You didn’t misread the title; there is grass in the sea! Seagrass is the only marine flowering plant. Grasses aren’t charismatic, but seagrass fights climate change by storing carbon. If asked, you’d probably say forests store more carbon than seagrass. But a global analysis found that seagrass meadows store 80,000 metric tons of carbon per compared to 30,000 for forests [1]. Excitingly, in the UK we have two amazing seagrass species.
Having seagrass around our coastlines is awesome! Seagrass acts as a natural barrier which reduces wave energy and protects coasts from flooding [2]. I’d definitely rather see more seagrass meadows than ugly concrete walls.
Seagrasses don’t live alone; they interact with many other organisms. Seagrass meadows are full of life, providing homes and predator protection for many, including economically important fish species like cod and plaice in the UK. Seagrasses also have mutually beneficial relationships hosting microbes. In return for a home, microbes help in many aspects of the seagrass’s life increasing available nutrients and preventing sulphur build up – something which is potentially toxic for the grasses.
Seagrass brings us, the environment and other animals so many benefits – but they are not indestructible. In fact, these rather delicate habitats are at risk from a common marine foe…
Plastic really is everywhere.
No corner of the planet is untouched from plastic. Plastic has been found pole to pole: in the deepest reaches of the sea [3], in ice, soils, rivers, lakes, streams, estuaries, oceans, on top of Mount Everest, in the atmosphere… and even within animals, from the smallest insects to the largest predators, including us [4].
The media may focus on turtles eating shopping bags; however plastic’s widespread reach means many species are impacted, even those which cannot physically consume plastic. We seldom talk about the dangers less charismatic species face. Dangerously, plastic breaks down into smaller and smaller pieces (called microplastics). You might think big pieces are most damaging, but microplastics are an unseen persistent threat that is harder to solve.
As a stationary plant, seagrass cannot escape from the onslaught of environmental stress as other, mobile species can. And there isn’t a single issue threatening seagrass: there are many, all at once. We humans threaten seagrass through coastal development, removing habitat, increasing temperatures from climate change, and nutrient pollution, such as from overuse of pesticides and other chemicals [i]. On top of this, seagrass is quite literally swamped by plastic of all sizes, from the large to the microscopic. Could this be affecting our local climate hero, and their minuscule microbial companions?
Bringing the coast inside...
To find out, Egea et al. decided to recreate the seagrasses’ watery world inside, where they could get a closer look at exactly what was going on.
In the ocean, seagrass growth rate can change for many reasons – like temperature, nutrients, hungry herbivores… or multiple reasons acting together. This makes it hard to know what caused the change. But by replicating the seagrass’ habitat in experimental tanks, the scientists could change one variable at a time (e.g. temperature), while keeping everything else the same. This meant that if the seagrass grew differently, they knew what exactly was behind it.
Within these tanks, the team lovingly planted pots containing seagrass, and measured their growth over the course of a month. Some tanks contained microplastics, some contained nitrogen from fertiliser pollution, some had both, and some were controls. If the control groups, having everything a seagrass might need, grew normally, the team would know that any unusual growth in the experimental conditions could be allocated to plastic or nitrogen pollution. They also identified which microbes were present on the roots and shoots of the seagrasses , to keep an eye on any changes to plant-microbe relationships.
Plant v. plastic.
While nitrogen pollution affected seagrass growth, microplastics were the worst culprit. Concerningly, seagrass polluted by microplastics were smaller above and below-ground with fewer new roots and shoots, shorter roots and slower canopy growth than those in unpolluted or even fertiliser-polluted tanks. The researchers realised that the microplastics were literally blocking root pores, reducing nutrient uptake. Smaller seagrasses are less beneficial for the environment, as they provide less habitat and wave protection whilst capturing less carbon.
The microplastics didn’t just impact the plants themselves; researchers discovered microplastic pollution altered seagrasses’ microbial communities. A high-microplastic environment favoured microbes which break down organic matter. The abundance of these species speeds up the decomposition rate of dead seagrass, with the result that less carbon is stored in the soil and is instead released into the atmosphere. It seems that unseen to the human eye, microplastic pollution is wreaking havoc on one of our most important assets in the fight against climate change!
How to find Another Way
The UK has lost over 44% of seagrass since 1936 [8]! It urgently needs protecting and replanting, especially as plastic pollution shows no sign of stopping. There are many wonderful initiatives restoring seagrass meadows across the UK from Hampshire to Shetland – and many of them need volunteers to help.
Anyone can help seagrass; you don’t need to live by the sea. Here are positive ways you can protect this climate hero:
Most coastal microplastic pollution comes from plastic fibres from textiles in laundry wastewater [9]. Adding a microfibre filter to your washing machine stops plastics entering nature directly at the source (make sure to dispose of the fibres responsibly following instructions from the filter manufacturer).
Similarly, avoid using a tumble dryer when possible and air dry clothes instead. As heat breaks down textile fibres faster, releasing more microplastics.
You could also join community beach cleans or litter pick wherever you are. This will stop plastic entering rivers or the ocean, keeping it safely away from seagrass and other marine species. A great way to socialise whilst making your community tidier and helping the environment!
Want to know more?
Chapter 7 Ocean Plastics: “Not the end of the world” by Hannah Ritchie. This engaging optimistic book looks at the bigger picture of climate change data. (Don’t be put off by the focus on statistics it contains some interesting ideas that will make you think).
Read more about seagrass on the Ocean Conservation Trust’s website: https://oceanconservationtrust.org/ocean-habitats/why-seagrass/
Inspired to volunteer? Project seagrass is a UK charity driven to protect seagrass across the world. https://www.projectseagrass.org/volunteer/
Or be involved in citizen science with Project Seagrass https://www.projectseagrass.org/seagrassspotter/
Primary paper:
Egea, L. et al. (2026) Microplastics threaten seagrass carbon sinks through microbial changes, Marine Pollution Bulletin 222 https://doi.org/10.1016/j.marpolbul.2025.118638
Other References:
[1] Fourqurean, et al. (2012). “Seagrass ecosystems as a globally significant carbon stock.” Nature Geosci. https://doi.org/10.1038/ngeo1477
[2] Forrester, J. et al. (2024) Seagrass as a nature-based solution for coastal protection. Ecological Engineering. https://doi.org/10.1016/j.ecoleng.2024.107316
[3] Jamieson, A., Brooks, L., Reid,W et al., (2019). “Microplastics and synthetic particles ingested by deep-sea amphipods in six of the deepest marine ecosystems on Earth”. R. Soc. Open Sci. https://doi.org/10.1098/rsos.180667
[4] Richard C. Thompson et al. (2024). Twenty years of microplastic pollution research—what have we learned? Science. DOI: 10.1126/science.adl2746
[5] Jiang, C., et al. (2018). Microplastics in Sediment and Surface Water of West Dongting Lake and South Dongting Lake: Abundance, Source and Composition. International Journal of Environmental Research and Public Health. DOI: https://doi.org/10.3390/ijerph15102164
[6] Cullen-Unsworth, L. and Unsworth, R. (2013) “Seagrass meadows, ecosystem services, and sustainability”. Environment. https://doi.org/10.1080/00139157.2013.785864
[8] Green, A. et al. (2021). “Historical Analysis Exposes Catastrophic Seagrass Loss for the United Kingdom.” Front. Plant Sci. https://doi.org/10.3389/fpls.2021.629962
[9] Gaylarde, C. et al. (2021). “Plastic microfibre pollution: how important is clothes’ laundering?” Heliyon. https://doi.org/10.1016/j.heliyon.2021.e07105
