For instance, in marine environments the halve life of a plastic bag made of low-density polyethylene (LDPE) with an average specific surface degradation rate of 15 µm per year is estimated to be in the range between 1.4 and over 2500 years 32. The aforementioned long lasting effects of MP are partly a consequence of their high persistence in natural systems which are estimated to last from hundreds to thousands of years 31. As a consequence of these negative ecological impacts, marine littering of plastic debris is considered to be part of a global crisis 30. In addition to this, MP may disturb the biogeochemical processes with which plankton capture CO 2 at the sea surface and sequester carbon in the deep oceans, although this is still poorly understood 29. For example, in 2015, plastics alone were responsible for 4.5% of the world´s greenhouse gas emissions 17.
On a global scale, plastics release CO 2, methane and an array of other greenhouse gases at each stage of their life cycle, thereby, contributing to climate change in a meaningful proportion 28. Ultimately, this could change population structure and abundance of certain species. Nanoplastics, which are defined as plastic particles with a size between 1 nm and 1 µm, are even able to pass biological membranes and, thus, can directly interact with genetic material and cellular organelles 27.
Once ingested by organisms, MP can cause physical damage of tissue, e.g., intestinal obstructions, inflammation processes, and might also influence animal behavior in a negative manner 26. In addition, MP has the potential to accumulate in marine food webs as a consequence of trophic transfer 24, 25. This chemically complex cocktail might be released at a constant, yet increasing rate due to plastic fragmentation, resulting in a so-called global plastic toxicity debt 23. For instance, MP act as both, sinks and sources of potentially toxic substances, i.e., persistent organic pollutants, metals, additives, plasticizers and antibiotics 18, 19, 20, 21, 22. Regardless of its formation, MP can undoubtedly induce adverse effects on marine wildlife and ecosystem services, down from the cellular level of a single individual up to an acceleration of climate changes 16, 17. Secondary MP originates from the fragmentation of larger plastic pieces 12. Pellet loss during production, transport, storage or waste management can act as an important input pathway for primary MP into marine habitats 14, 15. Primary MP is deliberately produced in this size range for many purposes, e.g., cosmetic and hygiene products or resin pellets for industrial usage 12, 13. There are two types of MP: primary and secondary MP. Recently, a special focus was set on plastics in the size range from 1 µm to 5 mm, which are called microplastics (MP) 9, 10, 11. It was also reported that about 275 million metric tons of plastic waste was generated in 192 coastal countries in 2010, with 4.8 to 12.7 million megaton entering the ocean 8. As a consequence, plastics can be found everywhere 6, with at least five billion plastic pieces that have entered the oceans, a major sink for plastic debris 7. Plastics have been discarded into the environment for at least five decades 4, 5. The usage of plastics has led to numerous improvements in everyday life 1, 2, but has recently been recognized as an environmental thread on a global scale that exceeds planetary boundaries due to high production rates and uncontrolled littering 3.
Future long-term studies should focus on underlying mechanisms and timescales of plastic degradation.
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These findings provide further evidence that plastic degradation under saltwater conditions is comparatively slow, whereas UV radiation and high temperatures on beaches are major drivers of that process. Contrary, beached plastics exhibited changes on all structural levels, which sometimes caused fragmentation. MP from inside the wreck showed no signs of degradation compared to pristine reference samples. Most pellets were made of low-density polyethylene, but a minor proportion also consisted of high-density polyethylene. Pellets from both sampling sites were analyzed by microscopy, X-ray tomography, spectroscopy, calorimetry, gel permeation chromatography, and rheology. Its sinking date enabled us to precisely determine how long MP rested in the wreck and a nearby beach, on which part of the load was washed off. We here present a novel study object that preserves information on plastic age: microplastic (MP) resin pellets from the wreck of the SS Hamada, a ship that foundered twenty-nine years ago at the coast of Wadi el Gemal national park, Egypt. This is partly caused by the lack of long-term studies and methods that determine weathering duration. Degradation and fragmentation of plastics in the environment are still poorly understood.
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