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Review

Progress in Research on Microplastic Prevalence in Tropical Coastal Environments: A Case Study of the Johor and Singapore Straits

by
Emily Curren
*,
Audrey Ern Lee
,
Denise Ching Yi Yu
and
Sandric Chee Yew Leong
St. John Island National Marine Laboratory, Tropical Marine Science Institute (TMSI), National University of Singapore, 18 Kent Ridge Road, Singapore 119227, Singapore
*
Author to whom correspondence should be addressed.
Microplastics 2024, 3(3), 373-389; https://doi.org/10.3390/microplastics3030023
Submission received: 3 April 2024 / Revised: 30 May 2024 / Accepted: 3 July 2024 / Published: 8 July 2024
(This article belongs to the Special Issue Microplastics in Aquatic Enviroments)
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Abstract

Microplastics are contaminants in marine ecosystems, posing great threats to biota and human health. In this work, we provide an overview of the progress made in understanding microplastic prevalence in tropical coastal environments, focusing on the Johor and the Singapore Straits as a case study. We examine the sources, distribution, transport, and ecological impact of microplastic pollution in this region through a systematic review. All papers relating to marine microplastics in Singapore’s sand and benthic sediments, seawater, and marine biota were used for analysis, from 2004 to 2023. In addition, we discuss the influence of envi-ronmental factors such as coastal morphology and anthropogenic activities on patterns of microplastic accumulation. We emphasize that microplastic pollution is more prevalent along the eutrophic Johor Strait compared to the Singapore Strait due to hydrological conditions. Rainfall is also a key factor that influences mi-croplastic abundance during the monsoon seasons. Furthermore, the bacterial and plankton assemblages of organisms on microplastic surfaces are diverse, with eutrophic waters enhancing the diversity of organisms on microplastic surfaces. Novel harmful cyanobacteria and bloom species of phytoplankton were also found on microplastic surfaces. By synthesizing existing research findings and highlighting regional characteristics, this paper contributes to ongoing efforts to mitigate microplastic pollution in tropical regions.

1. Introduction

The pervasiveness of microplastics in various ecosystems has emerged as a critical ecological concern in recent decades. These particles have been documented across many habitats, from land-based to ocean environments [1]. Microplastics measure 10 μm to 5 mm in diameter and can exist as primary or secondary pieces with particles below 500 μm considered small microplastics [2]. Primary microplastics are produced to be small and are usually used for pre-production purposes. Primary microplastics can be observed as glitter in cosmetics or microbeads in facial washes [3]. Secondary microplastics originate from the breakdown of larger plastic items such as fishing buoys or plastic bottles [4], and microplastic fibers, which are released from washing machine effluents [5]. In the natural environment, physical, chemical, and biological degradation aid the formation of secondary microplastics [6]. For instance, UV from sunlight facilitates the breakdown of microplastics, which aids further physical fragmentation of larger plastic items [6]. In marine environments, the presence of microplastics has had great consequences on marine organisms. As these particles are small, they potentially are a great threat to marine biota. Microplastics have been documented in a wide range of organisms, ranging from zooplankton such as cyclopoids [7] and calanoids [8] to filter feeders such as shrimp [9] and snails [10]. Microplastics also have been identified in larger marine organisms such as turtles [11] and whales [12]. Many reviews have recorded the harmful effect of microplastics on the growth, development, and reproduction of marine biota [13]. For instance, microplastic fibers can clog the guts of daphnids exposed to these plastics and cause mortality [14] and Japanese medaka Oryzias latipes exposed to low-density polyethylene (LDPE) pellets resulted in hepatotoxicity and degeneration [15]. However, microplastic concentration has to be taken into consideration on whether there are observable effects on the organism.
Microplastics are also known to be a habitat and mode of transport for a wide range of marine organisms, including toxic species. Reports of microplastics being a vector of transport for bloom-forming organisms have been published in Naik et al. [16], where the surfaces of microplastics contain biofilms, which disseminate harmful algal bloom (HAB) species in the form of vegetative cysts or cells. A study by Curren and Leong [17] showed the germination of dinoflagellates from the harmful genus Gymnodinium, following incubation of microplastic particles in filtered seawater [17]. The presence of toxic species on microplastics can act as seeds of future HABs when these particles are transported to new locations and germinate under suitable conditions. Plastics are known to exist in the marine environment and these plastics can range from polypropylene to polyester particles. A polyethylene terephthalate (PET) bottle takes an average of 400 years to degrade naturally and hence can persist in the marine environment for a very long time [18]. Hence, these present a suitable surface for the colonization of multiple groups of organisms.
Southeast Asia is a hotspot for microplastics, where rapid urbanization, industrialization, and population growth result in significant plastic pollution within the region’s water bodies. Following China’s ban on the import of mixed plastics, Southeast Asia experienced an influx of plastic waste trade from other countries, resulting in the contamination of water supplies, respiratory issues from burning hazardous plastic waste, and increased crime from illegal recycling factories [19]. In this region, the concentrations of microplastics in the water column can reach up to 11,000 pieces/L in Semarang, Tambak Lorok, Indonesia [20]. Other locations such as Bintan, Indonesia documented 450 pieces/L of microplastics in seawater [20]. Numerous studies have documented the pervasiveness of microplastics in the aquatic environments of Southeast Asia and other parts of the world, including estuaries and marine ecosystems [21,22]. Locations such as Santa Monica Bay and the Northeastern Atlantic Ocean document a microplastic abundance of 3920 and 2460 pieces/L of seawater, respectively [20]. Other reports have highlighted the role of hydrological processes such as tidal currents and monsoonal winds in dispersing microplastics across these waters [23,24]. The absence of proper waste management in many rural locations is a major contributor to plastic pollution in this region [25]. Singapore is an island nation that is situated at the southern tip of Peninsular Malaysia and is separated from Malaysia by the Johor Strait. To the south, Singapore is bounded by the Singapore Strait, which functions as a busy international maritime corridor, connecting the Pacific and Indian Oceans. Singapore experiences a semi-diurnal tidal cycle [26] and monsoon seasons all year round [27]. Singapore also has a growing population of 7 million citizens and with this, plastic consumption has been on the rise in the past decade. Since the pandemic in 2020, the total volume of plastic waste generated from Singapore increased by 20% to 1 million metric tons in 2022 [28]. During the COVID-19 pandemic in 2020, there was an extra 1334 tons of plastic consumed just during the 3-month period where dining out was disallowed [29]. This increase was contributed to by single-use plastic items during food takeaways. Intensified plastic consumption after the pandemic was also observed across many nations due to the use of single-use personal protective equipment such as gloves and masks [30]. Hence, with an increase in plastic consumption, it is essential to closely monitor and assess the impact of plastic waste, especially microplastics, on our coastal regions.
The aim of this study was to examine the progress in research regarding the issue of microplastic contamination in tropical coastal regions, focusing on the Johor and Singapore Straits. This also includes microplastic research conducted on commercially available seafood as microplastic consumption has implications for human health and food security [16]. This body of work will provide new insights and allow for a more in-depth examination of the sources and implications of marine microplastic pollution on organisms, ecosystems, and human health, for the formulation of effective mitigation strategies.

2. Methodology

This review was conducted using several international databases, namely PubMed and Google Scholar. PubMed is known to be a popular database for systematic reviews due to its broad reach and extensive coverage. This database is also known to provide access to a wide range of literature including ecological studies [31]. It was also important to use a supporting database, such as Google Scholar, to complement the primary database used for the literature search, to address any potential gaps and mitigate retrieval bias [32]. The keywords “marine”, “microplastics”, “beach sediments”, “water column”, “surface water”, “marine biota”, “Johor Straits” and “Singapore Straits” were used to obtain the relevant papers required for this review. We included the search term “beach sediments” in addition to “benthic sediments” as certain research articles that surveyed beach sediments did not include the keyword “benthic” in their work. We used the search term “marine biota” as it encompasses the group of living organisms in the marine ecosystem, which is necessary for our review. We also used the search term “marine” to include relevant searches in both coastal and offshore marine environments, such as the deeper waters beyond the continental shelf. Papers investigating the presence of microplastics in beach and benthic sediments, or seawater, and microplastic studies of organisms from both Straits or found in Singapore were included. Some articles publishing microplastic studies in marine environments did not contain the keyword “marine” so the keywords had to be searched separately. Studies that have not been peer-reviewed or published were not taken into consideration. A total of 7 peer-reviewed papers were used to compile information relating to marine microplastics for this study. A summary table was constructed to provide a comprehensive review of the relevant studies regarding microplastics from beach sediments, seawater, and marine biota from Singapore’s coastal waters (Table 1). Wherever possible, numbers were rounded to 1 decimal place and percentages were included for microplastic types. Fields with missing information had NA as inputs. The key microplastic sampling locations relevant to the studies of this paper are displayed in Figure 1.

3. Microplastics from Tropical Coastal Environments

3.1. Microplastics from Sediments

Microplastics were sampled from Singapore’s beach sediments, along different parts of the Johor and Singapore Straits (Figure 1), across various studies, as early as 2004 [24]. However, there was a large break in sampling efforts until 2018. This was the study by Curren and Leong [33], which examined beach sediment microplastics from the high and low-strand lines of Changi Beach (S7b), Sembawang Park (S1b), and Lazarus Island (LI1). Microplastics were in average concentrations of 59.9, 31.1, and 9.2 particles/kg.w.w. Samples were obtained during the first inter-monsoon period (IM1) and observed to be from 1 mm to 5 mm. These samples were not treated with any chemicals prior to enumeration. Although the low-strand lines had a greater number of microplastics at two sites, there was no significant difference in the microplastic abundance recorded from the high and low-strand lines from all samples. Foam was the most abundant microplastic type observed from sites of the Johor Strait, with sites S7b and S1b having prevalences of 54.1% and 66.4%, respectively (Table 1). Foam is a common contaminant along busy shipping routes. Fragments were most commonly observed at site LI1, which could have come from the illegal disposal of domestic solid waste from surrounding locations (35.7%; Table 1). Microplastic pellets, fibers, and film were commonly found together. It was observed that beach sediment microplastic concentration was greater at the Johor Strait compared to the Singapore Strait. This is attributed to the greater accumulation of microplastics in the Johor Strait due to slower water flow. This finding is corroborated in another study [34] in which an average of 333 ± 190 pieces/kg.d.w.s were found during the IM1 period at Sembawang beach, compared to that of East Coast Park on the Singapore Strait, which only had 23 ± 9 pieces/kg. d.w.s. The study by Jong et al. [34] examined microplastics from the coastal waters of Singapore from 2020 to 2021 across the Northeast Monsoon (NE monsoon), Southwest monsoon (SW monsoon), and the two inter-monsoon seasons. Across this time period, an average of 303.3 and 470.8 particles/kg. d.w.s. were found at Sembawang and East Coast Parks, respectively. At both sites, microplastics were mostly found to be smaller than 500 µm (>60%), as it is easier for smaller microplastics to migrate in the marine environment. In addition, fragment, fiber, and film microplastics were found at about the same abundance (28–32%) across sites from this study. Six polymer types were also identified from the two sites, with the most abundant being PP, PE, and PS [34]. In this study, samples were not treated with any chemical prior to enumeration but instead floated with a hypersaturated saline solution following filtration [33].
In the most recent study, beach microplastics from another part of the Singapore Strait were examined [35]. A section of beach along Bedok Jetty, East Coast Park was examined from the period of June 2022 to October 2023 (Figure 1). The peak densities of microplastics were recorded in September 2022 and August 2023 with densities ranging from 150–170 items/m2. The lowest density of microplastics recorded was in March 2023, with only 12 items/m2. Like previous studies, foam microplastics were the most abundant type of plastic on the beach [35]. The abundance of foam pieces from sites along the Johor Strait could be due to the multiple aquaculture farms, which use expanded polystyrene floats (EPS). These floats degrade with time to form secondary microplastics that then accumulate on the beach. From the same site, various microplastic polymers were previously examined via Fourier transform infrared spectroscopy (FTIR). Three plastic polymers were examined, as they represented the major microplastic types recorded from that site [36]. A blue microplastic fragment was identified to be PP, which could have come from the breakdown of larger colored items such as bottle caps or toys. A white microplastic foam piece was identified to be thermoplastic copolyester (TPC), which is commonly used as insulation for electrical cables. A semi-translucent pellet was identified to be polyethylene, and this is likely a pellet used for pre-production purposes [36].
Table 1. Reports of microplastics sampled from beach and benthic sediments along the Johor and Singapore Straits.
Table 1. Reports of microplastics sampled from beach and benthic sediments along the Johor and Singapore Straits.
Sampled MatrixLocationDate SampledTotal AbundanceMicroplastic TypeSizeColorReference
Beach sediment
Pasir RisJohor Strait20040–9 particles/kgPolystyreneNANA[24]
Changi0 particles/kgNA
Sembawang
East CoastSingapore Strait0–6 particles/kgPolyvinyl alcohol, Polypropylene
Sentosa Island0 particles/kgNA
Sembawang Park (S1b)Johor Strait201831.1 particles/kgFoam (66.4%), Fragment (28.0%), Pellet (3.4%), Fiber (1.1%), Film (1.1%)NANA[17]
Lazarus Island (LI1)Singapore Strait9.2 particles/kgFragment (35.7%), Foam (31.0%), Pellet (23.8%), Fiber (7.2%), Film (2.3%)
Changi Jetty (S7b)Johor Strait59.9 particles/kgFoam (54.1%), Fragment (39.6%), Fiber (4.2%), Pellet (1.4%), Film (0.7%)
Bedok Jetty Singapore Strait2022–202371.9 particles/m2Foam (44%), Fragment (23%), Fiber (17%)NANA[35]
Benthic sediment
Sembawang Johor Strait2020–2021303.3 particles/kg. d.w.sFragment (32%), Fiber (29%), Film (28%), Foam (8%), Others (3%)<500 µm (70%), 500–1000 µm (10%), 1–5 mm (20%)NA[34]
East Coast Singapore Strait 470.8 particles/kg. d.w.s<500 µm (60%), 500–1000 µm (30%), 1–5 mm (10%)
NA denotes no records.
Knowing the exact type of plastic polymer that pollutes different sites provides more information for source identification. This is because certain plastic polymers may be associated with specific industries, consumer products, or waste disposal practices. A study by Senathirajah et al. [37] assessed the hazardous properties of common plastic polymer types using a comprehensive framework and found that PVC, PP, and PS had the highest risk of harm to human health and the environment. Hence, the identification of plastic polymers is crucial for assessing the relevant risks associated with plastic pollution.

3.2. Microplastics from Water Column

Microplastics from Singapore’s coastlines were first examined by Ng and Obbard in 2004 [24], where they quantified 0–1 microplastics/L of surface seawater. The authors studied the surface and subsurface (1 m) layer of seawater and found a low concentration of 0–1 particles/L for both layers in Republic of Singapore Yatch Club (RSYC) at the Singapore Strait [24]. They also studied the seawater at Kranji, Johor Strait where the concentrations at the surface were only slightly higher, at 0–1 particles/L, compared to the subsurface, where it was 0–0.5 particles/L (Figure 1; Table 2). Various plastic polymers were also identified, with polyethylene and polystyrene identified from RSYC and the surface seawater of Kranji. Polypropylene microplastics were found in the subsurface layer of the Kranji samples ([24]; Table 2).
Surface water and depth (5 m) samples were also examined along the Johor and Singapore Straits from 2021 to 2022 [36]. Microplastics were observed from every sample at the surface and at depth, with size ranges from 10 µm to 5 mm. The Johor Strait recorded a greater abundance of microplastics at the surface and at depth (106–238 particles/mL) compared to that of the Singapore Strait (143–196 particles/mL). On average, microplastics were more abundant from surface samples of the Johor Strait (S1 and S9), and depth samples from the Singapore Strait (SW1, SJI, and SE2) had more microplastics in comparison. In this study, microplastic fragments were the dominant microplastic type observed (70%), followed by film (25%) and fiber (5%). Except for site SJI, samples from all sites contained microplastic fragments, film, and fibers (Table 2). Across the various sampling sites, a total of seven colors were recorded from all microplastic types, with transparent microplastics being the most commonly observed (64.9%), then black (25.1%) and blue (5.5%; [36]). The remaining colors were purple (2.3%), pink (1.7%), red (0.5%), and brown (0.04%). Microplastics were also characterized according to size, where particles smaller than 500 µm were most abundant (98.9%). Particles sized 500–1000 µm and 1–5 mm were only 1% and 0.1% abundant, respectively [36].
Microplastics from the coastal waters surrounding Singapore are comparable the pollution levels from regions such as the Northeastern Pacific Ocean (279,000 pieces/L; [38]) and the Arctic Ocean (0–18,000 pieces/L; [39] In other locations such as the South Yellow Sea and Hangzhou Bay, concentrations of microplastics are much lower at 4.5–67.5 pieces/L [40] and 1.4 × 10−3 pieces/L [41], respectively.

3.3. Spatiotemporal Variation in Microplastics

The Johor and Singapore Straits are major water bodies that function as busy international shipping routes. From the studies above, it is evident that sites from the Johor Strait had a higher microplastic abundance in comparison with those from the Singapore Strait. Two main reasons contribute to this, the first being that the Johor Strait is more polluted, due to smaller rivers such as Sungei Tebrau and the Johor River emptying into it [42]. Rivers are major pathways of microplastic transport from land-based areas to marine environments. The influx of freshwater carries its load of microplastics from upstream sources such as urban effluents, industrial zones, and agricultural lands [43]. It is estimated globally that rivers discharge 1.2–2.4 metric tons of floating plastic from terrestrial sources to the oceans [21].
The other major reason contributing to this phenomenon is the water flow in the Johor Strait. The Johor Strait is separated into the East (EJS) and West (WJS) Johor Strait due to an impermeable structure which is the causeway bridge that connects Singapore to Malaysia. This structure results in the obstruction of water flow, which is absent in the Singapore Strait. A model by He et al. [44] also showed that slow water velocities result in greater particle deposition and less overall transport of the microplastic particle. Hence the lower water flow in the Johor Strait promotes the retention and accumulation of microplastics, exacerbating this issue of microplastic pollution.
The abundance of microplastics in the Johor and Singapore Straits was influenced by oceanographic conditions such as the monsoon season, where higher abundances of microplastics were observed. This was seen from the study of Curren and Leong [36], in which multiple sites along the Singapore Strait recorded a 1.1–1.7× increase in microplastic abundance in the monsoon season compared to the inter-monsoon season. The study by Jong et al. [34] also showed increased abundances of microplastics during the southwest monsoon period, across both surface water and sediment samples in the Johor and Singapore Straits. During the monsoon season, there is a recorded increase in rainfall, and these months can have up to 50% more rainfall than months in the inter-monsoon season in Singapore (Hassim & Timbal, 2019). Similar studies from other regions such as the west coast of India [45], east Malaysia [46], and Thailand [47] also show an elevated abundance of microplastics during the wet season in comparison to the dry season. Heavy rainfall also increases sediment mixing of the benthic layer with the water column, resulting in more microplastics resuspending into the water column [48].
Previous models have shown that current and wind-induced wave flows are other key factors controlling the spatial distribution of microplastics at the water surface [49]. Changes in windspeeds can also influence the abundance and distribution of microplastics [50]. Elevated windspeeds can result in greater weathering and fragmentation of large plastic debris, generating more secondary microplastics. Greater windspeeds can also result in a wider dispersion of microplastics. A study by Bullard et al. [51] showed that microplastics are preferentially transported by wind compared to sand and soil and hence make atmospheric transport of microplastics more significant. These findings emphasize the complex interactions between meteorological factors and microplastic distribution in marine ecosystems.

3.4. Diversity of Organisms on the Plastisphere

The diversity of organisms on microplastics from beach sediments was also analyzed in two studies. In the study of Curren and Leong [33], the bacterial communities from these microplastics were examined using 16S rRNA (v3–v4) shotgun sequencing. An average of 443 OTUs were found from the three sites, with S7b having the greatest number at 472 OTUs. The three groups Proteobacteria, Bacteroidetes, and Firmicutes were dominant across all samples, with the families Erythrobacteraceae (19%), Rhodospirallaceae (22%), and Rhodobacteraceae (21%) most commonly observed from all sites. However, the structures of bacterial communities between the sites were distinctly different [33]. From site S7b, Erythrobacter sp. was the most abundant genus (21%), while from site S1b, Arcobacter sp. was the most abundant genus (6%), and from site LI1, Brachymonas and Pseudomonas sp. were the most abundant genera (5%; [33]). Furthermore, five species from the pathogenic genus Pseudomonas were also identified from all sites from the Johor Strait, and this included species such as Pseudomonas alcaligenes and Pseudomonas veronii [33], which have been associated with various human infections.
A follow-up study by Curren and Leong [17] profiled the plankton communities on these microplastic surfaces through the sequencing of the 18S rRNA (v4) region. An average of 781 OTUs were observed from the three sampling sites, with site S7b also having the greatest number of observed taxa (735). Site S7b also had the greatest diversity as observed from the Simpson’s index (0.892), and also the highest species richness as observed from the Chao1 index (785). The community structure of the plankton communities at each sampling site was found to be distinct, although Eukaryota was dominant at each location [17]. Furthermore, the major class of organisms at each site varied, with Dinophyceae (19%), Spirotrichea (14.9%), and Sordariomycetes (25.3%) being dominant at sites S7b, S1b, and LI1, respectively [17].
A fraction of the microplastics from this study were incubated with filtered seawater for two weeks. However, organisms were only observed to grow from foam microplastics and not other microplastic types [17]. Motile dinoflagellates from the toxic genus Gymnodinium emerged from foam microplastics after three days. Under the microscope, resting cysts of Gymnodinium sp. were attached to the sides of the foam piece, with multiple swimming dinoflagellates of this genus surrounding the microplastic. This is the first study that has demonstrated this phenomenon [17]. This also emphasizes that foam microplastics are a habitat for the transport of viable temporary dinoflagellate cysts, facilitating their dispersal across aquatic environments. The germination of motile dinoflagellates from microplastics highlights the role of microplastics as potential hotspots for the reactivation and proliferation of these organisms.
Cyanobacteria were also observed when foam microplastics were incubated with filtered seawater after one week. A total of six types of cyanobacteria were recorded from microscopic observation from both filamentous and coccoid genera. The cyanobacterial genera Microcoleus, Lyngbya, and Moorena were identified [17]. Lyngbya and Moorena are harmful cyanobacterial genera that have previously bloomed or formed benthic mats in Singapore’s coastal waters. Moorena producens (previously Lyngbya majuscula) was found to bloom in an enclosed boat canal off the Southern Strait in 2012 [52]. In addition, benthic mats of Lyngbya aestuarii have been isolated from the concrete walls of Singapore’s southern islands in a previous study [53]. Hence, cyanobacteria from these microplastic surfaces can detach and enter new environments and form mats under suitable conditions. The reverse of this process could also be true, where bloom-forming cyanobacteria detach from mats and attach to microplastic surfaces, where they raft and are transported to new ecosystems.
In the West Johor Strait, an incubation study was carried out recently on various plastic polymers in seawater to examine the phytoplankton communities dominating these plastics [54]. A total of six plastic polymers were used for experimentation, namely, polypropylene (PP), polyethylene (PE), polystyrene (PS), polyethylene terephthalate (PET), and polyvinyl chloride (PVC), with glass (GL) as the control. The 28S rRNA region was used with MinION sequencing for the identification of attached organisms. Preliminary results show that the toxic dinoflagellate Alexandrium leei was identified from all plastic types [54]. Other dinoflagellates such as Levanderina fissa were also identified from multiple surfaces: GL, PP, PE, PET, and PVC [54]. Understanding the types of organisms that are preferentially found on specific polymer types is important to understanding their fate and transport in aquatic ecosystems [55]. In addition, this knowledge will be useful for biofouling management strategies, where polymers less prone to colonization of specific taxa can be identified. This allows for better mitigation of biofouling on ships, offshore structures, and aquaculture equipment, reducing maintenance costs and environmental impact [56].
Each type of plastic polymer exhibits variations in surface properties, texture, and chemical composition. This influences the composition and abundance of phytoplankton colonizing them [57]. Various factors are found to influence the diversity of organisms across ecosystems. One factor is the level of eutrophication in water bodies. Eutrophication is characterized by elevated nutrient content such as phosphorus and nitrogen, which generally results in a lower species diversity in multiple environments [58]. For instance, eutrophic waters are known to lower the diversity of organisms such as fish populations [59] and plant species diversity in western European wetlands [60]. Contrary to these findings, the diversity of organisms on microplastics was greater in the eutrophic waters of the Johor Strait [30]. The Johor Strait is considered more eutrophic compared to the Singapore Strait due to the emptying of smaller rivers as previously mentioned [61]. These rivers transport urbanized and agricultural effluents into the Johor Strait, leading to higher nutrient levels, where phosphate and nitrogen content (nitrate and nitrite) can be as high as 0.90 µM-P and 6.47 µM-N, respectively [61]. Other factors such as salinity and temperature also affect the biodiversity of various organisms. In one study, salinity was found to be the primary factor influencing bacterial diversity on different plastic polymer surfaces [62]. Increased global temperatures over the next few decades are predicted to cause a decrease in species diversity among macroinvertebrates, such as the mayfly Drunella cryptomeria and freshwater isopod Asellus sp. [63].
Microplastics not only host a diverse community of organisms but also are a source of novel species with bloom and pharmaceutical potential. Recently, a new cyanobacterial genus and species Sphaerothrix gracilis from the family Nodosilineaceae was isolated from foam microplastics off Bedok Jetty, Singapore Straits [64]. This filamentous cyanobacterium has toxic potential and could cause harmful blooms in coastal waters. Through genome mining, S. gracilis was revealed to have antibiotic resistance to Bleomycin, Linearmycin, and Tetracycline. Multidrug resistance genes mdTA-C E and H were also identified [64]. Analysis via the CyanoPATH database also demonstrated the bloom potential of this cyanobacteria. There were 700 related core genes identified, for nitrogen fixation (22%), trace metal and vitamin assimilation (14%), and toxin production (13%; [64]). Furthermore, antiSMASH analyses showed the presence of non-ribosomal polyketide synthethases (NRPS), with close matches to the harmful metabolite Puwainaphycin, from the toxic cyanobacterium Cylindrospermum alatosporum CCALA988. Interestingly, S. gracilis also contained plastic-degrading enzymes, targeting various plastic polymers such as nylon, polyethylene, and polylactic acid [64]. S. gracilis is the first novel cyanobacteria to be characterized from its presence on microplastic surfaces, highlighting the potential for the discovery of more novel species from the plastisphere in the future.

3.5. Microplastics Ingested by Marine Organisms

Microplastics have been investigated in a few types of marine organisms in Singapore. The research in this area only began recently, in 2018, and existing research has been limited to shrimp, gastropods, and seagrass. A total of 93 individuals of shrimp from three species, namely, Pacific whiteleg shrimp Litopenaeus vannamei, Argentine red shrimp Pleoticus muelleri, and Indian white shrimp Fenneropenaeus indicus, were studied from different regions [9]. In this study, the digestive tracts of the shrimp were digested with 6.25% filtered sodium hypochlorite (NaClO) to break down soft tissue for 48 h at room temperature. Across these species, microplastics ranged from 13.4 to 7050 pieces/g.w.w. of shrimp (Table 3). Individuals of L. vannamei from Malaysia and Ecuador had an average of 21 ± 4 and 13 ± 1 pieces/g.w.w and had the lowest microplastic abundance among the species (Table 3). Shrimp P. muelleri from the Southwest Atlantic FAO41 region had the greatest abundance of microplastics out of the three species, at 7050 ± 418 pieces/g.w.w. (Table 3). F. indicus from the Indian Ocean FAO57 region had an average microplastic abundance of 5570 ± 100 pieces/g.w.w. (Table 3). Four main types of microplastics were identified from the shrimp: film, fiber, fragment, and spheres [9]. The composition of microplastic types also varied between the three species. Spheres were in greatest abundance in both F. indicus and P. muelleri, at 61.6% and 69.6%, respectively (Table 3). Film was dominant in L. vannamei shrimp from both Malaysia and Ecuador, at 98% and 93%, respectively (Table 3). Microplastics were observed from 10 µm to an upper limit of 5 mm.
Microplastics were also examined in gastropods, which are another group of commonly consumed seafood in Asian nations. Microplastics were investigated in the caenogastropod Laevistrombus turturella from the coastal waters of Singapore [10]. This gastropod is often sold in supermarkets and is considered a popular type of seafood. Samples were digested with 10% potassium hydroxide (KOH) solution for 72 h at room temperature [10]. An average of 273 pieces of microplastics were observed per individual, with fibers being the greatest (35%), followed by fragments (34%), and film (31%; Table 3). Certain fibers were observed to have their ends shredded, which is due to the action of the radula during gastropod grazing. In addition, most microplastics were smaller than 500 µm (74%), with some 501–1000 µm (17%) and 1–5 mm (9%) in size [10]. In the same study, six microplastic colors were observed in total, with clear microplastics being the most abundant (47%), then black (38%), blue (9%), and finally, pink and green with the same abundance (3%) [10].
Microplastics were also examined in the green mussel Perna viridis from five aquaculture sites from the Johor Strait ([65] Table 3). Mussel samples were treated with 30% hydrogen peroxide solution (H2O2) for 48 h at 50 °C. Microplastics were in greatest abundance among samples from Teluk Jawa (60 pieces/kg.d.w.), with the lowest being 29 pieces/kg.d.w. from the sampling site near the causeway. Five categories of microplastic types were observed: fragments, fibers, foam, film, and beads. Fragments, fibers, and beads were the most commonly seen, at 79%, 18%, and 2%, respectively [65]. The colors of these microplastics were also characterized, with a total of six colors identified. Black (32%) and red (32%) were the major colors recorded and transparent particles were also commonly observed (25%). These microplastics were also measured, and most particles were 0.1–0.5 mm (63%), and 0.1–0.5 mm (15%) in size [65]. Polyamide (PA) fibers and PP fragments were identified from mussels across sites.
In marine organisms, the abundance of microplastics is intricately linked to surrounding pollution levels [66]. Elevated microplastic levels in marine environments will generally result in a greater abundance of microplastics taken up and hence a greater concentration observed within the organism [67]. Filter feeders and scavengers such as shrimp, snails, and mussels indiscriminately uptake microplastics from their surroundings, resulting in relatively high abundances of microplastics observed. In the study by Zin et al. [65], P. viridis contained PA fibers that most probably originated from aged fishing gear such as fishing nets. These fibers enter the marine sediment after degradation and are indiscriminately taken up by the mussel during filter-feeding.
Since these groups of organisms reflect the level of microplastic contamination in their surroundings, they have been termed bioindicators of microplastic pollution in various reports. Gastropods are considered bioindicators of microplastic contamination in marine environments, where the average concentration of microplastics found in marine gastropods was 32.8 pieces/individual [10]. In the study by Li et al. [66], mussels were explored as a global bioindicator of coastal microplastic pollution, with studies in over 16 countries. Although there are multiple reports documenting the presence of microplastics in shrimp, this group of organisms has yet to be evaluated as a suitable bioindicator of microplastic pollution.
Microplastic contamination has also been examined on the surfaces of marine macrophytes [68]. Three species of seagrass: Cymodocea rotundata, Cymodocea serrulata, and Thalassia hemprichii, and two species of subtidal macroalgae, Padina sp. and Sargassum ilcifolium, were sampled at four sites along the Singapore Strait (Table 3). There was an average of 0.029 pieces/cm2 of microplastics on all five species, with the mean microplastic density of seagrass ranging from 0.036 to 0.051 pieces/cm2 [68]. C. serrulata had the highest microplastic density on its blades. Macroalgae Padina sp. and S. ilicifolium had a mean microplastic density of 0.012 and 0.007 pieces/cm2, respectively. The microfiber length was also measured, with mean lengths ranging from 1443 to 2632 µm among all species [68]. Among others, the nerite snail Clithon oualaniensis is known to graze on the seagrass Cymodocea rotundata [69]. The grazing of such contaminated macrophytes by marine herbivores is one way in which microplastics can enter the food chain.
The presence of microplastics in seafood is a route of exposure during consumption. In recent years, there has been increased attention focusing on plastic-contaminated seafood being a threat to human health and food safety. Tissues of the human body are found to translocate and accumulate microplastics differently, with larger microplastics transported by paracellular methods and smaller microplastics (<50 µm) internalized by endocytosis [70]. Microplastics can cause oxidative stress and can inhibit antioxidant enzymes, resulting in metabolic disorders and liver damage [71]. Microplastics have been found in many key organs of the human body, such as the colon (28.1 pieces/g; [72]) and spleen (1.1 pieces/g; [73]). The presence of microplastics in human fecal matter is already correlated with inflammatory bowel disease [74]. Given the widespread distribution and multiple implications of microplastics in the human body, it is crucial to conduct further studies on mammals, especially using human cell lines, to understand the mechanisms and targeted toxicities of microplastics in the human body.
Table 3. Reports of microplastics from marine organisms that are commercially available in Singapore.
Table 3. Reports of microplastics from marine organisms that are commercially available in Singapore.
Sampled MatrixLocationDate SampledTotal AbundanceMicroplastic TypeSizeColorReference
Shrimp
Fenneropenaeus indicusIndonesia, Eastern Indian Ocean, FAO57January 20205570 ± 100 pieces/g.w.w.Sphere (61.6%), film (25.4%), fragment (10.8%), fiber (2.2%)NANA[9]
Lithopenaeus vannameiMalaysia21 ± 4 pieces/g.w.w.Film (98%), fragment (0.8%), fiber (0.6%), sphere (0.6%)
Ecuador13 ± 1 pieces/g.w.w.Film (93%), fragment (4.7%), fiber (2%), sphere (0.3%)
Pleoticus muelleriArgentina, Southwest Atlantic, FAO417050 ± 418 pieces/g.w.w.Sphere (69.6%), fragment (21.5%), film (7.4%), fibers (1.5%)
Snail
Laevistrombus turturellaSingapore2022273 pieces/individualFiber (35%), fragment (34%), film (31%)<500 µm (74%), 500–1000 µm (17%), 1–5 mm (9%)Transparent (47%), black (38%), blue (9%), pink (3%), green (3%)[36]
Mussel
Perna viridisJohor Strait202129–60 pieces/kg. d.w.Fragment (79%), fiber (18%), beads (3%)NA Black (32%), red (32%), transparent (25%), blue (9%), white (1%)[65]
Seagrass
Cymodocea rotundataSingapore Strait20180.051 pieces/cm2Fiber (97.3%), fragment (2.7%)<500 µm (2.7%), 500–1000 µm (34.3%), >1 mm (63%)NA[68]
Cymodocea serrulata0.060 pieces/cm2
Thalassia hemprichii0.036 pieces/cm2
Macroalgae
Padina sp. 0.012 pieces/cm2
Sargassum ilicifolium0.007 pieces/cm2
NA denotes no records.
From this study, there are a few key findings that could be summarized (Figure 2). Microplastics in Singapore’s marine environments are mainly influenced by increased rainfall during monsoon seasons. Furthermore, microplastics are host to diverse organisms, including toxic phytoplankton and novel cyanobacteria species. The diversity of these species is also impacted by eutrophic waters. Lastly, microplastic contamination is evident in commonly consumed seafood sold in the supermarkets of Singapore, presenting an additional source of plastic contamination to the human body through consumption.

4. Data Gaps and Future Work

The lack of standardization in sampling and analysis pipelines has been raised in multiple reviews relating to microplastic studies [75]. While a few standardization protocols have been developed in the past few years, not all studies have adopted them, due to various budgeting or technical constraints. Although the resolution of sampling methods has been dealt with in these reviews, it is still important to assess these approaches to choose the appropriate methods, considering the advantages and disadvantages of each protocol. It is important that we emphasize the standardization of sampling and analysis methods to enable robust comparison between studies, ensure data reliability, and enhance our understanding of microplastic pollution across various marine environments. The use of international, open-access databases containing information on microplastic identification will be helpful for microplastic identification and analysis. To date, the National Oceanic and Atmospheric Administration (NOAA)’s National Centers for Environmental Information (NCEI) is a global database portal that provides open access to data on marine microplastics data [76]. It is important that these databases are open-access and include information that is not restricted to specific waters. Other existing databases such as the European Union’s EMODnet (European Marine Observation and Data Network) allow users to access and download microplastic data [77]. Another public database, the Marine Plastic Database was also developed to compare microplastic monitoring results across Europe [78]. Although these databases are public, the data within was only limited to reports from European waters. With microplastics-related publications on an exponential rise since 2014 [76], it is necessary to integrate research into a large-scale database to provide a more complete global understanding of the sources, distribution, and fates of microplastics in the ocean.
In recent years, Singapore has also made a greater effort to reduce the consumption of single-use plastics. A significant initiative was introduced in July 2023, in which five major supermarket chains in Singapore implemented a charge on plastic bag consumption. Annually, about 820 million plastic bags are taken from major supermarkets, amounting to about two to four plastic bags per shopper each trip [79]. This measure has been very effective, as reports mention a drop in plastic bag use of between 50% and 80% within the first month [80]. Furthermore, the government, associated agencies, and schools are encouraging individuals to further recycle, with educational campaigns and research initiatives. This is because the rate of recycling in Singapore is still relatively low, with only 6% of the 1 million metric tons of plastic waste being recycled in 2022 [81]. Continual efforts are essential to enhance recycling infrastructure and to foster a culture of sustainability in the nation.

5. Conclusions

This study gives an updated report on the progress of research on microplastics from the coastal regions of Singapore. Microplastics in the Johor and Singapore Straits are abundant on beach and benthic sediments and in the water column and marine organisms, including seafood and seagrass. Microplastics are low-density buoyant pieces that have their movement subjected to many coastal environmental factors such as waves, currents, and tidal movements. Rainfall variations during the monsoon season were a key factor in influencing microplastic abundance in both straits. Eutrophic waters also enhance the diversity of bacteria and phytoplankton on microplastic surfaces, instead of decreasing species diversity. Foam microplastics are also a possible hotspot for the discovery of novel, harmful cyanobacteria and other organisms that produce toxic secondary metabolites. These organisms, which are chemically rich, will have potential for future pharmaceutical applications. Furthermore, microplastic contamination was evident in popular seafood such as shrimp, snails, and mussels. This holds significant implications for the marine ecosystem and human health given that microplastics have been found in key human organs such as the colon, spleen, and liver. Plastic is lightweight, cheap, and durable. The issue herein does not lie with the material itself, but rather with how much individuals consume and how plastic waste is being disposed of. Hence, there is a growing need for comprehensive monitoring programs to track the levels of plastic pollution along coastlines. Researchers and governmental organizations should work collaboratively to characterize the composition, distribution, and ecological consequences of microplastics in marine environments. The establishment of standardized methodologies and protocols for microplastic sampling, analysis, and reporting is important to ensure data comparability and reliability across studies. This will enhance collaborative efforts. In addition, the use of international, open-access databases will facilitate data sharing, synthesis, and dissemination, accelerating progress in microplastic research. This will enable us to understand the long-term implications of microplastic pollution and develop sustainable solutions to address this pertinent issue.

Author Contributions

Conceptualization, E.C., A.E.L., and D.C.Y.Y.; methodology, E.C.; investigation, E.C, A.E.L., and D.C.Y.Y.; writing—original draft preparation, E.C.; writing—review and editing, E.C, A.E.L., D.C.Y.Y., and S.C.Y.L.; visualization, E.C.; supervision, S.C.Y.L.; funding acquisition, S.C.Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially supported by funds awarded to Sandric Leong through the National University of Singapore.

Data Availability Statement

Not applicable.

Acknowledgments

The help provided by Team HABs and St. John’s Island National Marine Laboratory is greatly appreciated.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Key sampling locations of microplastics around the coastal regions of Singapore, where X marks the sampling spots.
Figure 1. Key sampling locations of microplastics around the coastal regions of Singapore, where X marks the sampling spots.
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Figure 2. An overview of key findings from microplastic research from the coastal waters of Singapore.
Figure 2. An overview of key findings from microplastic research from the coastal waters of Singapore.
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Table 2. Reports of microplastics from seawater of different sites along the Johor and Singapore Straits.
Table 2. Reports of microplastics from seawater of different sites along the Johor and Singapore Straits.
Sampled MatrixLocationDate SampledTotal AbundanceMicroplastic TypeSizeColorReference
RSYC (0 m)Singapore Strait20040–1 particles/LPolyethylene,
Polystyrene		NANA[24]
RSYC (1 m)Polyethylene
Kranji (0 m)Johor Strait0–1 particles/LPolyethylene,
Polystyrene
Kranji (1 m)0–0.5 particles/LPolypropylene
Sembawang (0 m)Johor Strait2021–202295.5 pieces/mLFragment (69.2%), Film (22.3%), Fiber (8.5%)<500 µm (96.3%), 500–1000 µm (2.8%), 1–5 mm (0.9%)Black (90.3%), Purple (5.2%), Transparent (3.85%), Red (0.5%)[36]
Sembawang (5 m)10.3 pieces/mLFragment (56.8%), Film (31.2%), Fiber (12.0%)<500 µm (96.6%), 500–1000 µm (3.4%)Black (84.8%), Blue (8.3%), Transparent (3.5%), Purple (1.7%), Red (1.7%)
Coney S4B (0 m)71.4 pieces/mLFragment (64.8%), Film (33.8%), Fiber (1.4%)<500 µm (98.5%), 500–1000 µm (1.5%)Black (91.7%), Transparent (7.5%), Purple (0.8%)
Coney S4B (5 m)70 pieces/mLFragment (71.1%), Film (28.0%), Fiber (0.9%)<500 µm (99.4%), 500–1000 µm (0.6%)Black (77.3%), Blue (1.0%), Transparent (11.1%), Purple (9.9%), Red (0.7%)
Ubin S9 (0 m)119 pieces/mLFragment (70.4%), Film (24.2%), Fiber (5.4%)<500 µm (99.5%), 500–1000 µm (0.5%)Black (56.0%), Blue (17.0%), Transparent (12.0%), Purple (12.0%), Red (3.0%)
Ubin S9 (5 m)120 pieces/mLFragment (70.4%), Film (24.2%), Fiber (5.4%)<500 µm (100%)Black (60.5%), Blue (3.2%), Transparent (7.4%), Purple (23.5%), Red (5.4%)
Jurong SW1 (0 m)Singapore Strait76.2 pieces/mLFragment (54.1%), Film (44.8%), Fiber (1.1%)<500 µm (100%)Black (58.4%), Blue (26.9%), Transparent (11.0%), Purple (2.0%), Red (1.7%)
Jurong SW1 (5 m)66.5 pieces/mLFragment (65.5%), Film (28.7%), Fiber (5.8%)<500 µm (98.8%), 500–1000 µm (1.2%)Black (87.0%), Blue (2.3%), Transparent (3.9%), Purple (4.5%), Red (2.3%)
St. John’s Island (0 m)101 pieces/mLFilm (50.2%), Fragment (44.0%), Fiber (5.8%)<500 µm (100%)Black (50.0%), Transparent (37.5%), Purple (12.5%)
St. John’s Island (5 m)62.5 pieces/mLFragment (100%)<500 µm (100%)Black (100.0%)
East Coast (0 m)107 pieces/mLFragment (80.4%), Film (14.2%), Fiber (5.4%)<500 µm (98.4%), 500–1000 µm (1.6%)Black (84.9%), Transparent (7.5%), Red (0.9%), Blue (0.6%)
East Coast (5 m)88.5 pieces/mLFilm (54.3%), Fragment (43.0%), Fiber (2.7%)<500 µm (100%)Black (68.5%), Blue (12.3%), Transparent (17.3%), Purple (2%)
NA denotes no records.
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MDPI and ACS Style

Curren, E.; Lee, A.E.; Yu, D.C.Y.; Leong, S.C.Y. Progress in Research on Microplastic Prevalence in Tropical Coastal Environments: A Case Study of the Johor and Singapore Straits. Microplastics 2024, 3, 373-389. https://doi.org/10.3390/microplastics3030023

AMA Style

Curren E, Lee AE, Yu DCY, Leong SCY. Progress in Research on Microplastic Prevalence in Tropical Coastal Environments: A Case Study of the Johor and Singapore Straits. Microplastics. 2024; 3(3):373-389. https://doi.org/10.3390/microplastics3030023

Chicago/Turabian Style

Curren, Emily, Audrey Ern Lee, Denise Ching Yi Yu, and Sandric Chee Yew Leong. 2024. "Progress in Research on Microplastic Prevalence in Tropical Coastal Environments: A Case Study of the Johor and Singapore Straits" Microplastics 3, no. 3: 373-389. https://doi.org/10.3390/microplastics3030023

APA Style

Curren, E., Lee, A. E., Yu, D. C. Y., & Leong, S. C. Y. (2024). Progress in Research on Microplastic Prevalence in Tropical Coastal Environments: A Case Study of the Johor and Singapore Straits. Microplastics, 3(3), 373-389. https://doi.org/10.3390/microplastics3030023

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