And the new study, published in Royal Society Open Science, reveals the microscopic plastic fibres are polluting sediment in deep seas all around the world.
Marine plastic debris is a global problem, affecting wildlife, tourism and shipping. And yet the scientists in the study point out that monitoring over the past decades has not seen its concentration increase at the sea surface or along shorelines, despite experts knowing that more plastic is being created.
However, the current study indicates this may be because microplastics have sunk to the ocean floor, with the number of fibres recorded in the deep seas up to four times greater than in shallow and coastal waters.
"The puzzle for marine scientists has been to establish where plastic debris is going. Part of the answer is that much of this waste is breaking down into fibres invisible to the naked eye and sinking to the sea floor," said Dr Lucy Woodall, zoologist at the Natural History Museum. "It is alarming to find such high levels of contamination, especially when the full effect of these plastics on the delicate balance of deep sea ecosystems is unknown."
Few studies have looked at the impact of microplastics, but it is known that they can absorb pollutants and even be ingested by organisms.
The new study, which also involved the University of Barcelona, the University of Oxford and the Scottish Association for Marine Science, focussed on deep-sea sediment and coral samples collected by Dr Lucy Woodall and other scientists from 16 sites in the Mediterranean Sea, Atlantic and Indian Oceans.
Dr Woodall first spotted bright blue and red fibres in the cores of sediment she had extracted as part of a project to study seamounts in the southwest Indian Ocean.
She began to systematically pick through her samples for plastic fragments, sending them to the University of Plymouth for chemical analysis. A quick search showed that deep-sea microplastics have previously been undocumented in scientific literature.
As part of the seamount project Dr Woodall also took samples of corals, and she was surprised to find that they also hosted microplastics.
Analysis of the non-natural particles at Plymouth University confirmed microplastics were abundant in all the samples at a rate ranging from 1 to 40 pieces per 50ml of sediment. The plastic particles were commonly around 2-3mm in length and were mostly blue, black, green or red in colour.
Rayon – a manmade non-plastic polymer used in personal hygiene products and clothing – contributed to 56.9% of the total fibres seen, with polyester, polyamides, acetate and acrylic among the others recorded.
Professor Richard Thompson, Professor of Marine Biology at Plymouth University, coordinated the study and led the identification process. He said: "The deep sea habitat extends to more than 300 million km² globally, so the discovery of previously under-reported microplastics suggests there may be even greater accumulation than was previously suspected.
"A range of shallow water organisms are known to ingest microplastics, and the extent of their harmful effects will likely be influenced by their relative abundance. The discovery of substantial quantities in deep-sea sediments is of considerable relevance to our understanding of the potential of these particles to cause harm in the marine environment."
A vast accumulation of plastic particles including microplastics in the surface waters of the north Pacific, known as the 'Great Pacific garbage patch', had previously brought the world's attention to the problem of plastics in the marine environment.
However, the recent discovery of the previously unknown microplastic fibres found in deep-sea sediments may be a much bigger problem – the concentrations found by researchers are considerably larger than those found in the Great Pacific garbage patch.