**1. Introduction**

Global plastic production is on the rise wherein 1.3 million tons of plastics were produced in 1950, and 359 million tons of plastic waste were generated in 2018 [1, 2]. It is estimated that the increase in plastic waste will reach 250 million metric tons by 2025 [3]. This adds additional pressure on the plastic management system. At present, 9% of the plastic waste is recycled, 50% ends up in landfills, 19% is incinerated, and the remaining 22% ends up being discarded as litter (and is categorized as mismanaged plastic waste) [4, 5]. The mismanaged plastic waste is often dumped on terrestrial lands or in marine habitats [6]. It has been estimated that 10% of the mismanaged plastic waste ends up in the marine environment where it will persist and accumulate over the coming years [7]. The large fragments of plastic debris found in the environment are termed macroplastics [8], and they are known to harm turtles and sea birds via entanglement [9, 10].

Once these macroplastics enter the environment, they undergo degradation and decompose into smaller fragments known as microplastics (size < 5 mm) and/or nanoplastics (size < 1 μm) [11, 12]. Microplastics can be differentiated into primary microplastics and secondary microplastics, depending on their sources. Primary microplastics are the ones manufactured for direct applications such as microbeads in personal skin care products [13–15]. Secondary microplastics are the ones formed as a result of the degradation and decomposition of the macroplastics [16]. The most common sink for microplastics is the marine environment, including the sediment, deep sea [17, 18], shorelines [19, 20], oceans [21, 22], and interestingly coral reefs as well [23].

Globally, microplastics are recognized as pollutants, and the United Nations Sustainable Development Goals (UN SDG) has assigned Goal 14 specifically to conserve and sustainably use the oceans, seas, and marine resources [24]. The contamination by microplastics and nanoplastics has been an issue of concern over the past decade. Owing to their small size, micro/nano plastics are readily bioavailable for consumption by marine organisms [25]. Once ingested by smaller marine organisms (primary consumers), they will be further transferred to the secondary consumers (e.g., large fishes) and eventually reach the tertiary consumers (e.g., humans), thus disrupting the food chain [26].

Though the sources, degradation pathways, and sinks (specifically marine habitat) of the microplastics are often discussed, the fate of microplastics is elusive after perusing various articles and literature. Through this chapter, we aim to collate the environmental impact assessment of the microplastics in the marine habitat, focusing on the following main elements: (a) sources of microplastics, their transport to the marine environment, as well as their types; (b) degradation pathways including photodegradation, weathering, corrosion, or mechanical forces of water; (c) ecotoxicological impact on marine biota, since the fragmented microplastics can be readily digested by the marine biota and cause a threat to them; (d) fate of microplastic in the marine environment, including the modes of transport and deposition. This chapter aims to provide a platform for the development of microplastic risk management strategies and also to provide a deeper insight into the fate of microplastics once it enters the marine environment.
