The tube alga Vaucheria velutina is the alga of the year 2021 – but for an unpleasant reason: Although this introduced algae was first detected in the north German Wadden Sea in June 2020, it has now spread explosively. The extensive algae carpet catches the silt and clogs the tunnels of the lugworms. As a result, the seaweed is already changing the mudflats and could have unforeseeable ecological consequences.
Alien organisms are increasingly reaching the coasts of the German North and Baltic Seas. Due to the warmer water and the mild winters, these species, which often come from more southern regions, can increasingly settle with us. They include sea animals such as the comb jellyfish, bizarre sunfish, the beach crab introduced from Asia or the Pacific oysters. They are increasingly endangering the native species of the Baltic Sea and the Wadden Sea. Sea algae and other aquatic plants are also introduced into our regions.
New species of algae overgrow huge mud flats
One of these newcomers is now of particular concern. Mudflat researchers around Karsten Reise from the Helmholtz Center for Polar and Marine Research (AWI) discovered a previously unknown species of algae off the island of Sylt in the North Sea last summer and observed its spread over the course of the year. The amazing thing is that the alga now covers an area the size of more than 280 soccer fields. “In my almost 50 years as a mudflat researcher, I have never seen such a rapid spread of a new type of alga,” says Reise. “Near Sylt, she has already conquered huge areas that extend very far out in the mudflats to the horizon and where a spa guest rarely goes.”
To find out which type of algae it is, a team of researchers led by Nataliya Rybalka from the University of Göttingen analyzed gene samples from individual algae specimens that were up to ten centimeters long. To do this, the scientists examined a short characteristic section in the genome of the Sylt algae samples. This enabled a clear assignment to the hose algae, a subgroup of the yellow-green algae (Xanthophyceae). The researchers identified the species as Vaucheria velutina. “The DNA section of the gene called rbcL that we examined contains the blueprint for a protein found in chloroplasts,” explains Rybalka. “In this section, all Vaucheria samples from the Sylt Watt differ from those from nearby salt marshes.”
The algae threads in the tidal flats appear as clones all of the same mother algae, as the researchers suspect. Because tube algae typically multiply in two ways: Both through constricted pieces of the tube and through spores that are formed asexually. The algae reproduce sexually only rarely and under stressful conditions. The pieces of tubing and the spores can easily drift for several kilometers with the tidal currents, which, according to the scientists, could explain the formation of the three algae beds near Sylt. It is possible that the mother organism was brought in with imported Pacific oysters, which are “fattened” in mesh bags in the Sylt Wadden Sea for consumption, says Reise.
Algae are already influencing Wattleben
Due to their explosive multiplication in the mudflats, algae researchers from the German Botanical Society have now chosen Vaucheria velutina as Alga of the Year 2021. In doing so, they also draw attention to the as yet unforeseeable, ecological consequences of the algae invasion: According to Reise and his colleagues, the spread of tube algae is already having an impact on the mudflat ecosystem. So they found out that the alga converts wide sand flats into mud flats. Fine sediment particles that are washed in with the tide get stuck between the algae threads protruding closely from the ground.
In the Sylt Watt, for example, 20 times more algae threads are growing than there are stars in the Milky Way, says Reise. “In the course of just one summer, a soft pad of mud built up that is up to eight inches higher than the surrounding sandy mud flats,” he explains. “Below the surface, the soft silt is deep black and exhales foul-smelling hydrogen sulfide.” According to the scientists, should the silt continue to accumulate in the sand mud flats, this would affect the lugworms in particular.
“If the silt that gets stuck between Vaucheria blocks the tunnels of the lugworms, all life in the mudflat is affected,” the researchers say. “This can even have an impact on the ability of the Wadden Sea to adapt to rapidly rising sea levels in the face of climate change. The Wadden Sea World Natural Heritage is currently changing irreversibly, right in front of our eyes and by what is actually a very small alga. “It remains to be seen whether the algae lawn near Sylt will continue to grow after the growth break in winter and how the algae will continue to change the Wadden Sea.
Source: Georg-August University of Göttingen