Microplastics are quietly eroding the ocean's carbon absorption capacity.

Over 70% of the Earth's surface is covered by oceans. The "lungs" of this blue planet are not primarily forests, but rather tiny phytoplankton in the ocean—they perform nearly half of the Earth's photosynthesis. These single-celled algae, like miniature factories, convert carbon dioxide, water, and sunlight into oxygen and energy in the sun-drenched surface of the sea, building their own bodies and providing the foundation for the marine food chain. It is they that make the ocean a vital carbon sink for the Earth: 25% to 30% of the carbon dioxide emitted by human activities is absorbed by the ocean, with phytoplankton playing a crucial role.

However, these tiny algae now have a new "neighbor"—microplastics. Microplastics are plastic particles with a diameter of less than 5 millimeters, ranging from everyday plastic bags, bottles, and clothing fibers to industrial waste, ultimately ending up in the ocean in large quantities. From bustling coastal cities to the remote Arctic and Antarctic, they can be found in almost every sea area. They are ubiquitous and are quietly altering the marine ecosystem.

Researchers Franziska Verones and her team at the Norwegian University of Science and Technology are diligently studying this issue. They want to understand whether microplastics not only cause localized pollution but also affect the ocean's overall function as a "food source, carbon reservoir, and recreational space." Their research has found that microplastics interfere with phytoplankton growth in several ways: they float in the water like a thin mist, blocking sunlight; some plastics (such as polyvinyl chloride) release harmful substances; and plastic particles can abrade algal cells, causing oxidative damage within the cells.

The research team collected data on phytoplankton from different climate zones worldwide and, combined with laboratory experimental results, estimated the specific impacts of microplastics on algal growth and carbon absorption. The results show that the negative impact of microplastics on carbon absorption is most pronounced in arid and tropical regions. These areas are already the most active in marine carbon absorption but are also the most vulnerable to the damage caused by microplastics. Specifically, in arid regions, microplastics reduce carbon absorption by approximately 25,000 tons per year; in tropical regions, this figure reaches 48,000 tons. While this sounds like a significant amount, it is still relatively small compared to the approximately 2 billion tons of carbon absorbed by the ocean each year. Verones emphasized that we must recognize the trend. Humans continue to produce and discard plastics, and most of the plastics entering the natural environment eventually end up in the oceans, with microplastic concentrations continuing to rise.

Today's "small impacts" could become major problems in the future. The significant value of this research lies in incorporating the impact of microplastics on carbon absorption into the life cycle assessment of plastics. What is a life cycle assessment? Simply put, it's not just about the convenience of using plastic products, but a comprehensive examination of their environmental footprint from "cradle to grave": raw material extraction, energy and water consumption during production, impacts during use, and long-term environmental damage after disposal. Verones explained that they wanted to assess the impact of plastics on marine ecosystem services as comprehensively as possible. Ecosystem services are the various benefits the ocean provides to humanity, such as absorbing carbon dioxide, providing food for fish, and regulating the climate.

This work is part of a large-scale project funded by the European Union. This project focuses on three key impacts of plastics on the ocean: first, biodiversity, such as animals dying from entanglement or ingestion of plastic; second, the spread of invasive species, with plastic debris acting like "floating ships," carrying alien species around the world and damaging local ecosystems; and third, ecosystem services, such as the impact on carbon absorption capacity. This is the first time researchers have systematically incorporated the impact of plastics on global ocean carbon absorption into a life cycle assessment framework. Veronese points out that a key reason for using life cycle assessment is its ability to address the UN's "triple planetary crisis"—climate change, biodiversity loss, and pollution. These three crises are intertwined: climate change alters ocean temperatures and currents, affecting phytoplankton growth; pollution (including microplastics) directly harms marine life; and biodiversity loss weakens ecosystem resilience. Focusing on only one problem may lead to neglecting others. Finding truly sustainable solutions requires a holistic perspective.

Currently, the overall impact of microplastics on carbon absorption is relatively limited, but it reminds us that the ocean is not an unlimited garbage can. If plastic products are not properly recycled and disposed of after use, the ultimate cost may be borne by the entire global ecosystem. There are many things ordinary people can do: reduce the use of single-use plastic products, such as straws, plastic bags, and takeout containers; choose durable, recyclable products; support better plastic recycling systems and the development of alternative materials; and pay attention to and spread scientific knowledge to make more people aware that "invisible" microplastics are affecting every breath we take and climate stability. The ocean is the Earth's blue heart, silently absorbing large amounts of carbon dioxide and helping to mitigate global warming. Protecting the ocean is protecting our shared future.