International Journal of Marine Science, 2024, Vol.14, No.5, 304-311 http://www.aquapublisher.com/index.php/ijms 305 2 Major Marine Biogeochemical Processes 2.1 Carbon cycle and oceanic carbon sequestration The carbon cycle in marine environments is a complex interplay of biological, chemical, and physical processes that regulate the movement and storage of carbon. One of the key components of this cycle is the sequestration of carbon in the ocean, which involves the uptake of atmospheric CO2 by marine organisms and its subsequent storage in various forms. The PISCES-v2 model, for instance, simulates the lower trophic levels of marine ecosystems and the biogeochemical cycles of carbon and other nutrients, providing insights into how carbon is cycled and stored in the ocean (Aumont et al., 2015; Zhang et al., 2024). The chemoattraction of marine fauna to dimethyl sulfide (DMS) plays a crucial role in natural iron fertilization, which in turn enhances carbon sequestration in high-nutrient, low-chlorophyll (HNLC) areas. This process highlights the interconnectedness of the carbon, iron, and sulfur cycles in marine ecosystems. 2.2 Nitrogen and phosphorus cycles The nitrogen and phosphorus cycles are fundamental to marine biogeochemistry, influencing primary productivity and ecosystem dynamics. Nitrogen cycling, driven by microbial processes, includes nitrogen fixation, nitrification, and denitrification. These processes are sensitive to environmental changes such as ocean acidification, which can alter the rates of nitrogen transformations and impact microbial community composition (Wannicke et al., 2018). The eco-energetic strategies of chemolithoautotrophic microorganisms, which participate in the nitrogen cycle, are essential for understanding how these processes respond to global change. Phosphorus, on the other hand, is a limiting nutrient in many marine environments. Recent advances have revealed a more dynamic and interconnected phosphorus cycle than previously understood, with significant implications for marine productivity and ecosystem structure. The coupling of phosphorus with carbon, nitrogen, and metal cycles underscores its integral role in marine biogeochemistry. 2.3 Sulfur and iron cycles Sulfur and iron cycles are closely linked with other biogeochemical processes in marine environments. Sulfur cycling, primarily driven by sulfate reduction, is a major component of the microbial ecology in marine sediments. This process is interconnected with the cycles of carbon, nitrogen, and iron, influencing both cellular and ecosystem-level processes (Wasmund et al., 2017). The role of sulfur-transforming microorganisms in these cycles is critical for understanding the overall biogeochemical dynamics of marine sediments. Iron, a limiting nutrient in many ocean regions, is recycled by marine biota, enhancing carbon assimilation and linking the iron and carbon cycles (Savoca, 2018). The interaction between sulfur, iron, and carbon cycles is further exemplified by the chemoattraction of marine fauna to DMS, which triggers iron recycling and augments carbon sequestration in HNLC waters. These interconnected cycles highlight the complexity and interdependence of marine biogeochemical processes. 3 Ecosystem Evolution in Response to Biogeochemical Changes 3.1 Long-term shifts in marine ecosystem structure Marine ecosystems are undergoing significant structural changes due to various biogeochemical alterations. Ocean acidification (OA) and warming are primary drivers of these shifts, leading to a simplification of ecosystem structure and function. This simplification is characterized by reduced energy flow among trophic levels and limited acclimation potential for many species (Gamfeldt et al., 2015). Habitat-forming species such as coralligenous reefs and Posidonia oceanica meadows are experiencing biomass reductions, which in turn affect the entire marine community and ecosystem services (Zunino et al., 2021). These changes suggest a reorganization of energy flows and a decrease in ecosystem size, indicating a high degree of ecosystem development but potentially suboptimal conditions from an anthropocentric perspective. 3.2 Impact of ocean acidification on ecosystem dynamics Ocean acidification is profoundly impacting marine ecosystems by altering the biogeochemical cycles and the structure of biogenic habitats. The decline in pH and carbonate saturation affects habitat-forming organisms, leading to decreased biodiversity in coral reefs and mussel beds, while potentially increasing it in seagrass and
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