Characterization of phosphorus availability in response to radial oxygen losses in the rhizosphere of Vallisneria spiralis

Influence of different cyanobacterial treatment methods on phosphorus cycle in shallow lake microcosms

Cyanobacterial bloom is a pressing issue affecting water supply security and ecosystem health. Phosphorus (P) released from cyanobacterial bloom during recession is one of the most important components involved in the lake P cycle. However, little is known about the consequences and mechanisms of the P cycle in overlying water and sediment due to the anthropogenic treatments of cyanobacterial blooms. In this study, treatment methods using hydrogen peroxide (H2O2), polyaluminum chloride (PAC), and the feces of silver carp were investigated for their influence on the P cycle using microcosm experiments. Results showed that H2O2 treatment significantly increased the internal cycle of sediment-related P, while PAC treatment showed minor effects. H2O2 and PAC treatment suppressed the release of P from sediment before day 10 but promoted the release of P on day 20, while silver carp treatment suppressed the release of P during the whole experiment. The reductive dissolution of iron oxide-hydroxide was the major factor affects the desorption of P. Path analyses further suggested that overlying water properties such as dissolved oxygen (DO) and oxidation-reduction potential (ORP) play critical roles in the treatment-induced sediment P release. Our results quantify the endogenous P diffusion fluxes across the sediment-water interface attributed to cyanobacterial treatments and provide useful guidance for the selection of controlling methods, with silver carp being the most recommended of the three methods studied.

The effect of novel aquaculture mode on phosphorus sorption-release in pond sediment

The emergence of aquaculture modes has brought considerable changes to the aquaculture landscape and profoundly influenced environmental processes. However, there is limited research on nutrient cycling in emerging aquaculture modes. This study investigated the characteristics and mechanisms of sediment phosphorus (P) sorption-release in traditional earthen pond culture (TEP) and pond-tank culture mode (PTC), which represents novel aquaculture modes. The results showed that under higher nutrient load, the PTC did not show significant differences in nutrient concentration in water and sediments compared to TEP. Although there are no significant differences in overlying water P concentration between the modes throughout the entire aquaculture period, the trends of its variation over time are different, which significantly affected the P sorption-release characteristics of sediment. Additionally, correlation analysis suggested that calcium-bound P and hot NaOH-extractable organic P may affect the sorption-release characteristics of sediment as active P fractions. The change in redox condition caused by enzyme-mediated organic matter decomposition (such as protein and lipids) is also an important reason for sediment P release. However, the P fractions and organic matter content showed no significant differences between the two modes. Sediment microbial analysis showed that TEP exhibited a significant dominance of inorganic P-solubilizing bacteria, especially Actinobacteria and Bacilli classes. PTC had a higher proportion of organic P-solubilizing bacteria, primarily in the Bacteroidia class. The quantitative results of the key functional gene phoD in organic P decomposition also showed that the abundance in PTC was significantly higher than that in TEP. This suggested that microbial differences may be another reason for differences in P sorption-release behavior. This study revealed the differences in P sorption-release characteristics and mechanisms between the TEP and PTC, which holds positive implications for water quality and pollution management in novel aquaculture modes.

Long-term improvement of sediment in situ restoration and REDOX characteristics by Vallisneria natans coupling with carbon fiber

China is conducting ecological restoration work in urban water bodies. Under anoxic and anaerobic conditions, pollutants transform and produce odorous and black substances, deteriorating the water quality, which is a significant problem in urban water bodies. Vallisneria natans has received widespread attention for its applications in water treatment and restoration. However, the efficiency by which V. natans reduces water pollution and allows sediment remediation requires further improvement. Therefore, in this study, we investigated the effect of V. natans coupled with carbon fiber on the restoration of water bodies and sediment compared with the control group that grew V. natans without carbon fiber. The oxidation-reduction potential (ORP) was selected as the main evaluation index for the water and sediment. Dissolved oxygen in the water and total organic carbon and total nitrogen (TN) in the sediment were also evaluated. V. natans coupled with carbon fiber significantly increased the ORP; that of surface sediment increased by 50 % and that of the water body increased by 60 % compared with the sediment without any bioremediation. Chemical oxygen demand, total phosphorous, and TN in water decreased by 61.2 %, 22.9 %, and 48.3 %, respectively. These results indicate that planting V. natans with carbon fiber can reduce pollutants in water (including humus) and sediments, effectively improving ORP in water and sediment.

High-resolution imaging of O2 dynamics and metal solubilization in the rhizosphere of the hyperaccumulator Leersia hexandra Swartz

The mobilization of trace metals in the rhizosphere can be affected by the redox potential, which is closely related to the O₂ dynamics. This study examined the distributions of O₂ and trace metals in the rhizosphere of the subaquatic hyperaccumulator Leersia hexandra Swartz under chromium (Cr) stress using planar optodes and the diffusive gradients in thin films technique coupled with laser ablation inductively coupled plasma mass spectrometry. The O₂ concentrations and oxidized areas in the rhizosphere significantly increased with increases in the light intensity, air humidity, and atmospheric CO₂ concentrations (p < 0.05). The O₂ concentration first increased with increasing ambient temperatures, then decreased when the temperature increased from 25 to 32 ℃. The O₂ concentration in the rhizosphere was significantly decreased under Cr stress (p < 0.05), with a prolonged response time to the altered ambient temperature. Cr stress led to decreased mobilities of As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Sb, V, W, and Zn in the rhizosphere, which were negatively correlated with the concentrations of O₂. These results provide new insights into the role of changes in the O₂ concentration induced by the roots of hyperaccumulator plants in controlling the mobility of trace metals in soils.

Effects on the migration and speciation of heavy metals by combined capping and biochemical oxidation during sediment remediation

Capping and oxidation by lanthanum-modified bentonite (LMB) and calcium nitrate (CN) has a dual effect of deep phosphorus (P)/arsenic (As) clearance and surface P/As blockade. However, little information is available on the effect of LMB and CN on heavy metals. In this study, we hypothesize that LMB and CN exerted the same synergistic effect on heavy metals as P and As. We verified this through Rhizon samplers, diffusive gradients in thin films technology (DGT) and planar optode (PO) methods. The results showed that individual and combined LMB and CN treatments temporarily decreased but eventually increased the dissolved oxygen of the sediment-water interface (SWI). DGT-labile sulfide in the surface 110 mm sediment, soluble Fe(II) and DGT-labile Fe(II) in the surface 80 mm sediment were eliminated within 30 days by CN and LMB + CN treatments. A temporary sharp increase in soluble Fe, Mn, Co, and DGT-labile Mn, Co, Cu, and Ni was observed in CN and LMB + CN groups probably due to sulfide oxidation and carbonate dissolution. LMB + CN group showed a less-intense increase in DGT-labile metals and less metal release than the CN group (inferred from the total metal content). This indicates that LMB and CN had a synergistic effect on heavy metals. When using the LMB + CN treatment, LMB partly adsorbed and blocked metal release in sulfide and carbonate bound forms and finally transformed them into Fe and Mn oxides and residual forms. We suggest that CN should be combined with capping agents (at an appropriate pH) to compact sediments and block metal exchange at the SWI.

In situ remediation mechanism of internal nitrogen and phosphorus regeneration and release in shallow eutrophic lakes by combining multiple remediation techniques

Large anthropogenic inputs of N and P alter the nutrient cycle and exacerbate global eutrophication problems in aquatic ecosystems. This study in Lake Datong, China, investigates the remediation mechanism of multiple remediation technique combinations (dredging, adsorbent amendment, and planting aquatic vegetation) on sediment N and P loads based on two high-resolution sampling techniques (HR-Peeper and DGT) and P sequential extraction procedures. The results showed that high temperature and low dissolved oxygen considerably enhanced pore water dissolved reactive P (DRP) and NH₄⁺ concentrations attributable to abundant Fe-P and organic matter content in the sediment. Fe reduction is critical for regulating pore water DRP release and promoting N removal. Overall, for Lake Datong, combining multiple remediation techniques is more effective in controlling sediment P loads (pore water DRP, P fluxes, forms of P, and labile P), from a long-term perspective, than a single remediation. Lanthanum-modified bentonite (LMB) inactivation treatment can transfer mobile P in the surface sediment into more refractory forms over time, thereby reducing the risk of sediment labile P release. However, it is difficult to effectively remediate internal P loads owing to inappropriate dredging depths and low biomass of aquatic vegetation. Future lake restoration practices should optimize the selection of different remediation technique combinations based on internal N and P pollution characteristics, while reducing external wastewater input. These results are important for understanding the remediation mechanisms of internal N and P and provide suggestions for sediment management of shallow eutrophic lakes.

From macrophyte to algae: Differentiated dominant processes for internal phosphorus release induced by suspended particulate matter deposition

In shallow lakes, eutrophication leads to a shift of the macrophyte-dominated clear state towards an algae-dominated turbid state. Phosphorus (P) is a crucial element during this shift and is usually concentrated in the suspended particulate matter (SPM) in water. However, the dominant processes controlling internal P release in the algae- (ADA) and macrophyte-dominated (MDA) areas under the influence of P-concentrated SPM remains unclear. In this study, we conducted monthly field observations of P exchange across the sediment-water interface (SWI) with the deposition of SPM in the ADA and MDA of Lake Taihu. Results revealed that both algae- and macrophyte-originated SPM led to the depletion of oxygen across the SWI during summer and autumn. Redox-sensitive P (Fe-P) and organic P (Org-P) were the dominant mobile P fractions in both areas. High fluxes of P across the SWI were observed in both areas during the summer and autumn. However, the processes controlling P release were quite different. In MDA, P release was mostly controlled by a traditional Fe-P dissolution process influenced by the coupled cycling of iron, sulfur, and P. In the ADA, Org-P control was intensified with the deterioration of algal bloom status, accompanied with the dissolution of Fe-P. Evidence from the current study revealed that the dominant process controlling the internal P release might gradually shift from Fe-P to a coupled process of Fe-P and Org-P with the shift of the macrophyte- to an algae-dominated state in shallow eutrophic lakes. The differentiated processes in the MDA and ADA should be given more attention during future research and management of internal P loadings in eutrophic lakes.

Effects of vermiculite on the growth process of submerged macrophyte Vallisneria spiralis and sediment microecological environment

Ecological restoration is one of the hot technologies for the reconstruction of eutrophic lake ecosystems in which the restoration and propagation of submerged plants is the key and difficult step. In this paper, the effect of vermiculite on the growth process of Vallisneria spiralis and sediment microenvironment were investigated, aiming to provide a theoretical basis for the application of vermiculite in aquatic ecological restoration. Results of growth indexes demonstrated that 5% and 10% vermiculite treatment groups statistically promote the growth of Vallisneria spiralis compared to the control. Meanwhile, the results of ecophysiological indexes showed that photosynthetic pigment, soluble sugar content, superoxide dismutase (SOD), and catalase (CAT) activity of 5% and 10% group were increased compared with the control while the malondialdehyde (MDA) content exhibited the opposite result (p < 0.05), which illustrated that vermiculite can improve the resistance of plants and delay the aging process of Vallisneria spiralis. In addition, result of PCA (Principal Component Analysis) demonstrated 5% and 10% group has improved the sediment physical conditions and create more ecological niche for microorganisms directly, and then promoted the growth of plants. The dissolution results showed that vermiculite can dissolve the constant and trace elements needed for plant growth. Furthermore, the addition of vermiculite increased the diversity of microorganisms in the sediments, and promoted the increase of plant growth-promoting bacteria and phosphorus-degrading bacteria. This study could provide a technique reference for the further application of vermiculite in the field of ecological restoration.

Ecotoxicological responses and removal of submerged macrophyte Hydrilla verticillate to multiple perfluoroalkyl acid (PFAA) pollutants in aquatic environments

The ubiquitous existence of perfluoroalkyl acids (PFAAs) in aquatic environments might pose toxic potential to ecosystems. To assess the ecotoxicological responses and removal of submerged macrophyte to multiple PFAA pollutants in aquatic environments, a typical submerged macrophyte, Hydrilla verticillate, was exposed to solutions with 12 typical PFAAs in the present study. The results showed that PFAAs at concentrations higher than 10 μg/L had significantly passive effects on biomass, relative growth rates, chlorophyll contents, and chlorophyll autofluorescence. PFAAs could induce the accumulation of hydrogen peroxide and lipid peroxidation in H. verticillate. Significant upregulation of CAT was observed in treatments with more than 10 μg/L PFAAs (p < 0.05). The results also showed that 13.53-20.01% and 19.73-37.72% of PFAAs could be removed in treatments without plants and with H. verticillate, respectively. The removal rates of PFAAs were significantly correlated with perfluoroalkyl chain length in treatments with H. verticillate. The removal of PFAAs was suggested to be related to the uptake of plant tissues and biosorption of microbiota. Furthermore, the dominant microbiota and biomarkers were identified in water and biofilm. Betaproteobacteriales was the most dominant microbiota at the order level. The presence of PFAAs could significantly increase the relative abundance of Micrococcales, Verrucomicrobiales, Rhizobiales, Sphingomonadales, Roseomonas, Cyanobium_PCC_6307, and Synechococcales. Our results provide scientific basis for evaluating the ecotoxicological responses and removal of submerged macrophytes in response to multiple PFAA pollutants at environmentally relevant levels, thereby providing insights into PFAA management and removal.

Increase of P and Cd bioavailability in the rhizosphere by endophytes promoted phytoremediation efficiency of Phytolacca acinosa

The information about the spatial distribution of bioavailable phosphorus (P) and heavy metal (HM) in the rhizosphere could aid in the precise phytoremediation regulation. In this study, a rhizobox system was adapted to study soil-root interactions and used to access the endophyte inoculation variation on bioavailable P and cadmium (Cd) spatial distribution during phytoremediation of Cd contaminated soils. Results showed that endophyte PE31 Bacillus cereus inoculation enhanced Cd uptake of P. acinosa by 52.70% and 46.73% in low and high Cd contaminated soils, increasing the phytoremediation hotspot area from 45.78% and 15.29% to 60.97% and 21.80%, respectively. Available P and Cd significantly diminished because root activities depleted large amounts of bioavailable P and Cd concentrations. However, PE31 increased bioavailable P and Cd concentration in the rhizosphere soil. The bioavailable P enhancement in the rhizosphere was positively correlated to plant growth and Cd accumulation. Overall, endophyte inoculation compensated the diminution of bioavailable P and Cd in the rhizosphere to improve plant biomass and HM absorption, and thus promote phytoremediation efficiency. This study helped to better understand bioavailable P and Cd spatial distribution under endophyte inoculation, which could provide effective management strategies for the precise regulation of phytoremediation.

Polyethylene microplastics interfere with the nutrient cycle in water-plant-sediment systems

Increasing microplastic (MP) pollution and its effects on aquatic systems have become a global issue; however, the impact of MPs on biogeochemical cycles is poorly understood. A simulation study was performed to analyse the influence of polyethylene (PE) microplastics on the morphological, physiological, and stoichiometric (C, N, P) characteristics of submerged plants, and to investigate their effects on the nutrient cycle and microbial community in freshwater sediment. The results showed that PE-MPs treatments significantly decreased leaf nitrogen and carbon contents. Exposure to 1% PE-MPs suppressed the plant height, total biomass, root activity, and relative growth rate of Vallisneria natans. Decrease in dissolved oxygen (DO) concentrations (19.93-40.26%) were observed in the 1% PE-MPs treatment group compared to that in the control between 1 and 6 days. The activities of enzymes (ammonia monooxygenase and nitrate reductase) related to the nitrogen cycle were significantly altered by the addition of PE-MPs. We found that PE-MPs acted as obstacle disruptors, resulting in a reduction in the release of nitrogen and phosphorus from the sediment to the overlying water. This is because PE-MPs significantly alter the composition and metabolic properties of the microbial communities in sediments, the plant growth, and the nutrient cycle. These findings helped evaluate the impacts of PE-MPs on the water-plant-sediment system and on the biogeochemical cycles of the freshwater ecosystems.

Removal potential of multiple perfluoroalkyl acids (PFAAs) by submerged macrophytes in aquatic environments: Tolerance of Vallisneria natans and PFAA removal in submerged macrophyte-microbiota systems

Perfluoroalkyl acids (PFAAs) have emerged as a global concern in aquatic environment remediation due to their abundance, persistence, bioaccumulation, and toxicity. To comprehensively understand the removal potential of multiple PFAAs by submerged macrophytes in aquatic environments, systematic investigations into the tolerance of the typical submerged macrophyte Vallisneria natans to 12 typical PFAAs and the removal capacity to PFAAs in V. natans-microbiota systems were carried out. Results showed that although PFAAs could induce the accumulation of hydrogen peroxide and malondialdehyde, V. natans was overall resistant to multiple PFAAs with natural concentrations. Catalase is one of the main strategies of V. natans to alleviate PFAA stress. Microbiota can remove 18.10-30.84% of the PFAAs from the water column. 24.35-73.45% of PFAAs were removed from water in V. natans-microbiota systems. The uptake of plant tissues and the bioaccumulation of microbiota were proposed as the main removal processes. The removal rates were significantly correlated with the perfluorinated carbon atoms numbers (p < 0.05). PFAAs and V. natans increased the relative abundance of Betaproteobacteria, Nostocales, Microscillaceae, Sphingobacteriales, SBR1031, Chlamydiales, Phycisphaerae, Caldilineales, Rhodobacterales, and Verrucomicrobiales. The present study suggested that V. natans can be a potential species to remove multiple PFAAs in aquatic environments, and further providing insights into the PFAAs' remediation.

Mechanistic insights into trace metal mobilization at the micro-scale in the rhizosphere of Vallisneria spiralis

Mobilization of trace metals in the rhizosphere of macrophytes is controlled by root-driven chemical changes, especially the steep gradients of O₂ and pH from the rhizosphere to bulk sediments. Here, the O₂ and pH dynamics, and the distribution of trace metal, in the rhizosphere of Vallisneria spiralis were obtained using planar optodes and diffusive gradients in thin films, respectively. Radial O₂ loss (ROL) and acidification occurred on all visible roots of V. spiralis and exhibited highly spatiotemporal dynamics depending on the root growth and various environmental conditions. Trace metals showed different mobilization mechanisms in the rhizosphere. ROL and produced Fe(III) (oxyhydr)oxides decreased the mobility of Fe, As, Co, V and W in the rhizosphere. However, Mn, Ni and Cu exhibited greater mobility in the rhizosphere than bulk sediments as a result of the oxidation of metal sulfide and proton-induced dissolution of minerals. In particular, Co and Ni presented increased activity at the interface between rhizosphere and bulk sediment, which was attributed to the redox dissolution processes of Fe and Mn as a result of ROL and rhizosphere acidification. These results provide new insights into the roles of macrophyte root-induced O₂ and pH changes in controlling trace metal mobility in sediments.

Phosphorus removal from sediments by Potamogeton crispus: New high-resolution in-situ evidence for rhizosphere assimilation and oxidization-induced retention

Macrophytes are usually chosen for phytoremediation tools to remove P in eutrophic aquatic ecosystems, but the lack of test methods hinders the understanding of removal mechanism and application. In this study, we used the novel technologies combined of Diffusive gradients in thin films (DGT), Planar optode (PO), and Non-invasive micro-test technology (NMT) to explore P dynamics in water-sediment continuum and rhizosphere of Potamogeton crispus over time. Results of the high-resolution in situ measurement showed that labile P(LP_DGT) fluxes at the surficial sediment significantly decreased from approximate 120, 140, and 200 pg/ (cm²•sec) via 30 days incubation period to 17, 40, and 56 pg/(cm²•sec) via that of 15 days. Obvious synchronous increase of LP_DGT was not detected in overlying water, suggesting the intense assimilation of dissolve reactive P via root over time. PO measurement indicated that O₂ concentration around the rhizosphere remarkably increased and radially diffused into deeper sediment until 100% saturation along with the root stretch downwards. NMT detection of roots showed the obvious O₂ inflow into root tissue with the uppermost flux of 30 pmol/(cm²•sec) from surroundings via aerenchyma on different treatment conditions. Different from previous reports, gradually saturating O₂ concentrations around the rhizosphere was principally driven by O₂ penetration through interspace attributing to root stretch downward rather than root O₂ leakage. Increased O₂ concentrations in deep sediment over time finally induced the oxidization of labile Fe(II) into Fe(III) bound P and local P immobilization.

Submerged macrophytes successfully restored a subtropical aquacultural lake by controlling its internal phosphorus loading

Intensive aquaculture has largely changed the global phosphorus (P) flow and become one of the main reasons for the eutrophication of global aquatic ecosystem. Artificial planting submerged macrophytes has attracted enormous interest regarding the restoration of eutrophic lakes. However, few large-scale (>80 km²) studies have focused on the restoration of aquatic vegetation in the subtropical lakes, and the mechanism underlying the restrain of sediment P release by macrophytes remains unknown. In this study, field surveys and the diffusive gradients in thin films (DGT) technique were used to elucidate the effects of macrophytes on internal P loading control in a typical eutrophic aquacultural lake. Results showed that half of the P content in overlying water and sediments, particularly dissolved P in overlying water and calcium bound P (Ca-P) in sediment, were removed after restoration. Temperature, as well as dissolved oxygen (DO) and P concentration gradients near the sediment-water interface (SWI) jointly controlled the release of labile P from surface sediments. Submerged macrophytes can effectively inhibit the release of sediment P into the overlying water, which depended on DO concentration in the bottom water. Future restoration projects should focus on the temperature response of submerged macrophytes of different growth forms (especially canopy-forming species) to avoid undesirable restoration effects. Our results complement existing knowledge about submerged macrophytes repairing subtropical P-contaminated lakes and have positive significance for lake restoration by in situ phytoremediation.

Localized Intensification of Arsenic Release within the Emergent Rice Rhizosphere

Behavior of trace elements in flooded/lowland rice soils is controlled by root-zone iron oxidation. Insoluble iron species bind/capture toxic elements, i.e., arsenic. However, it was recently observed that within this territory of arsenic immobilization lies a zone of prolific iron release, accompanied by a significant flux of arsenic in close proximity to rice root apices. Questions still remain on how common this phenomenon is and whether the chemical imaging approaches or soils/cultivars used influence this event. Here, three types of ultrathin/high-resolution diffusive gradient in thin films (DGT) substrates were integrated with oxygen planar optodes in a multilayer system, providing two-dimensional mapping of solute fluxes. The three DGT approaches revealed a consistent/overlapping spatial distribution with localized flux maxima for arsenic, which occurred in all experiments, concomitant with iron mobilization. Soil/porewater microsampling within the rhizosphere revealed no significant elevation in the solid phase's total iron and arsenic concentrations between aerobic and anaerobic zones. Contrary to arsenic, phosphorus bioavailability was shown to decrease in the arsenic/iron flux maxima. Rice roots, in addition to their role in nutrient acquisition, also perform a key sensory function. Flux maxima represent a significant departure from the chemical conditions of the bulk/field environment, but our observations of a complete rhizosphere reveal a mixed mode of root-soil interactions.