Introduced Alga Now Transforming the Wadden Sea

The alga now covers an area equivalent to more than 280 football fields. “In nearly 50 years working as a researcher in the Wadden Sea, I’ve never seen a new alga spreading that fast,” says Reise, who previously led the AWI’s Wadden Sea Station on Sylt and used GPS to measure the extent of algal cover. “On Sylt, it has already conquered an enormous area, which reaches far out into the tidal zone and beyond, where vacationers rarely venture.”

Whether or not the algal mats near Sylt will continue to grow after the winter, and how this will change the ecosystem of the Wadden Sea, remains to be seen. But even this year, it became apparent that the alga was transforming broad stretches of sandflats into siltflats: fine silt particles, carried to shore with the tide, get trapped between the alga’s closely packed filaments, which protrude from the mud. According to Reise’s calculations, there are now 20 times more algal filaments growing in the mudflats near Sylt than there are stars in the Milky Way. “In the course of just one summer, a layer of soft silt has accumulated that is up to 20 centimetres higher than ambient sandflats. Beneath the surface, the soft silt is pitch black and emits noxious hydrogen sulfide.”  

Rybalka, an experimental phycologist in charge of the University of Göttingen’s collection of algae cultures, analysed a short but characteristic segment of the genome in the samples from Sylt. This allowed her to clarify the species identity of the six to ten centimetre long filaments. “The DNA segment from the rbcL gene that we examined contains the blueprints for a protein found in chloroplasts,” she explains. “In terms of this segment, all Vaucheria from the mudflats differ from those gathered in nearby salt marshes.” Accordingly, the experts currently assume that the alga stem from the same mother alga and constitute a newly introduced clone. “We were only able to determine this because, in the DFG Priority Programme Taxon-OMICS, we’re working to classify new algae species and to analyse the diversity, distribution and interrelations in the algae group Xanthophyceae (yellow-green algae), which also includes the species complex of Vaucheria velutina,” says Rybalka, who introduced new analytical methods for this purpose in her project.

Algae from the Vaucheria group are among the oldest life forms on our planet; geologists have found imprintsin rock dating back one billion years. What’s unique about them is their potential to reproduce at astonishing speed. Reise believes the alga may have been inadvertently introduced with imported Pacific oysters, which were brought to the Wadden Sea near Sylt and are kept in mesh bags for fattening before being sold as a delicacy. The native oyster species had been driven to regional extinction in the previous century.

If the silt accumulation in the sandflats continues unchecked, lugworms will likely be hardest hit. Taken together, every year the lugworms rework enough sand in the Wadden Sea to cover the entire city of Hamburg 15 metres deep. “When the silt trapped between the Vaucheria filaments blocks the feeding funnel of lugworm burrows, all life of the tidal flats will be affected,” Rybalka and Reise warn. “This could even change the potential of the Wadden Sea in adjusting to the rapidly rising sea in the wake of global warming. The World Heritage Site Wadden Sea is now being irrevocably changed, right before our eyes, by a tiny alga.”

You can find detailed information on this topic at the website of the Phycology Section of the Germany Society for Plant Sciences: www.dbg-phykologie.de/alge-des-jahres/alge-des-jahres-2021.