Microplastics: a growing threat to coral reefs

Following the incredible Blue Planet II , and even more recently Drowning in Plastic, the public response has been brilliant, pushing for reductions in single-use plastic globally. My friends who I used to row with back at home founded Row For the Ocean and are currently rowing the Atlantic to raise awareness of plastic pollution in collaboration with Surfers Against Sewage, with the aim of creating a plastic free Exeter by 2020. However, more threats of plastic to the oceans continue to be uncovered, such as with my MSc thesis, supporting evidence that coral reefs may be suffering from microplastic pollution too. I thought it would be good to share an overview of my MSc thesis work: The effects of microplastics on coral holobiont photosynthesis, respiration and bleaching susceptibility, which I am currently working towards getting published.
Source: PlasticsFacts.com

Over the last sixty years, plastic production has increased dramatically worldwide and is now recognised as a serious threat to the marine environment. Plastic is durable, lightweight and versatile, with low production costs and is therefore a favourable every-day material that has led to the “age of plastics”, with production increasing from 0.5 million tons/yr in 1960 to almost 335 million tons/yr in 2016. Yet it is these brilliant properties that have made plastic such a harmful material, being carried across the globe in the ocean by currents and winds.

The negative environmental impact of plastic pollution was overlooked for decades with the plastics industry failing to predict the rapid increase in production and use of plastics. Impacts include entanglement and death of marine wildlife, loss of aesthetic value, increase in economic costs, introduction of invasive species and spread of disease.


Microplastics, fragments defined as <5mm, are much more difficult to control than macroplastics due to their size and the vast nature of the world’s oceans, now being found even in Antarctica. Microplastics can come in the form of beads produced for industry, while the majority are produced through degradation of larger plastics.

Microplastics have been found within numerous organisms including phytoplanktonbivalvescrustaceansfishmarine mammals and birds, often having serious repercussions. Yet only a handful of work has looked at the effects of microplastics on coral reefs which form a vital part of food webs, supporting immense biodiversity and providing important ecosystem services to millions of people.

dead bird with rubbish inside
Source: National Geographic

Corals and microplastics

Scleractinian reef-building corals contain photosynthetic algae called zooxanthellae, usually of the genus Symbiodinium. The coral provides protection and COvia cellular respiration to the Symbiodinium, while the zooxanthellae provide nutrients to the coral as a result of photosynthesis, forming a symbiotic relationship as seen below. Corals provide the first trophic link for many food webs through their Symbiodinium symbiosis and offer the majority of the habitat structure for reef organisms. When the coral is stressed due to environmental factors such as increased water temperature, the Symbiodinium are expelled. The algae provide corals with their bright colours and therefore when the Symbiodinium are released corals will look white and have become bleached. As the coral is no longer being provided with nutrients created through the relationship with Symbiodinium, the coral will usually die.

Alterations to the surrounding environment such as pollution or rising sea temperatures can change the symbiotic association within corals and has been observed to reduce calcification and metabolism, altering entire reef dynamics. By monitoring the photosynthetic ability and respiration rates of corals, any physiological changes as a result of contaminant exposure can be observed.

The symbiotic relationship between coral host and Symbiodinium. Source Soguchi et al. 2013

There is growing concern that corals may be particularly at risk from microplastics, as they are in the size range that corals ingest. A recent study demonstrated how disease likelihood increases 20 times when corals are covered by plastic due to light deprivation, anoxia and toxin release, providing a pathway for pathogens to enter coral tissue. Little work has been carried out to find whether there are any physiological effects caused by microplastics on corals. However microplastic ingestion by corals can occur at a similar rate to plankton uptake, with corals even showing a preference towards ingesting microplastics.

Microplastic fibres have also been found tangled within the gut cavity of corals which may reduce the ability to ingest natural food and therefore prevent normal function. Microplastic fibres released through washing clothes are thought to be the most abundant type of microplastic in the marine environment. Therefore for my experiments I decided to look at two different types of microplastics: polystyrene spheres (typically from packaging and very common) and fibres collected from laundry lint (below).

Microplastic fibres collected from tumble dryer lint under the microscope

Microplastic exposure experiments

My experiments looked at two different coral species: Acropora spp., an important dominant reef-building genus and one of the most abundant and species-rich groups of corals in the world, typically sensitive to anthropogenic ecosystem change. The second was Seriatopora hystrixthought to be more resilient to environmental change. My experiments tested the effects of microplastics, at a concentration that has been found within the ocean, on coral function and whether the presence of microplastics may cause corals to be more likely to bleach under higher temperatures. Using a PAM fluorometer and 4-chamber respirometer I monitored coral photosynthesis and respiration.

It is important to understand the effects of microplastics on corals as:

  1. Corals are a vital part of numerous marine food webs and associated seascapes such as seagrass, providing nursery grounds for various species and economic benefits such as fisheries and tourism. Reduced function or size of coral reefs may have serious repercussions for entire ecosystems and human communities.
  2. Microplastic pollution will add another stressor to a ecosystem that is already considered vulnerable due to climate change and ocean acidification.
  3. Coral reefs are easily exposed to microplastics, especially at low tide where floating debris can come into contact with shallow reefs. Reef close proximity to the coast also means corals are at a higher risk from land sources of plastic pollution.
  4. Bleaching events as a results of higher sea temperatures are expected to increase as well as plastic pollution rising. It is therefore vital to understand whether microplastics may make corals less resilient to bleaching events.
A fragment of Acropora spp. coral ready for experiments

Without being too technical, this research found that:

  • Microplastics can lower photosynthetic capability of corals which may reduce coral function, growth and reproduction.
  • Microplastics can increase respiration rates in corals which may trigger the production of reactive oxygen species (ROS) and in turn cause damage to the coral Symbiodinium.
  • The relationship between the toxicological/physiological effects of microplastics is species dependent, with Acropora spp. being more sensitive compared to Seriatopora hystrix. This suggests different mechanisms for susceptibility and/or toxicity with more sensitive species being more susceptible to microplastic pollution. Microplastic may therefore alter species diversity in coral reefs.
  • This may have drastic repercussions for entire reef systems, including associated seascapes and human communities through loss of fishing and tourism.

Moreover, this may cause corals to be more susceptible to bleaching events and reduce resilience to natural disturbances, due to the added stressor of plastic pollutionClimate change is already pushing the limits of coral resilience, which may be further limited by microplastic pollution. This study demonstrates a need for more effective mitigation and provides information to push for regional and international marine plastic pollution polices to protect marine ecosystems, including corals reefs, that are vulnerable to such threats.

More work needs to be done in this area such as making long-term observations of the effects of microplastics on coral calcification and reproduction. I really enjoyed my time working in the Changing Oceans Lab at Edinburgh University and as much as I love field work, I can’t wait to be back in the lab again – I’m hoping to run some more exposure experiments as part of my PhD!

Originally posted on EarthExchange but has since been updated.

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