Plastic and Microplastic in the Environment. Группа авторов
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reach humans due to their extensive consumption and dependency on seafood (Santillo et al. 2017). In aquatic organisms, the uptake pathways of MPs are through their gills and gastrointestinal tracts (Franzellitti et al. 2019). The organisms are often confused as these colorful MP fragments look similar to plankton species on which they feed, and thus a significant portion of MPs reach and are accumulated in consumer organisms (Setälä et al. 2014). Sometimes these MPs are deposited on seaweed or algal blooms, and become part of the food to the organisms, where they enter the gastrointestinal tracts of organisms (Walkinshaw et al. 2020). Once these MPs are mistakenly consumed by smaller organisms in confusion of phytoplanktons, they make their way to successive trophic levels, as predators consume prey already having MPs in their guts. Polymers of rope, usually old fishing gear left deliberately or mistakenly in the ocean, can entangle marine creatures, suffocating them by restricting their mobility and unintentional killing them; this is referred to as “ghost fishing” (Gilman 2015). These polymers may also undergo reduction in size due to natural forces acting on them in the ocean, such as waves, water temperature, contact with other floating debris, or larger marine creatures nibbling the pieces into smaller fragments, which then contributes to MPs. Recent studies show the presence of MPs of an array of shapes and sizes in various organs of different organisms such as gills, liver, gut, muscles, etc. They cause disturbances in processes of metamorphosis, metabolism disorder, behavioral change, oxidative stress, genotoxicity, etc. (Rahman et al. 2021). This is becoming a food safety threat as these organisms are heavily consumed as food by humans as seafood, and contribute to a significant enough proportion of the daily diet of people in coastal areas; for example, India has a huge coastline of 7516.6 km.
MPs have a high density, and become settled on the ocean floor, both after entering into the oceans and through the feces of organisms. There it causes oxygen‐deficient conditions like anoxia and hypoxia, in which there is less availability of oxygen and nutrients. In this way, it causes harm to seaweed, corals, and planktons (Seeley et al. 2020). Deposition of layers of MPs on body surfaces of corals (in the tropics) cause their degeneration by lowering the absorption of essential nutrients from the surrounding environment.
However, plastics play an important role in food safety and security by providing safe and durable packaging, and is a great contribution to the pharmaceutical and medical industries for packing medicine and providing disposal medical equipment (Hui et al. 2020). Due to a lack of proper waste management, over 250 000 tons of plastic pieces are dumped into the oceans (Hahladakis 2020). Larger plastics sizes are undergoing slow degradation by integrated physical–chemical and biological processes. They mainly degrade due to photo‐ and thermo‐oxidative processes (Mierzwa‐Hersztek et al. 2019).
MPs are generally found in the form of pellets, fragments, or fibers. Some of them are denser than seawater and settle at the seafloor like polyamide, polyester, polyvinyl chloride (PVC), and acyclic, etc. In contrast, those found throughout the water column and floating on the sea surfaces are a lighter density than the sea surface, e.g. polyethylene, polypropylene, polystyrene (Hidalgo‐Ruz et al. 2012). Although plastic is treated as non‐toxic because of its less reactive nature (Hwang et al. 2020).
3.1.1 Microplastic in the Marine Food Web
Due to wide distribution and unmanaged dumping (some through rivers) in the ocean, MP is a common ailment. However, European countries are more prone to this problem because they have busy sea routes as well as industries near coastal areas. The MPs are added to the ocean from terrestrial sources along with the secondary MPs from larger submerged plastics. Ballast water from ships release huge amounts of MPs. Those MPs are hotspots of toxic chemicals, pathogens, harmful algal bloom, etc. (Naik et al. 2019). Microplastic is of serious concern due to its wide distribution from pelagic to benthic marine biota (Thompson et al. 2009). These are a great matter of concern as they are affecting every segment of the ecosystem. They become attached to the planktons and cause disturbances in performing photosynthesis, and make a film over the water surface and provide the breeding ground for bacterial pathogens. The transfer of MPs from one trophic level to the next is a big concern. Another serious threat is the biomagnification of MPs along with the associated chemicals to the successive trophic level (Walkinshaw et al. 2020). The associated chemicals of MPs have a large area‐to‐volume ratio, which absorbs hydrophobic pollutants from the surrounding marine environment (Figure 3.1) (Smith et al. 2018). The most serious threat is of bioaccumulation of heavy metal in the presence of micro MPs at every trophic level of food chain, and which may lead to biomagnification of toxic heavy metals among the higher‐level organism in food chain, which have a high probability of being eaten by the human population.
3.1.2 Toxic Impacts on Primary Producers
Phytoplankton are considered the main contributor to the primary productivity in the oceans, which fixes almost half of the carbon dioxide of the earth during photosynthesis process by using photosynthetic active radiation (PAR) from Sun and carbon dioxide (Uitz et al. 2010). The MP deposition over phytoplankton decreases the chlorophyll concentration, photosynthesis, cell growth, and morphology of phytoplankton. Microplastics deposited upon the phytoplankton penetrate the cell walls and interfere with the chlorophyll mechanisms in green algae (Nerland Bråte et al. 2014). Phytoplankton absorb persistent organic pollutants released during the degradation of MPs which is further transferred along the marine food web (Chandra et al. 2020). This hazardous chemical has the property of bioaccumulation in successive trophic levels, which causes toxicity to them. When MPs are deposited over harmful alga, they release phycotoxin, which is transferred to phytoplankton, bivalves, and crustaceans (Sharma & Chatterjee 2017). The toxins are then bioaccumulated in their bodies and move to the next trophic level. These are then consumed by humans, which may result in many health issues. The coral reef, which has the highest biodiversity in the tropical shallow parts of marine realm, is also badly impacted by MPs. These coral reefs have the mutual collaboration of algae and fungi. However, most of the time, algal partners depend on phytoplankton, zooplankton, etc., for their food requirements, but they are confused by the colorful MPs and consume them. The digestive tracts of coral reefs (coral polyps) cannot deal with MPs and they have very harmful impacts on their health.
Figure 3.1 Flow diagram of the fate of plastic entering the environment.
These phytoplankton are trapped as part of the marine snow and an important constituent of marine organic matter, this organic matter is taken as food by benthos and nektons. Marine algae aggregates over the floating MPs and settle down to the sediment water interface. This reduces the residence time of floating organic matter in the water column, which in turns lowers the food availability to those organisms residing in the water column. Furthermore, hetero‐aggregates of MP and phytoplankton are consumed by zooplankton and have harmful impacts to them. These MP contaminated zooplankton are bioavailable to the predator and transferred to successive trophic levels. In these ways MP potentially disturbs the food transfer and, most importantly, reduces the energy flow from primary producers level.
3.1.3 Toxic Impacts on Consumers
Free‐floating zooplankton are key members of the marine food web, as they are the connecting link in transferring energy from producer to consumer level in the food chain, and then to the higher trophic levels. They feed upon phytoplankton and the MPs present over the surface of the phytoplankton are accumulated into their bodies (Cózar et al. 2014). Zooplankton consist of many species and have different life cycle stages with a wide range of feeding mechanisms (Wirtz 2012). Studies from lab experiments revealed that these zooplankton are capable of absorbing tiny plastic latex beads (Cole et al. 2013) as MPs of <5 mm have been found in 15 different taxa of zooplankton, from copepods to jellyfish. Ciliated heterotrophic planktons engulf MPs by phagocytosis (Laist 1987). The zooplankton ingests MPs particles, which may either pass through their digestive tract or