Bioavailability orthophosphate

Sharpley, A.N. (1993) An innovative approach to estimate bioavailable orthophosphate in agricultural runoff using iron oxide-impregnated paper. J. Environ. Qual. 22 597-601... 

Given that the Fe and P bound to the DOM in humic lakes are probably not directly available to organisms, the DOM-Fe-P complex would have to undergo some form of transformation to yield bioavailable Fe and P species. In terms of biological uptake, in general it can be said that Fe is preferred in the dissolved free ionic form and P as orthophosphate (P04). Several known processes result in the transformation of Fe and P from the DOM-Fe-P complex into biologically available species. Two of the mechanisms are physical-chemical processes (1) UV-mediated photoreduction and (2) DOM-mediated chemical reduction or dark reduction. Other mechanisms are... 

In terrestrial soils and in the euphotic zone of lakes and the ocean the concentration of dissolved orthophosphate is typically low. When bioavailable phosphorus is exhausted prior to... 

Dietary iron level does not seem to affect the efficiency with which dietary iron is converted into hemoglobin when ferrous sulfate (Table 1) or when ferric orthophosphate (Table 2) is the primary source of dietary iron. This is also true for white bread (Table 2) however, the source of the iron in the enriched flour used in the bread is unknown. That the efficiency of converting food iron into hemoglobin is not affected by dietary iron concentration is important to bioavailability testing because it is often difficult to formulate diets with precise amounts of iron, especially when foods are the sources of iron. 

Among experiments, the variability of the efficiency of converting iron from ferrous sulfate into hemoglobin (Table 1) was much greater than when ferric orthophosphate (Table 2) was the iron source. This variability is disturbing since ferrous sulfate is commonly used as a reference source of iron for bioavailability experiments, as well as an iron supplement clinically. Typically, this variability is dealt with by expressing the hematinic responses of the unknowns relative to ferrous sulfate (Shah et al., 1979 Coccodrilli et al., 1976 Amine et al., 1972). 

This does not mean that the bioavailability of iron from all compounds containing both phosphorus and iron is low. Wood, et al. (12) and Theuer and his associates (7, 8) have found that the bioavailability of iron from sodium iron pyrophosphate and ferric pyrophosphate was greatly improved when the foods containing these salts were processed with heat and pressure (Table II). Such processing did not, however, improve the bioavailability of iron from ferric orthophosphate or ferrous sulfate. The reason for this effect is not known but sugars in the foods may have formed chelates with the iron that facilitated absorption. 

Many organisms can store orthophosphate or polyphosphates. In plants, inositol hexaphos-phate (IHP) can form a major storage compound for phosphorus, particularly in seeds. Although relatively low in concentration, diesters may contribute to short-term bioavail-able pool more than monoesters. Compared to monoesters, diesters are more accessible to microbial attack. This fraction was shown to be rapidly made available to plants. 

Major differences in size (filterable, particulate) and chemical reactivity (condensed, organic) of phosphorus forms in samples can be used as the basis for speciation, as shown in Table 1. Of these fractions, total phosphorus (TP) and FRP are perhaps the most commonly measured, although it is arguable that the understanding of the aquatic phosphorus cycle is somewhat lopsided because of that bias. For example, wastewater discharge licenses often specify a maximum permissible concentration of TP, and provide an indication of the maximum potentially bioavailable phosphorus discharged. However, FRP, comprising mostly orthophosphate, is a measure of the amount of most readily bioavailable phosphorus. 

Microalgae use phosphorus in metabolic processes like energy conversion and photosynthesis (Zhu et al., 2013). Like nitrogen, some forms are more easily absorbed than others, with the most easily absorbed form as orthophosphate. Phosphorus must be added to the culture in excess of the stoichiometric requirement because not all of it will be bioavailable (Chisti, 2007). However, if the concentration is too high, growth is inhibited (Zhu etal., 2013). 

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