Macro and microplastics threaten cold-water corals

Written by Coralie Barrier and Florina Jacob.

Acknowledgements to Leïla Ezzat.

 

Macro and microplastics represent a real threat to the oceans and associated organisms. However, much remains to be discovered about their impact on deep-sea ecosystems. What does scientific research tell us about the interaction between deep-sea corals and plastics?

 

The definition and fate of plastics in the deep sea

 

The distinction between macro and microplastics is made according to their size. But there is little consensus on the size thresholds for each category among the scientific community (Hartmann et al, 2019). One of the commonly used definitions identifies macroplastics as particles 1cm or larger, and microplastics as particles between 1 µm and 1000 µm (Hartmann et al, 2019). Other approaches have been proposed as well, such as Bermudez and Swarenski (2021) who present a classification based on the sizes used for the study of plankton. 

Despite these different definitions (which may pose difficulties for the comparison of studies), there is no doubt about the urgency of conducting more studies on the presence and future of plastic in underwater ecosystems.

89% of plastic waste found on the deep seafloor is single-use plastic from human activities on land such as bottles, bags, and food wrappers (Chiba et al, 2018). But plastics used at sea are also a source of pollution: expanded polystyrene, buoys or even fishing nets account for about 18% of the microplastics found in the seas and oceans (Lusher et al, 2011).

The submarine canyons of the Mediterranean, known to host temperate and cold-water corals, function as vectors for the transport of marine litter (Angiolillo et al, 2021), and the plastic debris present accounts for about 70% of the observed litter (Tubau et al, 2015).

The risks that plastic poses to marine wildlife are numerous: entanglement, ingestion, accumulation in the food web, transport of pollutants such as heavy metals, and transport of microbes, among others (Chae et al, 2017; Wright et al, 2013). In particular, the creation of biofilms on the surface of plastics by microbes can decrease the buoyancy of plastics, which will settle to the bottom more quickly, exposing bottom-dwelling organisms to plastic pollution (Wright et al, 2013; Taylor et al, 2016). 

Micro and macroplastics colonized by microbial colonies can also impact coral metabolism. For example, research on the impact of microplastics as disease vectors, shows that the likelihood of shallow-water corals being affected by disease, increases from 4 to 89% following contact with plastic (Lamb et al, 2018). For cold-water corals, research on this topic is still very limited.

 

 

The case of the reef-building coral Lophelia pertusa

 

A study focusing on the deep-sea coral species Lophelia pertusa, was conducted by a team from the Oceanological Observatory of Banyuls-sur-Mer and the Alfred Wegener Institute and published in 2018 in Scientific Reports. The team of researchers, coordinated by Dr. Leïla Chapron, aimed to determine the consequences of plastic pollution on the growth, feeding and behavior of deep-sea corals through exposure to macroplastics (10x10cm plastic films) and microplastics (500 μm micropellets) for 72 days in controlled conditions (aquaria).

 

Figure: Lophelia pertusa coral colony (Mississippi Canyon). Photo by: NOAA Ocean Explorer.

 

Following the first few days of exposure to either microplastics or macroplastics, researchers observed a decrease in the rate of coral polyp activity (tentacle movement) as well as in the rate of prey capture, compared to the control conditions, showing a rapid response to stress.

After 20 days of exposure to macroplastics, polyp activity increased significantly compared to the control conditions, while prey capture rate remained lower compared to the control conditions. These observations suggest that macroplastics may act as a barrier for polyp-prey encounters, as well as for the influx of oxygen-laden water to coral tissue, leading corals to increase movement to optimize water contact, as suggested by the researchers (Chapron et al, 2018). 

Corals exposed to microplastics responded differently. After 20 days of exposure, polyp activity increased slightly, still remaining minor compared to the control conditions. Prey capture rate, however, increased after 47 days even compared to the control conditions, surprisingly. These results contrast with those found for other coral species exposed to microplastics. For example, the Mediterranean coral Astroides calycularis showed a decrease in prey capture rate in the presence of microplastics, related to consumption and handling of them for long periods (Savinelli et al, 2020).

Concerning the coral species Lophelia pertusa exposed to plastic (macro or microplastic), Dr. Leïla Chapron and her team pointed out that the growth rate of the coral skeleton was minor, compared to the control conditions. This suggests a change in the feeding behavior of the corals, resulting in a decrease in nutritional intake that may influence coral skeleton production, and ultimately reef formation.

 

Conclusion

 

This study highlights the dangers that plastic pollution poses to the behavior, feeding, and skeletal formation of cold-water corals. As ecosystem builders that provide habitat and resources for marine biodiversity (an area of cold-water corals has 29 times more marine biodiversity than an area without corals), negative impacts on corals can extend beyond ecosystem boundaries, undermining marine biodiversity and human communities.  

 

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