Fertilized by tiny things in the roots: The success of the ecologically important seagrass meadows is apparently based on a highly developed symbiosis of these aquatic plants with nitrogen-fixing bacteria, according to a study. The system is similar to the exchange concept that gives some land plants access to atmospheric nitrogen. Thanks to the growth-promoting symbiosis, seagrass meadows can ultimately also make a contribution to binding the greenhouse gas carbon dioxide, the scientists emphasize.
Green underwater landscapes of great importance: Lush seagrass meadows grow on many of the world’s coasts – in total they cover up to 600,000 square kilometers, which roughly corresponds to the area of France. They provide a home for many organisms and, above all, a safe nursery for many fish species. In addition, the carpets of the aquatic plants protect the coasts from erosion and also absorb millions of tons of carbon dioxide every year, which remains bound in the biomass for a long time as so-called “blue carbon”. Due to its far-reaching importance, seaweed has already been the focus of some studies. It is clear that these plants, like all plants, need nutrients. One of the most important is nitrogen. Although the element is also abundantly available in seawater through the solution of molecular nitrogen from the air, plants cannot absorb nitrogen in this form.
Lush despite the lack of nutrients
Until now, it was believed that seagrasses only get the usable nitrogen compounds from the water and sediment. But at most locations, the supply from “normal” sources is severely limited. However, it has long been known that special types of bacteria can transform elemental nitrogen into a biologically usable form via certain metabolic processes. Some of these microbes also enter into a symbiosis with plants: They live in their roots, where they fix the nitrogen in the air and release it to their partners in the form of more usable compounds. In return, the plant supplies the bacteria with carbohydrates and other nutrients. This symbiosis is particularly highly developed in legumes: In root nodules, beans and co even use special hemoglobin substances to ensure a particularly effective oxygen supply to their microbial partners.
“In the case of seaweed, however, it was previously suspected that they only absorb nitrogen from free-living nitrogen-fixing bacteria that live around the roots of the plants in the sea floor,” says Wiebke Mohr from the Max Planck Institute for Marine Microbiology in Bremen. “We are now showing, however, that it is also about the intimate relationship in the truest sense of the word: These bacteria live in the roots of the seaweed. Until now, this form of symbiosis was only known from land plants, ”emphasizes the scientist.
For the study, Mohr and her colleagues examined seaweed of the genus Posidonia, which is widespread in the Mediterranean. The researchers used microscopy techniques to track down the bacteria in the roots of these aquatic plants, also known as Neptune grass, in which individual types of bacteria can be color-coded. In this way, they were able to show that a certain type of microbe occurs both in the interstices and in the plant cells themselves, sometimes in high density. The researchers named these microbes Celerinatantimonas neptuna – after their host, the Neptune grass.
On the trail of symbiosis
The team was able to prove that it is a nitrogen-fixing bacterium using genetic and molecular biological methods. According to this, genes are active in the microbes that are also known to be involved in the binding and processing of elemental nitrogen in other species. The researchers also found evidence of plant adaptations that promote symbiosis. They were also able to directly detect the uptake of fixed nitrogen by the plants. To do this, they supplied test systems with nitrogen gas that contained an excess of the 15N isotope. This tracer nitrogen could then be detected in the plant tissue. The results suggest a form of symbiosis that is similar to terrestrial systems, say the scientists.
When the seagrasses moved from the land into the sea about 100 million years ago, they apparently acquired a marine symbiont that enabled them to improve their nitrogen supply in this habitat. The symbiosis is especially important in summer, because the nutrients in the water and sediment may be sufficient in winter and spring. “The symbionts are then sporadically present in the roots of the plants, but they are probably not very active,” says Mohr. In summer, however, when the sunlight and thus the growth increase, nitrogen quickly becomes scarce. Then the symbionts apparently take over the supply of nutrients. The researchers explain that it is possible that the seagrasses show their greatest growth in the nutrient-poor summer time.
They assume that this is not just something special about the Neptune grass: “Our genetic analyzes indicate that such symbioses also exist in tropical seagrasses,” says Mohr. “The next thing we want to do is examine these new bacteria more closely. We want to isolate them in the laboratory in order to examine more closely how the symbiosis works and how it came about. The search for comparable systems in other regions and habitats will also be exciting, ”says the scientist.
Source: Max Planck Institute for Marine Microbiology, specialist article: Nature, doi: 10.1038 / s41586-021-04063-4