How microplastics affect carbon uptake in the ocean

You have probably heard it before: More than 70 per cent of our planet is covered by oceans.

Nearly half of all photosynthesis on Earth takes place in these oceans, largely thanks to microplankton. These are tiny, single-celled plants that drift freely in the sunlit upper layers of the water column.

When these tiny algae bloom, they do so by converting sunlight, water, and carbon dioxide into oxygen and sugars that are used to build new algae cells.

Microplastics are everywhere

The algae now have a new marine neighbour to contend with. Microplastics, tiny plastic particles, are currently found everywhere in the ocean. From the most densely populated coastal areas to remote waters in the Arctic and Antarctic.

“The ocean plays a crucial role in absorbing carbon from the atmosphere. Between 25 and 30 per cent of all human-generated CO₂ is absorbed by the ocean. Phytoplankton plays an important role in this process. So, what happens if marine microplastics affect the plankton?”

The person asking the question is researcher Francesca Verones.

A service the ocean provides

She and her colleagues at NTNU’s Department of Energy and Process Engineering are working to quantify the impact of plastic on the ocean.

Not only in terms of local pollution, but also in relation to everything the ocean means to us: as a source of food, as a carbon sink, and as a recreational area.

“We're investigating how plastic affects what we call ecosystem services – meaning the services we receive from ecosystems. For example, we can view the fact that the ocean absorbs CO₂ from the atmosphere as a service that the ocean provides for us,” Verones explains.

Plastic blocks sunlight

The researchers collected phytoplankton data from various climate zones around the world and used lab data to determine the extent to which microplastics limit algal growth.

Using this data, they calculated the average impact that a certain concentration of microplastics would have on algae in different regions and climate zones, as well globally.

“Microplastics affect algal growth in various ways. The toxicity of certain types of plastics, such as PVC, is a problem in its own right. But plastic can also reduce the amount of sunlight that penetrates deeper into the water column, causing physical damage or leading to oxidative stress in the algae cells,” the researcher says.

The researchers found that the negative impact on carbon uptake was greatest in dry and tropical regions.

Concentrations will keep increasing

These climate zones have the highest carbon uptake and are also the most vulnerable to the harmful effects of microplastics.

“In these areas, we found that microplastics could reduce carbon uptake by 25,000 and 48,000 tonnes, respectively, over the course of a year. Although it may sound a lot, it's not a huge percentage,” says Verones.

The total amount of carbon absorbed by the ocean in a year is estimated to be two billion tonnes.

“But we must bear in mind that the amount of microplastics in the ocean is increasing all the time. All the plastic that is discarded in the natural environment will eventually end up in the ocean. So the concentrations are going to keep on increasing,” she says.

We need to view plastic from a life cycle perspective

The aim of the study was to incorporate the effect of microplastics on carbon uptake into a life cycle assessment of plastics.

A life cycle assessment provides insight into the overall environmental impact of a product: From the moment it is created or manufactured, through its use, until it is eventually discarded and breaks down.

“Life cycle assessment is a method in which all the different stages of a product’s life cycle are examined, enabling the various impacts to be assessed simultaneously. There could be thousands of factors involved. The goal is for it to be as comprehensive as possible,” says Verones.

Animals become entangled in and eat plastic

This includes how much water is used in production, how much energy is consumed during use, the extent to which the product can be recycled, and not least, how the waste generated affects the environment.

The research is part of a broader EU-funded project that investigates how plastic affects the ocean from a life cycle perspective.

“We're studying how plastic affects biodiversity, for example when animals become entangled in or ingest plastic. We're also investigating how plastic affects the spread of invasive species, for example by species attaching themselves to plastic and being transported around the world. The third area focuses on how plastic affects ecosystem services, such as carbon uptake,” Verones explains.

This is a triple crisis

This is the first time researchers have investigated how plastic affects carbon uptake in the world’s oceans and incorporated the findings into a life cycle assessment, according to Verones.

"The reason we choose to do it this way is that life cycle assessments are one of the few methodologies capable of covering all aspects of what the UN calls ‘the triple planetary crisis’,” the researcher says.

The triple planetary crisis refers to the three most pressing, interrelated challenges facing humanity today:

Climate change, pollution, and loss of biodiversity.

Each of these challenges has its own causes and consequences, and according to the UN, all of them must be addressed if we are to have a viable future on this planet.

“There's a climate crisis, a nature crisis, and a pollution crisis. Life cycle assessments are able to address all of these aspects. If we're to find a solution, we need to look at the big picture,” says Verones.

Reference:

Song et al. Assessing the effect of microplastics on the marine ecosystem's carbon sequestration potential in life cycle assessment, Ecosystem Services, 2026. DOI: 10.1016/j.ecoser.2026.101824

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Read the Norwegian version of this article on forskning.no

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