive detection and localization of microplastic particles in a sandy sediment by complementary neutron and X-ray tomography. J Soils Sediments 21, 1476–1487. https://doi.org/10.1007/s11368-021-02882-6