When it comes to absorbing carbon dioxide, many people think of forests. In fact, most of the carbon dioxide on earth is absorbed by the ocean. The ocean covers more than 80% of the earth's area and stores about 90% of the world's carbon dioxide. It is the largest active carbon pool on the earth. Marine vegetation absorbs fixed carbon dioxide through photosynthesis, converts it into stable organic matter and stores it, forming a unique marine carbon pool called "blue carbon". Among them, the lush seagrass bed widely distributed in the shallow sea from temperate to tropical areas is one of the main forces of marine carbon sequestration. However, this unique ecosystem has been puzzling scientists: where do the nutrients of seagrass beds come from? If you want a good harvest, you need to supplement fertilizer, and the seagrass bed also needs nutrient support. However, most seaweeds grow in the shallow sea environment with lack of nutrients, especially nitrogen, which is the inorganic nutrient with the highest demand for plant growth. In such a barren land, seaweed can "cook without rice" and flourish. It must have an unknown "dry rice secret". Pick up the leak, find a partner to rub nitrogen Plant "dry rice" has a clever trick Before introducing how seagrass gets "nitrogen" in shallow water environment, let's take a look at how most green plants in nature get nitrogen. Although the content of nitrogen in the surface is high, most of them are inactive gaseous nitrogen, which can not be gnawed by plants. It is necessary to convert gaseous nitrogen into specific nitrogen-containing substances before plants can "eat". This process of "fixing" gaseous nitrogen is called "nitrogen fixation". Among them, some prokaryotic microorganisms (such as bacteria) can complete "biological nitrogen fixation", and these microorganisms that can fix nitrogen are collectively referred to as nitrogen fixing bacteria. Biological nitrogen fixation is the main way to retain nitrogen in the biosphere under natural conditions. In the face of such valuable resources, plants have many ways to use them: 1. Secretly pick up and leak the nutrition of "free nitrogen fixing bacteria" Among nitrogen fixing bacteria, some free-living bacteria can be self reliant: fix nitrogen by themselves, find organic food by themselves, "live like a team". Previous studies have found that there are many such free nitrogen fixing microorganisms in the sediment of seagrass bed. Therefore, people think that seagrass mainly "picks up leakage" from the environment and absorbs some "leftovers" of free nitrogen fixing bacteria. However, such scattered nitrogen sources do not seem to feed the lush seagrass bed, and the question has not been completely answered. △ picture source: references [5] 2. "Symbiotic nitrogen fixation model" with nitrogen fixing bacteria The efficiency of self operated nitrogen fixing bacteria is not high, and the amount of nitrogen fixation is also low. It is difficult for plants to meet them only. Therefore, some plants on the land have developed a further close cooperative relationship with some fungi: the roots of plants produce a specialized structure, which allows bacteria to "live" in their own body like a "house", plants feed bacteria, and bacteria fix nitrogen for plants. The most well-known symbiotic nitrogen fixation model is legumes and rhizobia. In root nodule symbiosis, root cells are closely contacted and incorporated into root nodule nitrogen fixing bacteria to form a nodular and efficient nitrogen fixing plant. △ nodule of soybean root: many rhizobia are contained in each nodule, forming an efficient terrestrial nitrogen fixation system (picture source: reference [8]) 3. Loose cooperation between plants and "endophytes" Legumes develop nodules in order to cooperate with rhizobia, which is a difficult ability. Most plants do not have this skill. Therefore, some plants choose a simpler way to accommodate nitrogen fixing bacteria: "open" the permission for bacteria to invade the root and let bacteria inhabit the cell gap or cell wall in the root. However, plants will not specially change the root shape and produce special structure. These microorganisms living in plants are collectively referred to as endophytes. Some non leguminous plants (such as sugarcane, wheat [2] and agave [3]) can recruit endophytic bacteria with nitrogen fixation ability, which not only meets the acquisition of nitrogen, but also does not need to prepare a special "house" for bacteria to maintain a loose joint relationship, which is simple but not simple. △ plant endophytic bacteria. This picture shows the intracellular symbiotic bacteria observed in the roots of Agave tequilana, and the bacteria has nitrogen fixing activity. (image source: reference [3]) The cooperation between terrestrial plants and microorganisms has a long history. When the earliest plants evolved from algae in the ocean to land plants, they could not do without the help of microorganisms. After that, several small families of land plants chose to return to the ocean in the history of evolution, and gradually adapted to the marine environment. This kind of grass like flowering plants that can live in the sea are collectively referred to as "seagrass". All kinds of seaweed grow on the shallow beach and look like pastures on land, so it is known as "seaweed bed". Since terrestrial plants will form a close symbiotic relationship with nitrogen fixing bacteria, do seaweed ancestors who used to be terrestrial plants also have similar "social skills"? Seaweed: "the root gate is wide open", welcome nitrogen fixing bacteria to settle in By studying the common Posidonia Oceanica in the Mediterranean, scientists found the clue to the mystery of the source of nitrogen fertilizer in the seagrass bed: seagrass also has a symbiotic nitrogen fixation system similar to land plants [4]. Oceania Nepenthes is widely distributed in the Mediterranean and is a local landmark vegetation. Because the annual carbon fixation efficiency of Poseidon grass bed is higher than that of Amazon rainforest with the same area, its valuable ecological and cultural value makes it listed as a world heritage by UNESCO [6]. △ grassland of Haishen grass (picture source: reference [6]) Professor Marcel Kuypers of the school of marine microbiology of Max Planck Institute in Germany tracked the distribution law of nitrogen in Neptune grass and found that Neptune grass roots can absorb gaseous nitrogen, and the fixed nitrogen will transfer to the aboveground, which is particularly obvious in the summer growth season. It can be seen that there are nitrogen fixing bacteria in the roots of Haishen grass! It is an unprecedented discovery that the symbiotic nitrogen fixation system of land plants also exists in the marine environment with completely different environmental properties. △ Figure 1: the fixed content of microbial nitrogen fixation in roots increased significantly in July and August, which matched the peak season of plant growth. Fig. 2: the change of nitrogen transfer content detected in plant leaves matches with the nitrogen fixation content of roots, and it transfers more when the nitrogen fixation content is high (July). (image source: adapted from reference [4]) Scientists then identified a new bacterial species in the root of ocean Neptune grass: Candidatus celerinantimonas neptuna (ca. C. neptuna) [7]. The bacteria is significantly related to the overall nitrogen fixation activity trend of the plant, and has a complete set of nitrogen fixation gene groups, which can perform complete nitrogen fixation function. Moreover, the "cooperative trade" between bacteria and seaweed also fully follows the code of goods exchange of land nitrogen fixation symbiosis: bacteria hand in nitrogen and plants hand in sugar. The cooperation mode of the two systems is the same. △ conceptual diagram of interaction between endosymbiotic nitrogen fixing bacteria and seagrass The left side of the figure is a cross-sectional view of the root, in which the pink is nitrogen fixing bacteria. Bacteria mainly colonize in the cortex of plant roots. Red arrows indicate that bacteria absorb nitrogen and then provide ammonium salts to plants; Black arrows indicate that plants provide sugar and oxygen to bacteria. (image source: adapted from reference [4]) Under the fluorescence microscope, we can determine the living position of bacteria: ca. C. neptuna is distributed in the root of Neptune grass, and the distribution position is closely related to the change of nitrogen concentration in the root. Ca. C. neptuna is also the dominant microorganism in the roots of Poseidon grass in the rapidly growing summer; The bacteria could hardly be found in other sea grass roots that did not show nitrogen fixation activity. △ figure D on the left shows the microorganisms in the root shown by fluorescence microscope. There are a large number of nitrogen fixing bacteria (Pink / blue) in the gap of plant cell wall (green). Figure E on the right shows the tracer of nitrogen isotope concentration. The more yellow the color, the more nitrogen, indicating that the more active the nitrogen fixation process is. Note the consistent correlation between bacterial clusters and nitrogen concentrations shown by the white arrows across the two figures. (image source: adapted from reference [4]) The gene analysis of ca. C. neptuna also reveals that it is fully prepared for endosymbiotic life. For example, they can take the initiative to follow the steps of plants, recognize the signals given by plants, "shake hands and make peace" with the plant immune defense system, and degrade the pectin in the cell wall to create shelter. Many information shows that this bacterium has a very similar lifestyle to nitrogen fixing bacteria on land, and its endophytic characteristics are found in marine microorganisms for the first time. What can we learn from symbiotic nitrogen fixation between bacteria and seaweed? The nitrogen fixation cooperation between Poseidon grass and ca. C. neptuna is like a replica of the cooperative relationship on land. The history of evolution always has echoes: maybe when Poseidon grass ancestors returned to the sea about 100 million years ago and were "lonely and helpless" away from their terrestrial microbial partners, the ancestors of ca. C. neptuna in the sea extended a helping hand to it. Completely different species groups have developed the same mode of cooperation under similar difficulties. This pair of new friends have opened up territory and written a new chapter under the sea. The discovery of this new symbiotic nitrogen fixation system of marine bacteria seaweed also brings more opportunities and challenges. For example, how do seaweed recognize and accept this bacterium? Do other seagrasses (such as Zostera marina, which is more widely distributed in China's sea areas) have similar symbiotic partners? What is the role of endogenous nitrogen fixing bacteria in promoting the evolution of seagrass into the sea? How did the ancestors of seagrass complete the transformation from terrestrial symbiotic bacteria to marine symbiotic bacteria? The answers to these questions are waiting for future scientists to reveal them one by one. In addition to the in-depth academic discussion, the discovery of this symbiotic bacteria also has more value for protecting the threatened seagrass bed ecosystem. At the same time, we may be able to develop some microbial "bacterial agents" based on these bacteria to consolidate the "blue carbon" inventory of seagrass bed and provide a new low-cost path to alleviate global change. (Xinhua News Agency)