Plastic pollution is one of the toughest challenges of our edge. In the marine environment, these materials cause damage to the ecosystem, harming flora and fauna. Once entered into the sea plastic can be weathered by biotic and abiotic factors causing the formation of (secondary) micro (MPs) and nanoplastics (NPs). These tiny particles can interact with marine fauna invading the trophic chain, contaminating marine life from plankton to higher eucaryotes: in particular fishes and mussels are widely studied because directly connected with the human diet. MPs have been consistently investigated in the last decades and have been detected in diverse marine animal tissues. NPs are more challenging to monitor due to their size and low concentration in natural samples which often make them undetectable by analytical instruments. However, despite that, they have been detected in all the environmental compartments (air, water, soil)^1–4 and now also in farmed mussels (mussel = M).

To provide a size distribution of micro and nanoplastics (MNPs) in the organisms, 5 mussel specimens from a local Apulian (Italy) fish market were bought and analyzed with the Thermal Desorption – Proton Transfer Reaction – Mass Spectrometry (TD-PTR-MS). This technique combines high sensitivity with high mass resolution, revealing plastic presence in environmental samples with a limit of detection of <10 ng, which makes it perfect for investigating these very tiny little analytes. On the other hand, this machine can be overloaded by the presence of organic matter and is not able to provide MNPs morphological information (size and shape).

Unfortunately, when dealing with environmental samples, the presence of organic matter is an important issue to manage. Especially in this case, where the micro and nanoplastics are completely embedded in the mussel tissues. Thus,  removing as much organic matter as possible from the animals, and size-sorting the particles in their tissues before the TD-PTR-MS analysis are mandatory steps. To digest mussels, an optimized protocol was used. This procedure removed 98% of the mussels’ body and allowed us to proceed with a six-step cascade filtration to sort the particles in six size fractions: from 2.2 µm up to 0.02 µm (2.2 -1.4 µm; 1.4 µm – 0.7 µm; 0.7 µm – 0.4 µm; 0.4 µm – 0.2 µm; 0.2 µm – 0.02 µm), covering both the micro and the nano-size range.

NPs in the smallest size range (20 – 200 nm ) were detected in all 5 specimens with highly alarming results: our data ranges from tents to hundreds ng NPs/mg DW and may mirror the real environmental contamination variability.  Polyethylene (PE), PVC, and Polystyrene (PS) were detected in all the analyzed individuals, while Polypropylene (PP) and PET were found in three out of five samples (Figure 1).

 The most contaminated organism was selected to analyze the entire size range. The highest amount of plastics was found to be bigger than 2.2 µm or with a size within 20 -200 nm. At this point, we decided to set a threshold at 700 nm and compare micro and nanoplastics data (Figure 2). Polymer distribution and composition resulted quite similarly: the most produced polymers worldwide (PE, PP, and PVC) are the most frequent and abundant in both the micro and nano ranges. This supports the theory of the release of secondary nanoplastics from the micro-sized fraction and underlines how strong the link between anthropogenic sources of pollutants and their presence in natural compartments can be.

What makes these findings particularly alarming is 1) that NPs can pass the cellular barrier and have already been detected in humans and 2) that mussels are one of the most consumed seafood worldwide. To have an idea of the amount of NPs that circulate in our trophic chain, we upscaled these data and estimated that, considering the global production, approx. 2 tons of NPs (DW) could be stored in farmed mussel tissues. Moreover, as if that wasn't worrying enough, based on the EU countries' mussels’ consumption: a European can consume slightly more than 2 mg of NPs (DW) (2.2 x 10^-6 Kg) per year. The need to establish guidelines for monitoring MNPs in the environment is real and increasingly urgent since contamination is rapidly spreading in the food chain with still unknown effects. There is still a lack of proper management of plastic contamination in the environment: even if the mussels must undergo a strict depuration process as recommended by HACCP regulation, this is not enough to clear up their tissues, as the extensive literature about MPs (and now also NPs) contamination in farmed mussels can highlight.

The investigation of MNPs presence in organisms should be addressed and encouraged (despite the challenging procedures). Policymakers and the scientific community should define, at a global and regional scale, common strategies and guidelines for the monitoring of micro and nanoplastics in the environment (in particular in biota tissues), to finally have comparable datasets and interoperable data.  Enriching the collection of available data is mandatory to set a benchmark for the evaluation of possible ecological consequences (also for the human population) due to the widespread distribution of these micro and nano pollutants in the marine environment. In conclusion, is fair to say that is necessary to properly inform people about the risks connected to the consequences of their choices: because in the end, if these mussels are over-seasoned, who can be responsible, if not us?

References

1. Wahl, A. et al. Nanoplastic occurrence in a soil amended with plastic debris. Chemosphere 262, 127784 (2021).
2. Materić, D., Ludewig, E., Brunner, D., Röckmann, T. & Holzinger, R. Nanoplastics transport to the remote, high-altitude Alps. Environmental Pollution 288, 117697 (2021).
3. Allen, D. et al. Microplastics and nanoplastics in the marine-atmosphere environment. Nat Rev Earth Environ 3, 393–405 (2022).
4. Kirchsteiger, B., Materić, D., Happenhofer, F., Holzinger, R. & Kasper-Giebl, A. Fine micro- and nanoplastics particles (PM2.5) in urban air and their relation to polycyclic aromatic hydrocarbons. Atmospheric Environment 301, 119670 (2023).