Occurrence of orthophosphate monoesters in lake sediments: significance of myo- and scyllo-inositol hexakisphosphate

Abstract

Orthophosphate monoesters often constitute a significant fraction of total phosphorus in lake sediments. The knowledge on the specific composition and recalcitrance of these compounds is however limited. The main aim was therefore to identify and quantify specific orthophosphate monoesters in sediment from 15 Danish lakes by solution (31)P NMR spectroscopy. The four most quantitatively important orthophosphate monoesters were myo-inositol hexakisphosphate (myo-IP(6)), scyllo-inositol hexakisphosphate (scyllo-IP(6)) α-glycerophosphate (α-GP) and β-glycerophosphate (β-GP). The compounds were identified in 9, 4, 8 and in all 15 lakes, respectively. In total these four components made up 46-100% of the orthophosphate monoester pool. The glycerophosphates (GPs) are most likely degradation products of phospholipids, created as an artifact by the alkaline extraction procedure used for (31)P NMR spectroscopy, while the inositol hexakisphosphates (IPs) are naturally occurring compounds. There was a significant positive correlation between myo-IP(6) and total aluminium in the sediment and a negative correlation between myo-IP(6) and lake water pH, suggesting that myo-IP(6) is stabilized in the sediment by adsorption at slightly acidic or neutral conditions. In three lakes, the depth distribution of the orthophosphate monoesters was investigated. The content of scyllo-IP(6) and myo-IP(6) was constant with sediment depth in two of the lakes while the content of myo-IP(6) decreased with depth in one of the lakes. In all cases the IPs seem to be preserved with sediment depth to a higher extent than the orthophosphate diesters and especially the GPs suggesting that IPs can be a sink for phosphorus in the lake ecosystem or at least delay P-recycling for years.

The Chemical Nature of Phosphorus in Subtropical Lake Sediments

“…Principal component analysis revealed a link between the phosphomonoester pool and P associated with humic and fulvic acids (i.e., stable organic P) plus labile organic P. This is consistent with phosphomonoesters occurring in both labile (e.g., sugar phosphates, mononucleotides) and stable (e.g., inositol phosphates) forms in sediments (e.g., Turner and Weckström 2009;Jørgensen et al 2011). In contrast, DNA and inorganic polyphosphates were correlated strongly with the microbial P pool.…”

“…5), as indicated by signals in the phosphomonoester region from myo-and scyllo-inositol hexakisphosphate, as well as signals upfield of orthophosphate that were recently assigned to neo-and D-chiro-inositol hexakisphosphate . Inositol phosphates are abundant in mineral soils and have been detected in a number of Scandinavian lakes (Jørgensen et al 2011). Their absence in subtropical wetland sediments in the Florida Everglades Turner et al 2006a) does not appear to be due to anaerobic conditions (e.g., Suzumura and Kamatani 1995), because myoinositol hexakisphosphate persisted for many years at depth in the anaerobic sediment of a Helsinki embayment (Turner and Weckström 2009).…”

Organic phosphorus in the aquatic environment

“…They found the IHP fraction in the sediments to be stable enough to suggest that it could be used as a paleo-indicator of phosphorus input to the bay. Similarly, Jørgensen et al [75] noted the preservation of IHPs in sediments of a freshwater lake. Taken together these results suggest that exposure to a saline environment is essential for IHP degradation -possibly through release of IHP from sediment particles with increasing salinity.…”

“…IHPs were not detected in the sediments of 6 out of 15 Danish lakes surveyed using 31 P NMR spectroscopy. [75] In the lake sediments that did contain IHPs, it accounted only for between 1 and 10 % (mean ¼ 4.4 %) of the total phosphorus pool in the top 1 cm of sediment. Similar levels of IHPs have been found in near shore coastal sediments.…”

“…In fact there is some evidence that the NaOH extractant hydrolyses phospholipids producing glycerophosphates that are observed in the spectra. [75] Quantification Quantitative studies on phospholipids in aquatic ecosystems, particularly those that also quantify total P and or total organic P, are not common. [92] In one study that compared dissolved and particulate phospholipids in coastal marine environments, dissolved phospholipids represented less than 1 % of total dissolved organic P and between 3 and 13 % of total particulate P. [96] It is not surprising that the particulate fraction contained more phospholipids than the dissolved fraction.…”