Molecular Microbiology Research 2024, Vol.14, No.4, 198-207 http://microbescipublisher.com/index.php/mmr 201 3.2.2 Ecological significance in marine environments Denitrification and anammox processes are ecologically significant as they contribute to the removal of fixed nitrogen from marine ecosystems, thus regulating nitrogen availability and primary productivity. These processes are particularly important in oxygen minimum zones (OMZs) and other anoxic environments where they can account for substantial nitrogen loss (Lam and Kuypers, 2011). In subterranean estuaries and other transition zones, denitrification and anammox activities are closely linked to redox gradients and the availability of electron donors and acceptors (Wu et al., 2021). 3.2.3 Interactions with other biogeochemical cycles Denitrification and anammox processes are interconnected with other biogeochemical cycles, particularly the carbon and sulfur cycles. For instance, denitrifying anaerobic methane oxidation (DAMO) links the nitrogen and carbon cycles by coupling methane oxidation to denitrification, thereby mitigating greenhouse gas emissions (Harb et al., 2021). Additionally, some denitrifiers can use reduced sulfur compounds as electron donors, linking the nitrogen and sulfur cycles (Suter et al., 2020). 3.3 Ammonia oxidation and nitrite reduction Ammonia oxidation, the first step of nitrification, is carried out by ammonia-oxidizing bacteria (AOB) and archaea (AOA), which convert ammonia to nitrite. This process is crucial for the subsequent steps of nitrification and denitrification (Pajares and Ramos, 2019; Xia et al., 2019). Nitrite reduction, on the other hand, involves the conversion of nitrite to nitrogen gas or other nitrogenous compounds through processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA) (Xiang et al., 2020). These processes are facilitated by a diverse array of microorganisms, including autotrophic and heterotrophic denitrifiers, which are adapted to various environmental conditions. 4 Role of Marine Microorganisms in the Sulfur Cycle 4.1 Sulfate reduction and sulfur oxidation Marine microorganisms play a crucial role in the sulfur cycle through processes such as sulfate reduction and sulfur oxidation. Sulfate-reducing bacteria (SRB) are key players in the anaerobic oxidation of methane (AOM), coupling the reduction of sulfate to sulfide with the oxidation of methane, thus linking the carbon and sulfur cycles. This process is particularly significant in anoxic marine sediments where SRB collaborate with anaerobic methanotrophic archaea (ANME) to mitigate methane emissions (Nie et al., 2021). Additionally, sulfide produced by SRB can be oxidized back to sulfate by sulfide-oxidizing autotrophic denitrifiers, which use nitrate or nitrite as electron acceptors, thereby preventing sulfide accumulation and maintaining the balance of the sulfur cycle. 4.2 Dimethylsulfoniopropionate (DMSP) production and degradation Dimethylsulfoniopropionate (DMSP) is a pivotal organosulfur compound produced in large quantities by marine phytoplankton and some bacteria. DMSP serves multiple ecological functions, including acting as an osmolyte, antioxidant, and cryoprotectant (Zhang et al., 2019; Gregory et al., 2020). Marine bacteria degrade DMSP through two main pathways: demethylation, which channels sulfur into the microbial food web, and cleavage, which releases dimethyl sulfide (DMS) into the atmosphere. The cleavage pathway is particularly significant as DMS is a climate-active gas that contributes to cloud formation and climate regulation The production and degradation of DMSP are influenced by various factors, including the concentration of DMSP and the presence of DMSP-producing microalgae, which create microscale hotspots that enhance bacterial DMSP degradation (Gao et al., 2020; O'Brien et al., 2022). 4.3 Sulfur-metabolizing microbes and climate regulation Sulfur-metabolizing microbes, particularly those involved in the production and degradation of DMSP and DMS, play a critical role in climate regulation. DMS produced from DMSP cleavage is ventilated to the atmosphere, where it undergoes oxidation to form sulfate aerosols, which can influence cloud microphysics and regional climate (Jackson and Gabric, 2022). The microbial oxidation of DMS to dimethylsulfoxide (DMSO) represents a
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