Novel insights into molecular composition of organic phosphorus in lake sediments

Organic phosphorus (P_o) plays a key role in eutrophication and ecological equilibrium in lake systems. However, characterizing the composition of P_o in lake sediments has been a bottleneck hindering further understanding of the biogeochemical cycle of P_o. Here, multiple methods of ³¹P NMR spectroscopy and molecular weight (MW) ultrafiltration were combined to detect P_o composition characteristics from a novel angle in ten lake sediments of China. The results showed that sediment P_o mainly consisted of monoester (mono-P, 14±8.8% of the NaOH-EDTA total P on average), diester (di-P, 1.4±1.4%) and phosphonate (phos-P, 0.1±0.1%), while the abundance of P_o was largely underestimated by ³¹P NMR methods. Some specific species of mono-P were successfully determined, and the contents of these species followed a decreasing order: inositol hexakisphosphate (IHP6) > RNA mononucleotides (RNA-mnP) > β-glycerophosphate (β-gly) > D-glucose 6-phosphate (Glu-6) > α-glycerophosphate (α-gly), which was largely dependent upon their bioreactivity. A significant relationship between MW and P_o components was observed despite the great differences among sediment samples. For refractory P_o components, IHP6 was mainly rich in the MW < 3 kDa while phos-P was almost only detected in the MW > 3 kDa, which largely attributed to their metal binding affinities and characteristics. The abundance of bioreactive P_o species (α-gly, β-gly, Glu-6, di-P) in high MW (HMW, > 3 kDa) were all higher than that of low MW (LMW, < 3 kDa) due to microbial degradation and self-assembly. If the HMW organic molecules were biologically and chemically more reactive than its LMW counterparts, the high percentage of α-gly, β-gly, glu-6 and di-P in the HMW portion would highlights their high reactivity from the perspective of MW. These insights revealed the dynamics of the MW distribution of P_o components and provide valuable information to better understand the P_o composition and bioreactivity in sediments.