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

Nearly half of all photosynthesis on Earth takes place in the ocean, and this is largely thanks to microplankton – tiny, single-celled plants that drift freely in the upper, sunlit layers of the water column.

When these tiny algae bloom, they do so by converting sunlight, water and carbon dioxide into oxygen and energy-rich sugars, the latter of which are used to build new algae cells.

Unfortunately, algae have now got 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. She and her colleagues at the Norwegian University of Science and Technology (NTNU’s) Department of Energy and Process Engineering are working to quantify the impact of plastic on the ocean – not merely in terms of localized pollution, but in relation to everything the ocean means to us: as a source of food, as a carbon sink and as a recreational area.

“We are 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,” explained Verones.

The researchers have collected phytoplankton data from various climate zones around the world and used laboratory data to determine the extent to which growth is limited by microplastics. They have then used this data to calculate the average impact that a certain concentration of microplastics will have on algae in different regions or climate zones, as well as on a global scale.

“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,” explained Verones.

The results of the study show that the negative impact on carbon uptake was greatest in arid and tropical regions. 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 is not a huge percentage,” emphasized Verones.

The total amount of carbon absorbed by the ocean in a year is estimated at 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 said.

The aim of the study was to incorporate the effect that microplastics have 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,” said Verones.

This may involve 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 broad EU-funded project that investigates how plastic affects the ocean from a life cycle perspective.

“The project has three focus areas. We are studying how plastic affects biodiversity, for example when animals become entangled in or ingest plastic. We are 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,” explained Verones.

“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. 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’,” said Verones.

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 is a climate crisis, a nature crisis and a pollution crisis. Life cycle assessments are able to address all of these aspects. If we are to find a solution, we need to look at the big picture,” concluded Verones.