When improperly discarded or released into the environment, solid waste can travel great distances, contaminating various ecosystems, spreading invasive species, creating socioeconomic challenges, harming human health and negatively affecting tourism (Hirata et al. 2017, Olivatto et al. 2018, Silva et al. 2018, Machado et al. 2021). Research has consistently shown that plastic is the most prevalent type of waste on beaches and coastal areas (Silva et al. 2018, Olivatto et al. 2019, da Silva et al. 2022). This is largely due to plastic’s durability, resistance and low cost, which have made it integral across numerous sectors such as single-use products, food packaging, cosmetics (e.g. scrubs and abrasives), medical equipment, construction materials and textiles, effectively replacing traditional materials in these industries (Dehaut et al. 2018, Olivatto et al. 2018, Machado et al. 2021).

According to Plastics Europe (2023) estimates, global plastic production reached 400 million t in 2022, including thermoplastics, highlighting its widespread use. Given its high production and disposal rates, plastic is considered a highly contaminating product and a global threat to environmental issues, food safety and human health (Sharma et al. 2017, Bessa et al. 2018, Peixoto et al. 2019, da Silva et al. 2022).

Once in the environment, plastics undergo weathering processes such as photodegradation, chemical breakdown and mechanical wear, leading to fragmentation and changes in properties such as colour. When these fragments measure between 5 mm and 0.1 µm, they are classified as microplastics, either secondary (from the breakdown of larger plastics) or primary (originally manufactured at this size, such as pellets and microspheres). Particles smaller than 0.1 µm are categorized as nanoplastics (Castro et al. 2016, Olivatto et al. 2018, Mercogliano et al. 2020). Studies have widely documented microplastics and nanoplastics across aquatic, terrestrial and aerial environments, highlighting their extensive dispersion and ingestion across trophic levels, from plankton to humans (Sobral et al. 2011, Hirata et al. 2017, Peixoto et al. 2019, Zurier et al. 2020).

Microplastics are known to carry pathogens and absorb various pollutants present in the marine environment, including persistent organochlorine pesticides, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, pharmaceuticals and polybrominated diphenyl ethers. When these pollutants detach from ingested microplastics, they can be absorbed by marine organisms, potentially compromising their health and survival. Furthermore, the risk of biomagnification poses a hazard to top predators in the food chain, including humans, as prolonged exposure to these contaminants accumulates over time through the food web (Araújo et al. 2021).

When microplastics come into contact with skin or are inhaled or ingested, they can trigger oxidative stress and inflammatory lesions, affecting various human bodily systems, including cardiovascular, renal, respiratory, reproductive, gastrointestinal and neurological systems. This exposure can lead to serious health issues such as cancer and diabetes (Prata et al. 2020, UNEP, 2021). As a result, the inadvertent consumption of microplastics by a range of marine organisms poses a potential health risk to humans, even if the viscera are removed before consumption (Olivatto et al. 2018, De-la-Torre, 2020, Kutralam-Muniasamy et al. 2020, Zurier et al. 2020).

Brazil’s average seafood consumption is around 5-10 kg per capita yearly (FAO, 2020). While mussels constitute a relatively small fraction of this consumption, their sessile and filter-feeding nature allows them to accumulate pollutants in the water, potentially amplifying health risks when ingested. These characteristics enable mussels to reveal environmental impacts that may otherwise go unnoticed (Resgalla et al. 2008, Pierri et al. 2016, Cho et al. 2018, Mercogliano et al. 2020, Machado et al. 2021). Li et al. (2019) note that numerous researchers have suggested using mussels as indicators for microplastics in coastal areas.

Given their prevalence along the Brazilian coast and their popularity as a food source, especially in the southern and southeastern regions, the Perna perna mussel was chosen for this study. These mussels are harvested from natural beds and cultivated through aquaculture. According to IBGE (2023), Brazil’s total mussel production reached 15700 t, with the southeastern region alone contributing 82896 kg of oysters, mussels and scallops. This highlights the significance of mussel farming in the region’s seafood industry.

Mussel farming in Brazil utilizes longline systems, which involve nylon cables ranging from 50 to 100 m anchored to floats, typically large plastic barrels, to support cultivation nets. This method can support approximately 4 t of mussels (Valenti et al. 2021). Once harvested, farmed mussels typically undergo a depuration process before being packaged for sale, ensuring they are clean and safe for consumption.

Given the prevalence of plastic waste in the environment and the plastic materials used in mussel farming, there is a critical need for research to assess the potential presence of microplastics in mussels available to consumers. We hypothesize that farmed mussels, which undergo a depuration process before sale, are less likely to be contaminated with microplastics than those harvested directly from natural sources.

Additionally, the findings from this study could inform future research aimed at establishing maximum permissible concentrations of these pollutants in water and mussels, ultimately reducing the risks associated with consuming these organisms.

The presence of microplastics in both wild and farmed mussels is widely documented in the literature (Li et al. 2015, Li et al. 2018, Barboza et al. 2018, Cho et al. 2018, Mercogliano et al. 2020), aligning with our findings. This occurrence can be attributed to the behaviour of mussels, which can filter up to 2 L of surrounding water per hour, as well as to the increasing prevalence of these pollutants in coastal waters, where natural beds and aquaculture farms are located (Van Cauwenberghe et al. 2015, Cho et al. 2018, Mayoma et al. 2020, Machado et al. 2021). Mussels feed on what is carried by currents, including plankton and pollutants such as microplastics.

As in our results, the literature generally reports higher amounts of microplastics in wild mussels than in cultivated ones. Some authors (Van Cauwenbergh et al. 2015, Catarino et al. 2018, Cho et al. 2018) suggest that this difference may be due to pollution in the surrounding waters and intertidal zones, where bivalves commonly settle, exposing them to suspended microplastic particles. In contrast, cultivated mussels are typically located at depths greater than one metre and remain submerged for extended periods (or constantly) as a result of aquaculture setups.

The slight difference in microplastic values between wild (0.145 particles g^-1) and cultivated mussels (0.120 particles g^-1) in our study may be primarily due to the low levels found in wild mussels compared with other regions. For instance, Li et al. (2018) reported concentrations of 0.7 to 2.9 particles g^-1 in mussels from United Kingdom coastal areas. Renzi (2018) also found higher levels in four Italian locations, ranging from 3.0 to 9.2 particles g^-1. Similarly, Li et al (2015) reported concentrations ranging from 2.1 to 10.5 particles g^-1 in nine Chinese commercial bivalve species, while another study reported 1.52 to 5.36 particles g^-1 (Qu et al. 2018). All these authors commonly suggest that the main source of microplastics is the contamination of nearby areas, which is directly related to human activities.

In Niterói, Birnstiel et al. (2019) recorded 6.67 particles g^-1 in Perna perna mussels collected from Eva Beach and 4.12 particles g^-1 from Jurujuba Beach. Although these beaches are close to Itaipu Beach, they are in sheltered coves within Guanabara Bay, a known polluted area with high microplastic loads (Castro et al. 2016, Birnstiel et al. 2019, Castro et al. 2020). According to Castro et al. (2020), the waters of Itaipu, where wild mussels were collected, exhibited relatively low microplastic concentrations, ranging from 1.2 particles m^-3 in winter to 2.86 particles m^-3 in summer. The Itaipu Beach area is significantly influenced by ocean waters, which promote the easy dispersal of microplastic particles due to local hydrodynamics, potentially explaining the lower concentration of mussels from this area.

Another possible explanation for the lower difference in values between wild and cultivated mussels in this study could be the depuration process, which the latter undergo after harvesting and before packaging. Birnstiel et al. (2019) demonstrated that microplastic concentrations in cultivated mussels in Niterói decreased by 28.95% after 93 hours of depuration. This process can reduce larger particle concentrations (Van Cauwenberghe and Janssen, 2014). These authors found that depuration with HNO₃ lowered the microplastic concentration by around 30% after three days, although smaller particles persisted, suggesting potential translocation through the intestinal wall into tissues and the circulatory system. This is corroborated by the continued presence of microplastics in organisms even post-depuration, but differs from our results, as we found a greater quantity of larger particles in cultivated mussels.

The values obtained in this study for cultivated mussels were below those reported by Van Cauwenberghe and Janssen (2014), who found microplastic concentrations in Mytilus edulis of 0.36±0.07 particles g^-1 before depuration and 0.24±0.07 particles g^-1 afterward, and in Crassostrea gigas (0.47±0.16 particles g^-1 before depuration and 0.35±0.05 particles g^-1 afterward). Likewise, Li et al. (2018) observed concentrations ranging from 0.9 to 1.4 particles g^-1 in mussels purchased from supermarkets and Cho et al. (2018) reported values up to 1.08 particles g^-1 in mussels from a fish market in southern Korea.

The lower value found in this study (0.12 items g^-1 in cultivated mussels) corresponds to approximately 2% of the average daily microplastic intake per person (100 to 140 particles/day according to Cox et al. 2019), assuming a daily seafood intake of 20 g in Brazil (FAO, 2020). These values are only estimates, as mussel consumption in Brazil constitutes only a portion of total seafood consumption, but in the south and southeast regions where this consumption is higher, this daily intake may represent a higher percentage, increasing the risk of consuming these molluscs.

It is important to note that the varying concentrations of microplastics reported in various studies may also be influenced by the methodologies used to dissolve organisms. There is no standardized protocol for recovering microplastics in seafood (EFSA, 2016, Catarino et al. 2018, Cho et al. 2018, Digka et al. 2018, Li et al. 2018). The choice of chemical agents and their concentrations can partially or completely damage microplastics, affecting their size and colour.

Our study used 5% NaOH, a concentration that falls outside the range specified by Lusher et al. (2017). This concentration is unlikely to damage the main types of microplastic particles frequently reported in the literature, including polypropylene, polyethylene, polyvinyl chloride and polyethylene terephthalate.

The microplastic particles shown in this study displayed chemical similarities to nylon (polyamide), chlorinated polyethylene and chlorosulphonated polyethylene, with fibres and films identified as the main morphological types. Plastic fibres and films are commonly found in marine environments and were predominantly described in Guanabara Bay, a region close to Itaipu, by Baptista Neto et al. (2019). Similarly, Brocardo (2022) reported that fibres (75%) and fragments (25%) were the predominant microplastic forms in mussels from farming sites on Santa Catarina Island, close to the area of the current study. Fibres found in our study were all characterized as polyamide and were the total samples from farmed mussels. Birnstiel et al. (2019), studying microplastics in a farming area in Jurujuba, Niterói, RJ, also reported a high prevalence of polyamide in fibre form. These authors proposed that these detected fibres may have secondary sources, potentially originating from synthetic fabrics or maritime ropes, given the higher density of polyamide relative to polyethylene, the fact that mussels are farmed at depths greater than one metre and remain submerged for long periods, and the use of nylon ropes in these aquaculture structures.

In the Itaipu area, where wild mussels were collected, there is a fishing community, and the presence of polyamide fibres in the mussels from this area can be explained by fibre contamination from fishing gear used by these fishermen. The films analysed in mussels from Itaipu were characterized as chlorinated polyethylene and chlorosulphonated polyethylene. These compounds were also described by Castro et al. (2020) when they analysed the waters of this location. These compounds are used in various products and provide these products with resistance and durability against environmental stressors such as UV degradation. These characteristics may explain their availability in Itaipu’s waters, influenced by Guanabara Bay, a region heavily contaminated with microplastics, and their subsequent ingestion by mussels from natural beds.

These results support the hypothesis that improper waste disposal and fishing materials are sources of plastic particle contamination in the marine environment.

In this study, the most prevalent colours of microplastic particles were black and transparent, with blue also being common in mussels from cultivated sources. The prevalence of blue particles aligns with findings from Digka et al. (2018) and Brocardo (2022), which indicate that blue is characteristic of plastics used in longline structures such as ropes and suspension buoys. This blue coloration was consistently observed in the studies by Birnstiel et al. (2019) and Castro et al. (2020) carried out in the same areas as the present study.

Regardless of their origin, our results show that mussels accumulate microplastics and most likely reflect the presence of these pollutants in their environment, highlighting their suitability for monitoring pollution levels. This reinforces the importance of using mussels as indicators of plastic contamination in marine ecosystems, providing valuable data that can inform both environmental management practices and public health concerns.

This study aimed to quantify and characterize microplastic particles and their chemical composition in Perna perna mussels sold in Niterói and sourced from aquaculture farms and natural habitats. According to previous studies conducted in the regions studied, the quantity, colour and chemical composition of the microplastics found in wild and cultivated mussels suggest a strong influence from the surrounding environment.

Although the values found in wild and cultivated mussels were lower than those reported in other studies, it is evident that the population consumes contaminated mussels, regardless of whether they are wild or cultivated, putting their health at risk. Management measures, especially in areas designated for mussel cultivation, must be implemented to prevent the presence of plastic waste in these environments. Additionally, given the use of plastic materials in cultivation structures, mandatory depuration of these mussels before commercialization should be enforced.