High Nitrogen Loads from Land & Air Foster World’s Largest Green Tide

Researchers determine fast-growing algae can also adjust their photosynthetic metabolism to increase growth rate

The macroalgae that make up the world’s largest seaweed bloom -- in the Yellow Sea off the coast of China -- make use of an unusually high supply of nitrogen discharged to the sea by multiple human activities, according to a recent study in Scientific Reports by MBL scientists and collaborators.

The Green Tide produced by these seaweeds grows at a remarkably fast rate, a phenomenon that the team set out to explain. The major seaweed species in the Yellow Sea, Ulva prolifera, can increase its mass by 25 percent each day. In the course of three weeks or so, the Yellow Sea's Green Tide extends its floating canopy to an area equivalent to that of Lake Michigan.

A team including MBL scientists Javier Lloret and Ivan Valiela and research assistants Daniella Hanacek and Kelsey Chenoweth investigated what nitrogen sources could possibly support the remarkably rapid growth of these seaweeds, and, second, what physiological adaptation might make the surprisingly fast growth possible.

Dongyan Lui and colleagues at East China Normal University collected samples of the algae from the Yellow Sea. The nitrogen and carbon signatures of the algae were measured In the Stable Isotope Laboratory in the MBL’s Ecosystems Center. The results showed that most of the nitrogen taken up by the algae originated in human waste, with lesser contributions from fertilizers leached from over-fertilized cropland, and from atmospheric deposition. The nitrogen loads entering the Yellow Sea created an unusually highly enriched water column, where seaweeds could find ample nitrogen supplies for growth.

The isotopic analysis also revealed that U. prolifera seemed to adjust its carbon acquisition mechanisms to grow faster in waters where nitrogen was abundant, using enzymes and biochemical pathways most frequently found in higher plants.

“They go from C3 metabolism—the most prevalent carbon fixation mechanism among plants and algae—to C4 metabolism, a more effective and active process characteristic of fast-growing, more productive plants such as corn and crabgrass,” says Valiela, MBL Distinguished Scientist and first author on the paper. The team plans to further explore this area in future studies.

The massive algae blooms can lead to significant environmental problems, many still poorly defined. These include shading of the marine water column below the Green Tide canopy, lowering oxygen in water when the seaweed canopy senesces and sinks, and massive stranding of seaweeds onshore that rot and impair use of coastal beaches and shallow waters.

Volunteer and military teams set out to clean up the algae each summer, work that has to be repeated each year that seaweed strandings take place. In addition, the large volumes of salty algal mass have to be dealt with on shore. Reducing algae in the Yellow Sea will depend on developing effective means for managing sources of nitrogen, thus reducing supply of its limiting nutrient.

Citation:

Valiela, Ivan et al. (2018) Stable isotopic evidence of nitrogen sources and C4 metabolism driving the world’s largest macroalgal green tides in the Yellow Sea. Scientific Reports 8: Article number 17437