International Journal of Marine Science, 2025, Vol.15, No.5, 255-267 http://www.aquapublisher.com/index.php/ijms 262 Mediterranean phytoplankton demonstrates significant biochemical adjustment capabilities. Sampling analysis showed that their intracellular phospholipid content was much lower than that of similar Atlantic populations, and instead increased the proportion of non-phosphorus membrane lipids (such as sulfhydryl lipids, glucoglycerides, etc.), which was seen as a strategy for phytoplankton to construct membranes with other elements in a low-phosphorus environment to reduce dependence on phosphorus. At the genetic level, plankton in the Eastern Mediterranean is enriched with many genes related to organophosphorus utilization. The study also found that the alkaline phosphatase activity in the eastern Mediterranean waters is much higher than that in the Atlantic Ocean sea area, indicating that the microbial community is actively decomposing and utilizing limited DOP (Van Wambeke et al., 2024). 6 Phosphorus and Marine Primary Productivity 6.1 Effects of phosphorus supply on algae growth and photosynthesis Sufficient phosphorus supply is one of the necessary conditions to ensure the vigorous growth of phytoplankton and efficient photosynthesis. Algae undergo photosynthesis to fix CO₂, and a large number of phosphorus-containing molecules are required to synthesize, so the environmental phosphorus concentration often determines the maximum photosynthetic yield and upper biomass limit of phytoplankton (Hong and Huang, 2025). In phosphorus-rich water bodies, phytoplankton can reproduce at a near maximum specific growth rate, forming a "green water" area with high chlorophyll and high primary productivity; both field observations and culture experiments have proved that when phosphorus changes from lack to abundant, the photosynthetic rate and cell oxygen production rate of phytoplankton will increase significantly. Seasonal changes are similar. When the concentration of phosphorus in the estuary drops, sufficient light is sufficient but the algae are not prosperous, it indicates that phosphorus becomes a limiting factor, while the increase in phosphorus in winter and spring runoff promotes the prosperity of phytoplankton in spring. These all indicate that the phosphorus supply status is closely related to the photosynthesis intensity of algae. 6.2 The role of phosphorus in the formation of red tides and harmful algae blooms (HABs) Red tides and harmful algae blooms are ecological abnormalities caused by the over-reproduction of certain phytoplankton algae, and their occurrence is often closely related to the nutrient condition. As one of the main fertilizer source elements, phosphorus plays an important role in the formation of red tides. Generally speaking, the increase in nitrogen and phosphorus load in eutrophied waters is the basic prerequisite for inducing red tides. Excessive nitrogen and phosphorus input will promote the rapid growth of nutrient-resistant algae, break the original ecological balance, and thus trigger algae blooms (Mackey et al., 2017). Especially in closed or semi-enclosed sea areas, when the concentrations of N and P in the water far exceed the needs of plants, high concentrations of algae are easily accumulated in the lower wind flow and develop into red tides. Many studies have shown that dinoflagellate red tide organisms are more competitive under high N/P and phosphorus restriction conditions, while diatom red tides tend to have slightly lower N/P ratio environments (Shen et al., 2022). This means that artificial changes in nutrient ratios may lead to a shift in red tide types. For example, along the East China Sea coast, the proportion of diatom-type red tides has been observed since the end of the 20th century, from about 70% to less than 30%, while the proportion of dinoflagellate red tides has increased from less than 20% to more than 50%. This trend is partly attributed to the increase in the N/P ratio and intensified phosphorus restrictions in nearshore waters, which has allowed dinoflagellates, which are good at snatching organic phosphorus, to gain the upper hand. 6.3 Indirect effect of phosphorus on carbon fixation and carbon cycle As one of the limiting factors for primary marine productivity, the impact of phosphorus on carbon cycle is mainly reflected in regulating carbon fixation rates and changing the carbon flow path of the food web. The supply of phosphorus limits the photosynthesis of phytoplankton and will reduce the fixed flux of carbon dioxide. The global oceans fix approximately 50 Pg of carbon every year through photosynthesis, of which a considerable proportion settles to the deep sea to achieve carbon sink function. Second, phosphorus affects community structure, thereby changing the flow of carbon in the food web. When phosphorus restriction is severe, small
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