International Journal of Marine Science, 2025, Vol.15, No.6, 292-302 http://www.aquapublisher.com/index.php/ijms 295 4 Transport and Transformation Mechanisms of Phosphorus in the Marine Environment 4.1 Transport pathways of dissolved and particulate phosphorus The migration of phosphorus in the ocean occurs through two pathways: dissolved state and particulate state. Dissolved phosphorus (including DIP and soluble DOP) spreads with the flow of seawater: Ocean currents and upwelling transport phosphorus-rich water masses to other areas, and vertical mixing carries deep dissolved phosphorus to the surface (Murphy et al., 2021). These hydrodynamic processes shape the large-scale distribution pattern of phosphorus. In contrast, granular phosphorus mainly migrates along the vertical direction through gravitational sedimentation. Organic debris particles formed by plankton sink downward, transporting phosphorus from the surface to the deep sea. This "biological pump" process causes surface phosphorus to continuously move out and accumulate in the deep sea (Browning et al., 2017). Some of the sinking particles are decomposed during the process, and the phosphorus assimilated re-dissolves into the surrounding water body. The undecomposed ones eventually sink into the seabed sediments. In addition, phosphorus-containing sediment suspended in coastal waters can also be carried by coastal currents and transported horizontally near the bottom layer. The long-distance transport in dissolved form and the sedimentation in granular form jointly determine the spatiotemporal redistribution of Marine phosphorus. 4.2 Microbially driven degradation and remineralization of organic phosphorus The degradation and remineralization of organic phosphorus in the ocean are highly dependent on microbial activities. Heterotrophic bacteria and other microorganisms secrete phosphatases that hydrolyze DOP into inorganic phosphates, converting the originally unusable organic phosphorus into an absorbable form. When phosphorus is scarce, many phytoplankton and microorganisms can significantly increase the production and activity of alkaline phosphatase (AP), "extracting" phosphorus from the surrounding organic matter and alleviating environmental phosphorus limitation. In the sediment, a large amount of buried organic phosphorus is also decomposed under the action of anaerobic microorganisms, releasing phosphorus into the pore water and then diffusing into the overlying water body. The degradation rate of microorganisms is affected by environmental conditions: the higher the temperature, the faster the decomposition of organic matter and the remineralization of phosphorus. Oxygen conditions determine the decomposition pathways and efficiencies of certain organophosphorus compounds (Duan et al., 2025). 4.3 Phosphorus release and regeneration at the sediment–water interface The sediment-water interface is an active exchange site in the phosphorus cycle. After phosphorus-containing particles sink to the seabed, some of the phosphorus in them can be re-released into the water body at the interface. On the one hand, the sedimentary organic matter is decomposed by anaerobic microorganisms, and phosphate is released into the pore water of the sediment and diffuses along the concentration gradient to the bottom seawater (Jin et al., 2024). On the other hand, when the bottom water is oxygen-deficient, the iron and manganese oxides that originally adsorbed phosphate in the sediment are reduced and dissolved, and the phosphorus bound to them is desorbed and released into the water. Under oxidative conditions, the opposite process occurs: phosphorus is readily adsorbed and fixed by metal oxides in sediments to form precipitates (Randolph-Flagg et al., 2023). Periodic disturbances (such as storms and benthic activities) can enhance interfacial exchange, enabling phosphorus released from sediments to enter the overlying water layer cycle more quickly. Phosphorus regeneration at the sedimentwater interface regulates the nutrient supply of bottom water and surface water at a local scale: in eutrophic waters, it can act as an internal phosphorus source, intensifying phosphorus accumulation in the water body. When in overall equilibrium, sediments tend to act as terminal sinks for phosphorus. 5 Key Environmental and Biological Factors Affecting Phosphorus Bioavailability 5.1 Regulatory roles of physicochemical conditions The physicochemical conditions of the Marine environment regulate the bioavailability of phosphorus by influencing its form and flow. The higher the temperature, the faster the rate of organic matter decomposition and phosphorus remineralization usually is, but the high stratification of seawater can also inhibit the transport of deep phosphorus to the surface. The pH value of water bodies affects the dissolution equilibrium of phosphorus: under
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