Research progress on occurrence characteristic and toxicity of microplastics in marine organisms

Plastic products are light, tough, resilient, inexpensive, corrosion-resistant and easy to work with, and have been extensively used in industry, agriculture and daily lives. These plastic debris are discarded and enter the ocean as a result of many different land- and sea-based activities. Microplastics (MPs), defined as plastic fragments ranging from a few micrometers to five millimeters in any dimension, play a key role in the world’s marine litter problem. They originate from two sources, manufactured products (such as abrasive micro-beads in face scrubber cosmetic, toothpastes and antifouling of boats) that contain microplastics (primary microplastics) and fragments released from larger plastic debris through photooxidation, mechanical action and biodegradation (secondary microplastics).The presence of microplastics in the marine environment may be distributed globally by way of currents and winds. Various studies have shown that MPs are now distributed in all oceans, occurring on shorelines, sediments, surface waters even in remote locations (i.e. Arctic) and at all depths. Once entering into ocean, microplastics may pose serious hazards to marine organisms, including phytoplanktons, zooplanktons, clams, mussels, crabs and fishes. Microplastics not only can cause physical harm to marine organisms by contact, uptake and ingestion, but also can provide the marine organisms with a potential pathway for exposure to organic pollutants, metals and pathogens adsorbed from the ambient environment or to chemicals leached by themselves. Microplastics may also pose risks to human health, because they may be transferred through the food chain and can facilitate the food web transfer of incorporated/ adsorbed toxic chemicals. Therefore, the effects of microplastics on the marine environment have become major concerns, marine ecological issues caused by microplastics are now at the forefront of marine ecology and environmental research. Based on the existing research results, this paper summarized the distribution of microplastics in typical marine organisms and its influencing factors. In addition, the toxic effects of microplastics, such as physiological function, oxidative stress, immune response, neurotoxicity and even reproductive genetic toxicity, were summarized. This paper also explained the toxic mechanism of microplastics, including the particle effect of microplastics as foreign bodies into the organism, the leaching of microplastics additives, and the combined toxicity of microplastics and environmental pollutants caused by the adsorption effect of microplastics. At the end of the paper, the problems of microplastics research methods were discussed, the importance of mature and perfect microplastics research system and standards was pointed out and the suggestion that design of future toxicity exposure experiments need to be closer to the real environment was proposed, providing ideas and references for future research on microplastics.