International Journal of Marine Science, 2025, Vol.15, No.6, 313-319 http://www.aquapublisher.com/index.php/ijms 133 exhibit extremely high diversity and flexibility in cyanobacteria and phytoplankton. This "adaptability" precisely reflects their long-term adaptation to CO₂ fluctuations and light changes (Lu et al., 2023; Jaffe et al., 2024). 3.2 Microbial carbon pump and remineralization of dissolved organic carbon (DOC) Primary production is only half the story. Microorganisms also play an indispensable role in the "post-treatment" aspect - especially in the conversion and long-term storage of carbon. The so-called microbial carbon pump (MCP) refers to the transformation of easily decomposable dissolved organic carbon (DOC) into a more stable and refractory form by microbial communities. These "stubborn molecules" can exist in the deep sea for thousands of years, providing unexpected support for carbon sequestration (Hach et al., 2020). During this process, heterotrophic bacteria and archaea are the main forces. They can not only efficiently degrade organic matter but also transform some DOC into long-term inventory through mutual feeding and metabolic collaboration in a "back-and-forth" manner. However, these mechanisms do not work unconditionally - community composition, environmental gradient and substrate source all affect the efficiency of remineralization (Kong et al., 2021). 3.3 Contributions of deep-sea microbes to carbon storage Compared with the surface area, the carbon cycle in the deep sea has always seemed more covert, but in fact, it cannot be ignored. Especially those rare but "resilient" prokaryotic microbial communities, they have a knack for converting unstable DOC into stable and highly oxidizing molecules. This stable DOC is an important component of deep-sea carbon inventory, and the "main force behind maintaining this inventory" is these low-key microorganisms. Furthermore, it is precisely their community succession and diverse metabolic capabilities that support the efficient operation of the microbial carbon pump (MCP) in the deep sea, making the deep sea truly a reliable carbon sink and also an important link influencing global climate regulation (LaBrie et al., 2022). 4 Roles of Marine Microorganisms in the Nitrogen Cycle 4.1 Nitrogen fixation and conversion of atmospheric nitrogen by microorganisms In the past, people always thought that only a few blue-green algae were responsible for nitrogen fixation in the sea. But the actual situation is not that simple. It has now been discovered that, apart from chain-like cyanobacteria, many single-celled cyanobacteria and heterotrophic bacteria can also "grab" nitrogen (N₂) from the air and convert it into ammonia. More importantly, they do not all live in the traditionally believed areas, but are more widely distributed and have more complex environments (Fernandez et al., 2011; Zehr and Capone, 2020). Although this process is quite energy-consuming, it is indeed an important "dark pipeline" for the ocean to replenish nitrogen sources. Especially in the context of denitrification and anaerobic ammonium oxidation consuming a large amount of nitrogen, this replenishment becomes even more crucial. Without these nitrogen-fixing microorganisms, Marine primary production might have run out of food long ago. 4.2 Microbial regulation of nitrification and denitrification processes Nitrogen is running around in the ocean. In fact, it is the microorganisms at both ends that are "fiddling" - on one side is nitrification, which oxidizes ammonia into nitrate, and on the other side is denitrification, which reduces nitrate back to nitrogen gas. It sounds like "reverting to the original state". But things in between were not so smooth. The driving forces of these two processes are ammonia-oxidizing archaea, nitrite-oxidizing bacteria, and various denitrifying bacteria and archaea. These microorganisms are distributed in areas with thin oxygen, such as sediments or Marine "oxygen zones" (Pajares and Ramos, 2019). These processes are greatly influenced by the environment. The amount of oxygen and organic matter will both affect their "working efficiency". In addition, incidentally, these two mechanisms also release nitrous oxide, a potent greenhouse gas, during the "operation" process (Wang et al., 2025). To understand exactly how nitrogen circulates in the sea, the spatiotemporal "coordination degree" of these two processes is an unavoidable issue (Muck et al., 2019). 4.3 Role of anaerobic ammonium oxidation (Anammox) in deep-sea nitrogen loss In those oxygen-deficient or even oxygen-free areas of the ocean, a portion of nitrogen will "disappear out of thin air", and the driving force behind this is anaerobic ammonia-oxidizing bacteria. Their operation method is rather "straightforward" - they directly combine ammonium and nitrite to form nitrogen gas and then volatilize it. This
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