Written by: Jasmine Haskell
Edited by: Danielle Moloney
Featured image by: https://conservation.reefcause.com/how-does-plastic-pollution-impact-coral-reefs/
Around 60-80% of the world’s litter is made up of plastic with nearly 10% of yearly plastic finding its way into the oceans via rivers, sewer discharges, and runoff from rainfall (Mendrik et al., 2021). Unfortunately, this plastic debris – specifically microplastics – can have detrimental effects on marine life, including coral reefs. Microplastics, tiny pieces of plastic less than 5mm in size, have been identified as a considerable threat to coral health (Mehra et al., 2021). One of the primary ways in which microplastics impact corals is through ingestion. Corals are opportunistic feeders and can capture small prey including zooplankton and other small species with their tentacles. Regrettably, microplastics fit into the optimal size category of coral prey, meaning that they ingest these microplastics along with their food. This ingestion can lead to microplastics accumulating and fragmenting inside coral tissue, ultimately deteriorating their health.
Several studies have shown that the ingestion of microplastics by corals can cause a range of physical and physiological effects on corals including a decline in growth and calcification rates, symbiotic disruptions and increased disease occurrence (Vencato et al., 2021). However, the concentrations of microplastics used in laboratory studies were often much higher than that found in natural coral reef ecosystems. A study by Mendrik et al. (2021) looked at the species-specific response of two different types of coral to both microplastic fibres and microplastic spheres at environmentally relevant concentrations at two different temperatures.
The researchers conducted two 12-day experiments to study the effect of microplastic spheres and microplastic fibres on the photosynthetic capability and respiration of the two different species of coral. The first experiment was conducted at 26°C, while the following experiment involved a gradual temperature increase, starting at 26°C and reaching a maximum of 32°C, increasing at a rate of 0.5°C per day. Of the corals exposed to the microfibers, Acropora spp. showed a significant decrease in the photosynthetic efficiency when compared to Seriatopora hystrix at ambient temperature. When exposed to the microplastic spheres, Acropora spp. had higher photosynthetic efficiency than the control corals that were not exposed to any type of plastic. The increase in temperature did not significantly affect the photosynthetic efficiency of either species of coral exposed to microplastics. Acropora spp. Also demonstrated higher rates of respiration compared to the control when exposed to either type of microplastic. The increase in respiration has been reported to occur in corals and anemones when they are experiencing stress.
A summary of the experiments conducted by Menrik et al. (2021) on Acropora spp. and Seriatopora hystrix. The panel in the middle highlights a significant difference in the photosynthetic ability of Acropora spp.between days 1 and 12 of the experiment at ambient temperature. The panel on the right stresses the need for further experiments needed to assess physiological stress experienced by corals at elevated temperatures following exposure to microplastics.
Results from this experiment demonstrate that microplastic exposure at ambient temperatures can affect the coral holobiont’s photosystem, and that this response is dependent on the species of coral. Although Acropora spp. is one of the most dominant types of coral on the reef, its more sensitive nature resulted in a more pronounced response to the microplastic exposure. Despite the growing evidence of the negative effects of microplastics on corals, there remain significant knowledge gaps in understanding how these pollutants impact natural coral reef ecosystems. Further research is needed to investigate the precise mechanisms by which microplastics impact coral physiology, as well as the long-term effects of chronic exposure to these pollutants.
The increasing amount of plastic waste highlights the need for immediate action at both the consumer and the industrial level to curb plastic waste and its impact. Reduction of waste and recycling of plastic products act as a start to addressing the issue. Tourism, industrial waste, and fishing nets discarded in the ocean contribute to the ever-increasing plastic waste problem that is damaging our coral reefs. The plastic waste crisis is a threat to the survival of these fragile ecosystems, and it is imperative that collective action produces long-term solutions to combat the issue. If we do not take action and prioritise global solutions to reducing plastic waste, we stand to lose one of the most important ecosystems on the planet as they become damaged with increasing microplastic pollution.
References:
Mehra, S., Sharma, K., Sharma, G., Singh, M., & Chadha, P. (2021). Sources, Fate, and Impact of Microplastics in Aquatic Environment. IntechOpen. 10.5772/intechopen.93805
Mendrik, F.M., Henry, T.B., Burdett, H., Hackney, C.R., Waller, C., Parsons, D.R., Hennige, S.J. (2021) Species-specific impact of microplastics on coral physiology. Environmental Pollution. https://doi.org/10.1016/j.envpol.2020.116238.
Vencato, S., Isa, V., Seveso, D., Saliu, F., Galli, P., Lavorano, S., Montano, S. (2021) Soft corals and microplastics interaction: first evidence in the alcyonacean species Coelogorgia palmosa. Aquat Biol 30:133-139. https://doi.org/10.3354/ab00747
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