International Journal of Marine Science, 2025, Vol.15, No.6, 313-319 http://www.aquapublisher.com/index.php/ijms 134 kind of path is particularly active in deep-sea sediments. Even in some areas, they contribute half of the nitrogen loss. Moreover, these types of bacteria not only have a large number but also strong adaptability. Many of them even hide in extreme environments and work quietly (Wang and Wang, 2025). For this reason, their position in the global nitrogen cycle cannot be ignored, and it also makes us rethink how much role the deep sea plays in the nitrogen balance of the entire earth. 5 Roles of Marine Microorganisms in the Sulfur Cycle 5.1 Sulfur-oxidizing bacteria and the oxidation of hydrogen sulfide The microorganisms in Marine systems are "well aware" that H₂S itself is toxic. Sulfur-oxidizing bacteria (SOBs) have been striving to convert these reduced sulfides, such as hydrogen sulfide, into milder and more oxidized sulfates (SO₄²⁻) - a form of sulfur that is both stable and practical for Marine ecosystems. However, it is not only the chemoautotrophic SOBs mentioned in textbooks that are involved in this work. Those often overlooked heterotrophic prokaryotes, especially certain proteobacteria, are also "very active" in the sediment. They use various enzymes, such as SQR (sulfide: quinone oxidoreductase) and PDO (persulfide dioxygenase), to gradually oxidize the sulfides. During this process, it will pass through "intermediate stations" such as zero-valent sulfur and sulfites, and ultimately generate sulfates that do not volatilize easily, avoiding H₂S directly escaping into the water body (Wang et al., 2019). Moreover, among these different types of microorganisms, sulfur intermediates can be transferred to each other. This "relay race" mechanism makes sulfur oxidation in sedimentary environments more efficient and stable (Figure 1) (Alamoudi et al., 2025; Chen et al., 2025; Ogola et al., 2025). Figure 1 Chemocline of Kebrit Deep BSI and associated Campylobacterota diversity (Adopted from Alamoudi et al., 2025) 5.2 Sulfate-reducing bacteria and metabolic pathways in anaerobic environments In anoxic environments, sulfate-reducing bacteria (SRB) always play a key role. This type of anaerobic bacteria can reduce sulfates to sulfides, completing the "final step" of organic matter decomposition. Especially in Marine sediments, this step is almost the end of the mineralization process. They are not confined to a certain group. In fact, they can also be found in various uncultured lineages, especially in surface sediments and deeper anoxic zones. SRB also has a rather diverse diet, with a wide variety of substrate species. Coupled with the fact that they often form biofilms in groups, this enables them to better withstand environmental changes and also participate in beneficial reactions such as heavy metal precipitation. Of course, sometimes it can also bring troubles like corrosion (Saxena et al., 2025). However, SRBS do not act alone. They often mix with methanogens and sulfur-oxidizing bacteria to jointly shape the appearance of the entire sulfur cycle and also affect the ecological stability of the system (Jørgensen et al., 2019). 5.3 Dimethyl sulfide (DMS) formation and its climatic implications Regarding DMS (i.e., dimethyl sulfide), it is often linked in discussions about climate regulation. This gas comes from DMSP, a small molecule synthesized by phytoplankton. After being released into seawater, it is "taken over" by some Marine bacteria and transformed into DMS. Once DMS volatilizes, it may participate in the formation of
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