Impact of Microplastic in Mexican Coastal Areas Using Mussels (Mytilus spp.) as Biomonitors

Abstract

Microplastics (MP) are plastic fragments smaller than 5 mm found in water columns and sediments, posing a threat to marine life due to their toxicological potential for the absorption and release of harmful compounds such as heavy metals. Mussels exhibit high filtration rates with the ability to bioaccumulate microplastics and are considered bioindicators of environmental contamination. The present study aims to evaluate the impact of microplastics in different geographical areas to identify their effects on ecosystems and potential damage to human health, focusing on the bioaccumulation capacity of MP in mussels. Samples of water, sediment, and mussels (Mytilus) from the Central Mexican Pacific were analyzed using optical microscopy and contamination indices. The levels of heavy metals detected at the sampling sites suggest low contamination, according to the Heavy Metal Evaluation Index (HEI). In Juluapan Lagoon, 93% of the MP particles found were fibers, with similar results in Barra de Navidad (75%). In Puerto Interior, Laguna Valle de Garzas, and Juluapan Lagoon, fibers accounted for 100% of the identified MP in water. The concentration of MP in water reached up to 7 MP/L, and 13 MP/mussel in Barra de Navidad. The presence of associated contaminants and MP suggests potential harmful effects on environmental health due to the high bioaccumulation of microplastics in mussels.

1. Introduction

Plastic has become a widely used material across industries and sectors, leading to increased plastic production that reached 370 million metric tons in 2021 [1]. By 2025, it is estimated that plastic production will reach 500 million tons. China currently represents the largest producer of plastic, followed by the European Union and North America [2]. The high production of plastic worldwide is due to its properties which allow for the fabrication of a large variety of durable, light, and corrosion-resistant materials at low production costs [3]. These characteristics have benefited society through the development of technological advances that result in a better quality of life [4]. However, the production of plastic materials results in large amounts of plastic garbage. Approximately 79% of the plastics produced are discarded into the environment, particularly due to consumption habits and urbanization, such as transportation activities, textiles, electronics production, and packaging materials, ultimately leading to the production of microplastics [5]. Around 33% of plastic materials produced are designed not to be reused and are discarded a short time after their manufacture, which contributes to the waste management environmental problem. Most countries do not have legislation for the correct management of waste, and the recycling of materials is usually expensive and requires specialized infrastructure, the reason why it is not an alternative in high demand [6, 7].

Plastics are formed by the polymerization of long chains of monomers through covalent bonds [6, 8]. Individual monomers are added to each other to form long chains through the C=C reaction of both monomers. Typical polymers are polypropylene and polyethylene. There are also condensation polymers which involve a chemical reaction between functional groups of the two monomer units; polyamide is an example of this type of polymer [9]. Plastics are classified into categories based on their polymeric composition and applications, as established by the American Society for Testing and Materials (ASTM). The ASTM assigns a code by the type of use (refer to Table 1). The following are types of plastic derived from petroleum and natural gas: polyethylene terephthalate (PET), high density polyethylene (HDPE), plasticized and rigid polyvinyl chloride (PVC), low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polycarbonate (O), polyamide (PA), and others (Table 1) [4, 9]. Polypropylene is the most produced polymer annually, with approximately 68 million tons, followed by vinyl chloride with 38 million tons and polyethylene terephthalate with 33 million tons per year [8].

Table 1.

Types of polymers and their applications.

Plastics are frequently classified based on their size, either by their original dimensions or by the dimensions they have acquired due to external factors [10]. The largest materials are known as megaplastics (>1 m), followed by macroplastics (1 m to 2.5 cm), mesoplastics (2.5 cm to 5 mm), microplastics (MP) (0.5 cm to 1 um), and finally nanoplastics (<1 um) (Figure 1) [9]. Plastic particles are also classified based on their shape, which can be acquired through manufacturing or degradation. They can be in the form of fibers, films, sheets, granules, fragments, etc. (Figure 1). The primary dimensions, such as length, width, and height, help to identify the type of shape in which they are classified. For example, spheres and films are classified based on their dimensions, while particles with irregular dimensions are classified as fragments [10]. However, these dimensions may change over time.

Figure 1.

Classification of plastics.

MP arises from a diverse range of sources and products. They are classified as primary or secondary, with the former being manufactured with a length < 5 mm and the latter arising from the fragmentation of larger plastics [11]. Primary MP are polymer particles that are produced in this size for use in industries such as pharmaceuticals, cosmetics, and the textile industry to produce synthetic fibers and abrasive particles [3, 5]. The primary source of MP is the gradual degradation of larger plastic fragments, known as secondary MP. These fragments are broken down into plastic particles by various factors, including solar exposure through photobiodegradation, UV damage, biodegradation by microorganisms, damage from high temperatures [4], and mechanical damage. Many plastics end up in marine ecosystems, such as rivers and lakes, where they persist in the water column and sediments for years [12]. An estimated 300 million tons of plastic waste are produced annually, with 13 million tons ending up in the oceans. Plastic polymers account for 60–80% of total marine litter [13].

Contamination by heavy metals and MP is a global problem that directly affects Sustainable Development Goals 11 and 14, urban and sessile communities, and marine life, respectively [13] and poses a potential risk to human health through ingestion of marine organisms contaminated with MP, which humans may consume directly or through secondary organisms in the trophic chain [11]. In recent years, research on MP and their effects has increased, with the aim of understanding the effects of these plastic particles in the environments where their presence has been detected and the consequences they have at the ecosystem level and on the health of the organisms that inhabit them. The presence of MP has been reported in different regions of the world, such as a study carried out in Galicia, Spain, which revealed the presence and composition of MP in water, reporting that 30% of the particles found were polyethylene (PE) [14]. Another relevant study was carried out in the bays of the Central Pacific in Mexico, where the concentration of MP ranged from 0.01–1.05 particles/m², with polypropylene being the most abundant MP [15].

MP and nanoplastics have the potential to harm marine life through the release of harmful compounds and the adsorption of pollutants on the surface of particles, such as heavy metals [16]. One pathway for heavy metal ingestion by marine organisms is due to MP being laden with these contaminants, which accumulate in the tissues of marine organisms. Additionally, they can leach heavy metals into sediments, representing a risk of exposure for organisms depending on their physiology, body size, and feeding habits, among others. Therefore, they represent a potential risk to biological processes, for example, genotoxic, reproductive, and metabolic risks have been reported [2, 17, 18]. To assess marine contamination by MP, bioindicator organisms are necessary to help understand pollution levels and the ecological risks of these contaminants. Among these organisms, bivalves stand out for their ability to concentrate contaminants above environmental levels. Being organisms with high filtration rates and low excretion rates, they can store contaminants for long periods of time [19].

Mussels (Mytilus) are sessile organisms that can provide information about the specific locality they inhabit, as they are surrounded by pollutants present in the water column on which they feed. As bivalves, they have been reported to accumulate MP found in sediment and water, as well as additives and other pollutants related to plastic materials, such as heavy metals [17]. They have also been suggested as optimal indicators of marine particulate MP in the marine environment [19]. The presence of MP in mussels has allowed the study of the absorption of heavy metals such as mercury (Hg) in their tissues by different routes, revealing that the gills are the organ where most metals are absorbed, which can be translocated to other tissues [17]. The accumulation of MP in the digestive gland has been reported; however, they are eliminated quickly but partially, which is a toxicological risk for aquatic organisms [18]. At the toxicological level, there are reports of MP damage to bivalve organisms at the cellular level, one of which is that of Parra [19] who evaluated the effect of exposure of MP and cadmium (Cd) to Corbicula fluminea, an Asian bivalve, by means of the oxidative stress response.

Due to the complexity of MP analysis, there are currently neither standardized protocols for evaluating MP nor applicable regulations. Therefore, research on these contaminants is important to understand their potential damage and impact on the environment and health. The goal of this study is to provide relevant information for the use of indicators and proposals in favor of sustainable development.

The primary objective is to determine the bioaccumulation capacity of MP and associated compounds such as heavy metals in mussels (Mytilus), for the establishment of an indicator in coastal areas of the Pacific Ocean and Atlantic Ocean. To achieve this objective, (1) water bodies and sediments are characterized through physicochemical parameters, (2) the presence of heavy metals and associated compounds is analyzed, and the polymeric composition of MP is identified using microscopy and spectroscopy techniques, and (3) the bioaccumulation of MP in mussels (Mytilus) to determine ecotoxicity and potential risks is evaluated.

2. Methodology

2.1 Characterization of the site and collection area

Physicochemical analyses were conducted at the sampling sites to determine the concentrations of heavy metals and cyanides, as well as parameters such as BOD, QOD, TDS, and TSS. The permissible concentrations were determined using NOM-001-SEMARNAT-2021 (Table 2).

Parameter	Methodology
Total Dissolved Solids (TDS), Total Suspended Solids (TSS), Total Solids (TS)	NMX-AA-034-SCFI-2015
Biochemical Oxygen Demand (BOD)	NMX-AA-028-SCFI-2001
Chemical Oxygen Demand (COD)	NMX-AA-030/2-SCFI-2011
Heavy metals by atomic absorption	NMX-AA-051-SCFI-2001
Total Cyanides (Cn)	NMX-AA-058-SCFI-2001

Table 2.

Physico-chemical parameters analysed at the sampling sites.

For the detection of heavy metals, the procedure followed the standard “Water analysis - Determination of metals by atomic absorption in natural, potable, waste, and treated waste waters (NMX-AA-051-SCFI-2001)” [20]. A 50 mL sample was taken, and 3 mL of nitric acid HNO₃ was added. The sample was placed in a Teflon beaker covered with a watch glass on a hot plate. The temperature was increased until vapor reflux occurred. Once digestion was complete, the sample was removed and cooled. Subsequently, the detection of heavy metals and cyanides was carried out using a graphite furnace spectrophotometer.

The detection of heavy metals in sediments was carried out following the method of acid digestion assisted by microwave EPA 3051A [21]. A preliminary digestion of the 0.5 g sample was performed, which was dried at 60°C for 2 hours, ground, and sieved. Subsequently, it was placed in a Teflon beaker, and 9 mL of HNO₃ was added. It was then placed in a sand bath at 4 to 40°C for 4 hours. Afterward, it was placed in the microwave system following the manufacturer’s instructions. Once finished, it was allowed to cool for 5 minutes.

The concentration of heavy metals is used to calculate the Heavy Metal Evaluation Index (HEI). This index indicates the water quality with respect to these elements. Based on the value of the index, the water quality is classified as having low contamination (HEI ≤ 9), medium contamination (HEI 9–18), or high contamination (HEI ≥18). The HEI is determined using the following equation:

$$HEI=\underset{}{\overset{}{{\displaystyle \sum _{i=1}^n=\frac{M_i}{S_i}}}}$$E1

where:

$M_i=measuredmetalconcentrationvalue.$

$S_i=permissibleconcentrationvalue$

$i=numberofmetalsmeasured$

2.2 Collection

The collection of mussels, sediment, and water for MP analysis was carried out in several sites along the coasts of the Central Mexican Pacific. These sites include the port of Barra de Navidad (BN) in the municipality of Cihuatlán, Jalisco, as well as the Juluapan Lagoon (LJ), Puerto Interior (PI), and Laguna Valle de Garzas (LGV) in the municipality of Manzanillo. For both zones, two sampling points were selected for the first two sites. The first point was located far from the area with the highest anthropogenic activity, while the second point was located near the populations.

Mussels, water, and sediment were collected from LJ in July 2023 at point 1 (19°06′39″N 104°24′12″W). This point is situated in an area with close proximity to human disturbance, as shown in Figure 2. Mussels (Mytilus) were collected from the roots of the white mangrove (Laguncularia racemosa) at surface level. Using a metal spatula, the mussel colonies were detached from the roots, and 12 organisms with a size of 3.0–4.0 cm were selected to ensure homogeneity in size and developmental stages. Careful handling was employed to avoid tearing off the byssus and to preserve the mussels in good condition. The samples were subsequently stored in aluminum and transported to the laboratory, where they were frozen at −18°C for later analysis. In the laboratory, identification was carried out using dichotomous keys. However, due to the lack of specific dichotomous keys for bivalves in the Mexican Pacific and previous records, identification was limited to the genus Mytilus as mentioned in the literature [22, 23, 24].

Figure 2.

Location of water, sediment, and mussel sampling points in Juluapan Lagoon, Colima. Mussels in the Juluapan Lagoon, Colima. Point 1 (p1): water sampling, sediment, and mussels; point 2 (p2): water and sediment sampling.

To collect the water samples, 1 liter was taken at a depth of 1 m using Niskin bottles, and another sample was taken from the surface (20 cm) using a zooplankton net with a pore size of 60 um. The net was rinsed with water from the lagoon to ensure that all particles were collected. The samples were then stored in bottles and transported to the laboratory for cold storage until further analysis. Sediment samples were collected using a 10 cm diameter core sampler at depths of 5 cm and 1 cm. The samples were then placed in aluminum containers and stored at a temperature of −18°C for further laboratory analysis.

Point 2 was located at 19°06′50″N 104°24′31″W (Figure 2). Due to the absence of mussels in the area, only water and sediment samples were collected. The samples were collected using an Equinox kayak that was stationary, following the methodology described above.

The collection in BN was conducted in July via motorboat. Mussel was collected in point 1.A (p1.A) 19°11′52”N 104°41′07”W by diving to a depth of 5 meters. The samples were then transferred to the laboratory and stored at −18°C until further analysis, and sediment and water were collected at point 1B (p1.B) 19°11′51″N 104°40′51″W. At point 2 (p2) (19°11′48″N 104°40′10″W), water and sediment samples were collected using the same methodology. This point represents an area with less human disturbance (Figure 3).

Figure 3.

Location of water and sediment sampling points in Barra de Navidad, Jalisco. Point 1.A (p1.A) is for mussel sampling, while point 1.B (p1.B) is for water and sediment sampling. Point 2 (p2) is for water and sediment sampling.

In the LVG, Colima, periodic water samples were taken at stations (E1–E8) during the first week of each month from January to August 2023, following the methodology described above (Figure 4).

Figure 4.

Location of the sampling stations of water and sediments (p1–p8) in Laguna Valle de Garzas in Colima, Mexico.

Water and sediment samples were collected at stations (E1-E5) in PI, Colima, on the first week of each month from January to August 2023, following the methodologies described in Figure 5.

Figure 5.

Location of the sampling stations of water and sediments (p1–p5) in Puerto Interior (PI) in Colima, Mexico.

The next step in the analysis of MP is to digest the matrix in which they are collected. It is important to choose the right technique because the matrices containing MP must be properly digested to obtain the plastic particles more easily. However, MP are found in various environmental matrices, such as air, sediment, animals, and human tissues [25]. Therefore, when seeking the presence of MP in samples, a significant amount of organic matter is also present, which poses a challenge in selecting the appropriate technique. Optimal digestion techniques are required to isolate the MP from the matrix, removing the organic matter while preserving the integrity of the plastic particles [26]. MP analysis involves digesting and filtering plastic particles using various chemical, thermal, physical, and mechanical processes [27].

2.3 Matrix digestion

The analysis of MP in water was performed by direct filtration using a vacuum filtration system with a Biobase pump (model GM-0.5 A). The contents were filtered using 110 mm grade 934-AH glass fiber filters with a pore size of 1.2 um. The filters were then left to dry at room temperature for 24 hours in a closed hood to prevent environmental contamination. To digest the Mytilus organisms, first, they were dissected using a scalpel, opening their shells to access the digestive glands and gills. These organs are of interest because they accumulate the highest amount of pollutants. The organs were kept at −18°C until digestion. To process the digestive glands and gills, a KOH (10% w/v) acid digestion was employed, following the modified methodology described by several authors [28, 29]. The gills and digestive glands of each mussel were placed in glass beakers. KOH (10% w/v) was added to the beakers in a volume three times higher than that of the samples. The beakers were covered with aluminum foil and left to stand at 21°C for 24 hours. Afterward, the samples were incubated for 6 hours at 60°C. At the end of the incubation period, filtration was performed under vacuum using the methdology described for water samples. The 10% (w/v) KOH was prepared in an Erlenmeyer flask by adding 100 g of KOH in lentils (KOH ACS Meyer brand) to 900 mL of ultrapure water (Type 1), subsequently stirring to achieve a homogeneous mixture. The MP in sediment samples were dried at 60°C and then crushed in a mortar with care not to apply too much force so as not to break the particles of interest. The sediment was immediately sieved to have a sample of homogeneous size. Subsequently, the digestion consisted of adding a specific amount of sediment and adding three times its volume of 30% H₂O₂ and then incubating for 6 h at 60°C. At the end of the incubation period, the filtration described above was carried out.

2.4 Quality control

As standardized protocols do not exist, quality control is paramount for MP analysis. Wide-mouth glass containers with purified water were used as procedural blanks. During water, sediment, and mussel sampling, the bottles were left uncovered and placed aside within the vessel. After sampling, they were covered and transported to the laboratory for analysis. The analysis involved filtering the water directly and observing the filters following the methodology described earlier to rule out MP presence due to contamination. If MP were found, they were classified and excluded from the samples collected at the same site. This process was conducted per sampling site.

In the laboratory, all samples were handled in enclosed spaces, with new nitrile gloves and cotton lab coats. All instruments used were made of metal or glass, and only one person performed the observations to reduce human error.

2.5 Observation and separation

MP observation is an essential part of every analysis, and the choice of equipment and technique is fundamental for obtaining accurate results. In this study, MP detection was solely performed using optical microscopy. After filtration, filters were left to dry for 24 hours and then stored in aluminum foil until observation. MP were observed using a Park Systems digital microscope at 800x digital zoom. They were identified and classified based on shape (fiber, fragment, granule, and film) and color (black, blue, red, white). Classified particles were picked up with metal tweezers, transferred to filter paper, and covered with aluminum for subsequent identification using more advanced spectroscopic and microscopic techniques.

2.6 Bioaccumulation of MP and toxicological indicators

The accumulation of MP in mussels in the gills and digestive glands was carried out by analyzing the concentration of MP found in the water column of the collection area relative to those found in the tissues [30].

$$Bioconcentration\left(BCFs\right)=\frac{Cbiota}{Cwater}$$E2

Cbiota: MP concentration in organisms (tissue and organs).

C water: MP concentration in the water to which the biota is exposed.

The concentration of MP was determined by the number of plastic particles per volume of sample collected. The abundace of MP in surface waters was expressed as the number of MP particles per liter (n/L), and the abundance in the sediments was expressed as the number of MP particles per kg (n/kg) [23].

$$MPconcentrationinwater=\frac{n}{L}$$E3

$$MPconcentrationinsediment=\frac{n}{kg}$$E4

3. Results and discussion

In LVG, copper (Cu) exhibited the highest concentration in March at station E8 with 8.83 mg/kg. Lead (Pb) was consistently the least abundant metal across all months, followed by mercury (Hg) which was absent in July. The lowest cyanide (CN) concentration was recorded in January across all stations, measuring 0.0026 mg/kg, followed by June which registered 0.63 mg/kg. Analysis of heavy metals and cyanides in PI (B) revealed copper to be the dominant metal in March, particularly at station E2. Lead, on the other hand, exhibited its lowest concentration in January across stations 1 and 2 (E1 and E2), with no reports from other stations (Figure 6).

Figure 6.

Concentration of heavy metals and cyanides (mg/kg) Laguna Valle de Garzas (LVG) and Puerto Interior (PI) at the sampling stations (E) As: arsenic; CN: cyanide; Cu: copper; Hg: mercury; Pb: lead; Zn: zinc.

Although heavy metal presence in the water bodies fell within permissible limits and indicated low contamination levels (Figure 6), anthropogenic activities surrounding the sample sites, such as wastewater discharge and the proximity to industrial and residential areas in LVG, and port-related activities in PI, contribute to the release of toxic compounds including heavy metals. Another important factor to consider is the adaptation of the MP surface to these contaminants, which depends on the exposure concentration due to the properties of its surface that allow the absorption and accumulation of heavy metals, as described in Liu’s et al.’s study [31].

In the case of LVG, the highest concentrations were found at the first stations, situated in areas with denser vegetation, influencing the presence of heavy metals due to their ability to concentrate them in water and sediment. During the dry season months of January–March, there was a higher concentration of copper, likely due to reduced water flow [32, 33]. However, in March, there was an increase in Cu concentration, suggesting industrial contamination, especially from the Specialized Container Terminal No. 2 (TEC 2), and reduced vegetation cover (mainly mangroves). Zinc concentrations were higher, indicating increased absorption by MP [33, 34]. In PI, the concentrations of heavy metals were similar to LVG and within permissible limits. The concentration values correspond to trace metals primarily related to activities such as vessel traffic and geochemical processes that increase with sediment resuspension and possible changes in physicochemical factors such as suspended matter [35] and salinity [36].

With respect to the heavy metal evaluation index (HEI), it can be observed that no heavy metal or cyanide was greater than 9, that is, they represent low contamination in water samples from LVG and PI (Figure 7). In LVG, the highest HEI values are reported in March and in station E8 with 3.48, in the month of May, in station E5, no concentrations of heavy metals were reported as can be seen in Figure 7. Regarding the HEI of PI, the highest value was in February, specifically at station A4.

Figure 7.

Heavy metals and cyanides (A) and heavy metal evaluation index (B) of Laguna Valle de Garzas (LVG) and Puerto Interior (PI).

By observing the filters through a digital microscope connected to a laptop, the photographs were taken. The total number of MP found in the mussels found in the LJ was 149 (Figures 8 and 9), of which 93% represented fibers (139), the most abundant being black with 109 MP (78%), followed by 17 blue fibers (19%), 5 fragments, and 5 granules (3.3% of the total elements found). Regarding the MP found in mussels from BN, there were a total of 163 MP (Figure 8), 75% representing fibers (122), the black fibers were the most abundant with 100 MP accounting for 82% of total, followed by red fibers with 18 MP (14.8%) and blue fibers with 4 MP (3.2%). The granules represented 24% of the elements found (39 MP), 36 black MP and 3 red MP were observed. The type of MP with the least presence was fragments, only 2 black (0.13%).

Figure 8.

Forms of MP found in the gills (B) and digestive glands (G) of mussels in Juluapan Lagoon (LJ) and Barra de Navidad (BN).

Figure 9.

The colors of MP found in the gills (B) and digestive glands (G) of mussels in Juluapan Lagoon (LJ) and Barra de Navidad (BN).

The variability of the type of MP accords with other similar works [11] in which fiber-type MP are the most abundant where 81% of the particles identified in water samples from the Ria Vigo were fibers in addition to coinciding in the color, followed by red and blue fibers, which is related to the origin of MP, which may be due to water derived from textile and industrial waste that is not adequately treated, which is why they end up being part of bodies of water. Likewise, the variety of types of MP suggests that there is a diversity of activities as sources of these particles, due to the activities carried out in the area. The accumulation of MP in mussel tissues depends on physicochemical interactions of the particles and the organs and the routes by which they accumulate. The abundance found in this study in the digestive gland suggests that the ingestion route allows them to be retained in the digestive tract [37]. The presence of P in mussel tissues also points to contamination by heavy metals due to their affinity, which entails risks for different types of organisms depending on the trophic chain, representing a risk to human health considering that we are the final consumers. In addition to the MP in the digestive gland, it is also observed in the gills, which are the organs that filter the food. However, due to the particle size of the MP, they are ingested by the mussels.

At a depth of 20 cm, black fibers were identified in the MP from water samples taken at point P1 and point P2 of LJ. Similarly, black fibers were found in PM samples taken at BN at both points at the same depth (5 and 2, respectively). At P2, one blue fiber was found at a depth of 1 m (Figure 10). The predominance of black-colored fibers is attributed to their origin, suggesting they are derived from tire waste or residues discarded in nearby communication pathways.

Figure 10.

Types of MP found in Juluapan Lagoon (LJ) and Barra de Navidad (BN) at various depths.

The sampling points selected for having less disturbance and human activity still showed the presence of MP, similar to the sites chosen for having greater sources of contamination. It was observed that the deeper the sampling point (specifically P2 in BN), the higher the concentration of MP. This could be attributed to water currents and boat activity in the area. The surface samples taken at both sampling points in both regions (Figure 10) recorded fibers, a type of MP that tends to float near the surface on coasts and remain in the water column [15].

Considering that fibers are particles that remain suspended in the water column for an extended period before settling, this aligns with the findings presented in Figure 10. It is worth noting that the collection was only conducted at the surface. The analysis of heavy metals in LJ and LVG, as well as the presence of PM, suggests a higher risk of negative effects on marine fauna. This is supported by previous studies [31], which have shown that ingestion by organisms in the environment can be particularly harmful.

51 fiber-type MP were found in the water samples from LVG (Figure 11), with black fibers being the most abundant (43 MP), followed by blue and red fibers (4 MP). Station E2 and station E6 had the highest presence of MP, with eight black fiber-type MP. The highest number of red fibers was found at station E8, and E1 had the lowest number of MP. July had the highest presence of MP with 11 black, 3 blue, and 1 red fibers, followed by August with 12 black fibers and 1 blue fiber.

Figure 11.

Types of MP discovered at the Laguna Valle de Garzas stations.

In PI (Figure 12), only 19 black fibers were found, with the highest abundance in July (13 MP). The findings from BN align with another study [15] that identified fibers as the most prevalent type of MP. BN is an area where wastewater is discharged without prior treatment, whereas LJ exhibits a lower concentration of MP. The quantity of MP found in PI and LVG was higher during the rainy season (starting from May) due to increased water runoff from surrounding areas into the water bodies. This runoff carries more debris and leads to greater circulation of currents. Additionally, surface water runoff results in increased transport of MP [32]. With the influx of water and movement, the sedimentation period decreases, allowing for a higher presence of MP during sampling in this period.

Figure 12.

Types of MP discovered at Puerto Interior stations.

The concentration of MP was determined by calculating the number of plastic particles per volume of sample collected, as shown in Table 3. The results indicate that individuals from BN had a higher number of plastic particles, with 13.58 MP/individual ±5.48, followed by individuals from LJ with 12.42 MP/individual ±5.72. The concentration of MP in water was found to be higher in P2 of BN, which coincides with the presence of MP in the mussels.

Matrix	Concentration
LVG water sample	1.50 particles/L
PI water sample	1.35 particles/L
P1 Laguna de Juluapan water sample	0.50 particles/L
P2 Laguna de Juluapan water sample	3.50 particles/L
P1 Barra de Navidad water sample	3.50 articles/L
P2 Barra de Navidad water sample	7.00 particles/L
LJ Mussel tissue	12.42 particles/organism ±5.72
BN Mussel tissue	13.58 particles/organism ±5.48

Table 3.

Concentration of MP in different matrices.

The accumulation present in organisms suggests that MP represent a risk to them; they can be translocated through the intestine or hemolymph, posing risks such as biochemical alterations [37], due to low elimination of MP and high persistence in gills through epithelial tissues, limiting the filtration mechanism. Considering the above, the presence of MP in water currents where filter-feeding organisms inhabit is important due to their high retention capacity. However, high proportions of MP excretion have been in Rivera-Hernádez et al.’s study [17], suggesting that bivalve organisms eliminate unrecognized particles. Therefore, during sampling, there may be a lower quantity of MP in the water due to the filtration and excretion process.

Various techniques for analyzing MP are reported in the literature. These techniques are divided into specific stages, beginning with the collection of MP in different matrices, which require different collection methodologies.

4. Conclusion

The concentration and presence of heavy metals suggest low contamination in the sampling areas. However, they pose a potential risk to the health of Mytilus mussels due to the affinity of heavy metals for absorption on the surface of MP, given their structure and feeding mechanism through water column filtration. The abundance of MP in the environmental matrices analyzed suggests high contamination from plastic waste and poor urban waste management, leading to their accumulation.

Fibers were the most abundant type of MP in all three matrices analyzed, likely due to the type of waste present. Additionally, the quantity of MP found depends on the season of sample collection, with higher concentrations during the rainy season due to the runoff from surrounding areas.

Regarding MP found in mussel tissues, they were more abundant compared to those found in the water column, indicating rapid particle ingestion and longer bioaccumulation time in organisms. Therefore, mussels are considered excellent environmental pollution monitors in their environment. This also poses a significant health risk to these organisms, ecosystem health, and human health as consumers of mussels, potentially causing adverse effects and alterations to biochemical, reproductive, and genetic mechanisms.

Innovative studies like this one are crucial, as they analyze the presence of MP in different environmental matrices, along with associated contaminants, in various geographic areas. This issue is not specific to one site but represents a global environmental challenge.

Regarding MP analysis, the chosen protocols were suitable for observing and classifying particles as there was no interference from excessive organic matter residues, allowing for easy and accurate observation of MP in all matrices.

For further investigation, this work will continue with the identification of the types of polymers found through spectroscopy and high-resolution microscopy (SEM, RAMAN, and FTIR), as well as conducting QSAR ecotoxicity indices and BPR (cumulative potential risk) analyses and biochemical analyses on organisms to determine the neurotoxic effect of MP by analyzing the activity of acetylcholinesterase (AChE).

Acknowledgments

This study was conducted through the collaboration of researchers and the use of facilities at the University of Colima, also with the participation of researchers from the University of Guadalajara. We appreciate the support of the Spanish Institute of Oceanography, which serves as an entailment institute for all the research.

Conflict of interest

The authors declare no conflict of interest.