Effects of speciation on equilibrium fractionations and rates of oxygen isotope exchange between (PO4)aq and H2O

The oxygen isotope composition of phosphate released from phytic acid by the activity of wheat and <i>Aspergillus niger</i> phytase

“…In the terrestrial environment, the oxygen isotope composition (δ 18 O) of phosphate has been used as a tracer in the terrestrial environment to study the cycling of P in soils (Zohar et al, 2010a and b;Tamburini et al, 2010Tamburini et al, , 2012Angert et al, 2011Angert et al, , 2012Gross and Angert, 2015), in plants (Young et al, 2009;Pfahler et al, 2013) and in aerosols (Gross et al, 2013). Under ambient conditions and in the absence of biological activity, the δ 18 O of phosphate does not change (Kolodny et al, 1983;O'Neil et al, 2003). However, biological uptake of phosphate leads to a substantial alteration of δ 18 O values (Paytan et al, 2002;Blake et al, 2005;Stout et al, 2014).…”

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“…Bright yellow precipitate forms immediately, and may turn black over time, possibly due to hydrogen reduction of the silver (Baxter and Jones 1910). Dettman et al (2001) found that the fast precipitation of silver phosphate gives the same isotopic composition as the slow precipitation method used in O'Neil et al (1994). At this stage, pH of the sample solution can be checked with universal indicator pH strips to ensure that the pH was approximately 7 (±0.5).…”

“…This approach, magnesium-induced coprecipitation (MagIC), removes P from solution via coprecipitation with Mg(OH) 2 and results in the quantitative concentration of phosphate from large volumes of water into more manageable volumes for analyses. Many methods have been developed for the purification of phosphate from apatite for isotopic analysis (Firsching 1961;Crowson et al 1991;O'Neil et al 1994), each of which involves dissolution of the solid sample into homogenous solution for precipitation of silver phosphate. However, they are not easily applied to DIP purification and conversion to silver phosphate from a salt-water matrix due to the abundance of chloride ions, which interfere with the silver phosphate precipitation.…”

“…There are several possible reasons for naturally occurring variations in δ 18 O of dissolved inorganic phosphate: inorganic hydrolysis and polymerization, organic (enzyme-mediated) hydrolysis and polymerization, and mixing of phosphate of different isotopic composition (source phosphate). The first of these possibilities is unlikely because at low temperatures and moderate pH, nonbiological isotope exchange between phosphate and water is known to be negligible (Lécuyer et al 1999;O'Neil et al 2003). Eliminating inorganic hydrolysis as a potential cause of isotopic exchange leaves enzyme-mediated hydrolysis and polymerization and/or mixing of different sources of isotopically distinct phosphate as the likeliest possibilities for observed natural variability.…”

“…To overcome some of these issues, Firsching (1961) introduced a method to precipitate the phosphate as silver phosphate, a pure, stable, nonhygroscopic compound (Baxter and Jones 1910). Subsequently, O'Neil et al (1994) developed a method that reacted silver phosphate with graphite in sealed silica tubes at high temperatures to produce CO 2 for isotope analysis as an alternative to the fluorination procedure (Vennemann et al 2002). Although these methods made analyses more efficient, these modifications still require a relatively large sample size (10 to 20 mg silver phosphate), which is not always practical or available, in particular for soluble reactive phosphate (SRP) in open ocean waters where phosphate concentrations could drop to less than 0.2 µM (GEOSECS).…”