International Journal of Marine Science, 2025, Vol.15, No.6, 292-302 http://www.aquapublisher.com/index.php/ijms 296 acidic conditions, phosphate adsorbed on particles is more likely to be released into water, and in alkaline environments, phosphate tends to combine and precipitate with cations such as calcium and magnesium (Wu et al., 2021). The REDOX state is even more crucial: under oxygenated conditions, phosphorus is easily adsorbed and fixed by iron and manganese oxides. Once the environment turns oxygen-deficient, these oxides are reduced and dissolved, and the phosphorus adsorbed on them is released in large quantities, resulting in an increase in the phosphorus concentration of the surrounding water. Furthermore, the mixing condition of water bodies determines the efficiency of nutrient re-supply. Strong vertical mixing (such as storms, winter convection) can bring deep phosphorus sources back to the surface and alleviate surface phosphorus limitation (Zhou et al., 2021); Long-term stable stratification leads to the depletion of surface phosphorus without replenishment. The combined effect of these physical and chemical factors shapes the effective supply level of phosphorus in different regions and seasons. 5.2 Binding effects of metal ions, mineral particles, and organic matter The activity of phosphorus in seawater is affected by its combination with various metals and particles. Phosphate has a strong affinity for metal oxides such as iron and aluminum, and is easily adsorbed on their surfaces or precipitated with calcium ions to form insoluble phosphate minerals, thereby removing phosphorus from the aqueous phase and turning it into a solid phase. This means that even if the total phosphorus content in the environment is not low, some of it may be fixed on particles and temporarily unavailable (Yan et al., 2022). Finegrained sediment and clay minerals can also adsorb phosphate ions, causing phosphorus to settle with the particles (Brady et al., 2022). Dissolved organic matter can affect the availability of phosphorus through multiple effects: some organic colloids can combine with phosphorus to form complexes, hindering its direct uptake by organisms; Some organic ligands, when combined with metal ions, weaken the fixation effect of metals on phosphorus, indirectly increasing the bioavailable ratio of phosphorus. 5.3 Metabolic regulation by microbial communities and phytoplankton When there is not enough phosphorus, Marine life will not just sit and wait to die. Phytoplankton usually "transform" themselves: reducing components with high phosphorus demands, such as replacing some membrane phospholipids with sulfur-containing lipids; It will also increase the production of phosphatase and "extract" some usable phosphorus from the surrounding DOP. Sometimes, when there is too much phosphorus, they will absorb a little more and stockpile polyphosphates to prepare for the subsequent "famine" (Fru et al., 2023). The appetite for phosphorus varies greatly among different organisms. Small phytoplankton, due to their large surface area and small volume, have a higher absorption efficiency and are often the winners in phosphorus-poor sea areas. Large algae grow fast but consume a lot. Once they lack phosphorus, they will fall behind. As for the internal part of the microbial community, it is not monolithic either. Bacteria can decompose organic matter and release phosphorus, providing nutrients for algae. But sometimes they also compete with algae for inorganic phosphorus, and this competition is no less intense (Zhang et al., 2025). Overall, both microorganisms and phytoplankton are making subtle metabolic adjustments to enable the ecosystem to respond flexibly to the amount of phosphorus, and as a result, the phosphorus utilization pattern of the entire ocean is constantly changing. 6 Relationship Between Marine Primary Productivity and Phosphorus Limitation 6.1 Effects of phosphorus limitation on phytoplankton growth and community structure When the available phosphorus in seawater is lower than the demand of phytoplankton, both their growth rate and community composition will change. Insufficient phosphorus can slow down the cell division and photosynthesis of phytoplankton and cause physiological changes, such as depletion of intracellular phosphorus reserves and a decrease in photosynthetic pigment content. Some large populations that rely on high phosphorus (such as macrodiatoms) gradually decline under the condition of continuous phosphorus deficiency, while some small algae and blue-green algae that tolerate low phosphorus have the advantage with lower nutrient requirements and higher absorption efficiency, resulting in the succession of community structure towards miniaturization and low diversity (Figure 1) (Browning and Moore, 2023). Meanwhile, under phosphorus restriction, the intracellular carbonphosphorus ratio (C:P) of phytoplankton cells increases, the nutritional quality declines, and the growth and reproduction of zooplankton may be hindered after feeding (Lin et al., 2023). It can be seen from this that the level
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