Responses of Microcystis Colonies of Different Sizes to Hydrogen Peroxide Stress

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.

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.

Transgenerational effects of extracts containing Microcystin-LR exposure on reproductive toxicity and offspring growth inhibition in a model organism zebrafish

Cyanobacteria cell lysates release numerous toxic substances (e.g., cyanotoxins) into the water, posing a serious threat to human health and aquatic ecosystems. Microcystins (MCs) are among the most abundant cyanotoxins in the cell lysates, with microcystin-LR (MC-LR) being one of the most common and highly toxic congeners. In this study, zebrafish (Danio rerio) were exposed to different levels MC-LR that from extracts of Microcystis aeruginosa. Changes in the MC-LR accumulations, organ coefficients, and antioxidant enzyme activities in the zebrafish were analyzed. Transgenerational reproductive toxicity of MC-LR in the maternal and paternal generations was further investigated, as well as the influences of extracts containing MC-LR exposures of the F1 on the growth of zebrafish. The study found that high levels of MC-LR could be detected in the major organs of adult zebrafish, particularly in spleen. Notably, concentration of MC-LR in the spermary was significantly higher than that in the ovarium. MC-LR could induce oxidative damage by affecting the activities of catalase and superoxide dismutase. Inherited from F0, MC-LR led to impaired development in the F1 generation. Difference in offspring survival rates could be observed in the groups with different MC-LR levels of maternal and paternal exposures. This study reveals transgenerational effects of MC-LR on the reproductive toxicity and offspring growth inhibition to the aquatic organisms, which should be emphasized in the future ecological risk assessment.

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.

Hydrogen peroxide as a mitigation against Microcystis sp. bloom

Microcystis sp. is a harmful cyanobacterial species commonly seen in earthen ponds. The overgrowth of these algae can lead to fluctuations in water parameters, including DO and pH. Also, the microcystins produced by these algae are toxic to aquatic animals. This study applied hydrogen peroxide (7 mg/L) to treat Microcystis sp. in a laboratory setting and in three earthen pond trials. In the lab we observed a 64.7% decline in Microcystis sp. And in our earthen pond field experiments we measured, on average, 43% reductions in Microcystis sp. cell counts within one hour. The treatment was found to eliminate specifically Microcystis sp. and did not reduce the cell count of the other algae species in the pond. A shift of the algae community towards the beneficial algae was also found post-treatment. Lastly, during the pond trials, the gill status of Tilapia and Giant tiger prawn were not affected by the H2O2 treatment suggesting this may be a good mitigation strategy for reducing cyanobacteria in pond aquaculture.

Differences in survivability and toxic potential among Microcystis colonies of different sizes in sediment

Microcystis colonies have the ability to persist for extended periods in sediment and function as a "seed bank" for the succeeding summer bloom in water column. The colonial morphology and toxin production ability of Microcystis are important for their population maintenance and life history. However, it is unclear about the influence of the colony morphology and toxic potential of Microcystis colonies on their benthic process. To address this question, we classified field Microcystis samples into three groups based on their size (< 150 μm, 150-300 μm, and > 300 μm) and compared their survivability and toxic potential during culturing in sediment. The results showed that Microcystis colonies in sediments disappeared quickly at 25℃ but survived for long periods at 5℃. The survivability of smaller Microcystis colonies (< 300 μm) was significantly higher than that of larger ones (> 300 μm). The activities of catalase (CAT) were significantly increased in large colonies compared to small colonies at 15℃ and 25℃. Real-time PCR indicated that smaller colonies had higher proportion of potential toxic genotype, and Microcystis colonies cultured at 15℃ and 25℃ showed higher percentage of microcystin-producing genotype. These results indicate that Microcystis colonies survived longer at low temperature and that larger Microcystis colonies are more susceptible to oxidative stress in sediments. The difference of toxic potential of Microcystis colonies of different sizes in sediments may be related to their survival ability in sediments.

Novel ecological implications of non-toxic Microcystis towards toxic ecotype in population-promoting toxic ecotype dominance at various N levels and cooperative defense against luteolin-stress

Microcystin (MC)-producing (MC+) and MC-free (MC-) Microcystis always co-exist and interact during Microcystis-dominated cyanobacterial blooms (MCBs), where MC+Microcystis abundance and extracellular MC-content (EMC) determine the hazard extent of MCBs. The current study elucidated intraspecific interaction between MC+ and MC-Microcystis at various nitrogen (N) levels (0.5-50 mg/L) and how such N-mediated interaction impacted algicidal and EMC-inhibiting effect of luteolin, a natural bioalgicide. Conclusively, MC+ and MC-Microcystis were inhibited mutually at N-limitation (0.5 mg/L), which enhanced the algicidal and EMC-inhibiting effects of luteolin. However, at N-sufficiency (5-50 mg/L), MC-Microcystis promoted MC+ ecotype growth and dominance, and such intraspecific interaction induced the cooperative defense of two ecotypes, weakening luteolin's algicidal and EMC-inhibiting effects. Mechanism analyses further revealed that MC+Microcystis in luteolin-stress co-culture secreted exopolymeric substances (EPSs) for self-protection against luteolin-stress and also released more EMC to induce EPS-production by MC-Microcystis as protectants, thus enhancing their luteolin-resistance and promoting their growth. This study provided novel ecological implications of MC-Microcystis toward MC+ ecotype in terms of assisting the dominant establishment of MC+Microcystis and cooperative defense with MC+ ecotype against luteolin, which guided the application of bioalgicide (i.e. luteolin) for MCBs and MCs pollution mitigation in different eutrophication-degree waters.

Recoverability of Microcystis aeruginosa and Pseudanabaena foetida Exposed to a Year-Long Dark Treatment

Cyanobacteria are a significant primary producer and pioneer species that play a vital role in ecological reconstruction, especially in aquatic environments. Cyanobacteria have excellent recovery capacity from significant stress exposure and are thus suggested as bioreserves, even for space colonization programs. Few studies have been conducted on the recovery capacity after experiencing stress. Long-duration darkness or insufficient light is stressful for photosynthetic species, including cyanobacteria, and can cause chlorosis. Cyanobacterial recovery after extensive exposure to darkness has not yet been studied. In this experiment, Microcystis aeruginosa and Pseudanabaena foetida were subjected to a year-long darkness treatment, and the change in recovery capacity was measured in monthly samples. Cyanobacterial growth, chlorophyll-a concentration, oxidative stress, and photosynthetic capacity were evaluated. It was found that the rapid recovery capacity of the two species remained even after one year of darkness treatment. However, the H2O2 content of recovered samples of both M. aeruginosa and P. foetida experienced significant changes at six-seven months, although the photosynthetic capacity of both cyanobacteria species was maintained within the healthy range. The chlorophyll-a and carotenoid content of the recovered samples also changed with increasing darkness. The results showed that long-term dark treatment had time-dependent effects but different effects on M. aeruginosa and P. foetida. However, both cyanobacteria species can recover rapidly after one year of dark treatment.

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.

Combined Effect of NZVI and H2O2 on the Cyanobacterium Microcystis aeruginosa: Performance and Mechanism

In order to eliminate the harmful cyanobacterium Microcystis aeruginosa and the algal organic matters (AOMs) produced by M. aeruginosa, the combined process of nanoscale zero-valent iron (NZVI) and hydrogen peroxide (H₂O₂) has been carried out, and the removal mechanism has also been clarified. As the initial cyanobacterial cell concentration is 1.0 (±0.05) × 10⁵ cells·mL^-1, all the treatments of NZVI, H₂O₂, and NZVI/H₂O₂ have inhibition effects on both the Chl a contents and photosynthetic pigments, with the Chl a removal efficiency of 47.3%, 80.5%, and 90.7% on the 5th day, respectively; moreover, the variation of ζ potential is proportional to that of the Chl a removal efficiency. The malondialdehyde content and superoxide dismutase activity are firstly increased and ultimately decreased to mitigate the oxidative stress under all the treatments. Compared with NZVI treatment alone, the oxidation of the H₂O₂ and NZVI/H₂O₂ processes can effectively destroy the antioxidant enzyme system and then inactivate the cyanobacterial cells, which further leads to the release of photosynthetic pigments and intracellular organic matters (IOM); in addition, the IOM removal efficiency (in terms of TOC) is 61.3% and 54.1% for the H₂O₂ and NZVI/H₂O₂ processes, respectively. Although NZVI is much more effective for extracellular organic matters (EOM) removal, it is less effective for IOM removal. The results of the three-dimensional EEM fluorescence spectra analysis further confirm that both H₂O₂ and NZVI/H₂O₂ have the ability to remove fluorescent substances from EOM and IOM, due to the oxidation mechanism; while NZVI has no removal effect for the fluorescent substances from EOM, it can remove part of fluorescent substances from IOM due to the agglomeration. All the results demonstrate that the NZVI/H₂O₂ process is a highly effective and applicable technology for the removal of M. aeruginosa and AOMs.

Changes in fatty acid composition as a response to glyphosate toxicity in Pseudomonas fluorescens

Excessive use of herbicides decreases soil biodiversity and fertility. The literature on the xenobiotic response by microorganisms is focused on herbicide biodegradation as a selective event. Non-degradation systems independent of selection could allow the survival of tolerant bacteria in contaminated environments, impacting xenobiotic turnover and, consequently, bioremediation strategies. However, it is uncertain whether the response based on these systems requires selective pressure to be effective. The objective here was to analyze non-degradation phenotypes, enzymatic and structural response systems, of Pseudomonas fluorescens CMA-55 strain, already investigated the production pattern of quorum sensing molecules in response to glyphosate, not present at the isolation site. One mode of response was associated with decrease in membrane permeability and effective antioxidative response for 0–2.30 mM glyphosate, at the mid-log growing phase, with higher activities of Mn-SOD, KatA, and KatB, and presence of fatty acids as nonadecylic acid, margaric and lauric acid. The second response system was characterized by lower antioxidative enzymes activity, presence of KatC isoform, and pelargonic, capric, myristic, stearic, palmitoleic and palmitic acid as principal fatty acids, allowing the strain to face stressful conditions in 9.20–11.50 mM glyphosate at the stationary phase. Therefore, the bacterial strain could modify the fatty acid composition and the permeability of membranes in two response modes according to the herbicide concentration, even glyphosate was not previously selective for P. fluorescens, featuring a generalist system based on physiological plasticity.

Community succession during the preventive control of cyanobacterial bloom by hydrogen peroxide in an aquatic microcosm

Hydrogen peroxide (H₂O₂) is an environmentally friendly algaecide with good prospects for cyanobacterial bloom control. In this study, 0.2-1.5 mg L^-1 of H₂O₂ was applied to an aquatic microcosm containing cyanobacteria, bacteria, and eukaryotic phytoplankton at the early cyanobacterial growth stage·H₂O₂ generated hormesis in cyanobacteria at 0.2 mg L^-1; significantly (p < 0.05) inhibited cyanobacterial growth, cyanobacterial photosynthesis, and microcystin production at 0.5-1.5 mg L^-1; and effectively prevented the formation of cyanobacterial bloom without generating adverse effects on eukaryotic phytoplankton at 1.0 and 1.5 mg L^-1. Application of 0.5-1.5 mg L^-1 H₂O₂ directly inhibited the abundance of five typical bloom-forming cyanobacterial genera (Microcystis, Anabeana, Synechococcus, Nostoc, and Oscillatoria), which were negatively correlated with four bacterial genera (Actinotalea, Flavobacterium, Fluviicola, and Exiguobacterium) and five eukaryotic phytoplankton genera (Cyclotella, Desmodesmus, Dinobryon, Fragilaria, and Mychonastes) and positively correlated with six proteobacterial genera (Brevundimonas, Devosia, Limnohabitans, Porphyrobacter, Pseudomonas, and Rhodobacter). After application of 1.0 and 1.5 mg L^-1 H₂O₂ for 15 days, H₂O₂-treated groups showed significantly (p < 0.05) different prokaryotic community structures from that of the control group at the bloom stage (15th day), while eukaryotic community structures in H₂O₂-treated groups remained stable and showed high similarity with that of the control group at a non-bloom stage (5th day). Application of low-dose H₂O₂ during the early cyanobacterial growth stage could effectively prevent the formation of cyanobacterial blooms without disrupting non-target organisms.

Efficient photocatalytic inactivation of Microcystis aeruginosa by a novel Z-scheme heterojunction tubular photocatalyst under visible light irradiation

The design of a photocatalyst for efficient algal inactivation under visible light is essential for the application of photocatalysis to the control of harmful algal blooms. In this study, a novel Z-scheme heterojunction tubular photocatalyst, Ag₂O@PG, was synthesized by chemically depositing silver oxide compounded with P-doped hollow tubular graphitic carbon nitride for the photocatalytic inactivation of Microcystis aeruginosa (M. aeruginosa). The photocatalytic algal inactivation experiments showed that the photocatalytic activity of Ag₂O@PG was influenced by the ratio of the composition of the obtained materials. The optimal algal inactivation efficiency was observed when using Ag₂O@PG-0.4 at a dosage of 0.2 g/L. It was able to achieve a 99.1 % M. aeruginosa inactivation at an initial concentration of 4.5 × 10⁶ cells/mL following 5 h' visible light irradiation. During the process, the cell membrane permeability and cell morphology changed. Furthermore, under the constant attack of superoxide radicals and holes caused by Ag₂O@PG, the superoxide dismutase, glutathione and malondialdehyde of algae cells increased during the experiments to alleviate oxidative damage. Eventually, the antioxidant system of algae cells was destroyed. To further validate the potential application of Ag₂O@PG-0.4 in real algal bloom environment, an experiment in real water samples was carried out. Overall, the Ag₂O@PG-0.4 as an efficient photocatalyst has a promising potential for emergency treatment measures to alleviate algal blooms.

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.

Oxidative Stress in Bacteria and the Central Dogma of Molecular Biology

Ever since the "great oxidation event," Earth's cellular life forms had to cope with the danger of reactive oxygen species (ROS) affecting the integrity of biomolecules and hampering cellular metabolism circuits. Consequently, increasing ROS levels in the biosphere represented growing stress levels and thus shaped the evolution of species. Whether the ROS were produced endogenously or exogenously, different systems evolved to remove the ROS and repair the damage they inflicted. If ROS outweigh the cell's capacity to remove the threat, we speak of oxidative stress. The injuries through oxidative stress in cells are diverse. This article reviews the damage oxidative stress imposes on the different steps of the central dogma of molecular biology in bacteria, focusing in particular on the RNA machines involved in transcription and translation.

Using hydrogen peroxide to control cyanobacterial blooms: A mesocosm study focused on the effects of algal density in Lake Chaohu, China

The application of hydrogen peroxide (H₂O₂) to control harmful algal blooms is affected by algal density and species. In the present study, a simulation field study was carried out to evaluate the removal of cyanobacteria with high algal density (chlorophyll a of approximately 220-250 μg/L) and low algal density (chlorophyll a of approximately 30-50 μg/L) using 10, 20 mg/L H₂O₂ and 5 mg/L H₂O₂. The dynamics of algal biomass, nutrients, microcystins, phytoplankton, and zooplankton were measured within 7 d. The results showed that 5 mg/L H₂O₂ effectively eliminated algal biomass (measured as chlorophyll a and phycocyanin) and inhibited 50% of the photosynthetic activity of the cyanobacteria at 7 d in the low algal cell density group, while the same inhibition rate was observed in the high algal cell density group when the H₂O₂ was 20 mg/L. However, using a high dosage of H₂O₂, such as 10 mg/L, to suppress cyanobacteria with high biomass could result in a dramatic increase in nutrients and microcystins in the water column. The portion of eukaryotic algae, such as Chlorophyta, Bacillariophyta and Euglenophyta, in the phytoplankton community increased with increasing H₂O₂ concentrations; moreover, the dominant species of cyanobacteria changed from the nontoxic genus Dactylococcopsis to the toxic genus Oscillatoria, which may result in acute toxicity to zooplankton. Our results demonstrated that the application of H₂O₂ to control cyanobacterial blooms at the early stage when algal cell density was low posed less potential ecological risks and may have increased the diversity of the phytoplankton community.

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.

α-Lipoic acid protects against microcystin-LR induced hepatotoxicity through regeneration of glutathione via activation of Nrf2

Microcystins (MCs), as the most dominant bloom-forming strains in eutrophic surface water, can induce hepatotoxicity by oxidative stress. Alpha-lipoic acid (α-LA) is a super antioxidant that can induce the synthesis of antioxidants, such as glutathione (GSH), by nuclear factor erythroid 2-related factor 2 (Nrf2). However, the potential molecular mechanism of α-LA regeneration of GSH remains unclear. The present study aimed to investigate whether α-LA could reduce the toxicity of MCs induced in human hepatoma (HepG2), Bel7420 cells, and BALB/c mice by activating Nrf2 to regenerate GSH. Results showed that exposure to 10 μM microcystin-leucine arginine (MC-LR) reduced viability of HepG2 and Bel7402 cells and promoted the formation of reactive oxygen species (ROS) compared with untreated cells. Moreover, the protection of α-LA included reducing the level of ROS, increasing superoxide dismutase activity, and decreasing malondialdehyde. Levels of reduced glutathione (rGSH) and rGSH/oxidized glutathione were significantly increased in cells cotreated with α-LA and MC-LR compared to those treated with MC-LR alone, indicating an ability of α-LA to attenuate oxidative stress and MC-LR-induced cytotoxicity by increasing the amount of rGSH. α-LA can mediate GSH regeneration through the Nrf2 pathway under the action of glutathione reductase in MC-LR cell lines. Furthermore, the data also showed that α-LA-induced cytoprotection against MC-LR is associated with Nrf2 mediate pathway in vivo. These findings demonstrated the potential of α-LA to resist MC-LR-induced oxidative damage of liver.

Effects of Light Intensity and Exposure Period on the Growth and Stress Responses of Two Cyanobacteria Species: Pseudanabaena galeata and Microcystis aeruginosa

Light is an important factor that affects cyanobacterial growth and changes in light can influence their growth and physiology. However, an information gap exists regarding light-induced oxidative stress and the species-specific behavior of cyanobacteria under various light levels. This study was conducted to evaluate the comparative effects of different light intensities on the growth and stress responses of two cyanobacteria species, Pseudanabaena galeata (strain NIES 512) and Microcystis aeruginosa (strain NIES 111), after periods of two and eight days. The cyanobacterial cultures were grown under the following different light intensities: 0, 10, 30, 50, 100, 300, and 600 μmol m−2 s−1. The optical density (OD730), chlorophyll a (Chl-a) content, protein content, H2O2 content, and the antioxidative enzyme activities of catalase (CAT) and peroxidase (POD) were measured separately in each cyanobacteria species. P. galeata was negatively affected by light intensities lower than 30 μmol m−2 s−1 and higher than 50 μmol m−2 s−1. A range of 30 to 50 μmol m−2 s−1 light was favorable for the growth of P. galeata, whereas M. aeruginosa had a higher tolerance for extreme light conditions. The favorable range for M. aeruginosa was 10 to 100 μmol m−2 s−1.

Comparison of algal harvest and hydrogen peroxide treatment in mitigating cyanobacterial blooms via an in situ mesocosm experiment

The use of short-term, fast-acting curative treatments to rapidly suppress the proliferation of upcoming cyanobacterial blooms without negative side effects on overall water quality is important for environmental regulatory agencies. A 15-day in situ mesocosm experiment was conducted to evaluate the effects of algal harvest at different intensities and the effect of hydrogen peroxide on the mitigation of cyanobacterial blooms, subsequent algal growth and phytoplankton community structure. The results indicate that filtration through a 30-μm-pore-size net could remove most of the Microcystis colonies, leading to a decline in algal biomass. However, algal harvest at 30% and 60% intensities tended to promote cyanobacterial growth under nutrient-replete conditions, and the mitigation effect only lasted a few days, since cyanobacteria biomass exhibited no significant difference between the control and those two treatments on Day 6. When the algal harvest intensity was 90%, the cyanobacterial biomass remained at a relatively low level for 15 days. The average Microcystis colony size rapidly returned to the initial level after an initial decline across all the algal harvest intensities, indicating that algal harvest should be repeatedly performed within a short time period to mitigate Microcystis blooms. Furthermore, removing Microcystis colonies by filtration led to increased diversity in the phytoplankton community, as the proportion of non-Microcystis cyanobacteria increased with harvest intensity. This result might pose a challenge for cyanobacterial bloom control over the long term if filamentous cyanobacteria become dominant. The 10.0 mg L^-1 H₂O₂ treatment selectively suppressed cyanobacteria throughout the experimental period, leading to succession from a cyanobacteria-dominated to a Chlorophyta-dominated community after Day 9. Overall, using hydrogen peroxide is more effective than algal harvesting as a one-time quick curative measure.

The possible role of microcystin (D-Leu1 MC-LR) as an antioxidant on Microcystis aeruginosa (Cyanophyceae). In vitro and in vivo evidence

Microcystins constitute a serious threat to the quality of drinking water worldwide. However, the eco-physiological role of them is not completely known and it is suggested that toxins can play a role in the antioxidant protection. The objective of this study was to evaluate the microcystin antioxidant capacity in vitro by Electronic Paramagnetic Resonance, highly specific for the different reactive oxygen species and in vivo by 7 days exposure of Microcystis aeruginosa to high (29 °C) temperature in addition to a 26 °C control condition. An effective in vitro antioxidant activity was observed for [D-Leu¹]MC-LR against hydrosoluble radicals. As far as we know, this is the first in vitro record of the role of MC as antioxidant. In addition, a significant increase in cellular biomass was observed under 26 °C in cultures with [D-Leu¹]MC-LR supplementation in coincidence with a significant decrease of reactive species. For cultures at 29 °C, the antioxidant role of toxins was inconclusive probably due to the presence of different reactive species generated during the experiment. Thus, MC could scavenge certain reactive species associated with the antioxidant role of CAT or the OH content by SOD activity (not measured) and then CAT activity could be lower in the presence of MC. Reinforcing our hypothesis, the [D-Leu¹]MC-LR consumption after 7 days was significantly higher in cells with [D-Leu¹]MC-LR supplementation in both 26 °C and 29 °C.When the production of reactive species was controlled by the scavenger activity of antioxidants plus MC, cells avoided the potential oxidative damage and started with exponential growth.

The response of Microcystis aeruginosa strain MGK to a single or two consecutive H2 O2 applications

Various approaches have been proposed to control/eliminate toxic Microcystis sp. blooms including H₂ O₂ treatments. Earlier studies showed that pre-exposure of various algae to oxidative stress induced massive cell death when cultures were exposed to an additional H₂ O₂ treatment. We examined the vulnerability of exponential and stationary-phase Microcystis sp. strain MGK cultures to single and double H₂ O₂ applications. Stationary cultures show a much higher ability to decompose H₂ O₂ than younger cultures. Nevertheless, they are more sensitive to an additional H₂ O₂ dose given 1-6 h after the first one. Transcript analyses following H₂ O₂ application showed a fast rise in glutathione peroxidase abundance (227-fold within an hour) followed by a steep decline thereafter. Other genes potentially engaged in oxidative stress were far less affected. Metabolic-related genes were downregulated after H₂ O₂ treatments. Among those examined, the transcript level of prk (encoding phosphoribulose kinase) was the slowest to recover in agreement with the decline in photosynthetic rate revealed by fluorescence measurements. Our findings shed light on the response of Microcystis MGK to oxidative stress suggesting that two consecutive H₂ O₂ applications of low concentrations are far more effective in controlling Microcystis sp. population than a single dose of a higher concentration.

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

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

•OH Inactivation of Cyanobacterial Blooms and Degradation of Toxins in Drinking Water Treatment System

Cyanobacterial blooms continue to serve as one of the most serious global issues threatening water supply and human health. During cyanobacterial bloom season, a large •OH-yield equipment was developed and installed after coagulation settling in a 12000 ton/day drinking water treatment system in Xiamen, China. An •OH concentration of 7.76∼57.8 μmol/L was formed by using the oxygen activated species generated by strong ionisation discharge combining with the effect of water jet cavitation. •OH pre-treatment at a dose of 1.0 mg/L inactivated cyanobacterial blooms in the process of conveying bloom water within only 20s, which were then removed by sand filtration. Under SEM observation, dominant Microcystis sp. colonies connected by mucilage were dispersed into individuals that still retained the cell integrity, indicating no release of intracellular organic matter (IOM). According to a flow cytometry analysis, the main cause of •OH inactivation was the breakage of DNA strands. Meanwhile, the •OH-mineralized microcystin-LR was by breaking the C=C conjugated diene bond and crucial opening the persistent benzene ring to carboxylic acid m/z 158.0. During •OH pre-treatment of 1.0 mg/L and NaClO disinfection of 0.5 mg/L, all water quality indexes and disinfection by-product (DBP) contents complied with the Chinese Sanitary Standards for Drinking Water. Therefore, the •OH based on the strong ionisation discharge showed great prospect for large-scale drinking water treatment in the removal of cyanobacterial blooms while retaining cell integrity as well as the degradation of toxins.

An integrated method for controlling the offensive odor and suspended matter originating from algae-induced black blooms

Potentially toxic algae-induced black blooms can trigger crises in urban water supplies and have fatal effects on aquatic ecosystems. Urgent disposal methods to mitigate the taste and odor are imperative for ensuring the safety of the drinking water supply. In this study, we tested three oxidants and two flocculants to improve water quality after the occurrence of a black bloom. The results indicated that a two-step integrated treatment process is efficient as an urgent disposal measure. The first step is removal of volatile organic sulfide compounds (VOSCs) through the addition of H₂O₂. A total of 50 mg/L of H₂O₂ can largely decrease the concentrations of dimethyl trisulfide and related alkyl sulfide compounds in the water column. The second step is the flocculation and sedimentation of black-bloom-induced black matter via a chitosan-modified clay. The addition of 1 g/L of an attapulgite clay plus 10 mg/L of chitosan can effectively deposit suspended matter on the bottom of the water column and have a positive effect on the removal of nutrients.

Hydrogen peroxide treatment promotes chlorophytes over toxic cyanobacteria in a hyper-eutrophic aquaculture pond

Controlling blooms of toxigenic phytoplankton, including cyanobacteria, is a high priority for managers of aquatic systems that are used for drinking water, recreation, and aquaculture production. Although a variety of treatment approaches exist, hydrogen peroxide (H₂O₂) has the potential to be an effective and ecofriendly algaecide given that this compound may select against cyanobacteria while not producing harmful residues. To broadly evaluate the effectiveness of H₂O₂ on toxigenic phytoplankton, we tested multiple concentrations of H₂O₂ on (1) four cyanobacterial cultures, including filamentous Anabaena, Cylindrospermopsis, and Planktothrix, and unicellular Microcystis, in a 5-day laboratory experiment and (2) a dense cyanobacterial bloom in a 7-day field experiment conducted in a nutrient-rich aquaculture pond. In the laboratory experiment, half-maximal effective concentrations (EC₅₀) were similar for Anabaena, Cylindrospermopsis, and Planktothrix (average EC₅₀ = 0.41 mg L^-1) but were ∼10x lower than observed for Microcystis (EC₅₀ = 5.06 mg L^-1). Results from a field experiment in an aquaculture pond showed that ≥1.3 and ≥ 6.7 mg L^-1 of H₂O₂ effectively eliminated Planktothrix and Microcystis, respectively. Moreover, 6.7 mg L^-1 of H₂O₂ reduced microcystin and enhanced phytoplankton diversity, while causing relatively small negative effects on zooplankton abundance. In contrast, 20 mg L^-1 of H₂O₂ showed the greatest negative effect on zooplankton. Our results demonstrate that H₂O₂ can be an effective, rapid algaecide for controlling toxigenic cyanobacteria when properly dosed.