Plastics are lightweight, low-cost synthetic hydrocarbons made from a wide range of polymers [1]. Plastics can be utilized over a wide range of temperatures and are biologically inert, corrosion resistant, cheap, possessing high specific strength, good electrical insulator and are durable [2].

About 10% of globally produced plastics ends up in the aquatic environment [3]. All major ocean system has been found with plastic debris [4], with an assessed value of over 4 to 12 million metric tons of plastic waste generated on terrestrial area entering the marine environment every year [5]. Plastics can enter the sea through surface runoff and anthropogenic activities in coastal beach such as offshore work, commercial fishing, and aquaculture. Furthermore, precipitation and storm, cyclone or earthquakes followed by tsunamis which washdown large quantities of land material, including plastic debris, into the oceans. 

Plastics have become world-wide issue adversely affecting the marine ecosystem [6-7]. Plastic materials such as plastic bags, are often reported to be ingested by dolphins as well as turtles who may mistake them for food [8]. In recent years, ingestion of plastics in marine fish has been evaluated in many studies because many species are consumed by humans [9]. In dolphins, ingestion of plastic occurs at a higher frequency in young than in adults [10]. Sea bird deaths has been happening because of entanglement in plastic debris and ingestion of plastics. Data from previous report states that>45% of 367 species of marine birds have ingested plastics [11]. Study results showed that, because of the feeding habit and diet, 55% of procellariform birds had ingested plastic particles or fragments [12].

The poor disposal ways of plastics leads to unintended loss, and breakdown of larger plastics cause increasing accumulation of small plastic particles and fibres in the environment called microplastic or MPs ranges from 0.001µm-5mm and mesoplastic ranges above 5mm to 10mm [13]. The plastics enter marine environment by two main sources, they are as follow; primary source of microplastic are manufactured as it is, for e.g. microbeads [14] meanwhile the source for secondary microplastic are from fragmentation, chemical, physical, biological degradation processes [15]. They have become significant threat to marine biota [16]. Because of their small size, microplastics may be ingested by organisms such as bivalves, shrimp, zooplankton, mussels, fishes, oysters, whales which are filter feeders. These fragments are reported to cause false satiation, complications in reproductions, pathological stress, diminished growth rate and oxidative stress [7,17]. Tropic transfer of microplastic occurs through indirect ingestion which is a naturally occurring process when the contaminated prey is consumed by predators [18] and accumulate in the food web and cause damage to higher organisms [19]. Since the qualitative and quantitative assessment of microplastics have carried out in different samples in different locations but such reports are not available in the most fertile and versatile ecosystem of the Gulf of Mannar Marine Biosphere Reserve and hence this study was carried out.

The Gulf of Mannar which lies between 8° 47' to 9° 15' N latitude and 78° 12' to 79° 14' E longitude, located in the Southeast coast of India, and runs from Rameswaram to Kanyakumari. It has a row of 21 deserted low-lying coral reef islands, which are divided into four groups namely Tuticorin group (4 islands), Vembar group (3 islands), Keezhakarai group (7 islands) and Mandapam group (7 islands) (Figure 1).

Figure 1: Map Shows the Islands of the Gulf of Mannar

These islands also developed reefs and reef-associated ecosystems [20]. In 1986, Tamil Nadu government declared these 21 islands and the surrounding waters of the Gulf of Mannar as Marine National Park for the purpose of protecting the lives of marine and their environment. In terms of size, shape, elevation and geomorphic features, these islands vary from one to another. These islands are dominated by salt runners (Sesuvium, Salicornia etc.), shrubs (Calotropis, opuntia etc.), trees (Acacia, Palmyra) and mangroves in marshy regions (Rhizophora, Avicennia etc.). Pollution by human action, leads to the loss of resources in these regions [21].

Sediments were collected by using core sampler, the plankton samples were collected by using plankton nets, seaweeds, seagrass, and corals. Sample were collected by using hand picking from 12 islands in shallow regions (Table 1). 

Table 1: Sampling Site, Location, and Collected Sample from Islands of Gulf of Mannar

All the samples except the sediment were washed thoroughly in seawater and then distilled water and then removed the adhering particles and associated macro-organisms. The sediment samples were collected without washing but the organisms present in it were carefully removed. All the collected samples were packed in a labeled container and transported to the laboratory in an ice chest for further analysis. Further, the seagrass, seaweeds, mangroves samples were grinded by using electric mixer and filtered, then the filtrate was collected. The coral polyp was collected by passing distilled water forcedly using a Millipore filter unit and crushed by using mortar and pestle and all the processed samples were subjected for the density separation of microplastics [22]. 

Separated plastics were examined by naked eyes and using a microscope. Microplastic of considerable size was sorted out directly and small size particles were observed under fluorescent microscope. They were visually determined based on the standardized color, brilliance.

After the confirmation for the presence of degraded plastics, the identification and quantification of microplastic were done by using FT-IR (Fourier transform infrared spectroscopy) by surface technique called “attenuated total reflectance” (ATR) FTIR in transmittance or absorbance mode [23] (Figure 2).

Figure 2: Process of Separating Microplastic from Collected Sample

The study reveals that, the microplastics are mostly derived from plastic bottles, plastic bags, fishing nets, buoys (Figure 3 and Table 2) and abundantly present in sediments in 10 islands followed by seagrass in Krusadai island and seaweed in Appa island (Table 3).

Table 2: Assessment of Microplastics in Islands of Gulf of Mannar

Table 3: Source of Fragmented Plastics from Islands of Gulf of Mannar

Figure 3: Source of Microplastic in Islands of the Gulf of Mannar

The plastic particles were the fragments of larger items, but small, spherical industrially produced microplastic particles, fragments of various shapes and colors with pointed to round edges were also identified (Figure 4).

Figure 4: Fluorescent Microscopic Imaging 100X of the Extracted Microplastic from the Sample

The FTIR analysis reveals that, the microplastic size ranged from 0.8 to 1.0mm in diameter and the derivatives are polyurethane, high density polyurethane and ethylene vinyl acetate (Figure 5).

Figure 5: FT-IR Graph Value of Some of the Extracted Plastic Particles

Textures of the tested fragments surface were like those typical of sand grains, adhering particles, and grooves. Dense liquid separation revealed that all sediment samples collected from above islands contain plastic fragments except Manali-putti Island, Vishal Island. And the examination of other samples such as seagrass, seaweeds, fish, coral, plankton, only the seagrass sample from Krusadai Island and seaweed samples from Appa Island has shown the presence of plastic fragments. The result shows that most of the Island sediments are polluted with small, fragmented plastics. Small plastic particles may have been derived from chemical weathering process. Degradation of plastic rates can be increased by oxidation, exposure to solar UV radiation and with additional breakage. It is concluded from the present study that fragmented plastics can reach the higher organisms via food chain when they ingested the contaminated food source. Intake of contaminated food with microplastic may leads to bioaccumulation and biomagnification which in turn paves the way for poor metabolism and poor reproduction which ultimately leads to loss of biodiversity and emergence of inherent variations of marine organisms. Further studies are highly warranted to find out the biomagnification of microplastics in different organisms in the food chain and food web of the Gulf of Mannar Biosphere Reserve. Also, the impact of microplastics on the morphology, reproduction, metabolism, and sex reversal if any must be thoroughly studied.

Micro and mesoplastics on these islands could be primarily derived from inland sources, but they appear to be equally distributed. Although beaches in these islands are scarcely populated, they contain abundant plastic litter, and they come from anthropogenic activities, particularly tourism. Increase in the concentration of debris may be a consequence of the clockwise rotating ocean currents. And hence, plastic degradation might be predominantly occurred on land than in the marine environment where exposure to UV radiation and mechanical erosion is minimal [24]. Further studies are highly warranted on the distribution of microplastics in the seashore along Southeast coast of India. The distribution of microplastics may vary among different location of the Gulf of Mannar based on the environmental cues and the methodology used for sampling [25]. Vidyasakar et al., [26] reported that, the microplastic in the sediments of Rameswaram group of islands in the Gulf of Mannar has large quantity of white-colored and asymmetrical-shaped plastic particles among them, s polypropylene followed by polyethylene, polystyrene, nylon, and polyvinyl chloride polymers are predominant. Patterson., [27] reported that, the sediments in coral reef areas of the Gulf of Mannar contain microplastic fragments of 3-5mm and 1-3mm of microplastic fibres. In contrary, James et al., [28] observed that, the size of the microplastic ranged from 1 to 5 mm and fragments of size smaller than 1mm were in large quantity in the surface water. The gut content analysis of the 613 fishes of 12 families brought out that, microplastic were more in littoral than in abyssal zone fishes, especially in Clupeidae family (42%). The study on benthic invertebrates (Sternaspiss cutata, Magelona cinta) from Kochi coastal water [29] states that thread and fibres like microplastic debris were found and identified to be polystyrene. [30] reported that, that the distribution of microplastics was abundantly present in high tide area (plastic fragments 47%) than in low tide area in which polyethylene was the dominant polymer type followed by polypropylene and polystyrene on the beaches of 25 locations along Tamil Nadu coast and the gut content analysis of commercially important fishes shows 10.1% of microplastic ingestion. Asian green mussels collected from the harbors of Chennai, possesses polystyrene polymer ranges from 25 µm-30µm in size [31]. It is believed that the presence of microplastic in gut among the marine organisms might be due to the ingestion of humus along with the prey. [32] reported that, 4 types of microplastic viz., plastic debris, microscopic fibres, microbeads, and frothed plastics from stream towards the coastal area. The evaluation of microplastic particles on beach sands along the Indian coast (Mumbai, Tuticorin and Dhanuskodi) detected 45±12 # MP kg−1 to 220±50# MP kg−1 of dry sand. Polyethylene was the dominant polymer type present in the samples followed by Polyethylene terephthalate and polystyrene [33]. Microplastics which get released into the planet’s oceans, increases the risk by accumulating in the coasts and to the organisms that abode in these environment. It has become one of the major ecological problem because of their microscopic size and its nature of combining with other chemicals that can be stored in the entity which rise to ailments and mortality. Organisms consume them directly from the contaminated water or accidentally from the infested food web. The consequences of microplastic pollution are extensive, affecting health of marine life and human, interfering the fishing activities, tourism, and economy [34].

Acknowledgement

The authors are gratefully acknowledged the authorities of Alagappa University, Karaikudi, Tamil Nadu, India, Bioscience Research Foundation, Chennai, Tamil Nadu, India.

Conflicts of Interest

“The authors declare no conflict of interest".

Funding

DST-NRDMS (No.NRDMS/CHRA/S.Ravikumar/Tamilnadu/e-06/2019 (C),12.07.2019), MHRD-RUSA 2.0 schemes (Letter No: F.24-51/2014-U Policy (TN Multi-Gen), Dept. of Edn. Govt. of India, Dt. 09.10.2018) and UGC STRIDE COMPONENT-I (No.F.2-5/2019 (STRIDE-I), Dt.03.12.2019) New Delhi for financial assistance.

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