International Journal of Marine Science, 2025, Vol.15, No.5, 255-267 http://www.aquapublisher.com/index.php/ijms 261 and subtropical oceans, such as the North Atlantic and North Pacific subtropical circulation, surface phosphates are always low to approach detection limits, while nitrates are also low on the surface, but can be partially supplemented by nitrogen fixation, so these areas are closer to phosphorus restriction or nitrogen-phosphorus co-limitation. Global analysis by Browning and Moore (2023) shows that nitrogen limits are widely present in the distinctly stratified low-latitude subtropical circulation areas, iron limits are common in high productivity upstreams and the Southern Ocean, while single phosphorus limits are relatively rare, but in some closed waters (such as the Eastern Mediterranean), phosphorus limits are indeed dominated by phosphorus limits. As a restrictive nutrient, phosphorus has an ecological significance that once the supply of phosphorus is insufficient, the growth of phytoplankton will not be sustainable, and even if other nutrients are sufficient, it will be useless. Therefore, phosphorus restriction often leads to a decrease in phytoplankton biomass, a decrease in productivity, and a transformation in community composition. 5.2 Effects of changes in nitrogen and phosphorus ratio on the structure of phytoplankton community Marine phytoplankton communities are very sensitive to changes in environmental nitrogen-phosphorus ratio (N:P). There are differences in the adaptability of phytoplankton in different groups to nutrient ratio imbalance, so changes in nutrient ratio often lead to succession of community structure. When the environment N/P is significantly higher (relatively lacking phosphorus), it is often favorable for some organophosphorus to be more efficiently utilized or stored phosphorus. For example, studies have shown that the relative advantage of dinoflagellates will increase under high nitrogen-phosphorus ratio conditions, because many dinoflagellates have strong organic phosphorus utilization and migration and predation ability, and can obtain the required phosphorus in a phosphorus-deficient environment. The nitrogen and phosphorus ratio also affects the internal stoichiometry and equilibrium of phytoplankton cells. When the N/P ratio increases, phytoplankton may accumulate more carbohydrates and reduce protein content in the body, which in turn affects its nutritional value and feeding rate, and thus affects the energy flow of the food web. Laboratory and field addition experiments have confirmed that the composition of phytoplankton community under different N/P supply will undergo significant changes (Redoglio and Radtke, 2022). 5.3 Adaptation strategies and gene regulation mechanisms under phosphorus restriction Faced with phosphorus limitations, marine plankton and microorganisms have evolved a variety of adaptation strategies to maximize the acquisition and utilization of limited phosphorus sources. These strategies include physiological, biochemical and molecular genetic aspects. First, in cellular physiology, many phytoplankton reduce the proportion of phosphorus-related components in the cell when phosphorus is deficient, such as replacing part of phospholipids with sulfur or nitrogen lipids to reduce the need for phosphorus (Sebastián et al., 2015). Secondly, at the community level, there are synergistic and complementary mechanisms between plankton, such as organic matter decomposition bacteria release phosphate for algae, while algae release organic substrate for bacteria to utilize, forming a reciprocal relationship and improving the overall phosphorus utilization efficiency. At the molecular and gene regulation level, plankton's adaptation to the environment of phosphorus lacks is reflected in the changes in the transcriptional regulatory network. Many algae and bacteria have phosphorus starvation response regulation systems (PHO regulation systems). When the external phosphorus concentration decreases, a series of phosphorus acquisition gene expression will be activated, including phosphate high affinity transporter, alkaline phosphatase, phosphorus metabolism-related enzymes, etc. (Jha et al., 2018). In cyanobacteria, the transcription factor PhoB is the core of this regulatory network. Its activation energy triggers changes in the expression of dozens of genes, improving the cells' absorption of phosphorus and the resistance to phosphorus deficiency. 5.4 Case analysis: adaptation of phytoplankton to low phosphorus in mediterranean Oligotrophic Areas The eastern Mediterranean is considered one of the most obvious sea areas in the world with phosphorus restriction. In terms of community composition, small cyanobacteria and pico-eukino algae dominate, such as Prochlorococcus and Synechococcus. These micro-photosynthetic autotrophs have a high surface area-to-volume ratio, which is conducive to improving phosphorus absorption efficiency at ultra-low phosphorus concentrations.
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