Lush seagrass meadows carpet the floor of the Mediterranean Sea off the Italian island of Elba. Seagrasses help form an important ecosystem in coastal environments, and they absorb a significant amount of carbon that’s eventually buried deep under the sediment. But researchers have long wondered how these marine grasses, which often grow in waters with sparse resources, obtain a key ingredient for any plant’s success: nitrogen. 

“There’s a nutrient-poor environment on one hand, and a very large amount of biomass being created on the other hand,” says biogeochemist Wiebke Mohr of the Max Planck Institute for Marine Microbiology in Bremen, Germany. “So that’s where the idea started, that there has to be some source [of nitrogen].”

The only organisms that can “fix” nitrogen—that is, pull it from the environment and combine it with other elements into a form that other living things can use—are microbes. On land, many plants, such as legumes, form an intimate symbiotic relationship with nitrogen-fixing bacteria inside their roots. To find out if marine plants do as well, Mohr’s colleagues plucked whole Neptune grass (Posidonia oceanica) from Fetovaia Bay off the southwest coast of Elba and brought them to the lab. 

First, they threaded seagrass leaves through a little hole in a latex glove and placed the plant’s roots in a bottle, sealing the glove over the rim. They injected water containing a nitrogen tracer through a port in the glove and covered the bottle with a black plastic bag to mimic the darkness of burial under sediment. The researchers then submerged the experimental plants in a saltwater aquarium. A day or two later, Mohr and her team dissected the plants and measured how much of the nitrogen tracer had been fixed within the roots. “This is an amazing system that they made,” says University of California, Davis, seagrass microbiome researcher Jonathan Eisen.

In plants harvested during their summer growing season, Mohr’s team saw newly fixed nitrogen in the roots and found that a fifth of it moved to the leaves within 24 hours. To identify the nitrogen-fixing microorganism, the researchers analyzed microbial sequences in the seagrass roots and found a prime suspect. “Nearly every time we measured higher rates of nitrogen fixation, we also saw higher abundances of this [one] microorganism,” says Mohr. When they tested plants collected in non-summer months, lower levels of newly fixed nitrogen in the roots also correlated with lower levels of this microbe. The bacterium’s genome suggested it was a new species, which the team dubbed Candidatus Celerinatantimonas neptuna.

NITROGEN SUPPLIER: A newly discovered bacterial species, Candidatus Celerinatantimonas neptuna, lives in the roots of a marine plant called Neptune grass. Ca. C. neptuna takes in nitrogen from the environment and converts it to ammonium, which it uses to make amino acids. The bacteria share both ammonium and amino acids with the seagrass, which provides sugar and GABA in exchange. WEB | PDF

Using fluorescently labeled probes in cross-sections of seagrass roots, the team found the bacterium both within and in between root cells. The team also showed that Ca. C. neptuna genes for nitrogenase, the enzyme that facilitates the fixation process, were heavily transcribed when nitrogen was being fixed, and that the nitrogen tracer became incorporated into the bacterium’s amino acids. Further analyses showed that the seagrass provides sugar and the amino acid GABA to the bacteria, much as some land plants do.

“What makes this a good paper is they did everything . . . to try and characterize this particular system,” says Eisen, calling the study “seriously comprehensive.” He adds that he hopes a better understanding of these plants will improve conservation and restoration of coastal ecosystems. “Nitrogen acquisition is a fundamentally important part of the functioning of these communities.”

W. Mohr et al., “Terrestrial-type nitrogen-fixing symbiosis between seagrass and a marine bacterium,” Nature, 600:105–109, 2021.