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

The secondary outbreak risk and mechanisms of Microcystis aeruginosa after H2O2 treatment

The secondary outbreak of cyanobacteria after algicide treatment has been a serious problem to water ecosystems. Hydrogen peroxide (H2O2) is an algaecide widely used in practice, but similar re-bloom problems are inevitably encountered. Our work found that Microcystis aeruginosa (M. aeruginosa) temporarily hibernates after H2O2 treatment, but there is still a risk of secondary outbreaks. Interestingly, the dormant period was as long as 20 and 28 days in 5 mg L-1 and 20 mg L-1 H2O2 treatment groups, respectively, but the photosynthetic activity was both restored much earlier (within 14 days). Subsequently, a quantitative imaging flow cytometry-based method was constructed and confirmed that the re-bloom had undergone two stages including first recovery and then re-division. The expression of ftsZ and fabZ genes showed that M. aeruginosa had active transcription processes related to cell division protein and fatty acid synthesis during the dormancy stat. Furthermore, metabolomics suggested that the recovery of M. aeruginosa was mainly by activating folate and salicylic acid synthesis pathways, which promoted environmental stress resistance, DNA synthesis, and cell membrane repair. This study reported the comprehensive mechanisms of secondary outbreak of M. aeruginosa after H2O2 treatment. The findings suggest that optimizing the dosage and frequency of H2O2, as well as exploring the potential use of salicylic acid and folic acid inhibitors, could be promising directions for future algal control strategies.

Cyanobacterial blooms control with CaO2 in different stages: Inhibition efficiency, water quality optimization and microbial community changes

This study explored the feasibility of calcium peroxide (CaO2) to inhibit cyanobacterial blooms of the outbreak and dormancy stages. Our previous studies have found that CaO2 has a high inhibitory effect on cyanobacteria. In order to explore the application effect of CaO2 in actual cyanobacteria lake water, we conducted this study to clarify the effect of CaO2 on inhibiting cyanobacteria in outbreak and dormancy stages. The results showed that CaO2 inhibited the growth of cyanobacteria in the outbreak and dormancy stages by 98.7% and 97.6%, respectively. The main inhibitory mechanism is: (1) destroy the cell structure and make the cells undergo programmed cell death by stimulating the oxidation balance of cyanobacteria cells; (2) EPS released by cyanobacteria resist stimulation and combine calcium to form colonies, and accelerate cell settlement. In addition to causing direct damage to cyanobacteria, CaO2 can also improve water quality and sediment microbial diversity, and reduce the release of sediment to phosphorus, so as to further contribute to cyanobacterial inhibition. Finally, the results of qRT-PCR analysis confirmed the promoting effect of CaO2 on the downregulation of photosynthesis-related genes (rbcL and psaB), microcystn (mcyA and mcyD) and peroxiredoxin (prx), and verified the mechanism of CaO2 inhibition of cyanobacteria. In conclusion, this study provides new findings for the future suppression of cyanobacterial bloom, by combining water quality, cyanobacterial inhibition mechanisms, and sediment microbial diversity.

Transcriptomic analysis of hydrogen peroxide-induced liver dysfunction in Cyprinus carpio: Insights into protein synthesis and metabolism

Hydrogen peroxide (H2O2), a prevalent reactive oxygen species (ROS) found in natural aquatic environments, has garnered significant attention for its potential toxicity in fish. However, the molecular mechanisms underlying this toxicity are not yet comprehensively understood. This study aimed to assess H2O2-induced liver dysfunction in common carp (Cyprinus carpio) and elucidate the underlying molecular mechanisms via biochemical and transcriptomic analyses. Common carp were divided into normal control (NC) and H2O2-treated groups (1 mM H2O2), the latter of which was exposed to H2O2 for 1 h per day over a period of 14 days. Serum biochemical analyses indicated that exposure to H2O2 resulted in moderate liver damage, characterized by elevated alanine aminotransferase (ALT) activity and lowered albumin (Alb) level. Concurrently, H2O2 exposure induced oxidative stress and modified the hepatic metabolic enzyme levels. Transcriptome analysis highlighted that 1358 and 1188 genes were significantly downregulated and upregulated, respectively, in the H2O2-treated group. These differentially expressed genes (DEGs) were significantly enriched in protein synthesis and a variety of metabolic functions such as peptide biosynthetic processes, protein transport, ribonucleoprotein complex biogenesis, oxoacid metabolic processes, and tricarboxylic acid metabolic processes. Dysregulation of protein synthesis is principally associated with the downregulation of three specific pathways: ribosome biogenesis, protein export, and protein processing in the endoplasmic reticulum (ER). Furthermore, metabolic abnormalities were primarily characterized by inhibition of the citrate cycle (TCA) and fatty acid biosynthesis. Significantly, anomalies in both protein synthesis and metabolic function may be linked to aberrant regulation of the insulin signaling pathway. These findings offer innovative insights into the mechanisms underlying H2O2 toxicity in aquatic animals, contributing to the assessment of ecological risks.

Synechococcus dominance induced after hydrogen peroxide treatment of Microcystis bloom in the Caloosahatchee River, Florida

Few field trials examining hydrogen peroxide as a cyanobacterial harmful algal bloom (cHAB) treatment have been conducted in subtropical and tropical regions. None have been tested in Florida, home to Lake Okeechobee and downstream waterways which periodically experience Microcystis bloom events. To investigate treatment effects in Florida, we applied a 490 μM (16.7 mg/L; 0.0015%) hydrogen peroxide spray to a minor bloom of Microcystis aeruginosa on the downstream side of Franklin Lock and Dam in the Caloosahatchee River. Although hydrogen peroxide decreased to background level one day post-treatment, succession was observed in phytoplankton community amplicon sequencing. The relative abundance of Microcystis decreased on day 3 by 86%, whereas the picocyanobacteria Synechococcus became dominant, increasing by 77% on day 3 and by 173% on day 14 to 57% of the phytoplankton community. Metatranscriptomics revealed Synechococcus likely benefitted from the antioxidant defense of upregulated peroxiredoxin, peroxidase/catalase, and rubrerythrin expressions immediately after treatment, and upregulated nitrate transport and urease to take advantage of available nitrogen. Our results indicated hydrogen peroxide induces succession of the phytoplankton community from Microcystis to non-toxic picocyanobacteria and could be used for selective suppression of harmful cyanobacteria.

Bacterial community shifts induced by high concentration hydrogen peroxide treatment of Microcystis bloom in a mesocosm study

Hydrogen peroxide has gained popularity as an environmentally friendly treatment for cyanobacterial harmful algal blooms (cHABs) that takes advantage of oxidative stress sensitivity in cyanobacteria at controlled concentrations. Higher concentrations of hydrogen peroxide treatments may seem appealing for more severe cHABs but there is currently little understanding of the environmental impacts of this approach. Of specific concern is the associated microbial community, which may play key roles in the succession/recovery process post-treatment. To better understand impacts of a high concentration treatment on non-target microbial communities, we applied a hydrogen peroxide spray equating to a total volume concentration of 14 mM (473 mg/L, 0.04%) to 250 L mesocosms containing Microcystis bloom biomass, monitoring treatment and control mesocosms for 4 days. Cyanobacteria dominated control mesocosms throughout the experiment while treatment mesocosms experienced a 99% reduction, as determined by bacterial amplicon sequencing, and a 92% reduction in bacterial cell density within 1 day post-treatment. Only the bacterial community exhibited signs of regrowth, with a fold change of 9.2 bacterial cell density from day 1 to day 2. Recovery consisted of succession by Planctomycetota (47%) and Gammaproteobacteria (17%), which were likely resilient due to passive cell component compartmentalization and rapid upregulation of dnaK and groEL oxidative stress genes, respectively. The altered microbiome retained beneficial functionality of microcystin degradation through a currently recognized but unidentified pathway in Gammaproteobacteria, resulting in a 70% reduction coinciding with bacterial regrowth. There was also an 81% reduction of both total nitrogen and phosphorus, as compared to 91 and 93% in the control, respectively, due to high expressions of genes related to nitrogen (argH, carB, glts, glnA) and phosphorus (pntAB, phoB, pstSCB) cycling. Overall, we found a portion of the bacterial community was resilient to the high-concentration hydrogen peroxide treatment, resulting in Planctomycetota and Gammaproteobacteria dominance. This high-concentration treatment may be suitable to rapidly end cHABs which have already negatively impacted the aquatic environment rather than allow them to persist.

Unleashing the power of acetylacetone: Effective control of harmful cyanobacterial blooms with ecological safety

Harmful algal blooms resulting from eutrophication pose a severe threat to human health. Acetylacetone (AA) has emerged as a potential chemical for combatting cyanobacterial blooms, but its real-world application remains limited. In this study, we conducted a 42-day evaluation of AA's effectiveness in controlling blooms in river water, with a focus on the interplay between ecological community structure, organism functional traits, and water quality. At a concentration of 0.2 mM, AA effectively suppressed the growth of Cyanobacteria (88 %), Bacteroidia (49 %), and Alphaproteobacteria (52 %), while promoting the abundance of Gammaproteobacteria (5.0 times) and Actinobacteria (7.2 times) that are associated with the degradation of organic matter. Notably, after dosing of AA, the OD680 (0.07 ± 0.02) and turbidity (8.6 ± 2.1) remained at a satisfactory level. AA induced significant disruptions in two photosynthesis and two biosynthesis pathways (P < 0.05), while simultaneously enriching eight pathways of xenobiotics biodegradation and metabolism. This enrichment facilitated the reduction of organic pollutants and supported improved water quality. Importantly, AA treatment decreased the abundance of two macrolide-related antibiotic resistance genes (ARGs), ereA and vatE, while slightly increased the abundance of two aminoglycoside-related ARGs, aacA and strB. Overall, our findings establish AA as an efficient and durable algicide with favorable ecological safety. Moreover, this work contributes to the development of effective strategies for maintaining and restoring the health and resilience of aquatic ecosystems impacted by harmful algal blooms.

Cooperative antioxidative defense of the blue-green alga Arthrospira (Spirulina) platensis under oxidative stress imposed by exogenous application of hydrogen peroxide

Hydrogen peroxide (H2O2) is an environmentally-safe algaecide used to control harmful algal blooms and as a disinfectant in various domestic and industrial applications. It is produced naturally in sunny-water or as a by-product during growth, and metabolism of photosynthetic organisms. To assess the impact of H2O2 on Arthrospira platensis, several biochemical components, and antioxidant enzymes were analysed. The growth and biomass of A. platensis were decreased under the effect of H2O2. Whereas, the concentration up to 40 μM H2O2 non-significantly induced (at P < 0.05) the Chl a, C-phycocyanin (C-PC), total phycobiliprotein (PBP), and the radical scavenging activity of A. platensis. The half-maximal effective concentrations (EC50) for H2O2 were 57, 65, and 74 μM H2O2 with regards to the biomass yield, Chl a, and C-PC content, respectively. While, the total soluble protein, and soluble carbohydrates contents were significantly induced. However, the higher concentrations (60 and 80 μM) were lethal to these components, in parallel to the initiation of the lipid peroxidation process. Surprisingly, the carotenoids content was non-significantly increased by H2O2. Despite the relative consistency of catalase (CAT), the activities of superoxide dismutase (SOD) and peroxidase (POD) enzymes were boosted by all of the tested concentrations of H2O2. The relative transcript abundance of selected regulatory genes was also investigated. Except for the highest dose (80 μM), the tested concentrations had almost inhibitory effect on the relative transcripts of heat shock protein (HSP90), glutamate synthase (GOGAT), delta-9 desaturase (desC), iron-superoxide dismutase (FeSOD) and the Rubisco (the large subunit, rbcL) genes. The results demonstrated the importance of the non-enzymatic and enzymatic antioxidants for the cumulative tolerance of A. platensis.

Advanced oxidation processes for synchronizing harmful microcystis blooms control with algal metabolites removal: From the laboratory to practical applications

Harmful algal blooms (HABs) in freshwater systems have become a global epidemic, leading to a series of problems related to cyanobacterial outbreaks and toxicity. Studies are needed to improve the technology used for the simultaneous removal of harmful cyanobacteria and algal metabolites. In this review, widely reported advanced oxidation processes (AOPs) strategies for removing major species Microcystis aeruginosa (M. aeruginosa) and microcystins (MCs) were screened through bibliometrics, such as photocatalysis, activated persulfate, H2O2, Ozone oxidation, ultrasonic oxidation, and electrochemical oxidation, etc. AOPs generate kinds of reactive oxygen species (ROS) to inactivate cyanobacteria and degrade cyanotoxins. A series of responses occurs in algal cells to resist the damaging effects of ROS generated by AOPs. Specifically, we reviewed laboratory research, mechanisms, practical applications, and challenges of HABs treatments in AOPs. Problems common to these technologies include the impact of algal response and metabolites, and environmental factors. This information provides guidance for future research on the removal of harmful cyanobacteria and treatment of algal metabolites using AOPs.

Phytoplankton interspecific interactions modified by symbiotic fungi and bacterial metabolites under environmentally relevant hydrogen peroxide concentrations stress

Hydrogen peroxide (H2O2), which accumulates in water and triggers oxidative stress for aquatic microbes, has been shown to have profound impacts on planktonic microbial community dynamics including cyanobacterial bloom formation. Yet, potential effects of H2O2 on interspecific relationships of phytoplankton-microbe symbiotic interactions remain unclear. Here, we investigated effects of environmentally relevant H2O2 concentrations on interspecific microbial relationships in algae-microbe symbiosis. Microbes play a crucial role in the competition between M. aeruginosa and Chlorella vulgaris at low H2O2 concentrations (∼400 nM), in which fungi and bacteria protect Microcystis aeruginosa from oxidative stress. Moreover, H2O2 stimulated the synthesis and release of extracellular microcystin-LR from Microcystis aeruginosa, while intracellular microcystin-LR concentrations remained at a relatively constant level. In the presence of H2O2, loss of organoheterocyclic compounds, organic acids and ketones contributed to the growth of M. aeruginosa, but the reduction of vitamins inhibited it. Regulation of interspecific relationships by H2O2 is achieved by its action on fungal species and bacterial secretory metabolites. This study explored the response of phytoplankton interspecific relationships in symbiotic phytoplankton-microbe interactions to environmentally relevant H2O2 concentrations stress, providing a theoretical basis for understanding the formation of harmful-algae blooming and impact of photochemical properties of water on aquatic ecological safety and stability.

Solar/periodate-triggered rapid inactivation of Microcystis aeruginosa by interrupting the Calvin-Benson cycle

Frequent outbreak of cyanobacteria is a serious problem for drinking water treatment. The microcystins released from Microcystis aeruginosa (M. aeruginosa) could cause irreversible damage to human health. Catalyst-free solar/periodate (PI) system has recently presented great potential for bacterial inactivation, whereas the application potential and underlying mechanisms of the effective M. aeruginosa control remain unclear. Our work delineated the key role of ROS that inactivating/harmless disposing M. aeruginosa in the simulated sunlight (SSL)/PI system. Singlet oxygen may specifically cause DNA damage but maintain membrane integrity, preventing the risk of microcystins leakage. The SSL/PI 300 μM system could also effectively inhibit M. aeruginosa recovery for >7 days and completely degrade microcystin-LR (50.0 μg/L) within 30 min. Non-targeted metabolomic analysis suggested that the SSL/PI system inactivated M. aeruginosa mainly by interrupting the Calvin-Benson cycle, which damaged the metabolic flux of glycolysis and its downstream pathways such as the oxidative PPP pathway and glutathione metabolism. Furthermore, the activated PI system exhibited an even better algal inhibition under natural sunlight irradiation, evidenced by the seriously damaged cell membrane of M. aeruginosa. Overall, this study reported the comprehensive mechanisms of algal control and application potentials of solar/PI systems. The findings facilitated the development of emerging algicidal technology and its application in controlling environmental harmful algae.

The harmful cyanobacterium Microcystis aeruginosa PCC7806 is more resistant to hydrogen peroxide at elevated CO2

Rising atmospheric CO2 can intensify harmful cyanobacterial blooms in eutrophic lakes. Worldwide, these blooms are an increasing environmental concern. Low concentrations of hydrogen peroxide (H2O2) have been proposed as a short-term but eco-friendly approach to selectively mitigate cyanobacterial blooms. However, sensitivity of cyanobacteria to H2O2 can vary depending on the available resources. To find out how cyanobacteria respond to H2O2 under elevated CO2, Microcystis aeruginosa PCC 7806 was cultured in chemostats with nutrient-replete medium under C-limiting and C-replete conditions (150 ppm and 1500 ppm CO2, respectively). Microcystis chemostats exposed to high CO2 showed higher cell densities, biovolumes, and microcystin contents, but a lower photosynthetic efficiency and pH compared to the cultures grown under low CO2. Subsamples of the chemostats were treated with different concentrations of H2O2 (0-10 mg·L-1 H2O2) in batch cultures under two different light intensities (15 and 100 μmol photons m-2·s-1) and the response in photosynthetic vitality was monitored during 24 h. Results showed that Microcystis was more resistant to H2O2 at elevated CO2 than under carbon-limited conditions. Both low and high CO2-adapted cells were more sensitive to H2O2 at high light than at low light. Microcystins (MCs) leaked out of the cells of cultures exposed to 2-10 mg·L-1 H2O2, while the sum of intra- and extracellular MCs decreased. Although both H2O2 and CO2 concentrations in lakes vary in response to many factors, these results imply that it may become more difficult to suppress cyanobacterial blooms in eutrophic lakes when atmospheric CO2 concentrations continue to rise.

Inactivation of Microcystis aeruginosa by H2O2 generated from a carbon black polytetrafluoroethylene gas diffusion electrode in electrolysis by low-amperage electric current

Frequent outbreaks of cyanobacterial blooms have seriously threatened aquatic ecological environments and human health. Electrolysis by low-amperage electric current is effective for algae inactivation; however, it has no selectivity. Hydrogen peroxide (H₂O₂) is considered to be an efficient and selective suppressor of algae. Therefore, it is necessary to develop an electrode that can generate H₂O₂ to improve electrolysis technology. In this study, a carbon black polytetrafluoroethylene gas diffusion electrode (C-PTFE GDE) with good stability was prepared by a simple adhesive coating method. Then, the inactivation of Microcystis aeruginosa was conducted with electrolysis by low-amperage electric current using Ti/RuO₂ as the anode and C-PTFE GDE as the cathode. When the electrode spacing was 4 cm, the current density was 20 mA cm^-2, and the gas flow was 0.4 L min^-1, 85% of the algae could be inactivated in 20 min. Comparing the inactivation effect of the electric field and electrogenerated oxidants, it was found that electrolysis more rapidly and strongly inactivated algae when an electric field existed. However, electrogenerated oxidants dominated algae inactivation. The concentration of H₂O₂ was as high as 58 mg L^-1, while the concentration of chlorines was only 0.57 mg L^-1, and the generation rate of H₂O₂ was 65 times that of chlorines. Consequently, electrogenerated oxidants dominated by H₂O₂ attacked photosystem II of the algae and caused oxidative damage to membrane lipids, affecting the photosynthetic capacity. Eventually, most of the algae were inactivated. The study suggested that C-PTFE GDE was promising for the inactivation of Microcystis aeruginosa in this electrochemical system.

Effective Early Treatment of Microcystis Exponential Growth and Microcystin Production with Hydrogen Peroxide and Hydroxyapatite

Mitigating cyanotoxin production is essential to protecting aquatic ecosystems and public health. However, current harmful cyanobacterial bloom (HCB) control strategies have significant shortcomings. Because predicting HCBs is difficult, current HCB control strategies are employed when heavy HCBs have already occurred. Our pilot study developed an effective HCB prediction approach that is employed before exponential cyanobacterial growth and massive cyanotoxin production can occur. We used a quantitative polymerase chain reaction (qPCR) assay targeting the toxin-encoding gene mcyA to signal the timing of treatment. When control measures were applied at an early growth stage or one week before the exponential growth of Microcystis aeruginosa (predicted by qPCR signals), both hydrogen peroxide (H2O2) and the adsorbent hydroxyapatite (HAP) effectively stopped M. aeruginosa growth and microcystin (MC) production. Treatment with either H2O2 (10 mg·L-1) or HAP (40 µm particles at 2.5 g·L-1) significantly reduced both mcyA gene copies and MC levels compared with the control in a dose-dependent manner. While both treatments reduced MC levels similarly, HAP showed a greater ability to reduce mcyA gene abundance. Under laboratory culture conditions, H2O2 and HAP also prevented MC production when applied at the early stages of the bloom when mcyA gene abundance was below 105 copies·mL-1.

Sensitivity of phytoplankton, zooplankton and macroinvertebrates to hydrogen peroxide treatments of cyanobacterial blooms

Addition of hydrogen peroxide (H₂O₂) is a promising method to acutely suppress cyanobacterial blooms in lakes. However, a reliable H₂O₂ risk assessment to identify potential effects on non-target species is currently hampered by a lack of appropriate ecotoxicity data. The aim of the present study was therefore to quantify the responses of a wide diversity of freshwater phytoplankton, zooplankton and macroinvertebrates to H₂O₂ treatments of cyanobacterial blooms. To this end, we applied a multifaceted approach. First, we investigated the 24-h toxicity of H₂O₂ to three cyanobacteria (Planktothrix agardhii, Microcystis aeruginosa, Anabaena sp.) and 23 non-target species (six green algae, eight zooplankton and nine macroinvertebrate taxa), using EC₅₀ values based on photosynthetic yield for phytoplankton and LC₅₀ values based on mortality for the other organisms. The most sensitive species included all three cyanobacterial taxa, but also the rotifer Brachionus calyciflores and the cladocerans Ceriodaphnia dubia and Daphnia pulex. Next, the EC₅₀ and LC₅₀ values obtained from the laboratory toxicity tests were used to construct a species sensitivity distribution (SSD) for H₂O₂. Finally, the species predicted to be at risk by the SSD were compared with the responses of phytoplankton, zooplankton and macroinvertebrates to two whole-lake treatments with H₂O₂. The predictions of the laboratory-based SSD matched well with the responses of the different taxa to H₂O₂ in the lake. The first lake treatment, with a relatively low H₂O₂ concentration and short residence time, successfully suppressed cyanobacteria without major effects on non-target species. The second lake treatment had a higher H₂O₂ concentration with a longer residence time, which resulted in partial suppression of cyanobacteria, but also in a major collapse of rotifers and decreased abundance of small cladocerans. Our results thus revealed a trade-off between the successful suppression of cyanobacteria at the expense of adverse effects on part of the zooplankton community. This delicate balance strongly depends on the applied H₂O₂ dosage and may affect the decision whether to treat a lake or not.

Self-floating capsule of algicidal bacteria Bacillus sp. HL and its performance in the dissolution of Microcystis aeruginosa

Algicidal bacteria is considered as an efficient and environmentally friendly approach to suppress Microcystis aeruginosa (M. aeruginosa). However, algicidal bacteria in natural water is limited during the practical application due to the interference of external factors and the low reuse capability. In this study, a bio-degradation capsule for M. aeruginosa is prepared by bio-compatible sodium alginate (SA) compositing with eco-friendly ethyl cellulose (EC) to improve the property and reuse capability of algicidal bacteria. Bacterial strain HL was well immobilized and the capsule was obtained with 2% of SA, 3% of calcium chloride (CaCl₂) and 3% of EC. It has been proved that capsules immobilizing bacteria HL shows considerable advantage over traditional bio-treatment systems (free-living bacteria) and good reusable performance. A better algicidal rate of 77.67% ± 1.14% at 7th day was obtained with the use of capsule embedding 50 mL of algicidal bacteria, enhanced by 11.05% comparing with same amount of free-living bacteria. Moreover, the algicidal rate of M. aeruginosa still reached 68.57% ± 2.88% after three times repetitive use. The effect of capsules on the fluorescence and antioxidant system of M. aeruginosa indicated that the photosystems were irreversibly damaged and the antioxidant response of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) were significantly induced. Overall, capsules prepared in this study can provide a desirable environment for algicidal bacteria HL and ensure algicidal bacteria to in-situ work well in inhibiting booms of algae.

Monitoring and control methods of harmful algal blooms in Chinese freshwater system: a review

Harmful algal blooms (HABs) are a worldwide problem with substantial adverse effects on the aquatic environment as well as human health, which have prompted researchers to study measures to stem and control them. Meanwhile, it is key to research and develop monitoring methods to establish early warning HABs. However, both the current monitoring methods and control methods have some shortcomings, making the field application limited. Thus, we need to improve current approaches for monitoring and controlling HABs efficiently. Based on the freshwater system features in China, we review various monitoring and control methods of HABs, summarize and discuss the problems with these methods, and propose the future development direction of monitoring and control HABs. Finally, we envision that it can combine physical, chemical, and biological methods to inhibit HAB expansion in the future, complementing each other with advantages. Further, we promise to establish a long-term strategy of controlling HABs with various algicidal bacteria co-cultivate for field applications in China. Efforts in studying algicidal bacteria must be increased to better control HABs and mitigate the risks of aquatic ecosystems and human health in China.

Sorption behavior of Pb(II) onto polyvinyl chloride microplastics affects the formation and ecological functions of microbial biofilms

Microplastics (MPs) are regarded as transport media for heavy metals in aquatic systems, whereas the effects of the heavy metal-enriched MPs on microbial biofilms are still unclear. In this study, Pb(II) sorption onto polyvinyl chloride (PVC) MPs and its effects on the formation and ecological functions of microbial biofilms were investigated. The results showed that the interaction between Pb(II) and PVC MPs was dominated by physisorption. The maximum sorption amount reached 1.25 mg/g. Afterward, microbial biofilms were exposed to the Pb(II)-enriched PVC particles. It is suggested that Pb(II)-enriched PVC exposure reduced productivities of polysaccharides and proteins in extracellular polymeric substances, which restricted the formation of microbial biofilms. Meanwhile, microbial community structure was reassembled accompanying the decline of capacities for nitrate and phosphate removal. Therefore, this study examines the ecological risk associated with the heavy metal-enriched MPs that can adversely affect microbial biofilms.

Evaluation of a Peroxide-Based Algaecide for Cyanobacteria Control: A Mesocosm Trial in Lake Okeechobee, FL, USA

A 72 h small-scale trial was conducted in enclosed mesocosms in the Lake Okeechobee waterway to evaluate the effectiveness of a USEPA-registered peroxide-based algaecide (formulated as sodium carbonate peroxyhydrate) for controlling a natural cyanobacteria population. Mesocosms were initially subjected to either no algaecide or the maximum label rate of 10 mg H2O2·L−1. A subset of mesocosms were then subjected to a sequential application of 5 mg H2O2·L−1 at 48 h after initial treatment. Following application, peroxide concentrations rapidly decreased and were undetectable by 48 h. At 24 h after treatment, significant decreases in all biomass indicators were observed (compared to untreated mesocosms), including extracted chlorophyll a, microscopic counts (total phytoplankton and total cyanobacteria), and cyanobacteria-specific 16S rRNA gene copies by over 71%. Although peroxide treatment reduced cyanobacteria biomass, there was no change in overall community structure and the remaining population was still dominated by cyanobacteria (>90%). After 48 h exposure, some biomass recovered in single application mesocosms resulting in only a 32–45% reduction in biomass. Repeated peroxide dosing resulted in the greatest efficacy, which had a sustained (60–91%) decrease in all biomass indicators for the entire study. While a single application of the peroxide was effective in the first 24 h, a sequential treatment is likely necessary to sustain efficacy when using this approach to manage cyanobacteria in the field. Results of this study support that this peroxide-based algaecide is a strong candidate to continue with scalable field trials to assess its potential future utility for operational management programs in the Lake Okeechobee waterway.

Hydrogen peroxide can be a plausible biomarker in cyanobacterial bloom treatment

The effect of combined stresses, photoinhibition, and nutrient depletion on the oxidative stress of cyanobacteria was measured in laboratory experiments to develop the biomass prediction model. Phormidium ambiguum was exposed to various photosynthetically active radiation (PAR) intensities and phosphorous (P) concentrations with fixed nitrogen concentrations. The samples were subjected to stress assays by detecting the hydrogen peroxide (H₂O₂) concentration and antioxidant activities of catalase (CAT) and superoxide dismutase (SOD). H₂O₂ concentrations decreased to 30 µmol m^-2 s^-1 of PAR, then increased with higher PAR intensities. Regarding P concentrations, H₂O₂ concentrations (nmol L^-1) generally decreased with increasing P concentrations. SOD and CAT activities were proportionate to the H₂O₂ protein^-1. No H₂O₂ concentrations detected outside cells indicated the biological production of H₂O₂, and the accumulated H₂O₂ concentration inside cells was parameterized with H₂O₂ concentration protein^-1. With over 30 µmol m^-2 s^-1 of PAR, H₂O₂ concentration protein^-1 had a similar increasing trend with PAR intensity, independently of P concentration. Meanwhile, with increasing P concentration, H₂O₂ protein^-1 decreased in a similar pattern regardless of PAR intensity. Protein content decreased with gradually increasing H₂O₂ up to 4 nmol H₂O₂ mg^-1 protein, which provides a threshold to restrict the growth of cyanobacteria. With these results, an empirical formula-protein (mg L^-1) = - 192*Log((H₂O₂/protein)/4.1), where H₂O₂/protein (nmol mg^-1) = - 0.312*PAR²/(50² + PAR²)*((25/PAR)⁴ + 1)*Log(P/133,100), as a function of total phosphorus concentration, P (µg L^-1)-was developed to obtain the cyanobacteria biomass.

Effects of hydrogen peroxide on Scenedesmus obliquus: Cell growth, antioxidant enzyme activity and intracellular protein fingerprinting

Hydrogen peroxide (H₂O₂) is an environmental-friendly algicide and it is widely used to control algal blooms in aquatic ecosystems. However, the response of algal cell metabolic characteristics and intracellular protein profile under H₂O₂ stress is still not well understood. In the present study, the green alga Scenedesmus obliquus was exposed to different concentrations of H₂O₂ (0, 2, 6, 8 and 10 mg L^-1) to evaluate the changes in algal morphological, physiological, and proteomic features to H₂O₂ exposure. The results showed that 8 mg L^-1 of H₂O₂ could effectively inhibit the cell growth and photosynthetic activity of S. obliquus including chlorophyll-a content and chlorophyll fluorescence parameters. The increased activities of superoxide dismutase (SOD) and catalase (CAT) observed in this study indicate that cells exposure to H₂O₂ caused oxidative stress. The metabolic activity of S. obliquus was significantly decreased by H₂O₂ treatment. In terms of proteomic analysis, 251 differentially expressed proteins (DEPs) were successfully identified. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis revealed significant protein enrichment in the metabolic pathways, photosynthesis, ascorbic acid, and alginate metabolism and phenylpropane biosynthesis of S. obliquus. The analysis of protein-protein interaction system shows that the pathways of photosynthesis and metabolic pathways of S. obliquus were essential to resist oxidative stress. Taking together, these results shed new lights on exploring the cell physiological metabolism and intracellular protein mechanisms of H₂O₂ inhibition on algal blooms.

Differential Effect of Hydroxen Peroxide οn Toxic Cyanobacteria of Hypertrophic Mediterranean Waterbodies

Cyanobacterial blooms have been known since ancient times; however, they are currently increasing globally. Human and ecological health risks posed by harmful cyanobacterial blooms have been recorded around the world. These risks are mainly associated with their ability to affect the ecosystem chain by different mechanisms like the production of cyanotoxins, especially microcystins. Their expansion and their harmful effects have led many researchers to seek techniques and strategies to control them. Among them, hydrogen peroxide could be a promising tool against cyanobacteria and cyanotoxins and it is well-established as an environmentally friendly oxidizing agent because of its rapid decomposition into oxygen and water. The aim of the present study was to evaluate the effect of hydrogen peroxide on phytoplankton from two hypertrophic waterbodies in Greece. The effect of hydrogen peroxide on concentration of microcystins found in the waterbodies was also studied. Treatment with 4 mg/L hydrogen peroxide was applied to water samples originated from the waterbodies and Cyanobacterial composition and biomass, phycocyanin, chlorophyll-a, and intra-cellular and total microcystin concentrations were studied. Cyanobacterial biomass and phycocyanin was reduced significantly after the application of 4 mg/L hydrogen peroxide in water treatment experiments while chlorophytes and extra-cellular microcystin concentrations were increased. Raphidiopsis (Cylindrospermopsis) raciborskii was the most affected cyanobacterial species after treatment of the water of the Karla Reservoir in comparison to Aphanizomenon favaloroi, Planktolyngbya limnetica, and Chroococcus sp. Furthermore, Microcystis aeruginosa was more resistant to the treatment of Pamvotis lake water in comparison with Microcystis wesenbergii and Microcystis panniformis. Our study showed that hydrogen peroxide differentially impacts the members of the phytoplankton community, affecting, thus, its overall efficacy. Different effects of hydrogen peroxide treatment were observed among cyanobacerial genera as well as among cyanobacterial species of the same genus. Different effects could be the result of the different resistance mechanisms of each genus or species to hydrogen peroxide. Hydrogen peroxide could be used as a treatment for the mitigation of cyanobacterial blooms in a waterbody; however, the biotic and abiotic characteristics of the waterbody should be considered.

Fabrication of H2O2 slow-releasing composites for simultaneous Microcystis mitigation and phosphate immobilization

Hydrogen peroxide (H₂O₂) is a widely accepted algicide in controlling cyanobacterial blooms. However, this method includes two disadvantages: 1) a low H₂O₂ concentration (<5 mg L^-1) is required; 2) H₂O₂-induced cell lysis causes phosphorus (P) contamination. To overcome the drawbacks, a H₂O₂ slow-releasing composite (HSRC) based on calcium peroxide (CaO₂) was fabricated to substitute liquid H₂O₂. According to the results, a higher CaO₂ dose increased H₂O₂ yield and releasing rate. H₂O₂ yield of 160 mg L^-1 CaO₂ in HSRC reached 32.9 mg L^-1 and its releasing rate was 0.407 h^-1. In addition, a higher temperature decreased H₂O₂ yield and increased H₂O₂-releasing rate. Besides, HSRC endowed with a remarkable ability to immobilize P. Higher CaO₂ dose, pH value, and temperature increased the rate of P immobilization. The highest rate was 0.185 h^-1, which occurred with 160 mg L^-1 CaO₂ in HSRC at 25 °C and pH 8.0. Toxicity assays showed that HSRC exerted sustaining oxidative stress on Microcystis aeruginosa. Accumulation of intracellular reactive oxygen species resulted in the disruption of enzymatic systems and inactivation of photosystem. Tracking the variations of cell growth and H₂O₂ concentration during HSRC treatments, it suggested that the lethal effect on Microcystis aeruginosa was achieved with a super-low H₂O₂ concentration (<0.3 mg L^-1). In addition, cell lysis did not cause a sudden rise in P concentration due to the P immobilization by HSRC. Therefore, HSRC successfully offsets the drawbacks of liquid H₂O₂ in mitigating cyanobacterial blooms. It may be a novel and promising algicide that not only kills cyanobacteria but also reduces eutrophication momentarily.

A critical review on operation and performance of source water control strategies for cyanobacterial blooms: Part I-chemical control methods

Cyanobacterial blooms produce nuisance metabolites (e.g., cyanotoxins and T&O compounds) thereby posing water quality management issues for aquatic sources used for potable water production, aquaculture, and recreation. A variety of in-lake/reservoir control measures are implemented to reduce the abundance of nuisance cyanobacteria biomass or decrease the amount of available phosphorous (P). This paper critically reviews the chemical control strategies implemented for in-lake/reservoir management of cyanobacterial blooms, i.e., algaecides and nutrient sequestering coagulants/flocculants, by highlighting (i) their mode of action, (ii) cases of successful and unsuccessful treatment, (iii) and factors influencing performance (e.g., water quality, process control techniques, source water characteristics, etc.). Algaecides generally result in immediate improvements in water quality and offer selective cyanobacterial control when peroxide-based alagecides are used. However, they have a range of limitations: causing cell lysis and release of cyanotoxins, posing negative impacts on aquatic plants and animals, leaving behind environmentally relevant treatment residuals (e.g., Cu in water and sediments), and offering only short-term bloom control characterized by cyanobacterial rebound. Coagulants/flocculants (alum, iron, calcium, and lanthanum bentonite) offer long-term internal nutrient control when external nutrient loading is controlled. Treatment performance is often influenced by background water quality conditions, and source water characteristics (e.g., surface area, depth, mixing regimes, and residence time). The reviewed case studies highlight that external nutrient load reduction is the most fundamental aspect of cyanobacterial control. None of the reviewed control strategies provide a comprehensive solution to cyanobacterial blooms.

Can Alkyl Quaternary Ammonium Cations Substitute H2O2 in Controlling Cyanobacterial Blooms-Laboratory and Mesocosm Studies

Mitigation of harmful cyanobacterial blooms that constitute a serious threat to water quality, particularly in eutrophic water, such as in aquaculture, is essential. Thus, in this study, we tested the efficacy of selected cyanocides towards bloom control in laboratory and outdoor mesocosm experiments. Specifically, we focused on the applicability of a group of cationic disinfectants, alkyltrimethyl ammonium (ATMA) compounds and H₂O₂. The biocidal effect of four ATMA cations with different alkyl chain lengths was evaluated ex situ using Microcystis colonies collected from a fish pond. The most effective compound, octadecyl trimethyl ammonium (ODTMA), was further evaluated for its selectivity towards 24 cyanobacteria and eukaryotic algae species, including Cyanobacteria, Chlorophyta, Bacillariophyta, Euglenozoa and Cryptophyta. The results indicated selective inhibition of cyanobacteria by ODTMA-Br (C18) on both Chroccocales and Nostocales, but a minor effect on Chlorophytes and Bacillariophytes. The efficacy of ODTMA-Br (C18) (6.4 μM) in mitigating the Microcystis population was compared with that of a single low dose of H₂O₂ treatments (117.6 μM). ODTMA-Br (C18) suppressed the regrowth of Microcystis for a longer duration than did H₂O₂. The results suggested that ODTMA-Br (C18) may be used as an effective cyanocide and that it is worth further evaluating this group of cationic compounds as a treatment to mitigate cyanobacterial blooms in aquaculture.

Characterization of allelochemicals from Pistia stratiotes extracts and their effects on the growth and physiology of Microcystis aeruginosa

Due to the public and environmental health impact of cyanotoxins, investigations have been focused on finding environmental friendly algaecides from aquatic plants. The present study had the objective to evaluate the population control and physiological response of Microcystis aeruginosa (Kützing) Kützing (strain BCCUSP232) exposed to Pistia stratiotes L. extracts. Aqueous and ethanolic extracts of P. stratiotes at different concentrations (10, 25, and 50 mg L^-1) were submitted to M. aeruginosa and reduced significantly (p<0.05) the cyanobacterium cell density. The ethanolic extract presented the greatest growth inhibition of the strain at the highest concentration. During exposure to P. stratiotes extracts, intracellular hydrogen peroxide levels, malondialdehyde content, and antioxidant enzymes (peroxidase, catalase, and glutathione S-transferase) activities increased in M. aeruginosa, while total protein concentration decreased when compared to the control group. Superoxide dismutase (SOD) activities presented a sharp decline, suggesting superoxide radical and peroxide accumulation. This implied that SOD was a target for bioactive substance(s) from aqueous and ethanolic extracts of P. stratiotes. Phytochemical screening of the extracts revealed that the ethanolic extract presented 93.36 mg gallic acid equivalent (GAE) per gram dry weight (g^-1 DW) total polyphenols and 217.33 mg rutin equivalent (RE) per gram dry weight total flavonoids, and for the aqueous extract, 5.19 mg GAE g^-1 DW total polyphenols and 11.02 mg RE g^-1 DW total flavonoids were detected. Gas chromatography (GC)/mass spectrometry (MS) analyses of the ethanolic and aqueous extracts presented palmitic acid ethyl ester as major allelochemical. In view of these results, it can be concluded that P. stratiotes showed potential in controlling M. aeruginosa populations.

Growth inhibition and oxidative damage of Microcystis aeruginosa induced by aqueous extract of different submerged macrophytes

With the increasing eutrophication of the aquatic environments, cyanobacteria blooms caused certain damage to the animals and plants in the aquatic environments. In this experiment, two species were selected from six species of submerged macrophytes, the experimental conditions were changed to achieve the best inhibitory effect on Microcystis aeruginosa, and oxidative damage analysis was carried out. The experiment results demonstrated that the inhibition rate of Vallisneria natans and Ceratophyllum demersum was nearly 100% at the concentration of 3 g/L after 15 days of anaerobic soaking extract. In addition, the longer the soaking time of the two submerged macrophytes, the weaker the photosynthesis effect, and the lower the chlorophyll fluorescence parameters, the more obvious the inhibition effect on M. aeruginosa. Lipid peroxidation injury of M. aeruginosa could be reflected by malondialdehyde (MDA) concentration. The MDA concentration in the experimental group was significantly higher than the control group. Results showed that V. natans and C. demersum could induce oxidative damage in M. aeruginosa. It was also observed that the secondary metabolites produced by V. natans were mainly fatty acids (e.g., the oxidative acid was 6.92 w/%, and the successful acid was 9.85 w/%) which inhibited M. aeruginosa in gas chromatography-mass spectrometry (GC-MS). The main secondary metabolites in C. demersum were hydroxyl acids (e.g., the 4-hydroxy-3-methoxyphenylacetic acid was 24.33 w/%), which could inhibit the algae through allelopathy. This study provided reference for submerged macrophytes to inhibit M. aeruginosa under different conditions.

Resilience of Microbial Communities after Hydrogen Peroxide Treatment of a Eutrophic Lake to Suppress Harmful Cyanobacterial Blooms

Applying low concentrations of hydrogen peroxide (H₂O₂) to lakes is an emerging method to mitigate harmful cyanobacterial blooms. While cyanobacteria are very sensitive to H₂O₂, little is known about the impacts of these H₂O₂ treatments on other members of the microbial community. In this study, we investigated changes in microbial community composition during two lake treatments with low H₂O₂ concentrations (target: 2.5 mg L⁻¹) and in two series of controlled lake incubations. The results show that the H₂O₂ treatments effectively suppressed the dominant cyanobacteria Aphanizomenon klebahnii, Dolichospermum sp. and, to a lesser extent, Planktothrix agardhii. Microbial community analysis revealed that several Proteobacteria (e.g., Alteromonadales, Pseudomonadales, Rhodobacterales) profited from the treatments, whereas some bacterial taxa declined (e.g., Verrucomicrobia). In particular, the taxa known to be resistant to oxidative stress (e.g., Rheinheimera) strongly increased in relative abundance during the first 24 h after H₂O₂ addition, but subsequently declined again. Alpha and beta diversity showed a temporary decline but recovered within a few days, demonstrating resilience of the microbial community. The predicted functionality of the microbial community revealed a temporary increase of anti-ROS defenses and glycoside hydrolases but otherwise remained stable throughout the treatments. We conclude that the use of low concentrations of H₂O₂ to suppress cyanobacterial blooms provides a short-term pulse disturbance but is not detrimental to lake microbial communities and their ecosystem functioning.

Cyanobacterial Harmful Algal Blooms in Aquatic Ecosystems: A Comprehensive Outlook on Current and Emerging Mitigation and Control Approaches

An intensification of toxic cyanobacteria blooms has occurred over the last three decades, severely affecting coastal and lake water quality in many parts of the world. Extensive research is being conducted in an attempt to gain a better understanding of the driving forces that alter the ecological balance in water bodies and of the biological role of the secondary metabolites, toxins included, produced by the cyanobacteria. In the long-term, such knowledge may help to develop the needed procedures to restore the phytoplankton community to the pre-toxic blooms era. In the short-term, the mission of the scientific community is to develop novel approaches to mitigate the blooms and thereby restore the ability of affected communities to enjoy coastal and lake waters. Here, we critically review some of the recently proposed, currently leading, and potentially emerging mitigation approaches in-lake novel methodologies and applications relevant to drinking-water treatment.

Effect of hydrogen peroxide on natural phytoplankton and bacterioplankton in a drinking water reservoir: Mesocosm-scale study

Cyanobacterial blooms are increasingly reported worldwide, presenting a challenge to water treatment plants and concerning risks to human health and aquatic ecosystems. Advanced oxidative processes comprise efficient and safe methods for water treatment. Hydrogen peroxide (H₂O₂) has been proposed as a sustainable solution to mitigate bloom-forming cyanobacteria since this group presents a higher sensitivity compared to other phytoplankton, with no major risks to the environment at low concentrations. Here, we evaluated the effects of a single H₂O₂ addition (10 mg L^-1) over 120 h in mesocosms introduced in a reservoir located in a semi-arid region presenting a Planktothrix-dominated cyanobacterial bloom. We followed changes in physical and chemical parameters and in the bacterioplankton composition. H₂O₂ efficiently suppressed cyanobacteria, green algae, and diatoms over 72 h, leading to an increase in transparency and dissolved organic carbon, and a decrease in dissolved oxygen and pH, while nutrient concentrations were not affected. After 120 h, cyanobacterial abundance remained low and green algae became dominant. 16S rRNA sequencing revealed that the original cyanobacterial bloom was composed by Planktothrix, Cyanobium and Microcystis. Only Cyanobium increased in relative abundance at 120 h, suggesting regrowth. A prominent change in the composition of heterotrophic bacteria was observed with Exiguobacterium, Paracoccus and Deinococcus becoming the most abundant genera after the H₂O₂ treatment. Our results indicate that this approach is efficient in suppressing cyanobacterial blooms and improving water quality in tropical environments. Monitoring changes in abiotic parameters and the relative abundance of specific bacterial taxa could be used to anticipate the regrowth of cyanobacteria after H₂O₂ degradation and to indicate where in the reservoir H₂O₂ should be applied so the effects are still felt in the water treatment plant intake.

Oxidative stress in the cyanobacterium Microcystis aeruginosa PCC 7813: Comparison of different analytical cell stress detection assays

Cyanobacterial blooms are observed when high cell densities occur and are often dangerous to human and animal health due to the presence of cyanotoxins. Conventional drinking water treatment technology struggles to efficiently remove cyanobacterial cells and their metabolites during blooms, increasing costs and decreasing water quality. Although field applications of hydrogen peroxide have been shown to successfully suppress cyanobacterial growth, a rapid and accurate measure of the effect of oxidative stress on cyanobacterial cells is required. In the current study, H₂O₂ (5 and 20 mg L^-1) was used to induce oxidative stress in Microcystis aeruginosa PCC 7813. Cell density, quantum yield of photosystem II, minimal fluorescence and microcystin (MC-LR, -LY, -LW, -LF) concentrations were compared when evaluating M. aeruginosa cellular stress. Chlorophyll content (determined by minimal fluorescence) decreased by 10% after 48 h while cell density was reduced by 97% after 24 h in samples treated with 20 mg L^-1 H₂O₂. Photosystem II quantum yield (photosynthetic activity) indicated cyanobacteria cell stress within 6 h, which was considerably faster than the other methods. Intracellular microcystins (MC-LR, -LY, -LW and -LF) were reduced by at least 96% after 24 h of H₂O₂ treatment. No increase in extracellular microcystin concentration was detected, which suggests that the intracellular microcystins released into the surrounding water were completely removed by the hydrogen peroxide. Thus, photosynthetic activity was deemed the most suitable and rapid method for oxidative cell stress detection in cyanobacteria, however, an approach using combined methods is recomended for efficient water treatment management.

Immune, inflammatory, autophagic and DNA damage responses to long-term H2O2 exposure in different tissues of common carp (Cyprinus carpio)

Hydrogen peroxide (H₂O₂) is a stable reactive oxygen species (ROS) in aquatic environment, and high concentration of ambient H₂O₂ may directly or indirectly affect aquatic animal health. However, the response mechanism of fish to ambient H₂O₂ has not been well studied yet. Therefore, the aim of the study was to investigate the immune, inflammatory, autophagic and DNA damage responses to long-term H₂O₂ exposure in different tissues of common carp. The results showed that H₂O₂ exposure induced a significant immune response, with alterations in the levels of immune parameters including AKP, ACP, LZM, C3, HSP90 and HSP70 in different tissues. The inflammatory response evoked by H₂O₂ exposure was associated with the activations of TLRs and NF-κB (P65) in the majority of tested tissues. The autophagy process was significantly affected by H₂O₂ exposure, evidenced by the upregulations of the autophagy-related genes in liver, gills, muscle, intestines, heart and spleen and the downregulations in kidney. Meanwhile, the mRNA level of atm, a primary transducer of DNA damage response, was upregulated in liver, gills, intestines and spleen, and the DNA damage was evidenced by increased 8-OHdG level in intestines after H₂O₂ exposure. Moreover, cell cycle regulation-related genes, including cyclin A1, B and/or E1, highly expressed in all tested tissues except heart after H₂O₂ exposure. Interestingly, IBR analysis exhibited that immune, inflammatory, autophagic and DNA damage responses to H₂O₂ exposure were in a dose-dependent and tissue-specific manner. These data may contribute to understanding H₂O₂ toxicity for fish and assessing potential risk of H₂O₂ in aquatic environment.

Interspecific protection against oxidative stress: green algae protect harmful cyanobacteria against hydrogen peroxide

Oceanographic studies have shown that heterotrophic bacteria can protect marine cyanobacteria against oxidative stress caused by hydrogen peroxide (H₂O₂). Could a similar interspecific protection play a role in freshwater ecosystems? In a series of laboratory experiments and two lake treatments, we demonstrate that freshwater cyanobacteria are sensitive to H₂O₂ but can be protected by less‐sensitive species such as green algae. Our laboratory results show that green algae degrade H₂O₂ much faster than cyanobacteria. Consequently, the cyanobacterium Microcystis was able to survive at higher H₂O₂ concentrations in mixtures with the green alga Chlorella than in monoculture. Interestingly, even the lysate of destructed Chlorella was capable to protect Microcystis, indicating a two‐component H₂O₂ degradation system in which Chlorella provided antioxidant enzymes and Microcystis the reductants. The level of interspecific protection provided to Microcystis depended on the density of Chlorella. These findings have implications for the mitigation of toxic cyanobacterial blooms, which threaten the water quality of many eutrophic lakes and reservoirs worldwide. In several lakes, H₂O₂ has been successfully applied to suppress cyanobacterial blooms. Our results demonstrate that high densities of green algae can interfere with these lake treatments, as they may rapidly degrade the added H₂O₂ and thereby protect the bloom‐forming cyanobacteria.

Removal of Dolichospermum circinale, Microcystis aeruginosa, and their metabolites using hydrogen peroxide and visible light

Frequent cyanobacterial blooms in reservoirs used for human supply increase the risk of noxious secondary metabolites, endangering human health and ecological balance, and requiring constant monitoring by water companies. Although hydrogen peroxide (H2O2) has been widely reported as an effective agent for the control of cyanobacteria, being Microcystis aeruginosa one of the most studied species, very limited data is available on its effects over Dolichospermum circinale. Therefore, this study aimed to evaluate the impact of H2O2 on D. circinale and comparing it to the effects over the M. aeruginosa. The treatment was performed in cyanobacterial cultures with the application of 2 and 5 mg L-1 of H2O2 under visible light. To measure the impact of the treatment, intact cells were counted and cell re-growth monitored. Geosmin and microcystin, cell pigments, color, and organic matter in water were also analyzed during the treatment. The results showed that even the smallest H2O2 concentration (2 mg L-1) was able to completely remove D. circinale cells. Although M. aeruginosa could only be completely removed using 5 mg L-1, the few cells remaining after the application of 2 mg L-1 were not viable and did not re-grew after 15 days. Total microcystin concentration increased after M. aeruginosa was exposed to H2O2, suggesting that oxidative stress may increase the detection of this metabolite when the cells are lysed. While 2 mg L-1 was able to significantly decrease total geosmin, the addition of 5 mg L-1 did not improve removal. Chlorophyll-a was readily degraded after cell rupture but the same did not happen to phycocyanin, demonstrating its high resilience to this oxidant. Color and organic matter increased for the M. aeruginosa but decreased for the D. circinale suspension, probably because the higher concentration of the M. aeruginosa yielded more extracellular content to the water which was not able to be degraded by the amount of H2O2 applied.

Water-lifting aerator reduces algal growth in stratified drinking water reservoir: Novel insights into algal metabolic profiling and engineering applications

Water-lifting aerator (WLA) which was developed by Professor Tinglin Huang at Xi'an University of Architecture and Technology, China has multi-functional water quality improvement that significantly inhibits the occurrence of harmful algal blooms (HABs) in deep drinking water reservoirs. However, the biological mechanism of WLA to the suppress algal growth has not been comprehensively understood. Here, the cellular mechanism that allows WLA to control HABs was explored based on the combination of both laboratory simulation and field investigation. Under simulated hydrodynamic conditions, the results showed that the cell density, chlorophyll a content, chlorophyll fluorescence parameters, and dehydrogenase activity in Microcystis aeruginosa all peaked under light conditions at 25 °C. The metabolic activity of M. aeruginosa varied significantly under low temperature at 6 °C and light conditions when cultured for 48 h. The extracellular organic matter (EOM) and intracellular organic matter (IOM) contents of M. aeruginosa were both resolved into three components. Moreover, the total fluorescence intensities from EOM and IOM both peaked under light conditions at 25 °C. The field investigation showed that the growth of algae was decreased significantly in Lijiahe drinking water reservoir with WLA application. The chlorophyll fluorescence parameters decreased significantly after vertical mixing, thereby indicating that the WLA weakened the photosynthetic ability and reduced the biological activity of algae in situ. In addition, the WLA significantly affected the vertical distribution of the phytoplankton community composition. Altogether, these results shed new lights on understanding the control of algal blooms by WLA in stratified drinking water reservoirs. WLA has broad prospect of engineering applications, which can control algal blooms of water supply resources in situ, therefore, reduce the content of disinfection by-products in drinking water treatment plants.

Properties of CaO2 for H2O2 release and phosphate removal and its feasibility in controlling Microcystis blooms

Calcium peroxide (CP) has been widely applied in environmental remediation, but few studies have reported its application in controlling Microcystis blooms. To recognize its feasibility for mitigating Microcystis blooms, the properties of CP in terms of hydrogen peroxide (HP) release and phosphate removal were investigated at different CP doses, temperatures, and initial pH values. HP release kinetics followed the Higuchi model. Batch experiments conducted in this study suggested that the HP yield and release rate were positively correlated with the CP dose. Increasing temperature decreased the HP yield but accelerated the HP release rate. The phosphate removal kinetics were well simulated by the pseudo-second-order model. The batch experiments suggested that an increased CP dose enhanced the phosphate removal capacity, but it did not affect the phosphate removal rate. Moreover, increased temperature accelerated both phosphate removal capacity and rate. However, the initial pH of low-buffer-capacity solutions did not notably affect HP release and phosphate removal. According to laboratory experiments, HP released from CP could impair photosynthetic activity, resulting in Microcystis mortality. Furthermore, the reduced phosphate concentration in the solutions suggested that CP could facilitate the control of eutrophication, which directly reduced bloom formation. Hence, our results confirmed CP as a promising algicide for Microcystis bloom control, and it is worthwhile to develop novel methods for bloom mitigation based on CP. Graphic abstract.

Efficient removal of U(VI) from aqueous solutions using the magnetic biochar derived from the biomass of a bloom-forming cyanobacterium (Microcystis aeruginosa)

The utilization of Microcystis biomass is an urgent issue in the mitigation of cyanobacterial bloom. In this study, Microcystis-derived biochar (MB) and Fe₃O₄-modified biochar (Fe₃O₄/MB) were fabricated for the U(VI) elimination. The results showed that U(VI) sorption process by either MB or Fe₃O₄/MB was pH-dependent and ionic strength-independent. The maximum sorption capacity of MB was higher than that of Fe₃O₄/MB. According to the analysis of X-ray photoelectron spectroscopy, U(VI) sorption on both MB and Fe₃O₄/MB was mainly ascribed to the surface complexation between U(VI) and oxygen-containing functional groups on the surface of MB. Fe₃O₄ particles on the surface of MB didn't provide extra active sites for the sorption of U(VI), but it enabled the adsorbent to be magnetically separated. Five consecutive sorption/desorption cycles verified the good reusability of Fe₃O₄/MB in this study. Therefore, the investigation is not only meaningful for the utilization of nuisance biomass from cyanobacterial blooms, but also provides novel adsorbents for the U(VI) removal from aqueous solutions.

Characteristics of Fluorescence Spectra, UV Spectra, and Specific Growth Rates during the Outbreak of Toxic Microcystis Aeruginosa FACHB-905 and Non-Toxic FACHB-469 under Different Nutrient Conditions in a Eutrophic Microcosmic Simulation Device

Microcystis aeruginosa is the dominant alga forming cyanobacteria blooms, the growth of which is limited by available nutrients. Thus, it is necessary to study cyanobacteria blooms and explore the growth of Microcystis aeruginosa under different nutrient conditions. In this paper, we take Microcystis aeruginosa, including toxic Freshwater Algae Culture of Hydrobiology Collection (FACHB)-905 and non-toxic FACHB-469 strains, into account. The strains were cultured using a simulation device under different nutrient conditions. Ultraviolet spectra, three-dimensional fluorescence spectra, and kinetic parameter indicators of the two species are studied. Compared to FACHB-469, the results show that the specific growth rate of FACHB-905 is much higher, in particular, FACHB-905 is the dominant species under low nutrient conditions. Furthermore, the UV spectral characteristics indicate that the molecular weight of dissolved organic matter in the culture tank of toxic FACHB-905 is greater than that of FACHB-469. Additionally, the humification index of toxic FACHB-905 is slightly higher as well, which suggests that it is more stable in the presence of dissolved organic matter during blooms. Therefore, the toxic Microcystis strain is more likely to become the dominant species in water blooms under lower eutrophic conditions and water blooms formed by the toxic Microcystis strain may be more difficult to recover from.

Proteomic mechanisms for the stimulatory effects of antibiotics on Microcystis aeruginosa during hydrogen peroxide treatment

Application of low dosage of H₂O₂ at early stage of cyanobacterial life cycle is a promising route for cyanobacterial bloom mitigation, which could minimize adverse effects on non-target organisms. Besides, influence of co-existing contaminants on cyanobacterial bloom mitigation under combined pollution conditions remains unclear. This study assessed the influence of a mixture of four frequently detected antibiotics (tetracycline, sulfamethoxazole, ciprofloxacin and amoxicillin) during H₂O₂ treatment of Microcystis aeruginosa at early growth stage. H₂O₂ significantly (p < 0.05) inhibited growth rate, chlorophyll a content, F_v/F_m and rETR_max in a dose-dependent manner at low doses of 0.25-1 mg L^-1, through downregulating proteins involved in cell division, cellular component organization, gene expression and photosynthesis. Although H₂O₂ increased microcystin content in each cyanobacterial cell through the upregulation of microcystin synthetases (mcyC and mcyF), total microcystin concentration in H₂O₂ treated groups was significantly (p < 0.05) reduced due to the decrease of cell density. Existence of 80 and 200 ng L^-1 mixed antibiotics during H₂O₂ treatment facilitated the scavenging of ROS by antioxidant enzymes and significantly (p < 0.05) stimulated growth, photosynthesis, microcystin synthesis and microcystin release in H₂O₂ treated cells, through the upregulation of proteins involved in photosynthesis, oxidation-reduction process, biosynthesis, gene expression and transport. Mixed antibiotics increased the hazard of M. aeruginosa during H₂O₂ treatment, through the stimulation of microcystin synthesis and release at the proteomic level. Each target antibiotic should be controlled below 5 ng L^-1 before the application of H₂O₂ for eliminating the interference of antibiotics on cyanobacterial bloom mitigation.