Molecular Microbiology Research 2024, Vol.14, No.4, 198-207 http://microbescipublisher.com/index.php/mmr 199 2 Role of Marine Microorganisms in the Carbon Cycle 2.1 Carbon fixation and primary production Marine microorganisms play a pivotal role in carbon fixation and primary production, which are fundamental processes in the global carbon cycle. Prokaryotic nanoorganisms, including those from the CPR and DPANN lineages, have been identified as significant contributors to carbon fixation across the world's oceans. These microorganisms possess genetic components for various carbon fixation pathways, suggesting their substantial impact on the carbon cycle (Lannes et al., 2020). Additionally, the presence of diverse microbial communities in marine environments, such as the Marinifilaceae family, highlights their role in organic matter mineralization and carbon cycling. These bacteria are involved in the degradation of plant and animal detritus, contributing to the ocean's carbon cycle (Li et al., 2022). 2.2 Microbial degradation of organic matter The degradation of organic matter in marine environments is a critical process for nutrient recycling. In the Black Sea's sulphidic zone, a diverse microbial community, including Cloacimonetes and Marinimicrobia, has been identified as key players in the degradation of dissolved organic matter (DOM) and protein extracts. These microorganisms interact with other groups like Deltaproteobacteria and Chloroflexi Dehalococcoidia to facilitate organic matter mineralization under anoxic conditions (Suominen et al., 2019). Furthermore, the Marinifilaceae family has been shown to dominate in organic-matter-rich environments, where different subgroups specialize in degrading various macromolecules such as polysaccharides, lignin, and proteins (Li et al., 2022). This metabolic versatility underscores the importance of microbial communities in the breakdown and recycling of organic carbon in marine ecosystems. 2.3 Methanogenesis and methanotrophy Methanogenesis and methanotrophy are crucial processes in the marine carbon cycle, particularly in anoxic sediments. In marine sediments with high sulfate concentrations, sulfate reduction is the dominant process for organic carbon oxidation, often surpassing methanogenesis. However, in environments with lower sulfate levels, methanogenesis becomes more significant. Methanogenesis in marine systems typically occurs using noncompetitive substrates like methylamines, which are not utilized by sulfate reducers. Archaea play a vital role in these processes, with methanogenesis and anaerobic methane oxidation being performed exclusively by anaerobic archaea. These archaea contribute to the carbon cycle by converting inorganic carbon sources into methane and subsequently oxidizing it, thus influencing greenhouse gas emissions (Offre et al., 2013). The involvement of ultrasmall prokaryotes in methane degradation further highlights the complexity and diversity of microbial communities in regulating methane dynamics in marine environments (Lannes et al., 2020). 3 Role of Marine Microorganisms in the Nitrogen Cycle 3.1 Nitrogen fixation and nitrification Marine microorganisms play a crucial role in nitrogen fixation, the process of converting atmospheric nitrogen (N2) into ammonia (NH3), which is essential for primary productivity in the ocean. This process is primarily carried out by diazotrophic bacteria and archaea, including cyanobacteria such as Trichodesmiumand symbiotic unicellular cyanobacteria like UCYN-A. These microorganisms are widely distributed across various marine environments, including nutrient-poor open oceans and coastal regions, and their activity is influenced by environmental factors such as the availability of phosphorus and iron (Zehr and Capone, 2020). Nitrification, the conversion of ammonia to nitrate (NO3 -), is another critical process in the marine nitrogen cycle. This process is mediated by ammonia-oxidizing bacteria (AOB) and archaea (AOA), which convert ammonia to nitrite (NO2 -), and nitrite-oxidizing bacteria (NOB), which further oxidize nitrite to nitrate (Pajares and Ramos, 2019). The presence of these microorganisms and their activity levels are influenced by factors such as salinity and oxygen availability.
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