In living things, phosphorus appears in the form of free phosphate ions in solution, called inorganic phosphate. In aquatic ecosystems, phosphate released from particulate organic matter (POM) dominates the phosphorus (P) cycle. However, the mechanism of P release from POMs remains poorly understood due to complex compositional and analytical challenges.
In a recent study, the release of dissolved inorganic phosphate (DIP) during POM photodegradation was assessed using excitation-emission matrix (EEM) fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results showed that POM in suspension was significantly photodegraded under light irradiation, while DIP was produced and released in aqueous solution. The chemical continuous extraction results showed that organophosphorus (OP) in POM participated in the photochemical reaction. In addition, FT-ICR MS analysis showed that the average molecular weight of P-containing molecules in POM decreased from 374.2 Da to 340.1 Da after illumination. P-containing molecules with a low degree of oxidation and unsaturation are preferentially photodegraded to generate oxygen-rich and saturated molecular compounds, such as protein-like and carbohydrate-like compounds, which are beneficial to the further utilization of P by organic matter. Reactive oxygen species (ROS) play an important role in the photodegradation of POM, and the excitation of triplet chromogenic dissolved organic matter (3CDOM*) is the main cause of POM photodegradation. These results provide new insights into the biogeochemical cycling of P and the photodegradation of POM in aquatic ecosystems.
First, the photodegradation experiment of POM in the suspension was carried out, and the results showed that with the light process, POM undergoes photodegradation and releases a large amount of DIP, which is an important potential source of P in water. In order to analyze the change of P component in POM before and after photodegradation, the P component in POM was analyzed by chemical continuous extraction method. The results showed that BD-P and NaOH-rP were not the source of DIP in the solution; the content of NaOH-nrP component (organic bound P) and Res-P decreased significantly after illumination, which fully indicated that organic phosphorus (OP) in POM was degraded under illumination.
Before illumination, most P-containing molecules were classified as lipid-like, lignin-like and carbohydrate-like, with relative abundances (RA) of 46.5%, 18.8% and 13.9%, respectively. After light irradiation, lipid-like, unsaturated hydrocarbon-like and lignin-like P-containing compounds were almost removed, and some protein-like and carbohydrate-like P-containing compounds with high O/C were newly generated. These results suggest that photochemical degradation tends to remove low-oxygen P-containing compounds, such as lipid-like and lignin-like compounds, and generate high-oxygen P-containing compounds, such as protein-like and carbohydrate-like P-containing compounds.
To illustrate the compositional changes that occur in each subcategory of P-containing molecules, photodegradable, photogenerated, and photoresistant P-containing molecules were analyzed: photodegradable P-containing molecules were mainly concentrated in the low O/C region (O/ C< 0.3) and high H/C zone (H/C>1.0). On the contrary, the photogenerated P-containing molecules are mainly concentrated in the region of O/C>0.3 and H/C>1.0. These results suggest that (i) less oxidized (i.e. lower O/C) and more aliphatic (i.e. higher H/C) compounds are preferentially degraded to produce oxygen-rich molecules, which explains the significant increase in O/C values and (ii) the large difference in the degradation of P-containing species with different O/C and H/C.
The study further analyzed the role of ROS in the release of DIP from POM photodegradation. After adding various free radical scavengers, the release of DIP showed different degrees of reduction. After adding TMP, the release of DIP decreased by 59.2%. The release amount was decreased by 38.8% and 32.6% after adding IPA and CAT, respectively.
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Reference
- New insights into the mechanism of phosphate release during particulate organic matter photodegradation based on optical and molecular signatures
Water Research, 2023, 236: 119954.
