BiodiversityNew habitat for countless thousands of species

Filter for pathogens and pollutants

Fish populations living in deeper waters also benefit from seagrass meadows – partly because their future prey grow among their leaves. But also because, in shallow waters, the grass carpets filter roughly 50 percent of all pathogens, as well as nutrients, sediments, pollutants and microplastic particles – transported there by rivers or the surface water – out of coastal waters. In other words, the meadows guarantee consistently high water quality.

Seagrasses absorb waterborne pollutants like heavy metals through their roots and leaves, primarily storing them in their network of roots. When this absorption and storage take place on a regular basis, the pollutants can accumulate in the seagrasses – a process that is not only harmful for the plants themselves but can also affect the health of the animals that feed on them.

Thankfully, seagrasses have developed their own means of combating various bacterial pathogens and marine fungi. These bioactive substances allow them to cut the amount of pathogens in the water in half, which protects their own health and contributes to that of nearby biotic communities. These defense mechanisms have been proven e.g. to benefit coral reefs in the immediate vicinity of the underwater meadows.

Nursery for many key edible fish species

The young of many key edible fish species grow amid the dense leaves of seagrass meadows. These include the Pacific herring, Atlantic cod, striped red mullet, gilt-head bream, and the heavily fished Alaska pollock, also sold in Europe as Alaska saithe. Other fish species frequent the meadows on the search for food – like rabbitfishes. Of the 25 most important edible fish species in the world, more than a fifth rely on seagrass meadows at some point in their lifecycle. Accordingly, the loss of seagrass-covered areas often leads to a decline in local fishery catches.

Vital support for shelled fauna

Corals, clams and crustaceans, as well as fauna with calcium carbonate shells or skeletons, also feel right at home in seagrass meadows. By intensively absorbing carbon dioxide during the day, the underwater meadows change the water chemistry and reduce, at least temporarily, the extent of climate-related acidification. That means the fauna species mentioned above suffer less from the acidification of the ocean and have to expend less energy repairing acid damage to their shells or skeletons. As such, healthy seagrass meadows not only offer safe havens for these calcifying creatures but also bolster their resistance to the worsening acidification of the ocean – which represents an invaluable contribution to preserving marine biodiversity and its host of services for human beings.

A select retinue of microorganisms

Just like land-based plants, seagrasses also offer a home for countless microorganisms. These chiefly include bacteria and archaea, but also fungi, microalgae and other single-celled organisms with nuclei. According to some estimates, every square centimeter of leaf is home to roughly one million single-celled organisms; in the roots, the number is between 100,000 and one million.

Researchers have used DNA sequencing to assess the biodiversity of these microorganisms. What they’ve determined to date is that the community of microorganisms living on or in the seagrasses clearly differs from communities in the water column or elsewhere on the ocean floor. That would indicate that seagrass meadows maintain their own retinue of single-celled organisms, consisting both of generalists and species adapted to specific seagrasses.

Whether or not these microorganisms are vital for seagrasses, making them an essential prerequisite for the successful reintroduction of large meadows, is one of the most important research questions addressed by the SeaStore project. In addition, the participating marine biologists are working to determine where and how seagrasses acquire their retinues of single-celled organisms, and whether the latter’s makeup changes over time.

What we already know: seagrasses certainly do make selections, e.g. by producing substances to fight off potentially harmful microorganisms and releasing carbon and nutrients like phosphorus, nitrogen and sulfur to attract useful bacteria.

These single-celled helpers support the seagrasses through fixing phosphorus and nitrogen, and with the mineralization of organic nutrients. They also protect the plants from oxidative stress and pathogens, detoxify certain nutrient components, and produce “plant hormones” for seagrasses, which promote growth, flowering, and the formation and germination of seeds.