The Efficacy of Hydrogen Peroxide in Mitigating Cyanobacterial Blooms and Altering Microbial Communities across Four Lakes in NY, USA

The synchronicity of bloom-forming cyanobacteria transcription patterns and hydrogen peroxide dynamics

Hydrogen peroxide is a reactive oxygen species (ROS) naturally occurring at low levels in aquatic environments and production varies widely across different ecosystems. Oxygenic photosynthesis generates hydrogen peroxide as a byproduct, of which some portion can be released to ambient water. However, few studies have examined hydrogen peroxide dynamics in relation to cyanobacterial harmful algal blooms (cHABs). A year-long investigation of algal succession and hydrogen peroxide dynamics was conducted at the Caloosahatchee River, Florida, USA. We aimed to identify potential biological mechanisms responsible for elevated hydrogen peroxide production during cHAB events through the exploration of the freshwater microbial metatranscriptome. Hydrogen peroxide concentrations were elevated from February to September of 2021 when cyanobacteria were active and abundant. We observed one Microcystis cHAB event in spring and one in winter. Both had distinct nutrient uptake and cyanotoxin gene expression patterns. While meaningful levels of microcystin were only detected during periods of elevated hydrogen peroxide, cyanopeptolin was by far the most expressed cyanotoxin during the spring bloom when hydrogen peroxide was at its yearly maxima. Gene expressions of five microbial enzymes (Rubisco, superoxide dismutase, cytochrome b559, pyruvate oxidase, and NADH dehydrogenase) positively correlated to hydrogen peroxide concentrations. Additionally, there was higher nitrogen-fixing gene (nifDKH) expression by filamentous cyanobacteria after the spring bloom but no secondary bloom formation occurred. Overall, elevated environmental hydrogen peroxide concentrations were linked to cyanobacterial dominance and greater expression of specific enzymes in the photosynthesis of cyanobacteria. This implicates cyanobacterial photosynthesis and growth results in increased hydrogen peroxide generation as reflected in measured environmental concentrations.

Microbial Community Response to Granular Peroxide-Based Algaecide Treatment of a Cyanobacterial Harmful Algal Bloom in Lake Okeechobee, Florida (USA)

Cyanobacterial harmful algal blooms (cyanoHABs) occur in fresh water globally. These can degrade water quality and produce toxins, resulting in ecological and economic damages. Thus, short-term management methods (i.e., algaecides) are necessary to rapidly mitigate the negative impacts of cyanoHABs. In this study, we assess the efficacy of a hydrogen peroxide-based algaecide (PAK® 27) on a Microcystis dominated bloom which occurred within the Pahokee Marina on Lake Okeechobee, Florida, USA. We observed a significant reduction in chlorophyll a (96.81%), phycocyanin (93.17%), and Microcystis cell counts (99.92%), and a substantial reduction in microcystins (86.7%) 48 h after treatment (HAT). Additionally, there was a significant shift in bacterial community structure 48 HAT, which coincided with an increase in the relative abundance of photosynthetic protists. These results indicate that hydrogen peroxide-based algaecides are an effective treatment method for cyanoHAB control and highlight their effects on non-target microorganisms (i.e., bacteria and protists).

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.

Shifts in phytoplankton and zooplankton communities in three cyanobacteria-dominated lakes after treatment with hydrogen peroxide

Cyanobacteria can reach high densities in eutrophic lakes, which may cause problems due to their potential toxin production. Several methods are in use to prevent, control or mitigate harmful cyanobacterial blooms. Treatment of blooms with low concentrations of hydrogen peroxide (H2O2) is a promising emergency method. However, effects of H2O2 on cyanobacteria, eukaryotic phytoplankton and zooplankton have mainly been studied in controlled cultures and mesocosm experiments, while much less is known about the effectiveness and potential side effects of H2O2 treatments on entire lake ecosystems. In this study, we report on three different lakes in the Netherlands that were treated with average H2O2 concentrations ranging from 2 to 5 mg L-1 to suppress cyanobacterial blooms. Effects on phytoplankton and zooplankton communities, on cyanotoxin concentrations, and on nutrient availability in the lakes were assessed. After every H2O2 treatment, cyanobacteria drastically declined, sometimes by more than 99%, although blooms of Dolichospermum sp., Aphanizomenon sp., and Planktothrix rubescens were more strongly suppressed than a Planktothrix agardhii bloom. Eukaryotic phytoplankton were not significantly affected by the H2O2 additions and had an initial advantage over cyanobacteria after the treatment, when ample nutrients and light were available. In all three lakes, a new cyanobacterial bloom developed within several weeks after the first H2O2 treatment, and in two lakes a second H2O2 treatment was therefore applied to again suppress the cyanobacterial population. Rotifers strongly declined after most H2O2 treatments except when the H2O2 concentration was ≤ 2 mg L-1, whereas cladocerans were only mildly affected and copepods were least impacted by the added H2O2. In response to the treatments, the cyanotoxins microcystins and anabaenopeptins were released from the cells into the water column, but disappeared after a few days. We conclude that lake treatments with low concentrations of H2O2 can be a successful tool to suppress harmful cyanobacterial blooms, but may negatively affect some of the zooplankton taxa in lakes. We advise pre-tests prior to the treatment of lakes to define optimal treatment concentrations that kill the majority of the cyanobacteria and to minimize potential side effects on non-target organisms. In some cases, the pre-tests may discourage treatment of the lake.

MIB-derived odor management based upon hydraulic regulation in small drinking water reservoirs: Principle and application

The musty odorant (2-methylisoborneol, MIB) is prevalent in source water reservoirs and has become one of the major challenges for drinking water quality. This study proposes an approach to control the growth of MIB-producing cyanobacteria in a small reservoir based on hydraulic regulation, according to the results of long-term field investigations, laboratory culture experiments, model construction, and field application. Field investigations found that longer hydraulic retention time (HRT) is a factor that triggers MIB episodes. The culture study revealed that the maximum cell density, growth rate of MIB-producing Planktothricoides raciborskii, and MIB concentration are determined by the HRT (R2= 0.94, p-value < 0.001) and can be minimized by decreasing the HRT to less than 10 d. On this basis, an HRT regulation model was constructed and validated by field investigation, and critical HRT values were evaluated for 14 cyanobacteria genera. By decreasing the HRT to 5.4 ± 0.8 d, which is lower than the critical value of 7.5 ∼ 15.0 d, an MIB episode was successfully terminated in ZXD Reservoir in 2021. The results suggest that the proposed principle can provide a scientific basis for HRT regulation, which has been proved to be effective and feasible. This approach avoids negative impacts on water quality, does not require extra investment in engineering infrastructure, and in some cases may be applied readily by changing existing operational procedures. Therefore, HRT-based regulation is a promising strategy targeting MIB control and possibly for other cyanobacterial-derived water quality problems in small reservoirs.

Effect of Hydrogen Peroxide on Cyanobacterial Biofilms

Although a range of disinfecting formulations is commercially available, hydrogen peroxide is one of the safest chemical agents used for disinfection in aquatic environments. However, its effect on cyanobacterial biofilms is poorly investigated. In this work, biofilm formation by two filamentous cyanobacterial strains was evaluated over seven weeks on two surfaces commonly used in marine environments: glass and silicone-based paint (Sil-Ref) under controlled hydrodynamic conditions. After seven weeks, the biofilms were treated with a solution of hydrogen peroxide (H2O2) to assess if disinfection could affect long-term biofilm development. The cyanobacterial biofilms appeared to be tolerant to H2O2 treatment, and two weeks after treatment, the biofilms that developed on glass by one of the strains presented higher biomass amounts than the untreated biofilms. This result emphasizes the need to correctly evaluate the efficiency of disinfection in cyanobacterial biofilms, including assessing the possible consequences of inefficient disinfection on the regrowth of these biofilms.

Evaluation of algaecide effectiveness of five different oxidants applied on harmful phytoplankton

Harmful algal blooms (HABs) in coastal areas similarly impact both ecosystems and human health. The translocation of phytoplankton species via maritime transport can potentially promote the growth of HABs in coastal systems. Accordingly, ballast water must be disinfected. The main goal of this study is to assess the effectiveness of different emerging biocides, including H₂O₂, peracetic acid (PAA), peroxymonosulfate (PMS), and peroxydisulfate (PDS). The effectiveness of these biocides is compared with that of conventional chlorination methods. Their effects on two ichthyotoxic microalgae with worldwide distribution, i.e., Prymnesium parvum and Heterosigma akashiwo, are examined. To ensure the prolonged effectiveness of the different reagents, their concentration-response curves for 14 days are constructed and examined. The results suggest a strong but shorter effect by PMS (EC50 = 0.40-1.99 mg·L^-1) and PAA (EC50 = 0.32-2.70 mg·L^-1), a maintained effect by H₂O₂ (EC50 = 6.67-7.08 mg·L^-1), and a negligible effect by PDS. H. akashiwo indicates higher resistance than P. parvum, except when H₂O₂ is used. Based on the growth inhibition performance and consumption of the reagents as well as a review of important aspects regarding their application, using H₂O₂, PAA, or PMS can be a feasible alternative to chlorine-based reagents for inhibiting the growth of harmful phytoplankton.

Comparative assessment of algaecide performance on freshwater phytoplankton: Understanding differential sensitivities to frame cyanobacteria management

Cyanobacterial bloom represent a growing threat to global water security. With fast proliferation, they raise great concern due to potential health and socioeconomic concerns. Algaecides are commonly employed as a mitigative measure to suppress and manage cyanobacteria. However, recent research on algaecides has a limited phycological focus, concentrated predominately on cyanobacteria and chlorophytes. Without considering phycological diversity, generalizations crafted from these algaecide comparisons present a biased perpective. To limit the collateral impacts of algaecide interventions on phytoplankton communities it is critical to understand differential phycological sensitivities for establishing optimal dosage and tolerance thresholds. This research attempts to fill this knowledge gap and provide effective guidelines to frame cyanobacterial management. We investigate the effect of two common algaecides, copper sulfate (CuSO₄) and hydrogen peroxide (H₂O₂), on four major phycological divisions (chlorophytes, cyanobacteria, diatoms, and mixotrophs). All phycological divisions exhibited greater sensitivity to copper sulfate, except chlorophytes. Mixotrophs and cyanobacteria displayed the highest sensitivity to both algaecides with the highest to lowest sensitivity being observed as follows: mixotrophs, cyanobacteria, diatoms, and chlorophytes. Our results suggest that H₂O₂ represents a comparable alternative to CuSO₄ for cyanobacterial control. However, some eukaryotic divisions such as mixotrophs and diatoms mirrored cyanobacteria sensitivity, challenging the assumption that H₂O₂ is a selective cyanocide. Our findings suggest that optimizing algaecide treatments to suppress cyanobacteria while minimizing potential adverse effects on other phycological members is unattainable. An apparent trade-off between effective cyanobacterial management and conserving non-targeted phycological divisions is expected and should be a prime consideration of lake management.

Repeated hydrogen peroxide dosing briefly reduces cyanobacterial blooms and microcystin while increasing fecal bacteria indicators in a eutrophic pond

This study explored the effects of H₂O₂ on Cyanobacteria and non-target microbes using fluorometry, microscopy, flow cytometry, and high throughput DNA sequencing of the 16S rRNA gene during a series of mesocosm and whole-ecosystem experiments in a eutrophic pond in NY, USA. The addition of H₂O₂ (8 mg/L) significantly reduced Cyanobacteria concentrations during a majority of experiments (66%; 6 of 9) and significantly increased eukaryotic green and unicellular brown algae in 78% and 45% of experiments, respectively. While heterotrophic bacteria declined significantly following H₂O₂ addition in all experiments, bacteria indicative of potential fecal contamination (Escherichia coli, Enterococcus, fecal coliform bacteria) consistently and significantly increased in response to H₂O₂, evidencing a form of 'pollution swapping'. H₂O₂ more effectively reduced Cyanobacteria in enclosed mesocosms compared to whole-ecosystem applications. Ten whole-pond H₂O₂ applications over a two-year period temporarily reduced cyanobacterial levels but never reduced concentrations below bloom thresholds and populations always rebounded in two weeks or less. The bacterial phyla of Cyanobacteria, Actinobacteria, and Planctomycetes were the most negatively impacted by H₂O₂. Microcystis was always reduced by H₂O₂, as was the toxin microcystin, but Microcystis remained dominant even after repeated H₂O₂ treatments. Although H₂O₂ favored the growth of eukaryotic algae over potentially harmful Cyanobacteria, the inability of H₂O₂ to end cyanobacterial blooms in this eutrophic waterbody suggests it is a non-ideal mitigation approach in high biomass ecosystems and should be used judiciously due to potential negative impacts on non-target organisms and promotion of bacteria indicative of fecal contamination.

Mitigation of Marine Dinoflagellates Using Hydrogen Peroxide (H2O2) Increases Toxicity towards Epithelial Gill Cells

Hydrogen peroxide (H₂O₂) has been shown to efficiently remove toxic microalgae from enclosed ballast waters and brackish lakes. In this study, in vitro experiments were conducted to assess the side effects of mitigating toxic and non-toxic dinoflagellates with H₂O₂. Five H₂O₂ concentrations (50 to 1000 ppm) were used to control the cell abundances of the toxic dinoflagellates Alexandrium catenella and Karenia selliformis and the non-toxic dinoflagellates Lepidodinium chlorophorum and Prorocentrum micans. Photosynthetic efficiency and staining dye measurements showed the high efficiency of H₂O₂ for mitigating all dinoflagellate species at only 50 ppm. In a bioassay carried out to test cytotoxicity using the cell line RTgill-W1, control experiments (only H₂O₂) showed cytotoxicity in a concentration- and time- (0 to 24 h) dependent manner. The toxic dinoflagellates, especially K. selliformis, showed basal cytotoxicity that increased with the application of hydrogen peroxide. Unexpectedly, the application of a low H₂O₂ concentration increased toxicity, even when mitigating non-toxic dinoflagellates. This study suggests that the fatty acid composition of toxic and non-toxic dinoflagellate species can yield toxic aldehyde cocktails after lipoperoxidation with H₂O₂ that can persist in water for days with different half-lives. Further studies are needed to understand the role of lipoperoxidation products as acute mediators of disease and death in aquatic environments.

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.

Evidence-Based Framework to Manage Cyanobacteria and Cyanotoxins in Water and Sludge from Drinking Water Treatment Plants

Freshwater bodies and, consequently, drinking water treatment plants (DWTPs) sources are increasingly facing toxic cyanobacterial blooms. Even though conventional treatment processes including coagulation, flocculation, sedimentation, and filtration can control cyanobacteria and cell-bound cyanotoxins, these processes may encounter challenges such as inefficient removal of dissolved metabolites and cyanobacterial cell breakthrough. Furthermore, conventional treatment processes may lead to the accumulation of cyanobacteria cells and cyanotoxins in sludge. Pre-oxidation can enhance coagulation efficiency as it provides the first barrier against cyanobacteria and cyanotoxins and it decreases cell accumulation in DWTP sludge. This critical review aims to: (i) evaluate the state of the science of cyanobacteria and cyanotoxin management throughout DWTPs, as well as their associated sludge, and (ii) develop a decision framework to manage cyanobacteria and cyanotoxins in DWTPs and sludge. The review identified that lab-cultured-based pre-oxidation studies may not represent the real bloom pre-oxidation efficacy. Moreover, the application of a common exposure unit CT (residual concentration × contact time) provides a proper understanding of cyanobacteria pre-oxidation efficiency. Recently, reported challenges on cyanobacterial survival and growth in sludge alongside the cell lysis and cyanotoxin release raised health and technical concerns with regards to sludge storage and sludge supernatant recycling to the head of DWTPs. According to the review, oxidation has not been identified as a feasible option to handle cyanobacterial-laden sludge due to low cell and cyanotoxin removal efficacy. Based on the reviewed literature, a decision framework is proposed to manage cyanobacteria and cyanotoxins and their associated sludge in DWTPs.

On the Efficacy of H2O2 or S2O82- at Promoting the Inactivation of a Consortium of Cyanobacteria and Bacteria in Algae-Laden Water

Harmful algal blooms in coastal areas can significantly impact a water source. Microorganisms such as cyanobacteria and associated pathogenic bacteria may endanger an ecosystem and human health by causing significant eco-hazards. This study assesses the efficacy of two different reagents, H2O2 and S2O82−, as (pre-)treatment options for algae-laden waters. Anabaena sp. and Vibrio alginolyticus have been selected as target microorganisms. With the objective of activating H2O2 or S2O82−, additional experiments have been performed with the presence of small amounts of iron (18 µmol/L). For the cyanobacterial case, H2O2-based processes demonstrate greater efficiency over that of S2O82−, as Anabaena sp. is particularly affected by H2O2, for which >90% of growth inhibition has been achieved with 0.088 mmol/L of H2O2 (at 72 h of exposure). The response of Anabaena sp. as a co-culture with V. alginolyticus implies the use of major H2O2 amounts for its inactivation (0.29 mmol/L of H2O2), while the effects of H2O2/Fe(II) suggests an improvement of ~60% compared to single H2O2. These H2O2 doses are not sufficient for preventing the regrowth of V. alginolyticus after 24 h. The effects of S2O82− (+ Fe(II)) are moderate, reaching maximum inhibition growth of ~50% for Anabaena sp. at seven days of exposure. Nevertheless, doses of 3 mmol/L of S2O82− can prevent the regrowth of V. alginolyticus. These findings have implications for the mitigation of HABs but also for the associated bacteria that threaten many coastal ecosystems.

Oxidation to Control Cyanobacteria and Cyanotoxins in Drinking Water Treatment Plants: Challenges at the Laboratory and Full-Scale Plants

The impact of oxidation on mitigation of cyanobacteria and cyanotoxins in drinking water treatment sludge was investigated at the laboratory and treatment plant scales. Two common oxidants, KMnO4 (5 and 10 mg/L) and H2O2 (10 and 20 mg/L) were applied under controlled steady-state conditions. Non-oxidized and oxidized sludge was left to stagnate in the dark for 7 to 38 days. Controlled laboratory trials show that KMnO4 and H2O2 decreased cell counts up to 62% and 77%, respectively. The maximum total MC level reduction achieved after oxidation was 41% and 98% using 20 mg/L H2O2 and 10 mg/L KMnO4, respectively. Stagnation caused cell growth up to 2.6-fold in 8 out of 22 oxidized samples. Microcystin (MC) producer orders as Chroococcales and Synechococcales were persistent while Nostocales was sensitive to combined oxidation and stagnation stresses. In parallel, two on-site shock oxidation treatments were performed in the DWTP’s sludge holding tank using 10 mg/L KMnO4. On-site shock oxidation decreased taxonomic cell counts by up to 43% within 24 h. Stagnation preceded by on-site shock oxidation could increase total cell counts by up to 55% as compared to oxidation alone. The increase of cell counts and mcyD gene copy numbers during stagnation revealed the impact of oxidation/stagnation on cyanobacterial cell growth. These findings show the limitations of sludge oxidation as a strategy to manage cyanobacteria and cyanotoxins in sludge and suggest that alternative approaches to prevent the accumulation and mitigation of cyanobacteria in sludge should be considered.

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.

Novel high throughput sequencing - fluorometric approach demonstrates Microcystis blooms across western Lake Erie are promoted by grazing resistance and nutrient enhanced growth

Cyanobacterial harmful algal blooms (CHABs) are a global public health threat. While CHABs are often promoted by nutrients, an important and often overlooked influence on bloom dynamics is zooplankton grazing. In the present study, zooplankton grazing and nutrient enrichment experiments were combined with next generation sequencing and fluorometric analyses to quantify differential grazing and nutrient effects on specific cyanobacterial genera across the western basin of Lake Erie. Grazing by two different sized daphnids, Daphnia magna and Daphnia pulex, was compared to protozooplankton grazing effects assessed via a dilution approach at sites within the Maumee and Sandusky Bays where Planktothrix, Microcystis, Synechococcus, and Dolichospermum were the dominant genera. Daphnid grazing significantly reduced Synechococcus net growth rates at most sites as well as Planktothrix net growth in Sandusky Bay and Dolichospermum in Maumee Bay. Dilution resulted in significant growth increase of Synechococcus at half of the sites and Planktothrix at most sites evidencing substantial grazing pressure by the protozooplankton community on these genera. In contrast, Microcystis populations were largely unaffected by daphnids and protozooplankton grazing but benefitted from nutrient enrichment more than other CHAB genera. When diatoms were present in moderate abundance, grazing rates by daphnids on diatoms were significantly greater than grazing rates on cyanobacteria. The novel approach used in this study established differences in grazing pressure and nutrient effects on differing taxa and revealed that, while many taxa were grazed by multiple classes of zooplankton (e.g. Planktothrix, Synechococcus, Dolichospermum, diatoms), the lack of grazing pressure on Microcystis coupled with nutrient-enhanced growth in western Lake Erie promotes the occurrence of CHABs of this genus.

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.

Removal of Cyanobacteria and Cyanotoxins in Waters

Harmful cyanobacterial algal blooms and cyanotoxins currently pose a major threat to global society, one that exceeds local and national interests due to their extremely destructive effects on the environment and human health [...].

A Stochastic Characterization of Hydrogen Peroxide-Induced Regulated Cell Death in Microcystis aeruginosa

Regulated cell death (RCD) encompasses the activation of cellular pathways that initiate and execute a self-dismissal process. RCD occur over a range of stressors doses that overcome pro-survival cellular pathways, while higher doses cause excessive damage leading to passive accidental cell death (ACD). Hydrogen peroxide (HP) has been proposed as a potential tool to control harmful cyanobacterial blooms, given its capacity to remove cyanobacterial cells and oxidize cyanotoxins. HP is a source of hydroxyl radicals and is expected to induce RCD only within a limited range of concentrations. This property makes this compound very useful to better understand stress-driven RCD. In this work, we analyzed cell death in microcystin-producing Microcystis aeruginosa by means of a stochastic dose response model using a wide range of HP concentrations (0, 0.29, 1.76, 3.67, 7.35, 14.70, and 29.5 mM). We used flow cytometry and unsupervised classification to study cell viability and characterize transitional cell death phenotypes after exposing cells to HP for 48 and 72 h. Non-linear regression was used to fit experimental data to a logistic cumulative distribution function (cdf) and calculate the half maximal effective concentration (EC₅₀). The EC₅₀ of M. aeruginosa exposed to HP were 3.77 ± 0.26 mM and 4.26 ± 0.22 mM at 48 and 72 h, respectively. The derivative of cdf (probability density function; pdf) provided theoretical and practical demonstration that EC₅₀ is the minimal dose required to cause RCD in 50% of cells, therefore maximizing the probability of RCD occurrence. 1.76 mM HP lead to an antioxidant stress response characterized by increased reactive oxygen species (ROS) levels and HP decomposition activity. The exposure of 3.67 mM HP induced a dose-related transition in cell death phenotype, and produced several morphological changes (a less dense stroma, distortion of the cell membrane, partial disintegration of thylakoids, extensive cytoplasmic vacuolation and highly condensed chromatin). The EC₅₀ and the stochastic cdf and pdf together with the multidimensional transitional phenotypic analysis of single cells contribute to further characterize cell death pathways in cyanobacteria.

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.

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.

Diversity Assessment of Toxic Cyanobacterial Blooms during Oxidation

Fresh-water sources of drinking water are experiencing toxic cyanobacterial blooms more frequently. Chemical oxidation is a common approach to treat cyanobacteria and their toxins. This study systematically investigates the bacterial/cyanobacterial community following chemical oxidation (Cl₂, KMnO₄, O₃, H₂O₂) using high throughput sequencing. Raw water results from high throughput sequencing show that Proteobacteria, Actinobacteria, Cyanobacteria and Bacteroidetes were the most abundant phyla. Dolichospermum, Synechococcus, Microcystis and Nostoc were the most dominant genera. In terms of species, Dolichospermum sp.90 and Microcystis aeruginosa were the most abundant species at the beginning and end of the sampling, respectively. A comparison between the results of high throughput sequencing and taxonomic cell counts highlighted the robustness of high throughput sequencing to thoroughly reveal a wide diversity of bacterial and cyanobacterial communities. Principal component analysis of the oxidation samples results showed a progressive shift in the composition of bacterial/cyanobacterial communities following soft-chlorination with increasing common exposure units (CTs) (0–3.8 mg·min/L). Close cyanobacterial community composition (Dolichospermum dominant genus) was observed following low chlorine and mid-KMnO₄ (287.7 mg·min/L) exposure. Our results showed that some toxin producing species may persist after oxidation whether they were dominant species or not. Relative persistence of Dolichospermum sp.90 was observed following soft-chlorination (0.2–0.6 mg/L) and permanganate (5 mg/L) oxidation with increasing oxidant exposure. Pre-oxidation using H₂O₂ (10 mg/L and one day contact time) caused a clear decrease in the relative abundance of all the taxa and some species including the toxin producing taxa. These observations suggest selectivity of H₂O₂ to provide an efficient barrier against toxin producing cyanobacteria entering a water treatment plant.

Removal of Positively Buoyant Planktothrix rubescens in Lake Restoration

The combination of a low-dose coagulant (polyaluminium chloride—‘Floc’) and a ballast able to bind phosphate (lanthanum modified bentonite, LMB—‘Sink/Lock’) have been used successfully to manage cyanobacterial blooms and eutrophication. In a recent ‘Floc and Lock’ intervention in Lake de Kuil (the Netherlands), cyanobacterial chlorophyll-a was reduced by 90% but, surprisingly, after one week elevated cyanobacterial concentrations were observed again that faded away during following weeks. Hence, to better understand why and how to avoid an increase in cyanobacterial concentration, experiments with collected cyanobacteria from Lakes De Kuil and Rauwbraken were performed. We showed that the Planktothrix rubescens from Lake de Kuil could initially be precipitated using a coagulant and ballast but, after one day, most of the filaments resurfaced again, even using a higher ballast dose. By contrast, the P. rubescens from Lake Rauwbraken remained precipitated after the Floc and Sink/Lock treatment. We highlight the need to test selected measures for each lake as the same technique with similar species (P. rubescens) yielded different results. Moreover, we show that damaging the cells first with hydrogen peroxide before adding the coagulant and ballast (a ‘Kill, Floc and Lock/Sink’ approach) could be promising to keep P. rubescens precipitated.