Characteristics of microplastics in Stichopus horrens
A total of 20 samples of sea cucumber Stichopus horrens were contaminated with 1446 microplastics. An approximate 4–5 g Stichopus horrens intestine contained 60–70 microplastics. With length range between 0.2 to 7.0 µm and average length of 0.5 to 2 µm, the microplastic length included four-length sizes; 0.5–1 µm (34.79%), 1–2 µm (27.73%), 2–5 µm (13.97%) and < 0.5 µm (23.51%).
Figure 1 exhibits the typical colours and shapes of microplastics found in Stichopus horrens intestine while Fig. 2 depicts percentage of microplastic colours. The rank of microplastic percentage according to their colour was as followed: black/grey (59.13%) > blue (27.04%) > red (9.9%) > green (0.96%) > transparent/ white (0.9%) > pink/ purple (0.83%) > brown (0.76%) > yellow/ orange (0.48%). Results were presented in Fig. 4. In terms of shape, the majority of the microplastics were filaments (90.87%), followed by fragments (8.23%) and film (0.9%) (Fig. 3).
The ingestion of microplastics by Stichopus horrens was first reported on the island of Malaysia, in particular, and in the Asian tropics, in general. The results of this study could be predicted from visual analysis, since the colour shown implies that different polymer types may be present. There are several types of polymers in Stichopus horrens, based on Fig. 1, by looking at the different colours of microplastic particles according to the classification of previous studies (Ibrahim et al., 2016).
Figure 2 shows the colour distribution of the microplastic particles isolated from the gastrointestinal tract of Stichopus horrens. The most abundant microplastic particles found in this sample were black particles, accompanied by blue and red particles. Figure 1 displays the picture of microplastics captured under a comparison microscope. The findings showed that black and blue fragments were the majority of plastic products in Stichopus horrens, accounting for 59.13% and 27.04%, respectively (Fig. 2). Black and blue particles were found in all samples suggesting fishing activities that may lead to the high concentration of transparent microplastics in marine ecosystems (Stolte et al., 2015).
Microplastics collected from the gastrointestinal tract (GIT) of Stichopus horrens were classified into three shapes; fibre, fragment and film. The distribution of these three types of microplastics in all the samples were shown in Fig. 3. Fibre was the majority shape found in this species. The dominant type of plastic debris contained in this sample, accompanied by fibre, fragments and film. In the digestive organs of sea cucumbers, the type of microplastic that is easily transported by waves and reaches the digestive tract accumulates, resulting in digestive tract blockages from the absorption of plastic utensils known as fibre (Taylor et al., 2016; Mohsen et al., 2020; Sayogo et al., 2019; Iwalaye et al., 2020). Besides, the abundance of fibre was possibly attributed to the common usage of fishing equipment, as plastic fibres were important raw materials for fishing nets and lines (Zhao et al., 2018). Fibre type comes from monofilament fragmentation (single fibre) from fishing nets, ropes, synthesis cloth or clothing fibres. In addition, fragments were also found in gastrointestinal tract (GIT) of Stichopus horrens. Fragments come from plastic objects that can be degraded for a long time such as bottles, buckets, or other objects made of PVC. The source of the fragments could be the decomposition of large plastics, such as plastic utensils, cleaning items and packaging materials (Kyrikou et al., 2011). A type of microplastic that originates from the manufacturing of plastic products and plastic waste is film (Sayogo et al., 2019).
Another common measurement parameter for microplastics is size, although there is currently no uniform standard for measuring the size of microplastics. It was not unexpected that the number of microplastics, less than 1 mm, was high, because they were similar to previous research (Zhu et al., 2019; Zhang et al., 2020). Microplastics with smaller sizes (0.5 to 2 mm) have been found more often than other sizes, as seen in Fig. 4. Many previous studies have also reported a similar finding with smaller particles in a dominant position (Van Cauwenberghe et al., 2013; Mohamed Nor & Obbard, 2014; Van Cauwenberghe et al., 2015). Thompson (2015) reported that the small size of microplastic was probably due to the degradation of large microplastics. Small particles with large specific surface areas may be less resilient than large particles. Moreover, the accumulation rate often increases with the continuous degradation of plastic particles. As Zhang et al. (2020) proposed, the distribution of microplastics in size was directly linked to their rate of formation and degradation. The changes in this rate, are currently not clear and require further analysis.
FTIR spectrum of possible microplastic presence in Stichopus horrens is shown in Fig. 5 and Fig. 6. Spectral characteristics of polyethylene (PE) with chemical formula of (C2H4)n were peaks with wavenumbers 2916.14 cm− 1 – 2917.14 cm− 1, 1457.13 cm− 1 − 1471.78 cm− 1, 717.29 cm− 1 – 718.14 cm− 1 as shown in Fig. 5. Spectral characteristics of polymethyl methacrylate (PMMA) with chemical formula of (C5O2H8)n were peaks with wavenumbers 2959.11 cm− 1, 1730.85 cm− 1, 1453.33 cm− 1, 1395.27 cm− 1, 1230.35 cm− 1, and 1152.54 cm− 1 as shown in Fig. 6.
The FTIR spectra associated with microplastics are shown in Fig. 5 and Fig. 6. The collected microplastic particles identified by FTIR spectroscopy analysis proposed that the microplastics found in gastrointestinal tract Stichopus horrens were mostly polyethylene (PE) (Fig. 5) and poly(methyl methacrylate) (PMMA) (Fig. 6). During this study, polyethylene FTIR spectra carrying all major peaks linked to the alkyl functional group were observed as seen in Fig. 5. Polyethylene has a similar finding by Jung et al. (2018) which plastic pieces ingested by Pacific sea turtles accounted for 96% were identified by ATR FT-IR as low density polyethylene (LDPE) and high density polyethylene (HDPE). Besides, the microplastic found in wild Scapharca cornea bivalves were polyethylene (PE) (Ibrahim et al., 2016). In Fig. 5, at peaks of 2916.14 cm− 1 to 917.14 cm− 1 the strong absorption band of C-H2 stretching can be observed. At peak of 1457.13 cm− 1 to 1471.78 cm− 1 the C-H2 bending deformation was shown. In the occurrence of a very strong peak at 717.29 cm− 1 to 718.14 cm− 1, the rocking deformation of the ethylene linkage was shown. The peaks around 1734 cm− 1 on a polyethylene sample are indicative of oxidised content and peak at 1715 cm− 1 associated with the formation of a ketone group (Khalik et al., 2018). It was thought to have arisen due to solar radiation, biological processes or thermal oxidation. Polyethylene is abrasion prone and is commonly used in fishing products. As one of the generic incomes for the local population in Pulau Pangkor is fishery activities, this material is not surprisingly found. The most prevalent isolated plastic polymer was polyethylene in the marine environment. This could probably be due to high polyethylene demand and manufacturing that lead to the disposal of this plastic polymer in aquatic ecosystems (PlasticsEurope, 2017). Polyethylene is one of the most commonly used engineering plastics. The uses of polyethylene range from plastic bags, pipes and bottles (Kasirajan & Ngouajio, 2012). For CH and CH2 groups, polyethylene has distinct absorption bands. For positive identification, peaks with wave numbers 2850 to 2960 cm− 1, 1450 to 1470 cm− 1 and 720 to 730 cm1 must be present (Ng & Obbard, 2006).
During this study, FTIR spectra of poly(methyl methacrylate) (PMMA) carrying all major peaks linked to the carboxylic ester functional group and similar to the PMMA spectra reported by Jung et al.(2018) were observed in their research. In Fig. 6, the strong C-H stretching absorption bands were observed at peak of 2959.11 cm− 1 showing intense absorption bands at peak 1730.85 cm− 1, associated with carbonyl group stretching (C = O), belonging to the PMMA polymer. The C-H2 bending deformation was shown at peak 1453.53 cm− 1. Other absorption bands were observed at peak 1395.27 cm− 1 (C-H3 bending vibration), 1230.35 cm− 1 and 1152.54 cm− 1 (C-O stretching, respectively). In products such as illuminated lights, shatterproof windows, aircraft canopies, the automotive industry (car windows, light fixtures and rear lamps), contact lenses, dental restoration, road lines and acrylic panels, PMMA is a translucent and rigid material that is often used as a replacement for glass (bathtubs) (Ali et al., 2015). There were no previous study on PMMA in any marine organisms.