There is a global
microplastic cycle Researchers now recognize that
plastic cycles through more than just the ocean. Te ocean absolutely is a sink for microplastics, but as ecologists learn more about these particles, they realize that the ocean is not always the final fate of microplastics. Rather, the ocean is one part of the global “microplastic cycle.” Scientists have detected microplastics in soils, biota, and Earth’s atmosphere leading to the realization that this material moves around much as other global biogeochemical cycles (nitrogen, carbon, and water). What are the sources of microplastics, and how do they transform as they move from one pool (e.g., a beach, inside an organism, or a river bed) to another? A recent study by Brahney et al. (titled “Plastic rain in protected areas of the United States;” Science 369[6496]) looked at atmospheric transport of microplastics in airborne dust, which settles to the ground (deposition) during dry and wet periods and in both urban and remote locations. Scientists have understood the global transport of dust for decades, but until recently, dust was not known to carry substantial amounts of microplastics. Long-range atmospheric transport, away from urban centers, leads to deposition in remote mountains, the oceans, and even in Arctic snow. Brahney et al. studied both global and regional transport of microplastics by comparing the size and shape of particles deposited in dry and wet weather. Tey found microplastics in 98% of their wet and dry samples collected in protected lands (national parks and wilderness areas, etc.) of the western United States; and estimate that more than 122 tons of microplastics are deposited annually to these areas appear pretty pristine at first glance. (Teir study was conducted in many of the most famous of our national parks including Canyonlands, Craters of the Moon, Rocky Mountain NP, Bryce Canyon, Grand Canyon, Joshua Tree, Wind River, Indian Peaks, and several others.) A key insight from the new work is that fundamental tools for studying global dust transport can be applied to microplastics. Like dust, most particles measured were within the size range typical of global transport
58 FUNGI Volume 14:4 Fall 2021
(<25 μm). However, microplastics are less dense than soil and therefore might travel longer distances than natural dust particles. Te story is much the same in the
oceans of the world: plastic can end up very distant from the original source. Indeed Kane et al. (Science 368[6495] found no relationship, really, between microplastic concentrations on the sea floor and distance from terrestrial plastic sources. We’ve all seen images in the news of vast rafts of plastic debris that floats out in the open oceans of the world, converging surface currents in oceanic gyres are responsible for the global distribution of plastics on the ocean surface. Tese ocean gyres effectively concentrate buoyant plastics into the now infamous “garbage patches.” Te size of these floating trash heaps is astounding when you consider that plastic has only been around for a few decades. And that’s just on the surface and easily observable. It turns out that sea surface accumulations only account for approximately 1% of the estimated global marine plastic budget. Most of the missing 99% of plastic ends up in the deep sea where it accumulates on the seafloor.
From plastic rain to
plastic rocks If you’re a beachcomber you are
accustomed to seeing all manner of flotsam mixed in among the sand particles of any beach. Much of the anthropomorphic junk is obviously plastic of course. But take a closer look … much of what appears to be natural particles of sand grains and even larger rocks is actually plastic. Tis is pyroplastic—a newly described form of plastic pollution that was transformed by fire. Andrew Turner, an environmental scientist at the University of Plymouth in England who described the substance in a recent edition of Science of the Total Environment (vol. 64;
doi.org/10.1016/j. scitotenv.2019.133610), writes that pyroplastics may be hiding in plain sight all over the world and has examined samples from Scotland to British Columbia. Where exactly pyroplastics originate is still a mystery. Tere could be many sources, from campfires—which have been implicated
in the formation of a plastic-rock hybrid called plastiglomerate in Hawaii—to old landfill sites. And the stuff can probably float for long periods of time and from very far away. Several of his samples contained worm tubes that appeared to be enriched in lead, suggesting animals can ingest the plastic and may be introducing heavy metals into the food chain.
Plastics move through ecosystems—and
organisms We have just seen how, on a truly large
scale, microplastics can move between different large-scale compartments, including the air, terrestrial habitats, rivers and other freshwater bodies, and the ocean, including its sediments. Now let’s take a look at recent discoveries have been found at the microscopic level. Microplastics are mostly composed of carbon, among other elements. Microplastic addition to ecosystems thus represents a source of carbon independent of photosynthesis and net primary production. Tis polymer carbon likely has a slow turnover, because the material is mostly inert; however, the behavior and residence time of microplastics in soil are currently unknown. We also do not know the input rate of microplastic- carbon into ecosystems itself, because research hitherto has largely focused on quantifying particle numbers and types, rather than on the microplastic- derived carbon itself. Originally, most of the carbon of plastics is of fossil origin (petroleum is the raw material), rather than having recently been fixed from the atmosphere. Because of the resistance of microplastic to decomposition, it would be expected to accumulate in soils. A recent article by Rillig and Lehmann (Science 368[6498]: 1430–1431) show that plastic particles will accumulate and alter the physical composition of soils (there will be ever more tiny chunks of plastic mixed among tiny chunks of soil), what becomes of plastic as it further degrades from micro particles to nano particles and ultimately back to molecules? (See image, courtesy of Science.) Many studies from a couple of decades ago looked at toxicology of plastics but most of what is plastic
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