International Journal of Marine Science, 2025, Vol.15, No.6, 313-319 http://www.aquapublisher.com/index.php/ijms 132 model studies, this research aims to comprehensively expound how Marine microbial processes shape Marine biogeochemistry and thereby influence the Earth system. 2 Diversity and Ecological Functions of Marine Microbial Communities 2.1 Classification of microorganisms: bacteria, archaea, and planktonic eukaryotes Whether it is seawater, sediment or suspended biofilms, there are indeed many types of microorganisms in the ocean, which are generally classified into three major categories: bacteria, archaea and plankplankton eukaryotes. Most of the metabolic functions you can think of can find their "specialized players" among them. Bacteria and archaea are very common in most Marine environments, especially in seabed sediments, where their types and functions are rich. In contrast, planktonic eukaryotes (such as some protozoa and microalgae), although not necessarily in large numbers, are one of the main forces maintaining primary production and nutrient cycles. These three types of organisms do not act independently of each other. Instead, they interact and influence each other in the food web, and their metabolic activities are linked together, supporting the material cycle of the entire Marine ecosystem (Herve et al., 2025). 2.2 Distribution patterns of microorganisms in different marine niches When it comes to where microorganisms are abundant, merely looking at seawater is not enough. Factors such as depth, salinity and temperature have a significant impact. Microorganisms living in open seas, nearshore areas, sediments or biofilms also vary. For example, in sediments, the types of microorganisms are usually more numerous than in water bodies. Some types are almost exclusively "sediment-specific", while others prefer open waters (UlHasan et al., 2019; Zhang et al., 2019). In terms of planktonic eukaryotes, their distribution is more significantly affected by nutrient ratio, mixed layer depth and pH. For instance, the high-yield upwelling region is their "dense area" (Liu et al., 2025; Yanez et al., 2025). Another point that is easily overlooked is that the types of microorganisms in biofilms are completely different from those in water bodies, which is also a "hidden corner" of Marine microbial diversity. 2.3 Functional redundancy and ecosystem stability in microbial communities In fact, the scientific community has long been debating whether microorganisms have a "backup mechanism". Functional redundancy refers to the situation where multiple species can perform the same task. In the early days, some people thought it was fine that the system could still function normally without one species. But recent studies have begun to challenge this "foolproof" idea - once the community structure changes, the functional performance may also change accordingly. Although functional traits are much more stable than taxonomic structures in different environments, which helps maintain ecological balance, the degree of "redundancy" actually varies by location and function (Song et al., 2022). The more complex the microbial community is, the stronger its theoretical ability to cope with external disturbances. However, it is not the case that the more complex, the better - too much interconnection and mutual disturbance may instead become an unstable factor of the system (Royalty and Steen, 2021; Lee et al., 2025). It is precisely for this reason that issues such as the diversity of Marine microorganisms, where they are distributed, and what each can do are worth continuous tracking. After all, the way they exist directly determines whether the Marine ecosystem can operate stably and withstand environmental pressure. 3 Roles of Marine Microorganisms in the Carbon Cycle 3.1 Carbon fixation mechanisms of photosynthetic microorganisms If we talk about who is the main force behind Marine carbon fixation, cyanobacteria and phytoplankton definitely have to be on the list - they contribute nearly half of the global photosynthesis output. But things are not as simple as they seem. Because there is not much dissolved carbon dioxide in seawater to start with, and enzymes like Rubisco have a relatively low "affinity" for CO₂, many microorganisms have long evolved their own carbon concentration mechanisms (CCMs) to address this "efficiency bottleneck". By actively absorbing bicarbonate (HCO₃⁻) and utilizing carbonic anhydrase, these mechanisms can help them "work efficiently" under various water conditions and are less likely to fail (Kupriyanova et al., 2023; Beer and Beardall, 2025). Interestingly, these CCMs
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