Microplastics: a new variable in the ocean carbon cycle

New research suggests microplastics may interfere with the growth of microscopic algae involved in the ocean carbon cycle

Microplastics, tiny plastic particles generally smaller than five millimetres, have been found in marine environments around the world, from densely populated coastlines to remote polar waters. Already known to harm marine life through ingestion and exposure to toxic substances, their negative influence may also stretch to the microscopic algae involved in the ocean carbon cycle.

A crucial part of the carbon cycle

Much like plants on land, the microscopic algae, or phytoplankton, convert sunlight, carbon dioxide, and water into energy through photosynthesis. When they die, some sink to the sea floor, carrying absorbed carbon with them. Part of this carbon eventually becomes trapped in marine sediments.

“Microalgae's photosynthesis forms an important part of the ocean carbon cycle,” explains Fei Song, a PhD student at Norwegian University of Science and Technology (NTNU) and lead author of the research.

Laboratory studies have previously shown that exposure to microplastics can inhibit the growth of some microalgae. The researchers wanted to know whether this reduced growth could, in turn, decrease the amount of carbon sequestered by the ocean.

A small but measurable effect

By combining laboratory data with information on microalgal distribution, regional carbon-sequestration potential, and estimated 2020 microplastic concentrations, the researchers estimated how reduced growth could affect marine carbon sequestration.

“We estimated a potential reduction in carbon sequestration of about 75 kilotonnes of carbon dioxide per year,” says Song.

That number is relatively small compared with billions of tonnes absorbed by the ocean each year. “At the global scale, the estimated reduction is indeed small compared with the total amount of carbon absorbed by the ocean,” says Song. “So, I would not say that our study shows that microplastics currently represent a major threat to the global ocean carbon sink.”

Song also notes that much of the evidence comes from laboratory experiments, while conditions in the real ocean are far more complex. In addition, there is uncertainty over how much microplastic is present in the ocean and how different phytoplankton species respond to it.

“I would describe our results as an estimate of potential impact rather than a direct measurement of carbon loss in the ocean,” says Song. “It is an important first step, but more field data will be needed to improve these estimates.”

Interestingly, the estimated effect was not evenly distributed. “When we considered both the sensitivity of microalgae and the carbon-sequestration potential of different marine regions, we found quite substantial differences,” says Song.

Different regions contain different communities of microalgae, and species may not respond to microplastic exposure in the same way. The largest potential reductions in carbon sequestration were estimated in arid and tropical regions.

Accounting for the wider impacts of plastic

The significance of the study goes beyond estimating carbon sequestration reductions.

“Our goal was to translate this effect into life cycle assessments, so that microplastic impacts on marine ecosystem services can eventually be considered when assessing the environmental impacts of products and activities,” says Song.

Life-cycle assessments examine environmental impacts across the production, use, and disposal of a product, including factors such as energy use, greenhouse gas emissions, water consumption and waste. They are particularly useful when environmental decisions involve trade-offs.

“Plastic products can provide benefits; for example, reducing food waste or making products and transportation lighter, but plastics can also cause environmental impacts when they enter the environment,” explains Song.

"At this stage, I see the result as having both scientific and practical relevance,” says Song. “Scientifically, it gives us a first framework for quantifying this impact. Practically, it allows this type of ecosystem-service damage to eventually be considered alongside climate change, biodiversity and other impacts in environmental decision-making."

With this research, the hope will be to better understand how microplastic impacts can be minimised, both in product design through to disposal and the entire life cycle.

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Main image: Collecting water samples contaminated with microplastics & plastic pollution

Credit: Shutterstock

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