Microplastics and Coral Health

We don't know all the impacts microplastics have, but what we're starting to find out doesn't look good for corals.

Did you know that we’re all consuming tiny pieces of plastic on a daily basis? Called microplastics, these plastic particles are less than 5mm in length and can be found in a variety of products ranging from clothes to plastic bottles. In fact, a research review published last year stated that Americans ingest at least 74,000 microplastic particles every year while another study conducted by researchers at the University of Newcastle in Australia estimated that people consume about 5 grams of plastic a week through food, water, and even air.

But it’s not just humans that have been affected. Many marine animals mistakenly feed on microplastics even though it contains numerous organic pollutants. The chemicals end up accumulating in the animal’s tissues and actually increase in concentration as the pollutants are transferred up the food chain. Yet, despite how this poses a serious problem for aquatic environments, there is still a lot more research that needs to be done – we don’t know the full extent of the consequences of this issue.

To learn more about microplastics in aquatic systems, we spoke to Dr. Jessica Reichert – a postdoctoral student at the Department of Animal Ecology & Systematics in Justus Liebig University. Her research mainly focuses on global change and its impact on tropical coral reefs, the effects of microplastics on corals, as well as morphological adaptation to environmental changes. If you’re interested, you can read her publications here.

Q: Could you briefly explain your research regarding microplastics?

A: We started to study the impact of microplastics on corals in 2015. Until then, only one publication showed that corals ingest microplastic particles, but nothing was known about the effects on the corals. With our background at Justus Liebig University Gießen to set up laboratory experiments and study the impact of different global change stressors, such as increasing temperatures, we decided to set up a series of experiments looking at the effects of microplastics on corals.

In our first study, we investigated the responses of the corals and could see that different coral species respond very differently to the exposure. We observed handling of the particles by the corals with their tentacles or mesenterial filaments (part of the corals’ digestive tract), ingestion as well as egestion of the particles, but also that the corals produce more mucus (which is known as a cleaning response) or that they have overgrown particles. Interestingly, some corals responded with bleaching and/or necrosis, which are signs of compromised health.

To better understand these negative impacts, we set up a second experiment in which we studied the impacts of microplastics on different physiological aspects of the corals. To assess the impacts realistically, the experiment was run for 6 months and we applied a concentration of 200 particles per liters, which could be expected in highly polluted areas by the year 2100. We saw that also under these conditions, again some coral species showed signs of compromised health, others had lower growth rates, and others seemed to be not affected at all. Besides these longer-term experiments we study other aspects, for example the feeding behavior on plastics or how this stressor interacts with other stressors, such as increasing temperature.

Q: Why do you think the problem of microplastics has been getting worse over the past few years?

A: When talking about the threat microplastics are posing on corals it is crucial to say that the impact elevated temperatures have on coral reefs are by far the worst stressor and have already decimated coral reefs worldwide. For microplastics we are only starting to understand the impacts, but we can clearly say that it does not decimate reefs in the same way marine heat waves currently do.

However, microplastics seem to pose an additional threat to the already stressed corals. The problem is getting worse as more plastic enters the ocean every day and the amount of particles is still increasing. In addition, the large plastic pieces that are already in the ocean are fragmenting into smaller particles, the so-called secondary microplastic, additionally increasing the amount of microplastic in the ocean. However, the problem seems to be increasing even more as we have only started to understand it and with every new study, we get new insights.

Also, the public awareness about this problem has risen over the last years, which helps to tackle the sources of the problem, but might also support the impression that the problem itself is getting worse.

Q: What is the environmental impact if corals suffer from long-term exposure to microplastics? How might other aquatic species be affected?

A: Microplastics might cause stress in different ways. First, if corals ingest the particles, these are unlikely to be a source of food and might block their digestive tract. In the meantime, the corals cannot feed on actual food and on top of that, they need even more energy to egest the particle if they manage to. Second, different studies have found that plastics can harbor very specific bacterial communities that differ significantly from the bacteria commonly living on natural substrates. So it might be possible that some of these bacteria living on the microplastics turn out to be pathogenic for the corals. Third, plastic particles are known to accumulate toxins from the water column. Once ingested or overgrown by the coral, the bacteria as well as the toxins that are attached to the plastic have overcome the coral’s natural defense mechanisms and might cause damage to the organism.

We observed reduced growth and in some cases even impacts on the coral health. Our hypothesis is that the corals that are in contact with the microplastic particles (sometimes more than 3h) might have reduced energy reserves, because of the latter reasons. This might have impacts on a range of physiological aspects of the coral.

For example, corals with reduced energetic reserves are more susceptible to bleaching (under elevated temperatures) or diseases. Also, although the observed effects might be small, these effects can accumulate over a coral’s lifetime and for example corals with lower growth rates might lose space in the reefs while other organisms, for example algae might take over this space.

Also knowing that different coral species are differently affected, microplastic pollution might cause community shifts in coral reefs. Especially those coral species seem to be affected by microplastics, which are also susceptible to other stressors, such as increasing temperatures. Therefore, microplastics might contribute to the already ongoing degradation in coral reefs. Similarly, other aquatic species might be affected. Especially sessile filter feeders, such as sponges might be particularly affected by microplastics, but we have not studied these species.

Q: Are there specific traits that make certain species of corals more susceptible to being affected by microplastics? What are these traits & why?

A: This is one of the most interesting questions. We are only beginning to understand why some coral species are more affected than others. From what we observed, the degree of heterotrophy (feeding on particulate food in contrast to relying on the energy derived from photosynthesis of the associated microalgae) seems to be an important factor.

The species most affected by the exposure, Pocillopora verrucosa, is also the species that we observed to handle and feed most frequently on the microplastics. In addition, this species is also very sensitive to other stressors such as elevated temperatures. However, this is only a first observation and more research needs to be done to better understand what factors affect the susceptibility to microplastics seeing how diverse the interactions of corals with microplastic particles can be, yet it is unclear in what way the negative effects are caused. Until further studies reveal more insights, the answer to that question remains very speculative.

Q: Do you think it’s possible to solve the issue of microplastics? What do you think people should be focusing on if they want to reduce the amount of microplastics in the ocean?

It is likely impossible to remove all the plastic from the ocean. However, microplastics are deposited in high amounts in environmental sinks, such as the deep sea or arctic sea ice. This means that they become – at least temporarily – unavailable to the organisms. The problem is that we constantly add more plastics to the oceans. Therefore, the most important thing we need to do in order to tackle the issue of microplastics is to reduce the amount of plastic that enters the ocean.

This we can do with better recycling systems and beach cleanups. What is more and what everyone of us can do, is reduce the amount of unnecessary single-use plastics – carry a reusable shopping bag, bring your own coffee cup, or fill up your reusable water bottle. No matter in what way, it is necessary to reduce the amount of plastic pollution, not only for corals, but for all organisms that are affected by micro- or macroplastics.

Without doubting the need of reducing our plastic-footprint, we should be careful not to forget the most urgent issue of reducing greenhouse gas emissions, in order to tackle the problem of global warming that has already destroyed large fractions of coral reefs worldwide.

Q: What are some other problems that are affecting the health levels of coral reefs?

A: In general it is important to understand that microplastic is only one of many stressors corals are facing in the Anthropocene. Microplastic alone would not kill coral reefs, but it adds up to all other stressors, and most prominent the increasing ocean temperatures. To reduce the overall pressure on coral reefs we have to work on a lot of different aspects, reduce plastic inputs as well as nutrient inflows, but most importantly reduce greenhouse gas emissions.

Special thank you to Dr. Reichert for taking the time to answer all of our questions!

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