**5. Ecotoxicological impact on marine biota**

Owing to their small size, microplastics have the potential to be ingested by an array of marine biota [112]. There are several studies indicating the ingestion and accumulation of microplastics in marine organisms, and most of the studies were conducted on fishes. **Table 2** lists a number of studies demonstrating the impact of microplastics on different marine organisms, categorized into fishes, invertebrates, and other miscellaneous biota. These studies indicated the accumulation of microplastics in various marine organisms including fishes (mackerel, *Scomber japonicus*), copepods (*Calanus helgolandicus*), and shorebirds (whimbrel, *Numenius phaeopus*), and pacific golden plover (*Pluvialis fulva*) [113–115]. When a microplastic accumulates in the organism's body tissues, it may influence the organism's health in numerous ways, including stunted growth, infertility, and impact on egg's hatching [114, 116]. Once ingested by the marine organisms, the microplastics can translocate through the food chain, starting from the primary consumers (e.g., planktons, small fishes), to the secondary (e.g., larger fishes, birds, turtles), and eventually to the tertiary ones (humans) [117]. Such a process is known as biomagnification, which may cause human health risks [32]. Moreover, microplastics can bind to various marine pollutants such as heavy metals, enhancing their accumulation in the marine environment [118]. In addition, marine invertebrates such as mollusks (e.g., mussels, oysters, clams) and crustaceans (e.g., shrimps, crabs, lobsters) do not possess the required digestive enzymes to break down the microplastics into simpler nontoxic

compounds. Therefore, these invertebrates would release the microplastics back into the water as fecal matters [119]. As a result, the microplastic might not have any toxic impact on the marine organisms once the ingested microplastics are egested. In certain cases, microplastics can act as a vector of co-pollutants present in the marine system and prevent its translocation to the marine organisms, thus exhibiting a positive impact on the organism. For example, in the presence of co-pollutant (zinc oxide) and microplastics (PE), marine microalgae (*Dunaliella salina*) showed higher growth than in the absence of PE. This is because PE could attach to zinc oxide, leading to its leaching and preventing its uptake by the microalgae [120]. The ecotoxicological risk and impact of microplastics on the marine environment can be categorized into physical, chemical, and biological damages. Physical damage to marine organisms includes gastrointestinal tract blockage and damage, leading to the organism's death and affecting the mortality rate [121]. Chemical damage includes the property of microplastics acting as carriers or vectors for pollutants such as heavy metals (e.g., Cr, Ni, Cd, Zn) that are eventually ingested by marine organisms [122]. For instance, PE was found to facilitate the sorption of chromium (Cr) in common Goby fish, which led to a decrease in acetylcholinesterase (AchE) enzyme activity and resulted in acute toxicity [123]. Lastly, biological damage to marine organisms includes gene manipulation and the evolution of microorganisms with antibiotic resistance genes and metal resistance genes [124]. However, more research is needed to confirm the impact of these damages on marine organisms.

**Table 2** also summarizes the ultimate marine sinks for the microplastics. The marine organisms impacted by the microplastics are primarily present in the following major oceans: the Pacific ocean, Atlantic ocean, and Indian ocean. Pacific ocean serves as a marine sink to microplastic generated from the United States (e.g., California [125]) and South America (e.g., Peru and Chile coastlines and Northern Patagonia in Chile [126, 127]). These examples represent the East Pacific Ocean as the marine sink for microplastics, where the main sources of these microplastics include plastic manufacturing industries in the United States (e.g., California [125]), and textile industries and domestic washing of clothing in South America (e.g., Peru and Chile [126, 127]). Likewise, the West Pacific Ocean serves as a microplastic source for marine habitats including zebrafish, rotifers, copepods, shrimps, scallops, crinoids, (China [128–130], gastropods, bivalves, and crabs (Hongkong [131]), as well as seabirds and turtles (China [115, 132]). Based on recent studies summarized in **Table 2**, the main source of microplastic in the West Pacific Ocean is China. The increased consumption of plastic in China is directly linked to its high population (1.41 billion [133]), plastic manufacturing industries, and mismanaged plastic wastes [134].

Similarly, increased fishing activities, tourism, and high population are the main reasons for the microplastic source of the Indian Ocean, including India (primarily the high population of 1.38 billion [133]) and Thailand (primarily the tourism activities and fishing activities, [135]). The source of microplastic to the Atlantic Ocean is due to the increased amount of plastic waste generated (e.g., United Kingdom), and the mismanaged plastic waste that makes it to the ocean [134]. For example, the East Atlantic Ocean serves as a microplastic source to marine organisms including common goby (Iberian coast, [123]), different pelagic and demersal species in the English Channel (UK, France [136–138]), gilthead seabream and European seabass (Murcia, Spain, [139]), mussels (Port Quinn Cornwall, UK, [140]), copepod (English Channel, UK, [114]), insects (Italy, [141]), whale (the Netherlands, [142]), and otters (Norway, [143]). Similarly, for the West Atlantic Ocean, the United States, and South America serve as a microplastic sink/source for different marine organisms including






#### *Marine Pollution - Recent Developments*



**Table 2.** *Impact of microplastic (MP) on various marine biota. Please note that for the laboratory simulated studies, the major ocean sink information has not been included.*

#### *Marine Pollution - Recent Developments*

commercial fishes, seabass, and mackerel found along the Portugal coast ([113, 144]), discus fish found in the Amazon basin, Brazil [145], sea cucumbers in Florida and Maine (the USA, [146]), and seals in Massachusetts (the USA, [147]).

Lastly, there are several studies conducted under laboratory conditions to understand the impact of microplastics on marine organisms. These include investigating the impact of PS and PE on goldfish [148], the effect of PS on fathead minnow [149], and the effect of PP on marine copepod [150]. Please note that for the laboratory simulated studies, the major ocean sink information has not been included in **Table 2**.
