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Do microplastics induce oxidative stress in marine invertebrates?
Summary
This review examined whether marine invertebrates exposed to microplastics show evidence of oxidative stress — a common cellular response to toxic injury — finding support for this effect across multiple species and polymer types. Oxidative stress is a key mechanism by which microplastics may harm marine organisms.
The global annual production of plastics increased 20-fold in the last five decades, reaching about 335 \nmillion tons in 2016. Concomitantly, environmental pollution by plastic litter became a rising issue. \nPlastics easily escape from landfills into the surrounding environment and rivers, which discharge huge \nquantities of litter into the oceans. Plastic debris spoil the coastlines or accumulate in subtropical \noceanic gyres. Marine plastic litter can have adverse effects on marine vertebrates and invertebrates. \nLarger items, such as lost fishing nets are particularly hazardous because many marine species may \nbecome entangled and immobilized. Limited predator avoidance, starvation or drowning may be the \nconsequences. Plastics are hardly biodegradable but subject to mechanical degradation by wave action \nand UV-radiation. Progressive fragmentation of larger plastic items generates smaller fragments, \nfinally yielding micro- or even nanoparticles. Upon ingestion, smallest plastic fragments may enter \norgans and even penetrate into cells where they may cause imbalances of the cells homeostasis. \nIn the present study, the ingestion of microplastics by marine invertebrates, the possible transfer into \ncells of the digestive tract, and the cellular effects in the midgut gland were studied. The Atlantic ditch \nshrimp (Palaemon varians) served as model species. It inhabits coastal regions, estuaries, and brackish \nwater systems, which are strongest exposed to anthropogenic pollution. The shrimps received fluorescent \npolystyrene microbeads of 0.1, 2.1, and 9.9 μm in diameter as food. Uptake of the microbeads \ninto the digestive organs and, particularly, into the midgut gland was analysed by fluorescent \nmicroscopy of cryostat sections of the digestive tract. Activities of antioxidant enzymes were measured \nto verify cellular stress responses. Amplification of NADPH-oxidase transcripts served as indicator for \nthe presence of a cellular defence system capable of generating reactive oxygen species. \nThe smallest particles penetrated into the midgut gland while the largest particles retained in the \nstomach. An increase in the cellular defence mechanism against oxidative stress was verified by the \nactivity of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT). Formation of reactive \noxygen species (ROS) indicated NADPH oxidase activity, a superoxide (O2-) generating enzyme. The \nexpression of NADPH oxidase in the midgut gland of P. varians was verified by PCR-amplification. \nThe midgut gland of P. varians showed an intense cellular reaction after exposure to microplastics. The \nrapid increase of the anti-oxidative enzymes, particularly SOD, indicates a significant liberation of \nreactive oxygen species, presumably as an immune reaction of the NADPH-oxidase system. Oxidative \nstress, in turn, can have adverse effects on various cell structures and cell functions by affecting membranes, proteins, or DNA. Finally, it causes a toxicological impact on organs and the whole organisms. \nFuture studies should address the direct effects of increased oxidative stress in terms of toxicity, i.e. \nby the occurrence of lipid peroxidation. Also the destination of particles inside the organisms, may help \nidentify corresponding mechanisms that lead to oxidative stress and contribute to the big picture of \nmicroplastics in the environment.
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