Microfibres inside deep-sea organisms were found from 334–1783 m depth in the equitorial mid-Atlantic and 954–1062 m in the SW Indian Ocean (Fig. 3). Previous studies have found microfibres in sediments down to 2000 m in the subpolar North Atlantic, 2200 m in the NE Atlantic, 3500 m in the Mediterranean and 5768 m in the West Pacific35. Most deep-sea organisms rely either directly or indirectly on the supply of organic detritus from the euphotic zone, often called ‘marine snow’. Our confirmation of biological integration of microplastics makes recent evidence of a shift towards smaller plastic size categories, equivalent to the ‘marine snow’ size38, something now particularly relevant for deep-sea organisms.

Figure 3 Microplastic presence by material and depth, (a) mid-Atlantic data from JC094; (b) SW Indian Ocean data from JC066; (c) depth of all other known deep-sea microfibres found in sediments represented by grey bars. Full size image

In the few instances where they have been studied in deep sea sediments, microplastics occur in similar concentrations as in inter-tidal and shallow sub-tidal sediments25. Enders, et al.39 recently modelled microplastic distribution to 250 m depth but there is no raw data from deep-sea water columns on the High Seas. We assume that microplastics in sediment represent a vertical accumulation from falling ‘marine snow’25. We observed that the suspension-feeding anemone, armoured sea cucumber and octocoral had no microfibre load, although fibres were found inside the suspension-feeding sea pen and zoanthid from the SW Indian Ocean (Table 1, Fig. 3). By contrast fibres were found in all predatory, deposit and detritivore feeders examined. If this general observation (albeit based on very few samples), of filter-feeders having lower microplastic loads, holds true more widely, the implication is that deposit-feeding organisms may be more vulnerable to microplastic ingestion than suspension feeders. Of course, load depends on a wide range of factors, such as an animal’s ability to avoid microplastic ingestion, any size or shape-selection of food particles etc. and the abundance and density of microplastics found in an organism’s environment. Knowledge of background microplastic load, systematic surveys with multiple replicates of sediment, seawater collections and sampling of deep-sea organisms across a range of feeding strategies would be required to test if feeding strategy alone impacts organism vulnerability to microplastic ingestion.

Despite microfibres being the majority of microplastic pollution40,41, including in the deep-sea25,35, most feeding experiments that have been undertaken thus far use microbeads and plastic shavings, with a few exceptions, Hämer, et al.42, Watts, et al.27, Au, et al.43. Our study shows for the first time that deep-sea organisms are ingesting microfibres in a natural setting, thus we suggest that experimental designs using fibres are needed to determine the potential long-term impact of microplastics for both shallow and deep marine organisms.

The range of plastic microfibres found ingested/internalised by organisms studied here included modified acrylic, polypropylene, viscose, polyester, and acrylic. Polypropylene has been found to adsorb PCBs (polychlorinated biphenyls), nonylphenol and DDE, an organochlorine pesticide7. Polyethylene, a type of polyolefin fibre whose chemical composition in part is the basis of some polyester fibres (e.g. polyethylene terephthalate), has been found to adsorb four times more PCBs than polypropylene44. Polypropylene has also been found to adsorb a range of metals in a marine environment; the concentrations of most of these metals did not saturate over a year period suggesting plastics in the oceans for long time periods accumulate greater concentrations of metals9.

Chemical contamination experiments are rare in the marine environment, and often present unrealistic experimental scenarios45. Yet with the chemical ingredients in 50% of plastics listed as hazardous (United Nations’ Globally Harmonized System of Classification and Labelling of Chemicals) such issues maybe just the start of long-term ecological and health problems associated with waste plastics in the environment46; impacts that have not been looked at in many marine animals6,10,11 and no deep-sea animals as yet.

Of course, ingestion, and any subsequent biological impacts, depend on many factors32 including characteristics of the microplastics themselves, such as size, shape, density, abundance (as seen in shallow water sea cucumbers28), colour, and importantly, differential adsorption of harmful substances7, as well as organism physiology, ecology and behaviour; this also includes whether microplastics accumulate in the organism, feeding method and/or prey of organisms, and where microplastics accumulate, or are egested and/or translocate within the organism. A final factor is whether transfer of the microplastic up the food chain is a possibility. All of these facets of the microplastic biological impact problem are relevant to deep-sea organisms however as science knows less about deep-sea biology and ecology (as there are fewer experimental opportunities in this challenging environment) these aspects of marine pollution will be relatively difficult to pursue.

Shallow-water experiments have found microplastic bioaccumulation e.g. lobster17, mussel and oyster47. Given that our data are a snapshot of the fibres within six organisms we cannot determine whether microfibres are bioaccumulating. Five microfibres was the most found within one organism (the hermit crab, JC066-702) and not in a ball as was seen in the lobster Nephrops17 and crab, Carcinus maenas27. This could suggest that microplastics are transient within the organisms studied. Or, this could be indicative of low densities of microplastics in the deep-sea feeding areas of organisms studied here, or that microfibres have different residency times to other more intensively studied microplastics (e.g. microbeads), or that the organisms here have different gut residency times to other organisms studied. It may also be that certain feeding strategies convey less suspectibility to microplastic bioaccummulation. Given the low number of organisms it was possible to sample here, and without concurrent environmental sampling, the link between background microplastic densities and microplastic abundance within organisms is not possible to establish.