Mechanisms of photosynthetic inactivation on growth suppression of Microcystis aeruginosa under UV-C stress

Optimizing UVA and UVC synergy for effective control of harmful cyanobacterial blooms

Harmful cyanobacterial blooms (HCBs) pose a global ecological threat. Ultraviolet C (UVC) irradiation at 254 nm is a promising method for controlling cyanobacterial proliferation, but the growth suppression is temporary. Resuscitation remains a challenge with UVC application, necessitating alternative strategies for lethal effects. Here, we show synergistic inhibition of Microcystis aeruginosa using ultraviolet A (UVA) pre-irradiation before UVC. We find that low-dosage UVA pre-irradiation (1.5 J cm-2) combined with UVC (0.085 J cm-2) reduces 85% more cell densities compared to UVC alone (0.085 J cm-2) and triggers mazEF-mediated regulated cell death (RCD), which led to cell lysis, while high-dosage UVA pre-irradiations (7.5 and 14.7 J cm-2) increase cell densities by 75-155%. Our oxygen evolution tests and transcriptomic analysis indicate that UVA pre-irradiation damages photosystem I (PSI) and, when combined with UVC-induced PSII damage, synergistically inhibits photosynthesis. However, higher UVA dosages activate the SOS response, facilitating the repair of UVC-induced DNA damage. This study highlights the impact of UVA pre-irradiation on UVC suppression of cyanobacteria and proposes a practical strategy for improved HCBs control.

Principles and research progress of physical prevention and control technologies for algae in eutrophic water

The abnormal reproduction of algae in water worldwide is prominent in the context of human interference and global climate change. This study first thoroughly analyzed the effects of physical factors, such as light, temperature, hydrodynamics, and operational strategies, on algal growth and their mechanisms. Physical control techniques are safe and have great potential for preventing abnormal algal blooms in the absence of chemical reagents. The focus was on the principles and possible engineering applications of physical shading, ultrasound, micro-current, and ultraviolet (UV) technologies, in controlling abnormal algal reproduction. Physical shading can inhibit or weaken photosynthesis in algae, thereby inhibiting their growth. Ultrasound mainly affects the physiological and biochemical activities of cells by destroying the cell walls, air cells, and active enzymes. Micro-currents destroy the algal cell structure through direct and indirect oxidation, leading to algal cell death. UV irradiation can damage DNA, causing organisms to be unable to reproduce or algal cells to die directly. This article comprehensively summarizes and analyzes the advantages of physical prevention and control technologies for the abnormal reproduction of algae, providing a scientific basis for future research. In the future, attempts will be made toward appropriately and comprehensively utilizing various physical technologies to control algal blooms. The establishment of an intelligent, comprehensive physical prevention and control system to achieve environmentally friendly, economical, and effective physical prevention and control of algae, such as the South-to-North Water Diversion Project in China, is of great importance for specific waters.

Oxidation effects on Microcystis aeruginosa inactivation through various reactive oxygen species: Degradation efficiency, mechanisms, and physiological properties

The study investigated the inactivation of Microcystis aeruginosa using a combined approach involving thermally activated peroxyacetic acid (Heat/PAA) and thermally activated persulfate (Heat/PDS). The Heat/PDS algal inactivation process conforms to first-order reaction kinetics. Both hydroxyl radical (•OH) and sulfate radical (SO4-•) significantly impact the disruption of cell integrity, with SO4-• assuming a predominant role. PAA appears to activate organic radicals (RO•), hydroxyl (•OH), and a minimal amount of singlet oxygen (1O2). A thorough analysis underscores persulfate's superior ability to disrupt algal cell membranes. Additionally, SO4-• can convert small-molecule proteins into aromatic hydrocarbons, accelerating cell lysis. PAA can accelerate cell death by diffusing into the cell membrane and triggering advanced oxidative reactions within the cell. This study validates the effectiveness of the thermally activated persulfate process and the thermally activated peroxyacetic acid as strategies for algae inactivation.

β-cyclocitral induced rapid cell death of Microcystis aeruginosa

β-cyclocitral (BCC) is an odorous compound that can be produced by bloom-forming cyanobacteria, for example, Microcystis aeruginosa. BCC has been proposed to explain the rapid decline of cyanobacterial blooms in natural water bodies due to its lytic effects on cyanobacteria cells. However, few insights have been gained regarding the mechanisms of its lethality on cyanobacteria. In this study, M. aeruginosa was exposed to 0-300 mg/L BCC, and the physiological responses were comprehensively studied at the cellular, molecular, and transcriptomic levels. The result indicated that the lethal effect was concentration-dependent; 100 mg/L BCC only caused recoverable stress, while 150-300 mg/L BCC caused rapid rupture of cyanobacterial cells. Scanning electron microscope images suggested two typical morphological changes exposed to above 150 mg/LBCC: wrinkled/shrank with limited holes on the surface at 150 and 200 mg/L BCC exposure; no apparent shrinkage at the surface but with cell perforation at 250 and 300 mg/L BCC exposure. BCC can rapidly inhibit the photosynthetic activity of M. aeruginosa cells (40%∼100% decreases for 100-300 mg/L BCC) and significantly down-regulate photosynthetic system Ⅰ-related genes. Also, chlorophyll a (by 30%∼90%) and ATP (by ∼80%) contents severely decreased, suggesting overwhelming pressure on the energy metabolism in cells. Glutathione levels increased significantly, and stress response-related genes were upregulated, indicating the perturbation of intracellular redox homeostasis. Two cell death pathways were proposed to explain the lethal effect: apoptosis-like death as revealed by the upregulation of SOS response genes when exposed to 200 mg/L BCC and mazEF-mediated death as revealed by the upregulation of mazEF system genes when exposed to 300 mg/L BCC. Results of the current work not only provide insights into the potential role of BCC in inducing programmed cell death during bloom demise but also indicate the potential of using BCC for harmful algal control.

Two different anti-algal control mechanisms in Microcystis aeruginosa induced by robinin or tannin rich plants

Phytochemical is considered an alternative method for cyanobacterial bloom control in aquatic environments. When cyanobacteria are treated with anti-algal materials produced from plant tissues, they tend to exhibit growth inhibition or necrosis of cells. These different anti-algal responses have not been well discussed, and thus, the modes of anti-algal action in cyanobacteria remain obscure. In this study, transcriptomic and biochemical researches were conducted to understand the mechanisms of cyanobacterial growth inhibition and necrosis in harmful cyanobacterial cells exposed to allelopathic materials. The cyanobacteria Microcystis aeruginosa was treated with aqueous extracts of walnut husk, rose leaf, and kudzu leaf. Walnut husk and rose leaf extracts induced mortality of cyanobacterial population with cell necrosis, whereas kudzu leaf extract exhibited poorly grown cells with shrunk size. Through RNA sequencing, it was revealed that the necrotic extracts significantly downregulated critical genes in enzymatic chain reactions for carbohydrate assembly in the carbon fixation cycle and peptidoglycan synthesis. Compared to the necrotic extract treatment, expression of several genes related to DNA repair, carbon fixation, and cell reproduction was less interrupted by the kudzu leaf extract. Biochemical analysis of cyanobacterial regrowth was performed using gallotannin and robinin. Gallotannin was identified as the major anti-algal compound in walnut husk and rose leaf affecting cyanobacterial necrosis, whereas robinin, which is the typical chemical in kudzu leaf, was associated with growth inhibition of cyanobacterial cells. These combinational studies using RNA sequencing and regrowth assays provided evidence supporting the allelopathic effects of plant-derived materials on cyanobacterial control. Furthermore, our findings suggest novel algicidal scenarios with different responses in the cyanobacterial cells depending on the type of anti-algal compounds.

Composition identification and UV-C irradiation growth inhibition effect of green shading on the greenhouse cover

Greenhouse cover pollution with green shading composed of dust, microalgae and bacteria is a severe problem in tropical areas. The shading results in lower greenhouse indoor light intensity reducing the yield and quality of protected horticulture crops. However, few studies have focused on environmentally efficient ways to remove green shading to increase greenhouse production. In this study, five purified microalgae were isolated from the green shading of three greenhouse roofs and were identified using morphological and molecular assessments. The effects of Ultraviolet-C irradiation (UV-C, 254 nm) at doses of 100, 200 and 300 mJ cm^-2 on the growth of GLY-1 microalgae were investigated. The results indicated that five purified microalgae all appeared to belong to the genus of Jaagichlorella. The purified microalgae cell density and chlorophyll content decreased respectively by 26.89-74.44 % and 42.02-77.31 % at 1-3 d after UV-C treatment with doses ranging from 100 to 300 mJ cm^-2. The inhibition of the growth rate of microalgae was significantly positively correlated with the UV-C irradiation dose and significantly negatively correlated with treatment time. In summary, UV-C irradiation treatment at 300 mJ cm^-2 and 3 d could substantially inhibit microalgae growth in green shading on greenhouse covers. UV-C irradiation could be an effective method for solving the problem of greenhouse cover pollution with microalgae.

Fabrication of g-C3N4@Bi2MoO6@AgI floating sponge for photocatalytic inactivation of Microcystis aeruginosa under visible light

In this work, a floating photocatalyst was constructed by loading g-C₃N₄@Bi₂MoO₆@AgI (GBA) nanocomposite on a modified polyurethane sponge via a simple dip-coating method and applied for the inactivation of Microcystis aeruginosa under visible light. GBA ternary photocatalyst was fabricated successfully and the morphology, structure, chemical state, and optical properties were characterized systematically. The floating catalyst achieved near 100% removal efficiency of algae cells under 6 h visible light irradiation and also could be retrieved and used at least three times repeatedly. The influences of various conditions on photocatalytic performance such as loading content of nanoparticles, algae density, and concentration of natural organic matters were also studied, which revealed that the GBA floating catalyst exhibited excellent photocatalytic performance of algae removal under different conditions. Furthermore, the physiological characteristics of algae cells during the photocatalytic process, including cell morphology, membrane permeability, Zeta potential, photosynthetic system, antioxidant system, and the metabolic activity were investigated. Results confirmed that the algae cells were severely damaged during the photocatalytic inactivation and the normal physiological functions were significantly affected, which resulted in the death of algae cells at last. Finally, a possible photocatalytic inactivation mechanism of algae cells was proposed. In summary, GBA floating catalyst can effectively inactivate Microcystis aeruginosa under visible light, which confirmed the high efficiency of the novel photocatalytic algae removal technology. Meanwhile, the recyclable floating material also makes the practical application in eutrophic waters of the algae removal technology possible.

Efficient inhibition of cyanobacteria M. aeruginosa growth using commercial food-grade fumaric acid

The control of cyanobacteria blooms is a global challenge. Here, we reported the efficient inhibition of M. aeruginosa by fumaric acid (FA), an intermediate metabolite of the tricarboxylic acid cycle. FA showed strong algicidal activity with an inhibition rate of 90.5% on the 8th day at a dose of 40 mg/L. The presence of FA caused severe membrane damage, as suggested by the fluorescence flow cytometry and morphology analysis. FA inhibited the formation of chlorophyll a, interrupting the photosynthesis system. It also induced oxidative stress in cells. Principal component analysis of the indicators suggested that the FA-treated sample had a significantly different inhibitory pattern than the acid-treated sample. Thus, the inhibitory effect was not solely caused by the pH effect. Untargeted metabolomic analysis revealed that 31 metabolites were differentially expressed in response to FA stress, which were mainly involved in the metabolite processes and the membranes. A commercial food-grade FA was able to inhibit the growth of M. aeruginosa similar to the analytical-grade FA. Our results suggest that FA can be potentially an efficient and low-risk chemical for inhibiting M. aeruginosa growth, which may find future applications in cyanobacteria bloom control.

Production of β-Cyclocitral and Its Precursor β-Carotene in Microcystis aeruginosa: Variation at Population and Single-Cell Levels

Bloom-forming cyanobacteria produce and release odorous compounds and pose threats to the biodiversity of aquatic ecosystem and to the drinking water supply. In this study, the concentrations of β-cyclocitral in different bacterial growth phases were investigated using GC–MS to determine the growth stage of Microcystis aeruginosa at high risk for β-cyclocitral production. Moreover, the synchronicity of the production of β-cyclocitral and its precursor β-carotene at both population and single-cell levels was assessed. The results indicated that β-cyclocitral was the main odorous compound produced by M. aeruginosa cells. The intracellular concentration of β-cyclocitral (C_β-cc) as well as its cellular quota (Q_β-cc) increased synchronously in the log phase, along with the increase of cell density. However, they reached the maximum values of 415 μg/L and 10.7 fg/cell in the late stationary phase and early stationary phase, respectively. The early stage of the stationary phase is more important for β-cyclocitral monitoring, and the sharp increase in Q_β-cc is valuable for anticipating the subsequent increase in C_β-cc. The molar concentrations of β-cyclocitral and β-carotene showed a linear relationship, with an R² value of 0.92, suggesting that the production of β-cyclocitral was linearly dependent on that of β-carotene, especially during the log phase. However, the increase in Q_β-cc was slower than that in β-carotene during the stationary phase, suggesting that β-cyclocitral production turned to be carotene oxygenase-limited when the growth rate decreased. These results demonstrate that variations of β-cyclocitral production on a single-cell level during different bacterial growth phases should be given serious consideration when monitoring and controlling the production of odorous compounds by M. aeruginosa blooms.

Physiological, biochemical and transcription effects of roxithromycin before and after phototransformation in Chlorella pyrenoidosa

Photodegradation is an important transformation pathway for macrolide antibiotics (MCLs) in aquatic environments, but the ecotoxicity of MCLs after phototransformation has not been reported in detail. This study investigated the effects of roxithromycin (ROX) before and after phototransformation on the growth and physio-biochemical characteristics of Chlorella pyrenoidosa, and its toxicity were explored using transcriptomics analysis. The results showed that 2 mg/L ROX before phototransformation (T0 group) inhibited algae growth with inhibition rates of 53.06%, 54.17%, 47.26%, 31.27%, and 28.38% at 3, 7, 10, 14, and 21 d, respectively, and chlorophyll synthesis was also inhibited. The upregulation of antioxidative enzyme activity levels and the malondialdehyde content indicated that ROX caused oxidative damage to C. pyrenoidosa during 21 d of exposure. After phototransformation for 48 h (T48 group), ROX exhibited no significant impact on the growth and physio-biochemical characteristics of the microalgae. Compared with the control group (without ROX and its phototransformation products), 2010 and 2988 differentially expressed genes were identified in the T0 and T48 treatment groups, respectively. ROX significantly downregulated genes related to porphyrin and chlorophyll metabolism, which resulted in the inhibition of chlorophyll synthesis and algae growth. ROX also significantly downregulated genes of DNA replication, suggesting the increased DNA proliferation risks in algae. After phototransformation, ROX upregulated most of the genes associated with the porphyrin and chlorophyll metabolism pathway, which may be the reason that the chlorophyll content in T48 treatment group showed no significant difference from the control group. Almost all light-harvesting chlorophyll a/b (LHCa/b) gene family members were upregulated in both T0 and T48 treatment groups, which may compensate part of the stress of ROX and its phototransformation products.

Inhibitory effects of ultralow-dose sodium hypochlorite on Microcystis aeruginosa and Chlorella vulgaris: Differences in sensitivity and physiology

Throughout the COVID-19 pandemic, the application of residual free chlorine has been emphasized as an effective disinfectant; however, the discharged residual chlorine is associated with potential ecological risk at concentrations even below 0.1 mg/L. However, the influence of free chlorine at ultralow-doses (far below 0.01 mg/L) on phytoplankton remains unclear. Due to limitations of detection limit and non-linear dissolution, different dilution rates (1/500, 1/1000, 1/5000, 1/10000, and 1/50000 DR) of a NaClO stock solution (1 mg/L) were adopted to represent ultralow-dose NaClO gradients. Two typical microalgae species, cyanobacterium Microcystis aeruginosa and chlorophyta Chlorella vulgaris, were explored under solo- and co-culture conditions to analyze the inhibitory effects of NaClO on microalgae growth and membrane damage. Additionally, the effects of ultralow-dose NaClO on photosynthesis activity, intracellular reactive oxygen species (ROS) production, and esterase activity were investigated, in order to explore physiological changes and sensitivity. With an initial microalgae cell density of approximately 1 × 10⁶ cell/mL, an inhibitory effect on M. aeruginosa was achieved at a NaClO dosage above 1/10000 DR, which was lower than that of C. vulgaris (above 1/5000 DR). The variation in membrane integrity and photosynthetic activity further demonstrated that the sensitivity of M. aeruginosa to NaClO was higher than that of C. vulgaris, both in solo- and co-culture conditions. Moreover, NaClO is able to interfere with photosynthetic activity, ROS levels, and esterase activity. Photosynthetic activity declined gradually in both microalgae species under sensitive NaClO dosage, but esterase activity increased more rapidly in M. aeruginosa, similar to the behavior of ROS in C. vulgaris. These findings of differing NaClO sensitivity and variations in physiological activity between the two microalgae species contribute to a clearer understanding of the potential ecological risk associated with ultralow-dose chlorine, and provide a basis for practical considerations.

Efficient integration of plasmonic Ag/AgCl with perovskite-type LaFeO3: Enhanced visible-light photocatalytic activity for removal of harmful algae

A novel plasmonic Ag/AgCl@LaFeO₃ (ALFO) photocatalyst was successfully synthesized by a simple in-situ synthesis method with enhanced photocatalytic activity under visible light for harmful algal blooms (HABs) control. The structure, morphology, chemical states, optical and electrochemical properties of the photocatalyst were systematically investigated using a series of characterization methods. Compared with pure LaFeO₃ and Ag/AgCl, ALFO-20% owned a higher light absorption capacity and lower electron-hole recombined rate. Therefore, ALFO-20% had higher photocatalytic activity with a near 100% removal rate of chlorophyll a within 150 min, whose kinetic constant was 15.36 and 9.61 times faster than those of LaFeO₃ and Ag/AgCl. In addition, the changes of zeta potential, cell membrane permeability, cell morphology, organic matter, total soluble protein, photosynthetic system and antioxidant enzyme system in Microcystis aeruginosa (M. aeruginosa) were studied to explore the mechanism of M. aeruginosa photocatalytic inactivation. The results showed that ALFO-20% could change the permeability and morphology of the algae cell membrane, as well as destroy the photosynthesis system and antioxidant system of M. aeruginosa. What's more, ALFO could further degrade the organic matters flowed out after algae rupture and die, reducing the secondary pollution and avoiding the recurrence of HABs. Finally, the species of reactive oxygen species (ROS) (mainly •O₂^- and •OH) produced by ALFO were determined through quenching experiments, and a possible photocatalytic mechanism was proposed. Overall, ALFO can efficiently remove the harmful algae under the visible light, providing a promising method for controlling HABs.

Optimization of remedial nano-agent and its effect on dominant algal species succession in eutrophic water body

A new method for algal community restructuring is proposed, where harmful algae growth is inhibited through the addition of remedial nano-agent, while probiotic algae growth is promoted or only affected indistinctively. In this paper, the inhibiting effects of five different nanomaterials on Microcystis aeruginosa (M. aeruginosa) and Cyclotella sp. were studied, and the optimal nanomaterial was served as algae-inhibition ingredient of the remedial agent. The effects of the remedial agent on algal growth and their physiological characteristics were investigated, and the restructuring of algal community in actual water samples was explored. The results indicated that the inhibition ratio of 10 mg/L nm-Cu₂O/SiO₂ on M. aeruginosa and Cyclotella sp. could reach 293.1% and 82.8% respectively, acting as the best candidate for algae-inhibiting ingredient. After adding the remedial nano-agent made with nm-Cu₂O/SiO₂, the content of chlorophyll a, protein, and polysaccharides of M. aeruginosa decreased sharply, while the physiological characteristics of Cyclotella sp. were not significantly affected. Besides, the total biomass and proportion of cyanobacteria dropped (P < 0.05), but the Bacillariophyta biomass increased significantly (P < 0.05). The uniformity index, Shannon-Wiener index, and richness index all increased significantly (P < 0.05). Meanwhile, the quality of actual water samples has been improved evidently (P < 0.001). Therefore, the prepared remedial nano-agent in this study can control the harmful algae bloom to a certain extent by restructuring the algal community in eutrophic water bodies.

Fabrication of heterostructured Ag/AgCl@g-C3N4@UIO-66(NH2) nanocomposite for efficient photocatalytic inactivation of Microcystis aeruginosa under visible light

In this work, a novel Ag/AgCl@g-C₃N₄@UIO-66(NH₂) heterojunction was constructed for photocatalytic inactivation of Microcystis aeruginosa (M. aeruginosa) under visible light. The photocatalyst was synthesized by a facile method and characterized by XRD, SEM, TEM, BET, XPS, FT-IR, UV-vis DRS, PL and EIS. The nanocomposite can not only provide lots of active sites, but also improve capacities to utilize visible-light energy and effectively transfer charge carriers, thus enhancing removal efficiencies of cyanobacteria (99.9% chlorophyll a was degraded within 180 min). Various factors in photodegradation of chlorophyll a were studied. Besides, changes on cellular morphologies, membrane permeability, physiological activities of M. aeruginosa during photocatalysis were investigated. Moreover, the cycle test indicated that Ag/AgCl@g-C₃N₄@UIO-66(NH₂) exhibits excellent reusability and photocatalytic stability. Finally, a possible mechanism of M. aeruginosa inactivation was proposed. In a word, Ag/AgCl@g-C₃N₄@UIO-66(NH₂) can efficiently inactivate cyanobacteria under visible light, thus providing useful references for further removal of harmful algae in real water bodies.

Synergetic suppression effects upon the combination of UV-C irradiation and berberine on Microcystis aeruginosa and Scenedesmus obliquus in reclaimed water: Effectiveness and mechanisms

The risk of harmful algal blooms (HABs) in the water recharged with reclaimed water is a bottleneck for water reuse. The suppression effects and mechanisms of the combination of UV-C and berberine on Microcystis aeruginosa and Scenedesmus obliquus in reclaimed water were investigated. Mono UV-C irradiation at 75 mJ cm^-2 could suppress the growth of M. aeruginosa for 7 d and that at 90 mJ cm^-2 could suppress the growth of S. obliquus for 5 d. UV-C irradiation combined with 0.2-2 mg L^-1 berberine lengthened the inhibition period of M. aeruginosa to 10- > 22 d and that of S. obliquus to 7- > 22 d and induced more rapid lethal effects on the harmful microalgal cells, in significant synergetic patterns. The combination of UV-C and berberine suppressed total, intracellular and extracellular microcystin-LR (MC-LR) more effectively and decreased the MC-LR quota significantly, which further reduced the risks of microcystin production and release. Furthermore, synergetic mechanisms of the combined treatments were systematically investigated from the aspects of photosynthetic system (photosynthetic activity and pigments), metabolic activity (ATP and membrane potential), oxidation stress (reactive oxygen species (ROS) and glutathione (GSH)), and apoptosis-like cell death (phosphatidylserine (PS) ectropion, caspase-3 activity and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) positive rate). The combination treatment provided a joint attack of UV-C and berberine on photosynthetic transport chain of photosynthetic system II (PS II), and a synergetic pathway to achieve more severe disruptions in energy metabolism as well as aggravated oxidative stress. The accumulated ROS enhanced increases in programmed cell death (PCD) indicators of both microalgal species, which contributed to the enhancement effects on growth suppression. The results showed that the combination treatment achieved lower dose requirements of both UV-C irradiation and berberine for inducing the same inhibition effects on microalgal cells, which was promising to be applied in the HABs control of reclaimed water.

Antioxidant responses in cyanobacterium Microcystis aeruginosa caused by two commonly used UV filters, benzophenone-1 and benzophenone-3, at environmentally relevant concentrations

Benzophenone-type ultraviolet filters (BPs) have recently been recognized as emerging organic contaminants. In the present study, the cyanobacterium Microcystis aeruginosa was exposed to environmentally relevant levels (0.01-1000 μg L^-1) of benzophenone-1 (BP-1) and benzophenone-3 (BP-3) for seven days. A battery of tested endpoints associated with photosynthetic pigments and oxidative stress was employed for a better understanding of the mode of action. The tested cyanobacterium could uptake the two BPs (27.4-54.9%) from culture media. The two BPs were able to inhibit the production of chlorophyll a (chl-a) and promote the accumulation of carotenoids, leading to unaffected chl-a autofluorescence. Slightly increased malondialdehyde (MDA) contents suggested that BP-1 and BP-3 caused moderate oxidative stress. BP-1 stimulated the activities of superoxide dismutase (SOD), glutathione reductase (GR) and glutathione S-transferase (GST) in M. aeruginosa while BP-3 increased the activities of SOD, GST, and glutathione (GSH), showing a concentration- and time-dependent relationship. The activities of other biomarkers, such as catalase (CAT) and glutathione peroxidase (GPx) fluctuated depending on exposure time and concentration. The overall results suggested that the two BPs can trigger moderate oxidative stress in M. aeruginosa and the tested cyanobacterium was capable of alleviating stress by different mechanisms.

Suppression of water-bloom cyanobacterium Microcystis aeruginosa by algaecide hydrogen peroxide maximized through programmed cell death

The global expansion and intensification of toxic cyanobacterial blooms require effective algaecides. Algaecides should be selective, effective, fast-acting, and ideally suppress cyanotoxin production. In this study, whether both maximum growth suppression and minimal toxin production can be simultaneously achieved was tested with a selective algaecide H₂O₂, through its ability to induce apoptosis-like programmed cell death (AL PCD) in a common bloom species Microcystis aeruginosa. Under doses of 1-15 mg L^-1, non-monotonic dose-response suppression of H₂O₂ on M. aeruginosa were observed, where maximal cell death and minimal microcystin production both occurred at a moderate dose of 10 mg L^-1 H₂O₂. Maximal cell death was indeed achieved through AL PCD, as revealed by integrated biochemical, structural, physiological and transcriptional evidence; transcriptional profile suggested AL PCD was mediated by mazEF and lexA systems. Higher H₂O₂ doses directly led to necrosis in M. aeruginosa, while lower doses only caused recoverable stress. The integrated data showed the choice between the two modes of cell death is determined by the intracellular energy state under stress. A model was proposed for suppressing M. aeruginosa with AL PCD or necrosis. H₂O₂ was demonstrated to simultaneously maximize the suppression of both growth and microcystin production through triggering AL PCD.

UV-C irradiation for harmful algal blooms control: A literature review on effectiveness, mechanisms, influencing factors and facilities

UV-C irradiation has drawn much attention in recent years as a candidate for controlling harmful algal blooms (HABs). In this review, we have collated the recent knowledge about the UV-C irradiation technique for suppressing HABs, including the effectiveness, mechanisms, influencing factors, growth recovery pattern, and UV-C irradiation facilities. Most microalgal species have been proved to be effectively suppressed by UV-C irradiation and the suppression effects had positive correlation with UV-C dose. However, the effectiveness on difference algal species varied dramatically. The understanding for growth suppression mechanisms upon UV-C irradiation has been significantly deepened beyond pyrimidine dimers. The suppression effects on algal cell density were the results of UV-induced damage on nucleic acid, light harvesting and electron transfer and transportation, nitrogen fixation and assimilation, toxin synthesis, settle ability, antioxidative capacity and cellular membrane integrity. While several influencing factors, such as algal sensitivities, UV transmittance (UVT), salinity, pH, and microalgal growth recovery should be paid attention to in practical application. UV-C facilities with high maturity, especially flow-through reactors, make it possible to develop ship-born UV-C facilities and put UV-C irradiation technique into real practice on controlling HABs.

The growth suppression effects of UV-C irradiation on Microcystis aeruginosa and Chlorella vulgaris under solo-culture and co-culture conditions in reclaimed water

Harmful algal blooms (HABs) are serious problems in landscape waters sourced from reclaimed water. In this study, the suppression effects of UV-C irradiation on microalgal growth were researched to find a possible preventive approach. Microcystis aeruginosa and Chlorella vulgaris were exposed to UV-C irradiation and then cultured in real reclaimed water for 7-18 d. UV-C irradiation at 50-200 mJ cm^-2 could inhibit the growth of M. aeruginosa, C. vulgaris, and both microalgae in co-culture for 3-14, 1-3, and 1-5 d respectively. In addition, UV-C irradiation could cause damage to the cell integrity. At 100-200 mJ cm^-2 UV-C, the proportion of microalgal membrane damage (P_md) in M. aeruginosa cells increased rapidly to 56%-76% from day 3, whereas that in C. vulgaris cells increased to 23%-62% within 3 d. The photochemical efficiency (represented by Y value) of the irradiated groups was negatively affected immediately after UV-C irradiation and recovered gradually during the incubation. The Y value of M. aeruginosa cells began to recover from days 3 to 14, whereas that of C. vulgaris recovered much more quickly, from days 0.1 to 1. Overall, the irradiation-induced suppressive effects on algal growth correlated positively with the UV-C doses. Because M. aeruginosa was more sensitive to UV-C irradiation, UV-C irradiation not only controlled the total biomass of the mixed algae but also selectively reestablished the dominance of the nontoxic C. vulgaris.

Inhibitory Effects of Cu2O/SiO2 on the Growth of Microcystis aeruginosa and Its Mechanism

In this study, a novel nanomaterial Cu₂O/SiO₂ was synthesized based on nano-SiO₂, and the inhibitory effects of different concentrations of Cu₂O/SiO₂ on the growth of Microcystis aeruginosa (M. aeruginosa) were studied. At the same time, the mechanism of Cu₂O/SiO₂ inhibiting the growth of M. aeruginosa was discussed from the aspects of Cu²⁺ release, chlorophyll a destruction, oxidative damage, total protein, and the phycobiliprotein of algae cells. The results showed that low doses of Cu₂O/SiO₂ could promote the growth of M. aeruginosa. When the concentration of Cu₂O/SiO₂ reached 10 mg/L, it exhibited the best inhibitory effect on M. aeruginosa, and the relative inhibition rate reached 294% at 120 h. In terms of the algae inhibition mechanism, Cu₂O/SiO₂ will release Cu²⁺ in the solution and induce metal toxicity to algae cells. At the same time, M. aeruginosa might suffer oxidative damage by the free radicals, such as hydroxyl radicals released from Cu₂O/SiO₂, affecting the physiological characteristics of algae cells. Moreover, after the addition of Cu₂O/SiO₂, a decrease in the content of chlorophyll a, total soluble protein, and phycobiliprotein was found, which eventually led to the death of M. aeruginosa. Therefore, Cu₂O/SiO₂ can be used as an algaecide inhibitor for controlling harmful cyanobacteria blooms.

Unstandardized UV-C dose used for killing harmful cyanobacteria may instead initiate accelerated growth in the target organisms

Although UV-C radiation has been in use for killing unwanted cyanobacteria, experiments with lower doses of UV-C radiation instead showed induction of growth related parameters and enhanced biomass production in the cyanobacterium Nostoc muscorum Meg1. When the cyanobacterial cultures were exposed to UV-C radiation of varying doses (6, 12 and 18 mJ/cm²), concentrations of various photo-absorbing pigments, RuBisCO and D1 protein of PSII; activities of oxygen evolving complex, nitrogenase and glutamine synthetase were significantly increased upon 6 and 12 mJ/cm² UV-C radiation exposures. Resulting higher photosynthetic performance was evident from the augmentation in carbohydrate content by ∼49% under single exposure to 6 mJ/cm² UV-C by fifteenth day. The increased performances of both RuBisCO and D1 proteins were in part also due to induction at the genetic level as seen from the increase in their mRNA and protein levels under treatment. Similar increase was also observed in protein (16%) and in lipid contents (43%) that reflected an upsurge in the total biomass. Highest biomass (463 mg/L/d) was noted in culture exposed to 6 mJ/cm² UV-C radiation, representing a ∼25% increase. Furthermore the possibility of this organism using part of the incident UV-C radiation as an additional source of energy was deduced from an experiment where the thylakoid membranes excited within UV (226-400 nm) range showed emission at longer wavelengths with an emission maximum at ∼640 nm. Thus this work provides evidence that lower UV-C doses can potentially augment cyanobacterial growth and use of unstandardized UV-C doses for restricting cyanobacterial growth may in fact produce contrary result.

The light-dependent lethal effects of 1,2-benzisothiazol-3(2H)-one and its biodegradation by freshwater microalgae

As 1,2-benzisothiazol-3(2H)-one (BIT) has been widely used in high concentrations for microbial growth control in many domestic and industrial processes, its potential eco-risk should be assessed. This study investigated the interaction between BIT and microalgae in aquatic environment as the mechanism of BIT lethal effect on microalgae was unclear and whether microalgae could efficiently remove BIT was unknown. It was found that Chlorella vulgaris could be killed by high concentrations of BIT, and this lethal effect was strongly enhanced when exposed to light. Inhibition of photosystem II electron transport followed by a decrease in cellular chlorophyll led to serious damage to algal photosynthesis. The excess accumulation of reactive oxygen species caused by the photosynthetic damage under light further increased the oxidative damage and promoted cell death. Under dark condition, however, the algae could tolerate higher BIT concentrations. BIT could be efficiently removed when the growth of Scenedesmus sp. LX1 was not completely inhibited. With an initial concentration of 4.5 mg/L, over 99% of BIT was removed during 168 hour cultivation. Microalgal biodegradation was the primary reason for this removal, and the contributions of BIT hydrolytic/photolytic degradation, microalgal growth, photosynthesis and sorption were negligibly small. These results pointed to the potential application of microalgae for efficient BIT removal from wastewater.

Growth suppression and apoptosis-like cell death in Microcystis aeruginosa by H2O2: A new insight into extracellular and intracellular damage pathways

H₂O₂ has been suggested and applied as effective algaecide for harmful cyanobacterial bloom control, however, the transport of exogenous H₂O₂ into microalgal cells, the subsequent intracellular damage pathway and dose-response variations were little studied. We addressed these questions in a bloom-forming cyanobacterium Microcystis aeruginosa with H₂O₂ at 0.1-1.5 mM. The results showed that H₂O₂ at 0.4 mM and above significantly suppressed M. aeruginosa growth for over two weeks, and induced apoptosis-like death in terms of membrane potential dissipation, caspase-3 activation, chromatin condensation, and lysis induction. However, the dose-response effects were not monotonic. H₂O₂ at 0.7 mM resulted in the severest growth suppression among 0.1-1.5 mM treatments, including the lowest biomass for 74% loss, the highest cell lysis ratio for 79%, and the highest utilization rate of H₂O₂ for 0.101 mM d^-1. Moreover, several evidence point to severer apoptosis-like cell death in 0.7 mM treatments, involving fastest and severest cell lysis, smallest cell size and wrinkled surface and lowest membrane potential. Therefore, the apoptosis-like cell death induced by H₂O₂ at moderate dosages should be a crucial cause for the non-monotonic dose-response effects on growth suppression. Additionally, intracellular H₂O₂ level increased rapidly within 20 min after exposure at 0.4 mM and above, directly confirming the transport of exogenous H₂O₂ into M. aeruginosa cells and the intracellular damages due to subsequent elevation in intracellular oxidative stress. The study demonstrates that H₂O₂ at moderate dosages could be a promising method for the biomass control, in a fast and efficient way, on M. aeruginosa blooms.

UV-C suppression on hazardous metabolites in Microcystis aeruginosa: Unsynchronized production of microcystins and odorous compounds at population and single-cell level

The effectiveness of UV-C towards the toxin and odor of M. aeruginosa at population and single cell levels were investigated in three ways. In the absence of UV-C, MC-LR and β-cyclocitral production show similar pattern of incremental rate with growth rate on population level shown as intracellular concentrations of MC-LR (C_MC) and β-cyclocitral (C_BCC), but the cellular quota of MC-LR (Q_MC) and β-cyclocitral (Q_BCC) at single-cell level reached the maximum values, 37.5 ± 1.2 fg cell^-1 and 4.3 ± 0.1 fg cell^-1, just after the early exponential phase. Second, upon UV-C irradiation, the C_MC consistently decreased by 10-41% with increase of UV-C dosage (50-200 mJ cm^-2) while C_BCC increased by 2-14%. Third, during the 14 days' post-UV incubation, UV-C at 75-200 mJ cm^-2 induced remarkable suppressing effects on both C_MC and C_BCC for 3-14 days. The suppressing effects on Q_MC and Q_BCC were induced by UV-C at 100 mJ cm^-2 and above, with shorter suppressing periods by 1-4 days and lower decremental rates by 21%-30% than that of C_MC and C_BCC, indicating interruptions on biosynthesis processes partially contribute to suppression effects of C_MC and C_BCC. The suppression effect on either C_BCC or Q_BCC, with higher decrement rates while lower recovery rates, was more severe than C_MC and Q_MC.

UV fluences required for compliance with ballast water discharge standards using two approved methods for algal viability assessment

This study investigates the extra UV fluence needed to meet the International Maritime Organisation's ballast water discharge standards for the 10-50 μm size-class using the approved vital stain (VS) method compared to the Most Probable Number (MPN) method for organism viability assessment. Low- and medium pressure UV collimated beam treatments were applied to natural algae collected in temperate and tropical water environments and analysed using both methods. About 10 times higher UV fluence was required to meet discharge standards when using VS compared to MPN. Implementing a dark-hold period after UV treatments decreased algal viability. Length of dark-hold period to meet discharge standards decreased with increasing UV fluence. No significant differences between temperate and tropical samples were observed. The results showed that UV treated algae assessed using the VS method could meet discharge standards by increasing fluence and/or introducing a dark-hold period.

Cyanobacterium removal and control of algal organic matter (AOM) release by UV/H2O2 pre-oxidation enhanced Fe(II) coagulation

Harmful algal blooms in source water are a worldwide issue for drinking water production and safety. UV/H₂O₂, a pre-oxidation process, was firstly applied to enhance Fe(II) coagulation for the removal of Microcystis aeruginosa [M. aeruginosa, 2.0 (±0.5) × 10⁶ cell/mL] in bench scale. It significantly improved both algae cells removal and algal organic matter (AOM) control, compared with UV irradiation alone (254 nm UVC, 5.4 mJ/cm²). About 94.7% of algae cells were removed after 5 min UV/H₂O₂ pre-treatment with H₂O₂ dose 375 μmol/L, FeSO₄ coagulation (dose 125 μmol/L). It was also certified that low residue Fe level and AOM control was simultaneously achieved due to low dose of Fe(II) to settle down the cells as well as the AOM. The result of L₉(3)⁴ orthogonal experiment demonstrated that H₂O₂ and FeSO₄ dose was significantly influenced the algae removal. UV/H₂O₂ induced an increase of intracellular reactive oxidant species (ROS) and a decrease in zeta potential, which might contribute to the algae removal. The total microcystins (MCs) concentration was 1.5 μg/L after UV/H₂O₂ pre-oxidation, however, it could be removed simultaneously with the algae cells and AOM. This study suggested a novel application of UV/H₂O₂-Fe(II) process to promote algae removal and simultaneously control AOM release in source waters, which is a green and promising technology without secondary pollution.

Enhanced toxicity to the cyanobacterium Microcystis aeruginosa by low-dosage repeated exposure to the allelochemical N-phenyl-1-naphthylamine

It has been puzzling whether and how a plant could exert a strong allelopathic inhibition to the target organisms by releasing low concentrations of allelochemicals. Plant allelochemicals have been proposed to be released continuously, however, direct evidence from specific allelochemicals is urgently required. In the present study, the toxicity of allelochemical N-phenyl-1-naphthylamine (NPN) towards the cyanobacterium Microcystis aeruginosa by two different exposure patterns was compared. One was low-dosage repeated exposure (LRE), in which 50  μg L^-1 NPN was repeatedly dosed to simulate the continual release of allelochemicals, and the other one was high-dosage single exposure (HSE) as per the routine toxicity assay. The results showed a significant growth inhibition to M. aeruginosa in the LRE group, where the inhibition rate reached above 90% from day 6 to day 9. The cell-membrane damage ratio increased from 64.05% on day 5 up to 96.60% on day 9. PSII photosynthesis activity expressed as Fv/Fm, Φ_PSII, NPQ and ETRmax was also thoroughly inhibited in this group. Whereas the growth and PSII photosynthesis activity of M. aeruginosa in the HSE group were inhibited initially, but recovered gradually from day 4 or 5, which was accompanied by a continuous reduction of NPN content in culture solutions. Although NPN content in the LRE group was relatively lower, it remained at a more stable level throughout the experiment. These results indicate that continual release of low-dosage allelochemicals by aquatic plants plays crucial roles in their potent inhibition against cyanobacteria. Low-dosage continual exposure pattern needs to be investigated further.

Proteomic Analyses of Changes in Synechococcus sp. PCC7942 Following UV-C Stress

UV-C's effects on the physiological and biochemical processes of cyanobacteria have been well characterized. However, the molecular mechanisms of cyanobacteria's tolerance to UV-C still need further investigation. This research attempts to decode the variation in protein abundances in cyanobacteria after UV-C stress. Different expression levels of proteins in the cytoplasm of Synechococcus sp. PCC7942 under UV-C stress were investigated using a comparative proteomic approach. In total, 47 UV-C-regulated proteins were identified by MALDI-TOF analysis and classified by Gene Ontology (GO). After studying their pathways, the proteins were mainly enriched in the groups of protein folding, inorganic ion transport and energy production. By focusing on these areas, this study reveals the correlation between UV-C stress-responsive proteins and the physiological changes of Synechococcus sp. PCC7942 under UV-C radiation. These findings may open up new areas for further exploration in the homeostatic mechanisms associated with cyanobacteria responses to UV-C radiation.

Effects of water quality on inactivation and repair of Microcystis viridis and Tetraselmis suecica following medium-pressure UV irradiation

The transfer of invasive organisms by ballast-water discharge has become a growing concern. UV treatment has become an attractive ballast water treatment technology due to its effectiveness, no harmful disinfection byproducts and easiness to handle. Two robust algae strains Microcystis viridis and Tetraselmis suecica were selected as indicator organisms to determine efficiency of medium-pressure (MP) UV-treatment on ballast water. Inactivation and potential repair of these two algae strains following MP UV irradiation were assessed under various turbidity, total organic carbon (TOC) and salinity conditions. The investigated range of UV doses was from 25 to 500 mJ/cm(2). For M. viridis, results indicated that disinfection efficiency was negatively correlated with all of these three factors at low doses (25-200 mJ/cm(2)). Photoreactivation and dark repair were promoted at high TOC levels (6-15 mg/L) with about 6-25% higher repair levels compared with those in distilled water, whereas no significant impacts were identified for turbidity and salinity on both of the photoreactivation and dark repair. For T. suecica, increased turbidity and TOC levels both hindered the performance of UV irradiation at high doses (200-500 mJ/cm(2)). Suppressive effects on photoreactivation and dark repair were consistently observed with changes of all of the three factors. In conclusion, generally these three factors resulted in repressive effects on UV disinfection efficiency, and TOC played a more significant role in the levels of reactivation than the other two. The responses of T. suecica to these three factors were more sensitive than M. viridis.

Effects of Dracontomelon duperreanum defoliation extract on Microcystis aeruginosa: physiological and morphological aspects

Harmful cyanobacteria bloom contributes to economic loss as well as the threat to human health. Agricultural waste products, particularly straw, have been used to control bloom while arbor plant is the potential candidate for limiting antialgal activity. This study investigated the use of Dracontomelon duperreanum defoliation extract (DDDE) to inhibit the activity of Microcystis aeruginosa. The primary goal of the research was to explore the solution to control cyanobacterial bloom. The photosynthetic activity, cell morphology, membrane integrity, and esterase activity of M. aeruginosa were determined using phytoplankton analyzer pulse amplitude modulation (Phyto-PAM) and flow cytometry before and after exposure to DDDE. The inhibitory rate of M. aeruginosa was about 99.6 % on day 15 when exposed to 2.0 g L(-1). A reduction of chlorophyll a (Chl-a) activity and changes in cell membrane suggested the algistatic property of DDDE. Inhibition of photosynthetic activity was reflected by changing mean Chl-a fluorescence intensity (MFI) which was about 52.5 % on day 15 when exposed to 2.0 g L(-1) DDDE as well as relative electron transport rates (rETRs) of algal cell. These changes might contribute to the suppression of M. aeruginosa. Algal cell exposed to DDDE may lead to cell volume reduction or slow growth. This resulted in a decreased proportion of normal or swollen granular cells after DDDE treatment.

The combined effects of UV-C radiation and H2O2 on Microcystis aeruginosa, a bloom-forming cyanobacterium

In order to get insight into the impacts of UVC/H2O2 on Microcystis aeruginosa, physiological and morphological changes as well as toxicity were detected under different UVC/H2O2 treatments. In the presence of sole UVC or H2O2, the net oxygen evolution rate decreased significantly (p<0.05) since activity of photosystem II (PSII) was inhibited. Meanwhile, increase of intracellular reactive oxygen species (ROS), degradation of microcystin (MC) and ultrastructure destructions were observed. Under sole UVC treatment, no changes happened in the activity of photosystem I (PSI), but the degradation of D1 protein was observed. Under sole H2O2 treatment, an increase of malondialdehyde, aggregation of D1 protein and deformation of the thylakoid membrane were observed. ROS content under H2O2 treatment was about 5 times than that under UVC treatment. Combined use of UVC and H2O2, as well as 20mJcm(-2) UVC and 60μM H2O2, showed high synergetic effects. Obvious damage to membrane systems, the marked degradation of MC and inhibition of the photosystems were observed. It could be deduced that UVC worked on intracellular membrane components directly and the damaged oxygen-evolving complex, which was followed by the D1 protein degradation. H2O2 oxidised the membrane lipids via an ROS-mediated process, with thylakoid injury and the aggregation of D1 protein being the lethal mechanisms, and both PSII and PSI being the attacking targets. With regard towards the effective inactivation of M. aeruginosa and high removal of MC, UVC/H2O2 proposed a novel practical method in controlling cyanobacterial blooms.

New insight into the residual inactivation of Microcystis aeruginosa by dielectric barrier discharge

We report the new insight into the dielectric barrier discharge (DBD) induced inactivation of Microcystis aeruginosa, the dominant algae which caused harmful cyanobacterial blooms in many developing countries. In contrast with the previous work, we employed flow cytometry to examine the algal cells, so that we could assess the dead and living cells with more accuracy, and distinguish an intermediate state of algal cells which were verified as apoptotic. Our results showed that the numbers of both dead and apoptotic cells increased with DBD treatment delay time, and hydrogen peroxide produced by DBD was the main reason for the time-delayed inactivation effect. However, apart from the influence of hydrogen peroxide, the DBD-induced initial injures on the algal cells during the discharge period also played a considerable role in the inactivation of the DBD treated cells, as indicated by the measurement of intracellular reactive oxygen species (ROS) inside the algal cells. We therefore propose an effective approach to utilization of non-thermal plasma technique that makes good use of the residual inactivation effect to optimize the experimental conditions in terms of discharge time and delay time, so that more efficient treatment of cyanobacterial blooms can be achieved.

Hormesis effects of amoxicillin on growth and cellular biosynthesis of Microcystis aeruginosa at different nitrogen levels

Coexisting antibiotic contaminants have potential to regulate cyanobacterial bloom, and the regulation is likely affected by nitrogen supply. A typical cyanobaterium Microcystis aeruginosa was cultured with 0.05-50 mg L(-1) of nitrogen and exposed to 100-600 ng L(-1) of amoxicillin for 7 days. Algal growth was not significantly (p > 0.05) affected by amoxicillin at the lowest nitrogen level of 0.05 mg L(-1), stimulated by 600 ng L(-1) of amoxicillin at a moderate nitrogen level of 0.5 mg L(-1) and enhanced by 100-600 ng L(-1) of amoxicillin at higher nitrogen levels of 5-50 mg L(-1). Amoxicillin affected chlorophyll-a, psbA gene, and rbcL gene in a similar manner as algal growth, suggesting a regulation of algal growth via the photosynthesis system. At each nitrogen level, synthesis of protein and polysaccharides as well as production and release of microcystins (MCs) increased in response to environmental stress caused by amoxicillin. Expression of ntcA and mcyB showed a positive correlation with the total content of MCs under exposure to amoxicillin at nitrogen levels of 0.05-50 mg L(-1). Nitrogen and amoxicillin significantly (p < 0.05) interact with each other on the regulation of algal growth, synthesis of chlorophyll-a, production and release of MCs, and expression of ntcA and mcyB. The nitrogen-dependent stimulation effect of coexisting amoxicillin contaminant on M. aeruginosa bloom should be fully considered during the combined pollution control of M. aeruginosa and amoxicillin.

Cellular and transcriptional responses in Microcystis aeruginosa exposed to two antibiotic contaminants

The responses of Microcystis aeruginosa under exposure to spiramycin and amoxicillin were investigated on both cellular and genetic levels through a 7-day exposure test. Algal growth was inhibited by spiramycin while promoted by amoxicillin at test concentrations of 0.6-1.8 μg L(-1), indicating a higher toxicity of spiramycin than amoxicillin. During the whole exposure period, the chlorophyll a content and expression levels of psbA, psaB, and rbcL were significantly inhibited by spiramycin at test concentrations of 1.2 and 1.8 μg L(-1) (p < 0.05) and stimulated by 0.6-1.8 μg L(-1) of amoxicillin (p < 0.05), with respective decreases of up to 26, 75, 72, and 82% compared to the control and respective increases of 20, 70, 135, and 55%. During the 4 to 7 days of exposure, the microcystin-LR content and expression levels of mcyB and grpE were reduced by up to 66, 47, and 72% in spiramycin-treated algal cells, respectively, and stimulated by up to 1.3-, 1.4-, and 1.5-folds in amoxicillin-treated algal cells, respectively. Elevated recA expression was only observed in 1.2 and 1.8 μg L(-1) of spiramycin-treated algal cells, indicating severe DNA damage due to the high toxicity. Target antibiotics were suspected to regulate the growth and microcystin-production in M. aeruginosa via the photosynthesis system.

Influence of coexisting spiramycin contaminant on the harm of Microcystis aeruginosa at different nitrogen levels

The influence of nitrogen on the effects of the antibiotic contaminant spiramycin on Microcystis aeruginosa was studied through a 7-day exposure test. At current contamination levels of 0.5-100 mg L(-1) for nitrogen and 0.1-0.4 μg L(-1) for spiramycin, the two factors significantly interacted with each other (p<0.05) on the synthesis of chlorophyll-a and protein, the production and release of microcystins as well as the expression of ntcA gene and mcyB gene in M. aeruginosa. Nitrogen significantly affected the toxicity of spiramycin on algal growth (p<0.05) via regulation of protein synthesis. The photosynthesis system including chlorophyll-a, the psbA gene, and the rbcL gene participated in stress responses to spiramycin. Coexisting spiramycin contaminant increased the harm of M. aeruginosa by stimulating the production and release of MCs at a nitrogen level of 0.5 mg L(-1), but alleviated M. aeruginosa pollution at higher nitrogen levels of 5-100 mg L(-1) by inhibiting algal growth, the production of microcystins and the expression of ntcA and mcyB. The nitrogen-dependent effects of spiramycin should be considered in the control of M. aeruginosa bloom in the presence of spiramycin.

Heritage materials and biofouling mitigation through UV-C irradiation in show caves: state-of-the-art practices and future challenges

Biofouling, i.e., colonization of a given substrate by living organisms, has frequently been reported for heritage materials and particularly on stone surfaces such as building facades, historical monuments, and artworks. This also concerns subterranean environments such as show caves, in which the installation of artificial light for tourism has led to the proliferation of phototrophic microorganisms. In Europe nowadays, the use of chemicals in these very sensitive environments is scrutinized and regulated by the European Union. New and environmentally friendly processes must be developed as alternative methods for cave conservation. For several years, the UV irradiation currently used in medical facilities and for the treatment of drinking water has been studied as a new innovative method for the conservation of heritage materials. This paper first presents a review of the biofouling phenomena on stone materials such as building facades and historical monuments. The biological disturbances induced by tourist activity in show caves are then examined, with special attention given to the methods and means to combat them. Thirdly, a general overview is given of the effects of UV-C on living organisms, and especially on photosynthetic microorganisms, through different contexts and studies. Finally, the authors' own experiments and findings are presented concerning the study and use of UV-C irradiation to combat algal proliferation in show caves. Both laboratory and in situ results are summarized and synthesized from their previously published works. The application of UV in caves is discussed and further experiments are proposed to enhance research in this domain.

Combined effects of two antibiotic contaminants on Microcystis aeruginosa

Combined toxicity of spiramycin and amoxicillin was tested in Microcystis aeruginosa. The respective 50% effective concentrations (EC50mix) expressed in toxic unit (TU) values were 1.25 and 1.83 for spiramycin and amoxicillin mixed at 1:7 and 1:1, suggesting an antagonistic interaction at the median effect level. Deviations from the prediction of concentration addition (CA) and independent action (IA) models further indicated that combined toxicity of two antibiotics mixed at 1:1 varied from synergism to antagonism with increasing test concentration. Both the EC50mix of 0.86 (in TU value) and the deviation from two models manifested a synergistic interaction between spiramycin and amoxicillin mixed at 7:1. At an environmentally relevant concentration of 800ngL(-1), combined effect of mixed antibiotics on algal growth changed from stimulation to inhibition with the increasing proportion of higher toxic component (spiramycin). Chlorophyll-a content and expression levels of psbA, psaB, and rbcL varied in a similar manner as growth rate, suggesting a correlation between algal growth and photosynthesis under exposure to mixed antibiotics. The stimulation of microcystin-production by mixed antibiotics was related with the elevated expression of mcyB. The mixture of two target antibiotics with low proportion of spiramycin (<50%) could increase the harm of M. aeruginosa to aquatic environments by stimulating algal growth and production and release of microcystin-LR at their current contamination levels.