Where do they come from, where do they go? The missing plastics phenomenon.

Written by Zoe Ruben

Edited by Bobbie Renfro

A Burgeoning Issue

Plastics. We can’t live without them, but it is becoming more and more questionable to live with them. They have infiltrated everything, from our roadways to our lakes and streams, and especially our oceans. A 2015 study estimated land-to-sea plastic debris from worldwide solid waste data and population statistics. They found that of the 275 million metric tons (MT) of plastic waste generated by 192 coastal countries in 2010, 4.8 to 12.7 million MT entered the ocean1. For a visual, this is equivalent to dumping 1,600,000 to 4,233,333 pickup trucks into our oceans annually (Figure 1).

Figure 1. Hopefully it never comes to this.

Of equal if not greater concern to typical plastic debris are its smaller counterparts, microplastics. According to the National Oceanic and Atmospheric Administration (NOAA), microplastics are small fragments of any kind of plastic that are less than 5 mm in length. They come from a variety of sources, including breakdown from larger plastics and use in cosmetics, clothing, and industrial processes. They pose a serious threat to marine environments as they are exceedingly difficult to remove and are often ingested by marine organisms. These plastics can then bioaccumulate in the food chain and have been shown to be damaging to human health.

The Question

So, we know where microplastics come from, but the question still stands: Where do they go? Some puzzling statistics have scientists scratching their heads: the concentration of plastics in the ocean is lower than the concentration of particles entering the ocean would suggest2,3,4. This indicates the possibility of a “missing” plastics phenomenon, and some scientists speculate that coral reefs may be acting as one of the environmental sinks for these microplastics5. An environmental sink aids in the redistribution, storage, processing, and absorption of human made waste by the environment (NASA).

The Process

Reichert et al5 set out to explore this, with four objectives:

  1. Quantify particle deposition rates in corals exposed to microplastics
  2. Test the permanent translocation of particles into the coral skeleton
  3. Identify the size, shape, and growth parameters of the coral colonies that could be affecting deposition
  4. Estimate total annual deposition rate of microplastic particles in tropical, shallow-dwelling reef-building corals on a global scale.

To begin, the researchers collected coral nubbins (fragments) from four common reef-building coral species covering a range of body shapes and growth rates (Figure 2).  The researchers exposed these nubbins to polyethylene microplastics in the lab for 18 months. These microplastic particles are of similar size to biological particles that reef-building corals commonly feed on. The microplastic concentrations used in the experiment were similar to concentrations found in polluted waters; so, the experiment represented a realistic scenario for what coral experience in the wild. Some nubbins were subjected to a continuous exposure to microplastics in their water while another treatment had periodic pulses of plastics released into their water.

Figure 2. The four coral study species. (A) staghorn coral (Acropora muricata), (B) cauliflower coral (Pocillopora verrucose), (C) Porites lutea, and (D) blue coral (Heliopora coerulea). Images courtesy of Corals of the World.

The Results

Microplastics were measured in both the coral tissue and the coral skeleton (Figure 3). All four study species contained microplastics at the end of the experiment. Nubbins that were subjected to the continuous additions of microplastics to their water had much higher microplastic concentrations than their pulse treatment counterparts. The coral skeleton also contained much higher particle numbers than the coral tissue across both treatments.

Figure 3. Quantification of microplastic particles in coral tissue and skeleton. (A) Tissue of Pocillopora verrucosa, where there are no visible particles. (B) Skeleton of Pocillopora verrucosa, where location and direction of black microplastic particles are indicated by white arrows. (C) Total microplastic particles per coral nubbin across all four study species. The different letters represent significantly different groups. Figure courtesy of Reichert et al 2021.

Reef-building corals vary in size, shape, and method of growth (Figure 4). The effects of things like size of the coral nubbin and the growth rate of each coral species that may affect microplastic particle deposition were examined using 3D scanning. Coral size and growth affected particle deposition while coral shape did not. Variation in particle amount in the coral skeleton was mostly explained by coral growth. Contrastingly, complexity of the coral surface determined most of the variation in overall particle density.

Figure 4. Major growth forms of reef-building corals arranged according to extension method. Figure courtesy of Pratchett et al 2015.

The data gathered provided an estimate for total annual deposition rate of microplastics in shallow-water reef-building corals on a global scale. It is estimated that there is a net deposition of 5.84 x 109 – 7.44 x 1015 microplastic particles in these corals annually, which corresponds to 0.09% – 2.82% of bioavailable microplastic particles in coral reef habitat.

The Answer?

This study is the first of its kind to experimentally quantify permanent deposition of microplastics in reef-building corals. Results confirmed that these natural reef systems can act as environmental sinks and explain some of the “missing plastics” phenomenon. Although more research is needed to determine how this may affect coral reefs in the long term, one thing is certain: these reef systems are playing a vital role in mitigating marine plastic pollution.

References:

  1. Jambeck, J., Geyer, R., Wilcox, C., Siegler, T. R., Perryman, M., Andrady, A., Narayan, R., & Law, K. L. (2015). Plastic waste inputs from land into the ocean. Marine Pollution, 347(6223), 768-. https://science.sciencemag.org/CONTENT/347/6223/768.abstract
  2. Cozar, A., Echevarria, F., Gonzalez-Gordillo, J. I., Irigoien, X., Ubeda, B., Hernandez-Leon, S., Palma, A. T., Navarro, S., Garcia-de-Lomas, J., Ruiz, A., Fernandez-de-Puelles, M. L., & Duarte, C. M. (2014). Plastic debris in the open ocean. Proceedings of the National Academy of Sciences, 111(28), 10239–10244. https://doi.org/10.1073/ PNAS.1314705111
  3. Eriksen, M., Lebreton, L. C. M., Carson, H. S., Thiel, M., Moore, C. J., Borerro, J. C., Galgani, F., Ryan, P. G., & Reisser, J. (2014). Plastic pollution in the world’s oceans: More than 5 trillion plastic pieces weighing over 250,000 tons afloat at sea. PLoS ONE, 9(12), e111913. https://doi.org/10.1371/journal.pone.0111913
  4. Law, K. L., & Thompson, R. C. (2014). Microplastics in the seas. Science, 345(6193), 144–145. https://doi.org/10.1126/science.1254065
  5. Pratchett, M.S., Anderson, K.D., Hoogenboom, M.O., Widman, E., Baird, A.H., Pandolfi, J.M., Edmunds, P.J., & Lough, J.M. (2015). Spatial, Temporal and Taxonomic Variation In Coral Growth — Implications For The Structure And Function Of Coral Reef Ecosystems. Oceanography and Marine Biology: An Annual Review, 53: 215-295
  6. Reichert, J., Arnold, A. L., Hammer, N., Miller, I. B., Rades, M., Schubert, P., Ziegler, M., & Wilke, T. (2021). Reef-building corals act as long-term sink for microplastic. Global Change Biology, 28, 33– 45. https://doi.org/10.1111/gcb.15920

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