The Emerging Threat to Reef-Building Corals   

By Emma Kropke

Coral reef biota are being infiltrated with microplastics, threatening not only our oceanic ecosystems but the corals themselves. After learning the ins and outs of what a microplastic is in the last blog post, it’s time to tackle the different threats microplastics impose on important reef-building corals.

The term reef-building coral refers to hard corals, also known as scleractinian or stony corals. Hard corals produce a rigid skeleton made of calcium carbonate that forms the 3D structure of coral reefs, supporting reef diversity, function, and coastal protection. Boulder coral species, like the star and brain coral, consist of hundreds to thousands of individual polyps, all connected by their calcium carbonate skeletons (3). The individual polyps can extend their tentacles into the water column to feed on different types of plankton and organic detritus. The polyp is commonly said to look like a miniature sea anemone or an upside-down jellyfish – both of which corals are closely related to! Branching coral species, like staghorn or elkhorn coral, are another important group that supports reef complexity. They too are a colony of individual polyps; however, instead of having a boulder-like shape, branching corals diverge or “branch” out in different directions. Both boulder and branching corals contain a symbiotic algae, known as zooxanthellae. Zooxanthellae live inside the coral tissue, providing nutrients for the coral by transforming sunlight into energy through photosynthesis. Without a symbiont present to provide the coral energy, the coral relies on catching prey with their tentacles. But, catching prey alone usually does not provide the corals with enough energy to survive. So, without their symbionts, the corals are at risk of starvation or death from lack of nutrition. 

You may be wondering, how does any of this pertain to microplastic pollution? Well, you and me both. As microplastics become more prevalent, a new field of research is emerging studying their effects on hard corals. Within different coral colonies, the individual polyp can range in size from just one to ten millimeters (1).  They can be as small as the tip of your sharpest #2 pencil or up to the size of the eraser on the opposite end. If we recall, a microplastic is defined as any piece of plastic less than five millimeters in size. You can see how this might be an issue. Corals, just like any other ocean animal, can mistake plastic for food.

Accidental and active ingestion are the first threats we will highlight today, which largely affect boulder corals with polyps closer in size to ten millimeters. Many microplastics are small enough to confuse the coral polyps and be accidentally consumed (2). For example, recent research on a type of star coral shows that, on average, 100 microplastic particles were found within each individual polyp due to accidental ingestion (4). This study on Astrangia poculata, a species of reef-building coral, was one of the first records of microplastic inside wild coral (4). Not only are these corals being threatened by accidental ingestion, but some are showing a taste for a microplastic dinner. In a separate lab experiment, the same species of star coral showed a preference for microplastics over a natural food source. To determine this, both microplastic and shrimp eggs were introduced to Astrangia poculata. It was found that the corals ate almost twice as many microplastic beads compared to the shrimp eggs, a natural food source commonly used for corals in captivity. Moreover, “after those polyps had filled their stomachs with microbeads, which have no [nutritional] value, they stopped eating the shrimp eggs altogether” (4). This is obviously very alarming. In addition to accidental consumption, the coral could be adding to its own stress by choosing to eat even more microplastic. Overall, any type of ingestion of microplastic will prevent the coral from capturing a nutritious meal. Lower energy levels will make it harder for the coral to recover from things like predation, disease, bleaching, and destructive storms. 

Plastic pollution is just one of the many threats our coral reefs face. For instance, rising sea temperatures are continuously threatening their survival, causing extreme mass mortality events as a result of coral bleaching. When corals are under heat stress they expel their symbiotic zooxanthellae, and because their symbionts give them their color, they turn completely white or “bleached”. This is a direct effect of rising sea temperatures and prolonged heat stress. Coral bleaching does not always lead to mortality but if infected with microplastics as well, it can be more difficult for the bleached coral to recover. Let me explain. Bleached corals rely solely on active feeding from the water column using their tentacles due to the lack of zooxanthellae. Remember, microplastics garner no nutrition for the coral; so, if consumed it will not provide the coral with the energy it desperately needs. Thus, increasing the likelihood of mortality. Moreover, bleached corals have been found to ingest more microplastics than natural food sources (6). This means bleached corals are more susceptible to accidental ingestion of microplastic and tissue injuries via surface adhesion. So, microplastics are both a direct threat to corals and an indirect threat as a hurdle to their recovery. 

Reducing our plastic consumption is one of the most important and direct actions we can take to fight against microplastics. According to the International Union for Conservation of Nature, the main source of primary microplastics found in the world’s oceans is synthetic textiles (8). To combat this, when possible, we can choose to purchase sustainable clothing made from organic materials, such as cotton, silk, wool, hemp, or other natural fibers. We can also change our current laundry routine. Using less water or air-drying clothing can help reduce the amount of microplastic fibers released when machine washing and drying clothing, respectively (8).  To prevent items from entering the ocean and breaking down into secondary microplastics, you can lower your use of single-use plastics like water bottles, plastic bags, straws, or plastic cutlery. These swaps can be simple and implemented into our lives on a daily basis. You can bring reusable grocery bags while shopping, or ask for paper instead of plastic bags. You can swap disposable plastic straws for reusable metal, glass, or bamboo straws. You can purchase a refillable water bottle. Small changes like these can help protect our reef-building corals from microplastic exposure.

Relative to a human’s existence on this planet, plastic is an eternal substance. The plastic we use today will exist during the lifetimes of our children, our grandchildren, and even their children. Plastic pollutes our drinking water, our food, our oceans, and our planet. Coral reefs are the heart of ocean health and diversity while also supporting coastal communities and driving local economies. While corals face many challenges, plastic pollution worsens their outlook. Coral bleaching, predation, and disease are all made worse by both microplastic ingestion and adhesion. Not to mention, microplastics travel up the reef food web impacting not only our beloved sea creatures but our families as well. The first step in fighting against plastic pollution starts with us. It starts with me. It starts with you. We can help preserve our coral reefs and the many marine organisms that call it home. We can make a choice to say no to synthetic and single-use plastics to protect the natural environment from the consequences of convenience.  I can’t talk to the coral like I can talk to you. I can’t explain to them that consuming microplastic can be harmful, injure their tissue, or lower appetite. What I can do is share this. I can ask that we all work together and take action to dismantle the plastic pollution pipeline. 

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