International Journal of Marine Science, 2025, Vol.15, No.6, 292-302 http://www.aquapublisher.com/index.php/ijms 297 of phosphorus supply directly affects the productivity and composition of phytoplankton and has a chain effect on higher trophic levels through food web transmission. Figure 1 Experimentally derived nutrient limitation patterns on a background of estimated nitrate upwelling (Adopted from Browning and Moore, 2023) 6.2 Nutrient competition and adaptive mechanisms under phosphorus-limited conditions In a phosphorus-deficient environment, various organisms fiercely compete for the limited phosphorus resources and simultaneously evolve multiple adaptive strategies. Phytoplankton have developed a high-affinity phosphorus uptake system and a "luxury absorption" mechanism: once there is a brief phosphorus input into the environment, they quickly absorb it and store it in the form of polyphosphates for later use (Wang, 2025). Heterotrophic bacteria gain a competitive edge under low DIP conditions by virtue of their ability to utilize organic phosphorus. They obtain phosphorus from dissolved organic matter, reducing their reliance on inorganic phosphorus. The competition between algae and bacteria depends on conditions: in phosphorus-poor water bodies rich in organic matter, bacteria often have the upper hand; In environments with abundant sunlight and fleeting DIP, algae seize the initiative (Zhang et al., 2022). To adapt to phosphorus deficiency, many algae secrete more phosphatases to break down environmental DOP or replace certain cellular components with phosphorus-free substances to reduce the phosphorus requirement. Some nitrogen-fixing cyanobacteria also reduce their nitrogen-fixing activity when phosphorus is insufficient, prioritizing the limited phosphorus for basic growth (Lin et al., 2023). Through these competitive and adaptive mechanisms, ecosystems can still maintain certain functional operations under severe phosphorus constraints. 6.3 Regional differences in phosphorus utilization across oceanic areas Due to the different nutritional supply conditions, there are significant differences in the utilization strategies of phosphorus by plankton in various sea areas. In high-productivity seas such as coastal and upwelling areas, nitrogen is often the main limiting factor, while phosphorus is relatively abundant. Phytoplankton tend to grow rapidly and store excess phosphorus, and are not sensitive to phosphorus restrictions (Huang and Han, 2025). However, in the oligotrophic circulation centers of the ocean, the long-term low-phosphorus environment has created highly efficient "phosphorus-saving" communities: those plankton have an extremely high absorption affinity for trace phosphorus, can fully utilize DOP, and repeatedly recover phosphorus through the microbial loop, enabling the communities to operate at extremely low phosphorus concentrations (Jin et al., 2024). In addition, in some regions, due to high exogenous N input and a relatively large N-P ratio (such as the Mediterranean), phytoplankton, in order to adapt to the relatively phosphorus-deficient environment, will rely more on organic phosphorus and phosphatase pathways to obtain phosphorus.
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