International Journal of Marine Science, 2025, Vol.15, No.5, 255-267 http://www.aquapublisher.com/index.php/ijms 258 3 The Form and Transformation of Phosphorus in the Ocean 3.1 Distribution and utilization of inorganic phosphorus (phosphate) Dissolved inorganic phosphorus (DIP) usually exists in the form of phosphate ions and is the most direct and easy-to-use phosphorus source for most marine organisms. The typical distribution of DIP in the ocean is the exhaustion of the surface and the concentration increases with the increase of depth. This is due to rapid uptake of phosphate for growth in the surface eutrophication layer, resulting in concentrations often as low as nanomolar levels (McLaughlin et al., 2013). In the oligotrophic sea areas in the middle of the ocean, surface DIP is often at the lower limit of detection for a long time, and phosphorus has become one of the restrictive nutrients (Liang et al., 2022). The surface phosphate concentrations in the nearshore and upflow areas are relatively high due to exogenous recharge and deep water upsurge. Phytoplankton has a high affinity and rapid uptake of phosphates to cope with the scarcity of phosphorus in the environment. In the classic "poor nutrition-rich" ecological condition conversion, phytoplankton usually feels phosphorus-limiting pressure when the environmental DIP concentration drops below about 0.1 μmol/L, and activates a series of physiological response mechanisms. Phosphate, as the preferred source of phosphorus for plankton, will be immediately absorbed by cells once it is sufficient and used to synthesize nucleic acids, phospholipids and energy molecule ATP. This is also why in some nearshore waters affected by rivers or pollution emissions, increased phosphate concentrations often directly drive the jump in primary productivity and even induce algae blooms. 3.2 Organophosphorus compounds and their degradation pathways Dissolved organophosphorus (DOP) in seawater consists of various organophosphorus compounds, including nucleic acids, phospholipids, phosphoproteins, and specific metabolites. DOP accounts for a considerable proportion of the surface ocean phosphorus reservoirs, and even exceeds the content of inorganic phosphorus in the surface ocean water. However, most DOPs cannot be directly utilized by phytoplankton and need to be converted into inorganic phosphates first. Microbial communities play a key role in DOP degradation: they secrete various hydrolytic enzymes to break down organophosphorus compounds into available small molecules and phosphates (Adams et al., 2022). Common degradation pathways include: phosphorus in nucleic acids is released through nuclease reduction, phospholipids are hydrolyzed under the action of phospholipase, and compounds containing phosphoester bonds are cut off by phosphomonoesterase and phosphodiesterase, respectively. Recent studies have found that marine microorganisms have evolved into efficient strategies for decomposing phytic acid: by secreting phytase, gradually hydrolyzing hexaphosphate bonds, and phytic acid releases inorganic phosphorus. Studies have shown that marine macroalgae and seaweed are rich in inositol phosphoric acid such as phytic acid. Microorganisms can drive the metabolism of phytic acid and convert the phosphorus in it into phosphate for cell use (Sosa et al., 2019). This suggests that "hidden" organic phosphorus sources such as phytic acid may have the importance of being ignored in phosphorus-constrained sea areas. In addition, some marine bacteria also have a special pathway to utilize organic phosphonates, namely converting them into useful inorganic phosphorus through C-P lyase. 3.3 Enzymatic effects of microorganisms on phosphorus transformation Microorganisms act as "enzyme library" in the marine phosphorus cycle, accelerating the morphological transformation and regeneration of phosphorus through the secretion of various enzymatic reactions. The most typical of these is alkaline phosphatase (AP), an enzyme that can hydrolyze organic phosphorus into inorganic phosphates under alkaline conditions. Both phytoplankton and heterotrophic bacteria express alkaline phosphatases, and AP inducible expression is activated when the external DIP concentration drops below a certain threshold (usually on the order of nanomolar). Therefore, alkaline phosphatase activity (APA) is often negatively correlated with the degree of environmental phosphorus deficiency and is considered as a valid indicator of phosphorus restriction status (Ma et al., 2019). Some studies compared the relative importance of AP and PDE to DOP hydrolysis in the upper water bodies of the North Pacific. The results showed that the two had their own emphasis when decomposing different types of organophosphorus compounds, and contributed to the DOP cycle in both the upper and middle water bodies. When the supply of phosphorus in different morphologies changes in the environment, these microorganisms are able to dynamically regulate the expression of related genes, thereby
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