Bacterial communities on seagrass leaves suppress waterborne pathogens

Seagrasses are very special: they are the only flowering plants that have returned to sea from land. They are also known as the “lungs of the oceans” because of their ability to photosynthesize. And with the exception of Antarctica, they can be found on all continents, where they form extensive underwater meadows that generate and sustain healthy coastal regions.

Great ecological and economic importance

Seagrass meadows are key ecosystem engineers that directly benefit humans and animals, hence are of enormous ecological and economic importance. They are spawning grounds for economically important fish, hiding places for juvenile fish and habitats for mussels, snails and crabs, making them one of the most productive and diverse ecosystems on earth, along with coral reefs and rainforests. They protect our coasts by stabilising the sediment. They also store carbon dioxide very quickly and effectively.

Seagrass meadows as natural water purifiers

A few years ago, another remarkable ecosystem service of seagrasses was discovered: seagrass meadows reduce the load of pathogenic bacteria in the water around them. A 2017 study showed that the relative abundance of harmful bacteria, including human fecal bacteria and pathogens dangerous to marine animals and humans, was significantly (50%) lower in Indonesian seagrass meadows than in the water outside the meadows. Subsequent studies, including one at GEOMAR, have confirmed the reduction of pathogens such as Escherichia coli, enterococci, Salmonella and Vibrio species in the vicinity of seagrass beds.

Scientists of the Research Unit Marine Natural Product Chemistry at the GEOMAR Helmholtz Centre for Ocean Research Kiel have been investigating multiple mechanisms behind this sanitation effect for several years. The results of the first part of their study have recently been published in the journal Science of the Total Environment.

How do seagrasses combat pathogens?

“The elimination of pathogens from the water is a very complex phenomenon involving physical, (micro)biological and chemical mechanisms" says Dr. Deniz Tasdemir, professor of marine natural product chemistry and senior author of the study. The researchers started first analysing the cultivable microbiome of Zostera marina, a common seagrass species in the Baltic Sea, and natural molecules they produce. To do this, they isolated almost 90 bacteria and fungi from the surface and internal tissues of the seagrass leaves (and roots) and tested their extracts for antibiotic activity. These tests were carried out against a large group of aquatic, human and plant pathogens, including Vibrio species, which can cause serious diseases and even death when transmitted to humans by raw or undercooked seafood, or through skin damage during recreational activities.

This study showed that particularly the bacteria from healthy leaf surfaces have strong, broad-spectrum antibiotic activity, in some cases even outperforming commercial antibiotics. “This confirmed our hypothesis,” says Prof. Tasdemir. In addition to few known antimicrobial compounds, the team also discovered the presence of many new ones in these bacteria. These new molecules will now be isolated, in other words chemically purified, their chemical structures will be identified and their potential as future marine antibiotics will be assessed. “This is only the tip of the iceberg for us. We now heavily work, with an international team, on other chemical and microbiome-related mechanisms and how they may contribute to the hygiene effect of seagrasses in the laboratory and in the ocean settings”, says Prof. Tasdemir.

Antibiotics from the sea: the potential of the seagrass microbiome

The climate change-related ocean warming is increasing the load of pathogens, such as Vibrio species, in coastal waters during summer months. This is also a great public health concern for German Baltic Sea, as death is being reported among holiday makers. Therefore, the protection and restoration of seagrass meadows is essential for the health of oceans and humans more than ever. On the other hand, the seagrass microbiome holds great potential for the discovery of new antibiotics for other human infections, which is of enormous importance in the fight against rising antibiotic resistance.

Original publication:

Tasdemir, D., et al. (2024). Epiphytic and endophytic microbiome of the seagrass Zostera marina: Do they contribute to pathogen reduction in seawater? Science of the Total Environment, 908, 168422.