Mechanisms of the photochemical release of phosphate from resuspended sediments under solar irradiation

Benthic primary production decreases internal phosphorus loading from lake sediments under light supplement

In aquatic ecosystems, light penetrating the sediment surface in shallow lakes may regulate the internal phosphorus (P) release through benthic primary production, which subsequently affects oxidation, pH levels, and alkaline phosphatase activity in the upper sediment. To study the effects of light exposure on the P dynamics at the sediment-water interface under eutrophic conditions, a two-month mesocosm experiment was conducted in twelve cement tanks (1000 L each). The tanks were equipped with Light-Emitting Diode (LED) lights, and surface sediments collected from eutrophic Lake Nanhu (China) were exposed to four different light intensities (0, 50, 100, 200 μmol m-2 s-1). The results revealed that: 1) Both the total phosphorus concentration and the phosphorus release flux from the sediment were lower in the light treatments (mean value, 0.59-0.71 mg L-1 and 0.00-0.01 mg m-2 d-1, respectively) than in the control treatment (0.77 mg L-1 and 0.01 mg m-2 d-1, respectively), indicating that light supplement could decrease the internal P release. 2) Benthic primary production promoted by light directly absorbed soluble reactive phosphorus and decreased the internal P release. The resulting improved production could also increase dissolved oxygen concentrations at the sediment-water interface, thus indirectly inhibiting internal P release. 3) The relative contributions of direct absorption and indirect inhibition on the internal P release ranged between 23% to 69% and 31% to 77% depending on the light intensity.

New insights into the mechanism of phosphate release during particulate organic matter photodegradation based on optical and molecular signatures

Phosphate release from particulate organic matter (POM) dominates phosphorus (P) cycling in aquatic ecosystems. However, the mechanisms underlying P release from POM remain poorly understood because of complex fractionation and analytical challenges. In this 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). POM in suspension was significantly photodegraded under light irradiation, concomitantly with the production and release of DIP in the aqueous solution. Chemical sequential extraction revealed that organic phosphorus (OP) in POM participated in photochemical reactions. Moreover, FT-ICR MS analysis revealed that the average molecular weight of P-containing formulas decreased from 374.2 to 340.1 Da. Formulas containing P with a lower oxidation degree and unsaturation were preferentially photodegraded, generating oxygen-enriched and saturated formula compounds, such as protein- and carbohydrate-like P-containing formulas, benefiting further utilization of P by organisms. Reactive oxygen species played an important role in the photodegradation of POM, and excited triplet state chromophoric dissolved organic matter (³CDOM*) was mainly responsible for POM photodegradation. These results provide new insights into the P biogeochemical cycle and POM photodegradation in aquatic ecosystems.

The response of aquatic ecosystems to the interactive effects of stratospheric ozone depletion, UV radiation, and climate change

Variations in stratospheric ozone and changes in the aquatic environment by climate change and human activity are modifying the exposure of aquatic ecosystems to UV radiation. These shifts in exposure have consequences for the distributions of species, biogeochemical cycles, and services provided by aquatic ecosystems. This Quadrennial Assessment presents the latest knowledge on the multi-faceted interactions between the effects of UV irradiation and climate change, and other anthropogenic activities, and how these conditions are changing aquatic ecosystems. Climate change results in variations in the depth of mixing, the thickness of ice cover, the duration of ice-free conditions and inputs of dissolved organic matter, all of which can either increase or decrease exposure to UV radiation. Anthropogenic activities release oil, UV filters in sunscreens, and microplastics into the aquatic environment that are then modified by UV radiation, frequently amplifying adverse effects on aquatic organisms and their environments. The impacts of these changes in combination with factors such as warming and ocean acidification are considered for aquatic micro-organisms, macroalgae, plants, and animals (floating, swimming, and attached). Minimising the disruptive consequences of these effects on critical services provided by the world's rivers, lakes and oceans (freshwater supply, recreation, transport, and food security) will not only require continued adherence to the Montreal Protocol but also a wider inclusion of solar UV radiation and its effects in studies and/or models of aquatic ecosystems under conditions of the future global climate.

The effect of iron oxide types on the photochemical transformation of organic phosphorus in water

Iron oxides play an important role in the transport and transformation of organic phosphorus in aquatic environments. However, the effect of different types of iron oxide on the environmental fate of organic phosphorus has remained unclear. In this study, the photodegradation of the organic phosphorus compound adenosine triphosphate (ATP) via the activity of crystalline (goethite) and amorphous (ferrihydrite) iron oxides was investigated. It was found that ATP was photodegraded by goethite, resulting in the release of dissolved inorganic phosphate under simulated sunlight irradiation. The concentration of ATP on goethite decreased by 75% after 6 h of simulated sunlight irradiation, while the concentration of ATP on ferrihydrite decreased by only 22%. ATR-FTIR spectroscopy revealed that the intensity of the peaks for the P-O and PO stretching vibrations in the goethite-ATP complex decreased significantly more after simulated sunlight irradiation than did those for the ferrihydrite treatment. Combined with the higher TOC/TOC₀ values for the goethite treatment, the results indicate that a more vigorous photochemical reaction took place in the presence of goethite than with ferrihydrite. Reactive oxygen species analysis also showed that hydroxyl and superoxide anion radicals were generated when goethite was exposed to simulated sunlight irradiation, while ferrihydrite did not exhibit this ability. Overall, this study highlights that the type of iron oxide is an important factor in the transformation of organic phosphorus in aquatic environments.

Molecular insight into the release of phosphate from dissolved organic phosphorus photo-mineralization in shallow lakes based on FT-ICR MS analysis

Dissolved organic phosphorus (DOP) is a key factor in the water eutrophication process because of its high potential bioavailability and inorganic phosphate (Pi) compensation ability through bio- and photo-mineralization. However, the research on the characterization and transformation of DOP is insufficient owing to their complex composition. This study investigates the release of dissolved Pi from DOP photo-mineralization in Lake Dong based on Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis. The results showed that the photo-release of dissolved Pi is spatially heterogeneous in Lake Dong and is consistent with the distribution of DOP concentration. The FT-ICR MS results showed that the simulated irradiation decreased the relative abundance (RA) of the DOP molecular formulae with higher molecular weight (MW) and higher double bond equivalence values (DBE), while the RA of DOP molecular formulae with lower MW and lower DBE value increased or remained. Besides, the total RA of lipid-like formulae increased from 49.09% to 55.90%, while the oxy-aromatic-like formulae decreased from 50.91% to 44.10%, suggesting that simulated irradiation would influence the potential bioavailability of DOP. As the main photolysis medium during DOP photo-mineralization, the hydroxyl radicals (∙OH) are mainly derived from dissolved organic matter (DOM) compared to the nitrate (NO₃^-) and iron ion (Fe³⁺) in Lake Dong. These results are important in understanding the ability and mechanism of DOP photo-mineralization and provide suggestions for cycling phosphorus in eutrophic shallow lakes.

Photogeochemistry of particulate organic matter in aquatic systems: A review

Photochemical transformation of natural organic matter in aquatic environments strongly impacts the environmental behaviors of carbon, nutrients, and pollutants by affecting their solubility, toxicity, bioavailability, and mobility. However, the role of particulate organic matter (POM) in environmental photogeochemistry has received much less attention than that of dissolved organic matter (DOM). In this study, a systematic overview was conducted to summarize the photodissolution and photoflocculation of POM in aquatic systems. The photodissolution of various POM, such as resuspended sediments and algal detritus, could be a potential and important source of DOM in the overlying waters, and these photoreleased DOM were dominated by humic-like components. The photogeochemistry of POM is thought to proceed via direct photochemical reactions and reactive radical-dominated indirect processes. Photodissolution can modify the bioavailability of organic matter and influence the biogeochemical cycling of nutrients, heavy metals, and organic pollutants. In addition, the photo-induced flocculation of DOM to POM could also influence the transport and transformation of organic matter and its associated pollutants. The photochemistry of POM can be significantly influenced by several environmental factors, including irradiation wavelength and intensity, organic matter properties, and radical oxygen species. POM photogeochemistry is one of the most important components of the global cycling of natural organic matter. Further studies regarding photogeochemistry should be conducted to overcome the potential problems arising from the concurrent photodegradation of organic matter and to further develop more filed investigations and analytical methods.

Simultaneous removal of cyanobacterial blooms and production of clean water by coupling flocculation with a rotary drum filter

A mechanical harvesting technology based on coupling flocculation with a rotary drum filter (RDF, 35-μm) was applied to remove cyanobacterial blooms and produce clean water in Lake Caohai, a sub-lake of Lake Dianchi (Kunming, China). After treatment with a shipboard RDF and cationic polyacrylamide (CPAM, 0.5-2 mg/L) flocculation, > 95% of cyanobacterial biomass was removed. The chlorophyll-a (Chl-a) concentration and turbidity in the effluent were < 8 μg/L and < 3 NTU, respectively. Nutrient concentrations were also markedly reduced, with a permanganate index (PI) of < 2 mg/L and total phosphorus concentration of < 20 μg/L. The total nitrogen concentration was reduced from 2.75 to 1.65 mg/L, and most of the residual nitrogen was nitrate. Although powerful for the removal of suspended particles and an enhanced water transparency, the combined technology showed no significant reduction in inorganic nutrients and only a slight reduction in dissolved organic matter (DOM). The concentrations of protein and polysaccharide were significantly reduced, while that of humic matter did not change during the process. After flushing with the effluent of the RDF, a 20,000-m³ enclosure of lake water became clear when the volume of the effluent was 1.8 times that of the water enclosure. The electrical energy per order (EE/O) was calculated to be 0.053kWh/m³, which is lower than that of transferring water from more than 10 km away (0.058 kWh/m³). Thus, a shipboard RDF coupled with CPAM flocculation is a promising approach to remove harmful cyanobacterial blooms and improve the water environment of eutrophic lakes.

Mechanisms of photochemical release of dissolved organic matter and iron from resuspended sediments

Photochemical reactions can alter the transformation of sedimentary organic matter into dissolved organic matter (DOM) and affect its ultimate fate in water ecosystems. In the present study, the photorelease of DOM and Fe from resuspended lake sediments was investigated under different O₂ and NO₃^- concentration conditions to study the mechanisms of DOM and Fe photorelease. The amount of photoreleased Fe, which ranged from 0.22 to 0.70 μmol/L, was significantly linearly correlated with the amount of photoreleased DOM. O₂ and NO₃^- could promote the photochemical release of DOM and Fe, especially during the initial 4 h irradiation. In general, the order of the photorelease rates of DOM and Fe under different conditions was as follows: NO₃^-/aerobic > aerobic ≈ NO₃^-/anaerobic > anaerobic. The photorelease rates of DOM and Fe were higher for the initial 4 hr irradiation than these for the subsequent 8 hr irradiation. The photorelease of DOM and Fe is thought to proceed via direct photodissolution and indirect processes. The relative contributions of indirect processes (>60%) was much greater than that of direct photodissolution (<40%). The photoproduced H₂O₂ under aerobic and anaerobic conditions indicated that hydroxyl radicals (•OH) are involved in the photorelease of DOM. Using •OH scavengers, it was found that 38.7%, 53.7%, and 77.6% of photoreleased DOM was attributed to •OH under anaerobic, aerobic, and NO₃^-/aerobic conditions, respectively. Our findings provide insights for understanding the mechanisms and the important role of •OH in the DOM and Fe photorelease from resuspended sediments.

The photochemical release of dissolved organic matter from resuspended sediments: Insights from fluorescence spectroscopy

Sediments exposed to sunlight can serve as an important source of dissolved organic carbon (DOC) and nutrients to overlying waters. However, the photochemical release processes of dissolved organic matter (DOM) from resuspended sediments and the characteristics of photoreleased DOM are not fully understood. In this study, excitation-emission matrix fluorescence combined with parallel factor analysis (EEMs-PARAFAC) was used to study the photochemical release of DOM qualitatively and quantitatively. The EEMs-PARAFAC demonstrated that the photoreleased DOM is dominated by humic-like substances, and the photorelease process could be consist of the photoproduction and photodegradation of DOM. The concurrent photodegradation may result in the underestimation of photoreleased DOM. Moreover, the significant increases in DOC content and fluorescence intensity of humic-like components along with increasing nitrate and ferric ion indicated that nitrate and ferric ion could facilitate the photoproduction of DOM through the photochemical produced hydroxyl radical. However, the decreases in DOC and fluorescence intensity were also observed at high concentration of nitrate and ferric ion, owing to the photodegradation of DOM by redundant hydroxyl radical. All of these results suggest that EEMs-PARAFAC is an effective and sensitive analytical technique for evaluating DOM photoreleased from suspended lake sediments and previous studies may underestimate photochemical release of DOM from sediments due to the overlook of the subsequently photodegradation of these released DOM. Thus, the photochemical release of DOM and its associated pollutants from suspended particles in shallow and eutrophic lakes should be more significant then should be paid more attention.

Photo-induced phosphate release during sediment resuspension in shallow lakes: A potential positive feedback mechanism of eutrophication

Dissolved phosphate (P_i) can be released during resuspended sediments exposed to sunlight. However, the significance of this phenomenon in the process of eutrophication is not clear. In this study, the behavior of photo-induced P_i release during sediment resuspension in shallow lakes with the different trophic states was investigated. The amount of photo-induced P_i release in the sediment resuspension from Lake Liangzi, Lake Dong, Lake Tangxun and Lake Longyang in China was 0.013, 0.019, 0.032, and 0.048 mg/L, respectively, and increased as the trophic states of the lakes increased. The results of phosphorus speciation analysis showed that the phosphate monoester in the particulate phosphorus is the organic phosphorus species participated in the photochemical reaction. The steady-state concentration of hydroxyl radical (OH) in the sediment resuspension also increased along with the trophic states of lakes increased and dissolved organic matter (DOM), nitrate, and Fe³⁺ presented in sediment resuspension were the main photosensitizers for OH production. All these results indicate that the increase of trophic states of lakes leads to the accumulation of organic phosphorus and OH, resulting in more dissolved phosphate photo-released, which accelerate the eutrophication process in a form of positive feedback.

Distribution of organic phosphorus species in sediment profiles of shallow lakes and its effect on photo-release of phosphate during sediment resuspension

In this work, the distribution of organic phosphorus (P_o) species in sediment profiles of five shallow lakes was analyzed and its effect on the photo-release of dissolved phosphate (P_i) was investigated during sediment resuspension under simulated sunlight irradiation. The results show that P_o was highly enriched in the surface sediment and gradually decreased as sediment depths increased: 33.10 ± 2.55-96.71 ± 7.60 mg/kg, 33.55 ± 2.34-142.86 ± 5.73 mg/kg, 57.50 ± 3.46-149.68 ± 7.67 mg/kg, 55.18 ± 4.67-168.73 ± 8.31 mg/kg, 98.75 ± 7.56-275.74 ± 10.70 mg/kg for Lake Hou, Lake Tuan, Lake Tangling, Lake Guozheng and Lake Miao, respectively. The photo-release amount of dissolved P_i in the resuspension composed of surface sediments was also higher than that of deep sediment during sediment resuspension under the simulated sunlight irradiation for 9 h. The potential reasons for these results are: (1) difference in morphology and composition of sediments at different depths: the mean particle size of sediment decreased first and then increased as sediment depths increased; (2) difference in composition of P_o species with depths in the sediment profiles: more photolytic P_o species existed in surface sediments confirmed by sequential extraction and ³¹P NMR analysis; and (3) more OH production in the resuspension composed of surface sediment under simulated sunlight irradiation, which directly influence the photo-release of dissolved P_i from photodegradation of organic phosphorus. All of these results indicate that the distribution of organic phosphorus species in the sediment profiles plays an important role in P cycle and its photodegradation during sediment resuspension may be one of the potential pathways for phosphate supplement in shallow lakes.

Eutrophication triggers the shift of nutrient absorption pathway of submerged macrophytes: Implications for the phytoremediation of eutrophic waters

Ecologically restoring eutrophic water bodies by using submerged macrophytes is an economical, effective and sustainable technology worldwide. However, current understanding on the nutrient absorption pathway of submerged macrophytes in freshwater ecosystems, especially under different trophic states, is still limited. In this study, two strategically designed systems were established to form isolated units for preventing nutrient exchange amongst Potamogeton crispus, water column and sediments. Results showed that, in oligotrophic state, P. crispus mainly relied on their roots to absorb nutrients from sediments for maintaining stable growth, with the maximum average height, fresh weight and relative growth rate of 12.85 cm, 4.86 g ind^-1 and 0.062, respectively. However, the eutrophic conditions (TN of 4 mg L^-1 and TP of 0.3 mg L^-1) triggered the shift of the nutrient absorption pathway from the roots to the shoots to some extent, that is, the shoots of P. crispus gradually became a remarkable pathway to directly absorb nutrients from the water column. Approximately 49.85% and 18.35% of total nitrogen (TN) and total phosphorus (TP) from overlying water were allocated to the shoots of P. crispus, but had no effects on the growth, photosynthesis and ecological stoichiometric differences (p > 0.05). Sediments acting as a nitrogen (N) source supported nearly 11.56% of TN for shoot uptake and simultaneously received around 13.33% of TP subsidy from the overlying water. The no longer sole dependence of submerged macrophytes on their root system to absorb N and phosphorus nutrients indicated that the ability of shoots to absorb nutrients increased with the gradual increase in nutrients in water column. These findings imply that the large specific surface area of shoots is beneficial for restoring eutrophic waters.

Photomineralization of Effluent Organic Phosphorus to Orthophosphate under Simulated Light Illumination

Organic phosphorus (OP), one of the main forms of phosphorus in effluent from biological wastewater treatment plants, may contribute to the bioavailable phosphorus pool as well as water eutrophication. However, little is known about the photomineralization of OP or the possible impacts on the phosphorus cycle in water bodies. Herein, the photomineralization of effluent OP was investigated. An increase in orthophosphate concentration was observed under illumination. The ³¹P liquid nuclear magnetic resonance spectra demonstrated that the release of orthophosphate resulted from photomineralization of OP. Furthermore, the photoproduced hydroxyl radicals (·OH) were proved to play a dominant role in the OP photomineralization. Nitrate, effluent organic matter (EfOM), and Fe(III) presented in effluent were the main chromophores for ·OH photoproduction, and their contributions to ·OH production and photomineralization of OP followed the order: nitrate > EfOM > Fe(III). Additionally, the carbonate (or bicarbonate) in the effluent and high pH were unfavorable for OP photomineralization. The present study revealed the photomineralization behavior of OP in actual effluent, suggesting that photomineralization of OP might contribute to eutrophication and may play a non-negligible role in phosphorus turnover in water bodies.