Reservoir regulation-induced variations in water level impacts cyanobacterial bloom by the changing physiochemical conditions.

Gaining insight into the impact of reservoir regulation on algal blooms is essential for comprehending the dynamic changes and response mechanisms in the reservoir ecosystem. In this study, we conducted a comprehensive field investigation linking physiochemical parameters, and phytoplankton community to different water regimes in the Three Gorges Reservoir. Our aim was to explore the effects of reservoir regulation on the extinction of cyanobacterial blooms. The results showed that during the four regulatory events, the water levels decreased by 2.02-4.33 m, and the average water velocity increased 68 % compared to before. The average total phosphorus and total nitrogen concentrations reduced by up to 20 %, and the cyanobacterial biomass correspondingly declined dramatically, between 66.94 % and 75.17 %. As the change of water level decline increasing, there was a significant increase of algal diversity and a notable decrease of algal cell density. Additionally, a shift in the dominant phytoplankton community from Cyanobacteria to Chlorophyceae was observed. Our analysis indicated that water level fluctuations had a pronounced effect on cyanobacterial extinction, with hydrodynamic changes resulting in a reduction of cyanobacterial biomass. This research underlined the potential for employing hydrodynamic management as a viable strategy to mitigate the adverse ecological impacts of cyanobacterial blooms, providing a solution for reservoir's eco-environmental management.

Cadmium-Induced Physiological Responses, Biosorption and Bioaccumulation in Scenedesmus obliquus.

Cadmium ion (Cd2+) is a highly toxic metal in water, even at low concentrations. Microalgae are a promising material for heavy metal remediation. The present study investigated the effects of Cd2+ on growth, photosynthesis, antioxidant enzyme activities, cell morphology, and Cd2+ adsorption and accumulation capacity of the freshwater green alga Scenedesmus obliquus. Experiments were conducted by exposing S. obliquus to varying concentrations of Cd2+ for 96 h, assessing its tolerance and removal capacity towards Cd2+. The results showed that higher concentrations of Cd2+ (>0.5 mg L-1) reduced pigment content, inhibited algal growth and electron transfer in photosynthesis, and led to morphological changes such as mitochondrial disappearance and chloroplast deformation. In this process, S. obliquus counteracted Cd2+ toxicity by enhancing antioxidant enzyme activities, accumulating starch and high-density granules, and secreting extracellular polymeric substances. When the initial Cd2+ concentration was less than or equal to 0.5 mg L-1, S. obliquus was able to efficiently remove over 95% of Cd2+ from the environment through biosorption and bioaccumulation. However, when the initial Cd2+ concentration exceeded 0.5 mg L-1, the removal efficiency decreased slightly to about 70%, with biosorption accounting for more than 60% of this process, emerging as the predominant mechanism for Cd2+ removal. Fourier transform infrared correlation spectroscopy analysis indicated that the carboxyl and amino groups of the cell wall were the key factors in removing Cd2+. In conclusion, S. obliquus has considerable potential for the remediation of aquatic environments with Cd2+, providing algal resources for developing new microalgae-based bioremediation techniques for heavy metals.

Insights into the Enhanced Photogeneration of Hydroxyl Radicals from Chlorinated Dissolved Organic Matter.

Free available chlorine has been and is being applied in global water treatment and readily reacts with dissolved organic matter (DOM) in aquatic environments, leading to the formation of chlorinated products. Chlorination enhances the photoreactivity of DOM, but the influence of chlorinated compounds on the photogeneration of hydroxyl radicals (•OH) has remained unexplored. In this study, a range of chlorinated carboxylate-substituted phenolic model compounds were employed to assess their •OH photogeneration capabilities. These compounds demonstrated a substantial capacity for •OH production, exhibiting quantum yields of 0.1-5.9 × 10-3 through direct photolysis under 305 nm and 0.2-9.5 × 10-3 through a triplet sensitizer (4-benzoylbenzoic acid)-inducing reaction under 365 nm LED irradiation. Moreover, the chlorinated compounds exhibited higher light absorption and •OH quantum yields compared to those of their unchlorinated counterparts. The •OH photogeneration capacity of these compounds exhibited a positive correlation with their triplet state one-electron oxidation potentials. Molecular-level compositional analysis revealed that aromatic structures rich in hydroxyl and carboxyl groups (e.g., O/C > 0.5 with H/C < 1.5) within DOM serve as crucial sources of •OH, and chlorination of these compounds significantly enhances their capacity to generate •OH upon irradiation. This study provides novel insights into the enhanced photogeneration of •OH from chlorinated DOM, which is helpful for understanding the fate of trace pollutants in chlorinated waters.

Damage mechanism of calcium peroxide on Microcystis aeruginosa PCC7806 and its potential application.

Calcium peroxide (CP) is an oxidizing agent that can gradually release hydrogen peroxide (HP) to achieve selective killing of cyanobacteria in water blooms, and reduce the phosphorus content in the water column. Despite the potential of CP for use in cyanobacterial water bloom disposal, there is a lack of research on the mechanism of oxidative damage on cyanobacterial cells by calcium peroxide. Further studies are required to comprehend the underlying scientific principles and potential risks and benefits of applying this approach to cyanobacteria disposal. In this investigation, we employed varying doses of CP for the treatment of Microcystis aeruginosa (M. aeruginosa), which resulted in the following findings: (1) the HP released from CP can damage the photosystem II of M. aeruginosa, reduce cell photosynthetic pigment content, intensify the degree of membrane lipid peroxidation, and increase the extracellular protein content; (2) CP significantly increased the soluble extracellular polysaccharide (sEPS) and bound extracellular polysaccharide (bEPS) content of cells (p < 0.05), causing the cells to exist as agglomerates and effectively allowing them to flocculate and precipitate, reducing the turbidity of the water body; (3) The increased dose elevated the pH and calcium ions significantly decreased the orthophosphate content, resulting in an increase in extracellular alkaline phosphatase activity, but possibly increasing the total extracellular nitrogen content. These results suggested that CP is an effective chemical algaecide for cyanobacteria, and has the potential to be applied to dispose of cyanobacterial blooms while reducing the phosphorus content of the water column and further inhibiting the growth and proliferation of cells.

Molecular complexation properties of Cd2+ by algal organic matter from Scenedesmus obliquus.

A detailed understanding the metals binding with algal organic matter (AOM) is essential to gain a deeper insight into the toxicity and migration of metals in algae cell. However, the molecular complexation mechanism of the metals binding with AOM remains unclear. In this study, cadmium ion (Cd2+) binding properties of AOMs from Scenedesmus obliquus, which included extracellular organic matter (EOM) and intracellular organic matter (IOM), were screened. When Cd2+ < 0.5 mg/L, the accumulation of Cd2+ could reach 40%, while Cd2+ > 0.5 mg/L, the accumulation of Cd2+ was only about 10%. EOM decreased gradually (from 8.51 to 3.98 mg/L), while IOM increased gradually (from 9.62 to 21.00 mg/L). The spectral characteristics revealed that IOM was richer in peptides/proteins and had more hydrophilic than EOM. Both EOM and IOM contained three protein-like components (containing tryptophan and tyrosine) and one humic-like component, and their contents in IOM were higher than that in EOM. The tryptophan protein-like substances changed greatly during Cd2+ binding, and that the tryptophan protein-like substances complexed to Cd2+ before tyrosine protein-like substances in IOM was identified. Moreover, the functional groups of N-H, O-H, and CO in AOM played an important role, and the N-H group was priority to interacts with Cd2+ in the complexing process. More functional groups (such as C-O and C-N) were involved in the metals complexing in EOM than in IOM. It could be concluded that Cd2+ stress promoted the secretion of AOM in Scenedesmus obliquus, and proteins in AOM could complex Cd2+ and alleviate its toxicity to algal cell. These findings provided deep insights into the interaction mechanism of AOM with Cd2+ in aquatic environments.

Assembly processes of eukaryotic plankton communities in the world's largest drinking water diversion project.

The largest engineered water diversion project-the Middle Route of the South to North Water Diversion Project (MRP), is of strategic importance in solving the problem of the northern water shortage in China. Eukaryotic plankton are important to the water quality stability in the MRP, but little has been reported about their dynamics and assembly processes, especially for abundant and rare communities. In this study, amplicon sequencing was used to investigate the eukaryotic plankton communities. The results revealed both abundant and rare communities exhibited similar distance-decay patterns, but abundant communities were particularly subject to environmental heterogeneity and played an important role in determining seasonal differences in eukaryotic plankton communities and alpha diversity. In the MRP, with its strong hydrodynamic exchange, abundant and rare communities were mainly affected by stochastic processes, especially homogenizing dispersal. In addition, abundant communities were subject to moderate variable selection (25 %) and rare communities were affected by a higher proportion of dispersal limitation (27 % vs. 10 %). The variation in water temperature and water velocity led to a shift from a stochastic to a deterministic process dominating the assembly of abundant communities. This study extends insights into the dynamics and assembly processes of abundant and rare eukaryotic plankton communities in the large, engineered drinking water diversion project, which is also useful for the management and regulation of the MRP.

Urbanization and agriculture intensification jointly enlarge the spatial inequality of river water quality.

Rivers are severely polluted by multiple anthropogenic stressors. An unevenly distributed landscape pattern can aggravate the deterioration of water quality in rivers. Identifying the impacts of landscape patterns on the spatial characteristics of water quality is helpful for river management and water sustainability. Herein we quantified the nationwide water quality degradation in China's rivers and analyzed its responses to spatial patterns of anthropogenic landscapes. The results showed that the spatial patterns of river water quality degradation had a strong spatial inequality and worsened severely in eastern and northern China. The spatial aggregation of agricultural/urban landscape and the water quality degradation exhibits high consistency. Our findings suggested that river water quality would further deteriorate from high spatial aggregation of cities and agricultures, which reminded us that the dispersion of anthropogenic landscape patterns might effectively alleviate water quality pressures.

Mixotrophic Cultivation Optimization of Microalga Euglena pisciformis AEW501 for Paramylon Production.

Euglena, a flagellated unicellular protist, has recently received widespread attention for various high-value metabolites, especially paramylon, which was only found in Euglenophyta. The limited species and low biomass of Euglena has impeded paramylon exploitation and utilization. This study established an optimal cultivation method of Euglena pisciformis AEW501 for paramylon production under mixotrophic cultivation. The results showed that the optimum mixotrophic conditions were 20 °C, pH 7.0, and 63 µmol photons m-2∙s-1, and the concentrations of sodium acetate and diammonium hydrogen phosphate were 0.98 g L-1 and 0.79 g L-1, respectively. The maximal biomass and paramylon content were 0.72 g L-1 and 71.39% of dry weight. The algal powder contained more than 16 amino acids, 6 vitamins, and 10 unsaturated fatty acids under the optimal cultivation. E. pisciformis paramylon was pure ß-1,3-glucan-type polysaccharide (the purity was up to 99.13 ± 0.61%) composed of linear glucose chains linked together by ß-1,3-glycosidic bonds. These findings present a valuable basis for the industrial exploitation of paramylon with E. pisciformis AEW501.

Health risks from groundwater arsenic on residents in northern China coal-rich region.

Shanxi Province of northern China is a typical mining concentration and arsenism area. Years of mining activities have resulted in serious regional groundwater problems in Shanxi. Therefore, it is of great significance to know the health risk of groundwater arsenic on residents under the background of mining activities. Kriging interpolation was used to illustrate the spatio-temporal dynamics of the health risks on groundwater arsenic based on a ten-year investigation. The groundwater arsenic concentrations decreased over time and the distribution of high arsenic concentrations shrank. High arsenic concentrations were mainly distributed in the northern and middle basin areas. The forecasted area of high risks in coal mining areas was 5623 km2, which was larger than that in non-coal mining areas. The residents living around mining areas were more vulnerable to exposure to groundwater arsenic. Further, the output map outlines the high-risk zones in order to protect the safety of drinking water for residents. This study may be helpful for the policy-makers to adopt a lower limit for groundwater arsenic to the worst affected regions and groups.

Land-use change caused by anthropogenic activities increase fluoride and arsenic pollution in groundwater and human health risk.

Groundwater pollution is becoming a more serious issue because of various anthropogenic activities. A large proportion of the population living in the urbanized and industrialized world is exposed daily to hazardous materials. However, despite the knowledge that protecting groundwater is necessary, little is known about the role of land-use change for human health risks. In this study, we analyzed the temporal and spatial variation of groundwater fluoride (F) and arsenic (As) during 2010-2018 in Shanxi Province of Northern China. Distribution areas of high F and As increased from 2010 to 2018 and spread over time. We assessed human health risk by calculating carcinogenic risk and non-carcinogenic risk. The results showed that F exposure, frequency of high concentration, and risk from 2016 to 2018 were higher than that in 2010-2015, and similar results were obtained for As exposure. Further, land-use change caused by anthropogenic activities increased F and As pollution in groundwater and placed humans at a higher health risk. Our study sheds light on anthropogenic activities that could increase human health risks caused by groundwater F and As via changing land-use. The study provides supports and suggestions for policy-makers to reduce groundwater pollution and prevent adverse health risks to residents.

Optimization method for Microcystis bloom mitigation by hydrogen peroxide and its stimulative effects on growth of chlorophytes.

Hydrogen peroxide (HP) is a feasible algicide to control cyanobacterial blooms, but its application in the waters with strong reductive power is still a problem. The rapid decomposition rate of HP results in a short residence time in the waters, which renders the failure of bloom mitigation. In this study, the damage of Microcystis aeruginosa (M. aeruginosa) by HP, the optimization method for Microcystis bloom control and its field effects were investigated. Results of microcosm experiments indicated M. aeruginosa was vulnerable to HP. The HP-induced damage was mainly attributed to the impairments of HP detoxification pathways and photosystem. Repetitive additions of HP, which could prolong the residence time, were conducted in the mesocosm experiments. HP concentration ranged from 96 µM to 165 µM for 2 h could successfully mitigate Microcystis bloom, even though HP decomposition rate reached 109 µM per h. Besides the removal of M. aeruginosa, contents of total dissolved nitrogen, total dissolved phosphate, dissolved organic carbon and chromophoric dissolved organic matter in water column increased significantly (p < 0.05). The enrichment of nutrients promoted the growth of chlorophytes but the growth of M. aeruginosa couldn't be observed. The dominant species thrived in the HP-treated waters was Chlamydomonas sp. Results in this study confirmed that HP was a promising algicide for cyanobacterial blooms control. The optimization method further demonstrated that repetitive additions of HP could favor the mitigation of cyanobacterial blooms.

Effects of the Three Gorges Dam on spatiotemporal distribution of silicon in the tributary: evidence from the Xiangxi River.

In order to get insight into the impact of Three Gorges Dam construction on silicon distribution pattern due to the altered hydraulic and environmental conditions, the Xiangxi River was chosen as the delegate of the tributaries in the Three Gorges Reservoir; dissolved silica (DSi), biogenic silica (BSi), and lithogenic silica (LSi) were investigated monthly from November 2015 to October 2016 and the hydrodynamic conditions and environmental parameters were addressed synchronously. DSi, BSi, and LSi ranged from 56.07 to 106.07 µmol/L, 0 to 5.64 µmol/L, and 0.49 to 11.47 µmol/L, with the average concentration of 81.84 ± 14.65 µmol/L, 1.11 ± 0.69 µmol/L, and 2.68 ± 1.97 µmol/L, respectively. DSi was significantly lower in the wet season than the dry season (P < 0.05), but BSi and LSi showed a converse trend. DSi was the dominant component in the total silicon (> 90%) and it has a higher concentration in the midstream than other sites. While BSi and LSi exhibited a decrease trend from the upstream to the downstream. Statistical analysis showed that DSi and LSi was primarily controlled by discharge. BSi concentration was affected by algal growth since it was positively correlated with Chla. The backwater area retained 3.67% total silicon. It was concluded that the spatiotemporal heterogeneity of silicon distribution related to hydrodynamics was determined by the regulation of dam; permanent backwater area was the main deposition area for silicon.

The response of phosphorus uptake strategies of Microcystis aeruginosa to hydrodynamics fluctuations.

The mechanisms of hydrodynamic fluctuation that affect cyanobacterial physiological aspects are still unclear. This study was conducted to screen the effects of hydrodynamic fluctuation on phosphorus (P) uptake by Microcystis aeruginosa. Four levels of circumfluence (0, 100, 200, and 300 rpm) and three concentrations of P (0.5, 2.0, and 5.0 mgL-1) were used, and the specific growth rate, cell viability, P uptake kinetic, and alkaline phosphatase activity (APA) were determined. Results showed that M. aeruginosa had a higher specific growth rate, cell viability, and P uptake rate at low and moderate circumfluence, APA increased with a decrease of the cellular P quota (Qp). Turbulence at 300 rpm negatively affected the metabolic activity: the P uptake rate and affinity were reduced significantly (p < 0.05), APA decreased due to the high Qp (p < 0.05), which led to a decline in the specific growth rate and cell viability. It revealed that hydrodynamics could change P uptake and growth of M. aeruginosa. The P absorption capacity and affinity decreased under high turbulence, which led to the inhibition of cyanobacterial growth. It was deduced that high level of artificial turbulence could be used to decrease the risk of cyanobacterial bloom. On the other hand, increasing additional P loading could enhance the adaptability of M. aeruginosa to turbulence, which weakened the inhibition effect of high turbulence on algal growth, and reduced the effects of artificial turbulence on water bloom.