Replacing Spartina alterniflora with northward-afforested mangroves has the potential to acquire extra blue carbon.

Climate change provides an opportunity for the northward expansion of mangroves, and thus, the afforestation of mangroves at higher latitude areas presents an achievable way for coastal restoration, especially where invasive species S. alterniflora needs to be clipped. However, it is unclear whether replacing S. alterniflora with northward-afforested mangroves would benefit carbon sequestration. In the study, we examined the key CO2 and CH4 exchange processes in a young (3 yr) northward-afforested wetland dominated by K. obovata. We also collected soil cores from various ages (3, 15, 30, and 60 years) to analyze the carbon storage characteristics of mangrove stands using a space-for-time substitution approach. Our findings revealed that the young northward mangroves exhibited obvious seasonal variations in net ecosystem CO2 exchange (NEE) and functioned as a moderate carbon sink, with an average annual NEE of -107.9 g C m-2 yr-1. Additionally, the CH4 emissions from the northward mangroves were lower in comparison to natural mangroves, with the primary source being the soil. Furthermore, when comparing the vertical distribution of soil carbon, it became evident that both S. alterniflora and mangroves contributed to organic carbon accumulation in the upper soil layers. Our study also identified a clear correlation that the biomass and carbon stocks of mangroves increased logarithmically with age (R2 = 0.69, p < 0.001). Notably, both vegetation and soil carbon stocks (especially in the deeper layers) of the 15 yr northward mangroves, were markedly higher than those of S. alterniflora. This suggests that replacing S. alterniflora with northward-afforested mangroves is an effective long-term strategy for future coasts to enhance blue carbon sequestration.

Upstream nitrogen availability determines the Microcystis salt tolerance and influences microcystins release in brackish water.

The occurrence of large Microcystis biomass in brackish waters is primarily caused by its downward transportation from the upstream freshwater lakes and reservoirs through rivers rather than due to in situ bloom formation. Factors that determine the survival of freshwater cyanobacteria in brackish waters have not been well investigated. Here, we studied the spatiotemporal variability of inorganic nitrogen in an upstream lake and conducted laboratory and in-situ experiments to assess the role of nitrogen availability on the salt tolerance of Microcystis and the release of microcystins. A series of field experiments were carried out during bloom seasons to evaluate the salt tolerance of natural Microcystis colonies. The salt tolerance threshold varied from 7 to 17 and showed a positive relationship with intracellular carbohydrate content and a negative relationship with nitrogen availability in water. In August when upstream nitrogen availability was lower, the Microcystis colonies could maintain their biomass even after a sudden increase in salinity from 4 to 10. Laboratory-cultivated Microcystis that accumulated higher carbohydrate content at lower nitrogen availability showed better cell survival at higher salinity. The sharp release of microcystins into the surrounding water occurred when salinity exceeded the salt tolerance threshold of the Microcystis. Thus, Microcystis with higher salt tolerance can accumulate more toxins in cells. The obtained results suggest that the cell survival and toxin concentration in brackish waters depend on the physiological properties of Microcystis formed in the upstream waters. Thus, the life history of Microcystis in upstream waters could have a significant impact on its salt tolerance in downstream brackish waters, where the ecological risk of the salt-tolerant Microcystis requires special and careful management in summer at low nitrogen availability.

Denitrification shifted autotroph-heterotroph interactions in Microcystis aggregates.

Denitrification is the most important process for nitrogen removal in eutrophic lakes and was mostly investigated in lake sediment. Denitrification could also be mediated by cyanobacterial aggregates, yet how this process impacts nitrogen (N) availability and the associated autotroph-heterotroph relationships within cyanobacterial aggregates has not been investigated. In this study, incubation experiments with nitrate amendment were conducted with Microcystis aggregates (MAs). Measurement of nitrogen contents, 16S rRNA-based microbial community profiling and metatranscriptomic sequencing were used to jointly assess nitrogen turnover dynamics, as well as changes in microbial composition and gene expression. Strong denitrification potential was revealed, and maximal N removal was achieved within two days, after which the communities entered a state of severe N limitation. Changes of active microbial communities were further promoted both with regard to taxonomic composition and transcriptive activities. Expression of transportation-related genes confirmed competition for N sources by Microcystis and phycospheric communities. Strong stress response to reactive oxygen species by Microcystis was revealed. Notably, interspecific relationships among Microcystis and phycospheric communities exhibited a shift toward antagonistic interactions, particularly evidenced by overall increased expression of genes related to cell lysis and utilization of cellular materials. Patterns of fatty acid and starch metabolism also suggested changes in carbon metabolism and cross-feeding patterns within MAs. Taken together, this study demonstrated substantial denitrification potential of MAs, which, importantly, further induced changes in both metabolic activities and autotroph-heterotroph interactions. These findings also highlight the key role of nutrient condition in shaping autotroph-heterotroph relationships.

Reducing the water residence time is inadequate to limit the algal proliferation in eutrophic lakes.

The eutrophication problem now threatens many lakes and reservoirs. To avoid the occurrence of algal blooms, some cities try to increase the flow rate or directly choose lakes or reservoirs with a short water residence time (WRT) as drinking water sources. However, up to now, whether such a strategy can achieve its goal is still unclear. In this study, a newly restored lake with a WRT of approximately 3 days was chosen to investigate algal growth potential as well as its responses to external nitrogen (N) and phosphorus (P) inputs. The results suggested that although the water quality of the lake could generally meet the environmental quality standards for surface water, dissolved inorganic nitrogen reached a high level with an average value of 1.58 mg/L. Meanwhile, a considerable increase in Chl-a concentration was observed across the flow direction. Especially, in July, Chl-a concentration at the site near the outlet was 8.1 times higher than that at the inlet, and cyanobacteria became the dominant species accounting for 83% of the total cell density. Nutrient enrichment experiments showed that algae could grow rapidly within 3 days with average specific growth rates (µ) of 0.36-0.42 d-1. The addition of N and P furtherly promoted the algal growth, and µ values of the treatments with P addition were the highest at 0.67-0.83 d-1. These results indicated that even if the WRT was reduced to 3 days, the risk of the occurrence of algal blooms still exists, and this undesirable trend would be enhanced by the short-term external nutrient input. Our findings indicated that the hydrodynamic control measures may not be entirely successful in protecting the drinking water source from algal blooms, especially when its influent has already been under eutrophication.

Nitrogen availability affects the dynamics of Microcystis blooms by regulating the downward transport of biomass.

Nitrogen availability is one of the key factors affecting the dynamics of non-diazotrophic cyanobacterial blooms in eutrophic lakes. While previous studies mainly focused on the promoting effect of nitrogen on the growth of cyanobacteria, this study aimed to investigate the role of nitrogen availability in the downward transport of biomass and its effects on the dynamics of Microcystis blooms. We performed field enclosure experiments which demonstrated that nitrogen availability negatively affects the downward transport of biomass. With a nitrogen loading of 0.02 g N m-2 d-1, the Microcystis biomass in the water column decreased by 56.2% over a 4-day period. During the same period of time, the average sinking ratio was 0.23 d-1; moreover, the termination of biomass growth was detected. At the notably higher nitrogen loading of 0.5 g N m-2d-1, the downward transport of biomass could still compensate for the biomass growth, although the average sinking ratio was lower at 0.16 d-1. Additional laboratory culture experiments demonstrated that the increase in the downward transport of Microcystis occurred in parallel to an increase in the carbohydrate content and a decrease in gas vesicle content. Further proteomic analysis indicated that the carbohydrate accumulation induced by nitrogen deficiency was a result of the slowing down of catabolic consumption, especially the downregulation of glycolysis. Thus, our study suggests that increased intracellular carbohydrate accumulation at low nitrogen availability causes a higher sinking ratio of Microcystis, indicating that nitrogen limits the duration of Microcystis blooms; thus, decreased nitrogen availability may lead to increased sinking of biomass out of the water column, accelerating the dissipation of Microcystis blooms.

Restoring wetlands outside of the seawalls and to provide clean water habitat.

In this study, we reported a practice at northern Hangzhou Bay, southeast China aimed at restoring coastal wetlands within the intertidal zone outside of the seawalls. The principle idea is protecting the site and helping the marsh establishment by engineering measures, and thereafter, relieving the protections to encourage the self-organization of the restored ecosystem. The results of this implementation showed the marsh reached an average vegetation cover of 70% in the first year. The excess nitrogen was removed by an ecological recirculating treatment system, which was coupled in the wetland. The long-term performance of the wetland suggested that it could resist disturbances such as hurricanes and algal blooms, and provided clean water habitat for aquatic fauna. By presenting the case of Hangzhou Bay, we call for more novel coastal restoration implementations that aim to create new boundaries with engineering features and self-organization, which benefit both human and nature.

The nitrogen reduction in eutrophic water column driven by Microcystis blooms.

During the bloom seasons, the dissolved inorganic nitrogen declines, which results in the occurrence of nitrogen limitation. It is unclear where the nitrogen goes. Our enclosure experiments and batch tests suggested that Microcystis blooms could significantly reduce the nitrogen in water bodies and the key mechanisms for the nitrogen reduction in different layers were different. The assimilation was the main pathway for nitrogen reduction in the surface layer, while denitrification played an important role both at the sediment-water interface and in the overlying water. Stable nitrogen isotope experiments showed that the nitrate reduction efficiency at sediment-water interface was enhanced by Microcystis, reaching to 76.5∼84.7 %. Dissimilation accounted for 63.8∼67.3 % of the nitrate reduction, and the denitrification rate was 7.4∼8.5 times of DNRA rate. In the water column, the Microcystis bloom facilitated the formation of dark/anoxic condition, which favored the denitrification. The Microcystis aggregates collected from the field showed a great potential in removing nitrogen, and the TN in the overly water was reduced by 3.76∼6.03 mg L-1 within two days. This study provided field evidences and deeper insights into the relationship between Microcystis blooms and nitrogen reduction in the whole water column and gave more details about the enhancing effects of Microcystis on nitrogen reduction.

Nitrogen limitation affects the sinking property of Microcystis by promoting carbohydrate accumulation.

Nitrogen limitation has been proven to inhibit Microcystis proliferation, and the significant decline in Microcystis blooms in late summer or autumn has been considered to be related to the nitrogen depletion in water. Sinking loss is another factor that influences the dynamics of cyanobacteria in lakes. However, to date, it is still unclear how the sinking property of Microcystis responds to nitrogen availability. Our results suggest that nitrogen limitation would directly influence sinking property of Microcystis, through a significant increase in the specific density of cells. In the short term, carbohydrate accumulation was mainly responsible for the high specific density, showing a high correlation among the NO3--N concentration, specific density and carbohydrate content. Furthermore, carbohydrates could rapidly accumulate after one light/dark cycle, which was mainly due to the reduction in carbohydrate consumption in the darkness under nitrogen limitation. Under nitrogen-light coupling conditions, the specific density ranged from 1.060 to 1.068, except for the treatment with high-nitrogen plus low-light, which showed the value of 1.032. More importantly, when coupled with low nitrogen, the low light did not decrease the carbohydrate content and the specific density, which implied that the sinking cells could not migrate back to the surface. Accordingly, a hypothesis was proposed that the carbohydrate accumulation induced by low nitrogen availability caused an increase in specific density, which invalidates the buoyancy regulation, and cells sink continually out of the water column. This study explores a new understanding on the disappearance mechanisms of Microcystis blooms in the late summer and fall.

The secretion of organics by living Microcystis under the dark/anoxic condition and its enhancing effect on nitrate removal.

Recent studies indicated that the algal decomposition produces particulate and dissolved organic carbon (DOC), and can enhance denitrification in eutrophic lakes. However, the effects of the living cyanobacteria on nitrogen cycling in eutrophic lakes were still an unknown question. This study explores a new underlying mechanism of nitrate removal which is driven by living Microcystis. The results suggested that living Microcystis significantly enhanced the nitrate removal at sediment-water interface, with a nitrate removal rate of 0.54 d-1, which was 2.57 times higher than the nitrate removal rate in the treatment without the addition of Microcystis. Measurements of Chl a and Fv/Fm confirmed that Microcystis was tolerant to the dark/anoxic condition, and the recovery experiments suggested that Microcystis could survive under such stress conditions for at least seven days. Meanwhile, DOC secreted by living Microcystis reached to 4.55 mg C mg-1 Chl a. These secretions were biodegradable hydrophilic and contained carbohydrates and proteins. Our study indicated that during blooms, sinking Microcystis cells could directly provide DOC as carbon source, then consequently enhanced the denitrification at sediment-water interface, and the interactive relationship between living cyanobacteria and permanent nitrate removal should be taken into account while studying nitrogen cycling in aquatic ecosystem.

The production of cyanobacterial carbon under nitrogen-limited cultivation and its potential for nitrate removal.

Harmful cyanobacterial blooms (CyanoHABs) represent a serious threat to aquatic ecosystems. A beneficial use for these harmful microorganisms would be a promising resolution of this urgent issue. This study applied a simple method, nitrogen limitation, to cultivate cyanobacteria aimed at producing cyanobacterial carbon for denitrification. Under nitrogen-limited conditions, the common cyanobacterium, Microcystis, efficiently used nitrate, and had a higher intracellular C/N ratio. More importantly, organic carbons easily leached from its dry powder; these leachates were biodegradable and contained a larger amount of dissolved organic carbon (DOC) and carbohydrates, but a smaller amount of dissolved total nitrogen (DTN) and proteins. When applied to an anoxic system with a sediment-water interface, a significant increase of the specific NOX--N removal rate was observed that was 14.2 times greater than that of the control. This study first suggests that nitrogen-limited cultivation is an efficient way to induce organic and carbohydrate accumulation in cyanobacteria, as well as a high C/N ratio, and that these cyanobacteria can act as a promising carbon source for denitrification. The results indicate that application as a carbon source is not only a new way to utilize cyanobacteria, but it also contributes to nitrogen removal in aquatic ecosystems, further limiting the proliferation of CyanoHABs.

Pressurized Microcystis can help to remove nitrate from eutrophic water.

The aim of this study was to investigate the feasibility of using harmful cyanobacterium Microcystis to help remove nitrate from eutrophic water. The results showed that after treatment by pressurization at 0.4MPa, Microcystis quickly sank to the bottom. Pressurization did not significantly affect the viability of Microcystis and this cyanobacterium maintained high viability over three days under dark/anoxic conditions. Meanwhile, the amount of dissolved organic carbon (DOC) secreted from living Microcystis cells reached 2.48mgCmg-1 Chl a, and a significant enhancement of pressurized Microcystis on nitrate removal at the sediment-water interface was observed, with a 2.85-fold increase in the specific NOX--N removal rate. The results of this study support the novel idea that harmful Microcystis could be converted to a carbon source for removing nitrate from eutrophic water by a simple pressurization measure.

Sinking loss should be taken into account while studying the dynamics of Microcystis under light-availability control.

In-situ light-availability control is commonly used to suppress Microcystis blooms in nutrient-rich water resources. It has been suggested that the reduction of column cyanobacterial biomass could mostly be attributed to the inhibition of photosynthesis. However, sinking loss may be another factor influencing the column cyanobacterial biomass. To further investigate the mechanism of this reduction, a mixing-static water column experimental apparatus was designed to simulate the reduction of Microcystis biomass under light-availability control. Under light-shading plus mixing, the reduction of Microcystis in the water column was attributed to both intrinsic biomass loss and sinking loss. Comparatively, under light-shading without mixing, the Microcystis accumulated in surface water, maintaining a continuously increase of intrinsic biomass. Meanwhile, the sinking loss increased as the water column became static, even exceeding the increase of intrinsic biomass, suggesting that sinking loss was the main mechanism for the reduction under light-shading. Further investigation indicated that both intrinsic growth rate and sinking loss rate varied in response to available light. Accordingly, a hypothesis is represented that the loss of column biomass and the shift in dominant species under light-availability control are primarily attributed to the combined effects of intrinsic biomass change and sinking loss, which both respond to available light.

Effects of HRT and water temperature on nitrogen removal in autotrophic gravel filter.

Organic Carbon added to low ratio of carbon to nitrogen (C/N ratio) wastewater to enhance heterotrophic denitrification performance might lead to higher operating costs and secondary pollution. In this study, sodium thiosulfate (Na2S2O3) was applied as an electron donor for a gravel filter (one kind of constructed wetland) to investigate effects of hydraulic retention time (HRT) and water temperature on the nitrate removal efficiency. The results show that with an HRT of 12 h, the average total nitrogen (TN) removal efficiencies were 91% at 15-20 °C and 18% at 3-6 °C, respectively. When HRT increased to 24 h, the average TN removal increased accordingly to 41% at 3-6 °C, suggesting denitrification performance was improved by extended HRT at low water temperatures. Due to denitrification, 96% of added nitrate nitrogen (NO3(-)-N) was converted to nitrogen gas, with a mean flux of nitrous oxide (N2O) was 0.0268-0.1500 ug m(-2) h(-1), while 98.86% of thiosulfate was gradually converted to sulfate throughout the system. Thus, our results show that the sulfur driven autotrophic denitrification constructed wetland demonstrated an excellent removal efficiency of nitrate for wastewater treatment. The HRT and water temperature proved to be two influencing factors in this constructed wetland treatment system.

Enhancement of nitrate removal at the sediment-water interface by carbon addition plus vertical mixing.

Wetlands and ponds are frequently used to remove nitrate from effluents or runoffs. However, the efficiency of this approach is limited. Based on the assumption that introducing vertical mixing to water column plus carbon addition would benefit the diffusion across the sediment-water interface, we conducted simulation experiments to identify a method for enhancing nitrate removal. The results suggested that the sediment-water interface has a great potential for nitrate removal, and the potential can be activated after several days of acclimation. Adding additional carbon plus mixing significantly increases the nitrate removal capacity, and the removal of total nitrogen (TN) and nitrate-nitrogen (NO3(-)-N) is well fitted to a first-order reaction model. Adding Hydrilla verticillata debris as a carbon source increased nitrate removal, whereas adding Eichhornia crassipe decreased it. Adding ethanol plus mixing greatly improved the removal performance, with the removal rate of NO3(-)-N and TN reaching 15.0-16.5 g m(-2) d(-1). The feasibility of this enhancement method was further confirmed with a wetland microcosm, and the NO3(-)-N removal rate maintained at 10.0-12.0 g m(-2) d(-1) at a hydraulic loading rate of 0.5 m d(-1).

Adsorption of bisphenol A from water by surfactant-modified zeolite.

Zeolite synthesized from coal fly ash (ZFA) was modified with hexadecyltrimethylammonium (HDTMA) and was examined for the adsorption of bisphenol A (BPA) from water. Two ZFAs were prepared in our laboratory and were characterized to obtain chemical and mineralogical composition, surface area, and total and external cation-exchange capacity among other parameters. HDTMA was confirmed to form bilayer micelles on external surfaces of zeolites. Results indicate that, while ZFA had no affinity for BPA, the surfactant-modified ZFA (SMZFA) showed greatly enhanced adsorption capacity. Uptake of BPA was greatly influenced by pH, increasing at alkaline pH conditions which enable the deprotonation of BPA to form organic anions. The SMZFA with higher BET area and higher amount of loaded HDTMA showed greater retention for BPA. Uptake of BPA by SMZFA was improved slightly in the presence of NaCl, and was enhanced at a low temperature. We propose that BPA anions interact strongly with the positively charged heads of HDTMA, with the two hydrophobic benzene rings of BPA pointing to the inside of HDTMA bilayers. The adsorption of uncharged BPA probably involved hydrophobic partitioning into HDTMA bilayers and the coordination of the oxygen atoms of BPA with positively charged heads of HDTMA.

Laboratory investigation of reducing two algae from eutrophic water treated with light-shading plus aeration.

The occurrence of harmful algal bloom in water source poses a serious water safety problem to local water supply systems. In order to ensure the raw water quality, the feasibility of reducing harmful algae by light-shading plus aeration was investigated. The batch test showed that algal biomass reduced rapidly under light-shading condition, and the reduction efficiency was further increased when light-shading was accompanied by aeration. The continuous flow experiment showed that the algal reduction efficiency increased with the increase of residence time. At residence time of 5 d, when treated with light-shading plus aeration, algal biomass could be reduced by more than 65%, with raw water quality improved simultaneously. Furthermore, considering that some harmful algae such as Microcystis tend to float upwards under light-limited condition, an integrated light-shading system consisting of pre-separation process and light-shading plus aeration treatment was suggested to treat naturally high algal water. The result showed that pre-separation process could remove more than 40% of algal biomass, and the total reduction efficiency of the integrated system increased to above 80%.

Application of zeolitic material synthesized from thermally treated sediment to the removal of trivalent chromium from wastewater.

Zeolitic materials were synthesized from thermally treated sediment by alkali treatment using different NaOH/sediment ratios. Characterization of the materials was done by XRD, FTIR, cation exchange capacity and specific surface area. Use of high NaOH/sediment ratio favored the formation of zeolite. The potential value of the zeolitic materials for the retention of trivalent chromium from water was examined. The maximum of Cr(III) sorption by the zeolitic materials, determined by a repeated batch equilibration method, ranged from 38.9 to 75.8 mg/g which was much greater than that of the thermally treated sediment (6.3 mg/g). No release of sorbed Cr(III) by 1.0M MgCl(2) at pH 7 was observed but Cr(III) desorption by ionic electrolyte increased with decreasing pH. The zeolitic materials could completely remove Cr(III) from wastewater even in the presence of Na(+) and Ca(2+) with high concentrations with a dose above 2.5 g/L. The pH-dependent desorption behavior and the high selectivity of zeolitic material for Cr(III) were explained by sorption at surface hydroxyl sites and formation of surface precipitates.

Phosphate removal and recovery through crystallization of hydroxyapatite using xonotlite as seed crystal.

Xonotlite was synthesized and tested for phosphate removal and recovery from synthetic solution in a batch mode. The effects of pH, initial calcium concentration, bicarbonate concentration on phosphate removal through crystallization were examined. The morphology and X-ray diffraction (XRD) pattern of xonotlite before and after crystallization confirmed the formation of crystalline hydroxyapatite. The results indicated that the crystallization product had a very high P content (> 10%), which is comparable to phosphate rock at the dosage of 50-200 mg xonotlite per liter, with a maximum P content of 16.7%. The kinetics of phosphate removal followed the second-order reaction equation. The phosphate removal ability increased with increasing pH. The precipitation of calcium phosphate took place when pH was higher than 7.2, whereas the crystallization occurred at pH 6.0. A high calcium concentration could promote the removal of phosphate via crystallization, while a high bicarbonate concentration also enhanced phosphate removal, through that the pH was increased and thus induced the precipitation process. When xonotlite was used to remove phosphate from wastewater, the removal efficiency could reach 91.3% after 24 h reaction, with removal capacity 137 mg/g. The results indicated that xonotlite might be used as an effective crystal seed for the removal and recovery of phosphate from aqueous solution.

Phosphate immobilization from aqueous solution by fly ashes in relation to their composition.

Phosphate sorption capacities of 15 Chinese fly ashes were determined and related to their composition. The data of P sorption were best fitted to Langmuir equation, and the calculated sorption maxima of phosphate (Qm) ranged from 5.51 to 42.55 mg/g. The Qm value showed a significantly positive correlation with total Ca content (r=0.9836**) and total Fe content (r=0.8049**), but negative correlation with total Si and total Al content. Correlation coefficients of CaO (r=0.9647**) and CaSO4 (r=0.9399**) were much greater than that of CaCO3 (r=0.6361*). Correlation coefficients of Qm with Fe2O3d and Al2O3d were much higher than those of total Fe and total Al contents, respectively. Fractionation of P sorbed by fly ash revealed that loosely bound P fraction and/or Ca+Mg-P fraction were the dominant form of immobilized phosphate. Ca content was strongly correlated with the Ca+Mg-P fraction instead of Mg content, whereas Fe content was highly correlated with Fe-Al-P fraction compared with Al content. The loosely bound P was correlated well with both Ca and Fe content. The greatest removal of phosphate occurred at alkaline conditions for high calcium fly ash, at neutral pH levels for medium calcium fly ash, while low calcium fly ash immobilized little phosphate at all pH values. This behavior was explained by the reaction of phosphate with Ca and Fe related components. It was concluded that P immobilization by fly ash was governed by Ca ingredient (especially CaO and CaSO4) and Fe ingredient (especially Fe2O3d).