Dark adaptation and ability of pulse-amplitude modulated (PAM) fluorometry to identify nutrient limitation in the bloom-forming cyanobacterium, Microcystis aeruginosa (Kützing).

Harmful algal blooms in inland waters are widely linked to excess phosphorus (P) loading, but increasing evidence shows that their growth and formation can also be influenced by nitrogen (N) and iron (Fe). Deficiency in N, P, and Fe differentially affects cellular photosystems and is manifested as changes in photosynthetic yield (Fv/Fm). While Fv/Fm has been increasingly used as a rapid and convenient in situ gauge of nutrient deficiency, there are few rigorous comparisons of instrument sensitivity and ability to resolve specific nutrient stresses. This study evaluated the application of Fv/Fm to cyanobacteria using controlled experiments on a single isolate and tested three hypotheses: i) single Fv/Fm measurements taken with different PAM fluorometers can distinguish among limitation by different nutrients, ii) measurements of Fv/Fm made by the addition of DCMU are comparable to PAM fluorometers, and iii) dark adaptation is not necessary for reliable Fv/Fm measurements. We compared Fv/Fm taken from the bloom-forming Microcystis aeruginosa (UTEX LB 3037) grown in nutrient-replete treatment (R) and N-, P-, and Fe-limited treatments (LN, LP, LFe, respectively), using three pulse-amplitude modulated (PAM) fluorometers and the chemical photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and evaluated the effects of dark adaptation prior to PAM measurement. There were significant differences in Fv/Fm estimates among PAM fluorometers for light- versus dark-adapted cell suspensions over the whole experiment (21 days), which were all significantly higher than the DCMU-based measurements. However, dark adaptation had no effect on Fv/Fm when comparing PAM-based values across a single nutrient treatment. All Fv/Fm methods could distinguish LN and LP from R and LFe treatments but none were able to resolve LFe from R, or LN from LP cultures. These results indicated that for most PAM applications, dark adaptation is not necessary, and furthermore that single measurements of Fv/Fm do not provide a robust measurement of nutrient limitation in Microcystis aeruginosa UTEX LB 3037, and potentially other, common freshwater cyanobacteria.

Detrimental effect of UV-B radiation on growth, photosynthetic pigments, metabolites and ultrastructure of some cyanobacteria and freshwater chlorophyta.

BACKGROUND: Global warming directly influencing ozone layer depletion, which eventually is increasing ultraviolet radiation penetration having far-reaching impacts on living biota. This particularly influences the primary producer microalgae which are the basic unit of food webs in the aquatic habitats. Therefore, it is necessary to concentrate the research at this micro-level to understand the harmful impact of increased UV-B radiation ever before. Consequently, the present attempt aimed to focus on the influence of UV-B on growth criteria, photosynthetic pigments, some metabolites, and ultrastructure of the freshwater cyanobacteria, Planktothrix cryptovaginata (Microcoleaceae), Nostoc carneum (Nostocaceae), Microcystis aeruginosa (Microcystaceae), the Chlorophyte Scenedesmus acutus (Scenedesmaceae), and the marine Cyanobacterium Microcystis (Microcystaceae). METHODS: The cultures of investigated algae were subjected directly to different duration periods (1, 3, 5, and 7 h) of artificial UV-B in addition to unirradiated control culture and allowed to grow for 10 days, after which the algal samples were analyzed for growth, photosynthetic activities, primary metabolities and cellular ultrastructure. RESULTS: A remarkable inhibitory influence of UV-B was observed on growth criteria (measured as optical density and dry weight) and photosynthetic pigments of P. cryptovaginata, N. carneum, M. aeruginosa, S. acutus, and marine Microcystis. Where increasing the exposure time of UV-B was accompanied by increased inhibition. The variation in carbohydrate and protein contents under UV stress was based on the exposure periods and the algal species. The variation in algal ultrastructure by UV-B stress was noticed by an Electron Microscope. Cells damage and lysis, cell wall and cell membrane ruptured and release of intracellular substances, loss of cell inclusion, plasmolysis and necrosis, or apoptosis of the algal cells were observed by exposure to 7 h of UV-B. CONCLUSION: Exposure to UV-B has a marked harmful impact on the growth, pigments, and metabolic activity, as well as the cellular ultrastructure of some cyanobacteria and chlorophytes.

Antibiotic contaminants reduced the treatment efficiency of UV-C on Microcystis aeruginosa through hormesis.

Antibiotic contaminants exert stimulatory hormetic effects in cyanobacteria at low (ng L-1) concentrations, which may interfere with the control of cyanobacterial bloom in aquatic environments exhibiting combined pollution. This study investigated the influence of a mixture of four popular antibiotics (sulfamethoxazole, amoxicillin, ciprofloxacin, and tetracycline) during the application of UV-C irradiation for controlling the bloom of Microcystis aeruginosa. In the absence of antibiotics, 100-500 mJ cm-2 UV-C irradiation reduced cell density, growth rate, chlorophyll a content, Fv/Fm value and microcystin concentration in M. aeruginosa in a dose-dependent manner through the downregulation of proteins related to cell division, chlorophyll synthesis, photosynthesis and microcystin synthesis. UV-C irradiation stimulated microcystin release through the upregulation of the microcystin release regulatory protein (mcyH). The presence of 40 ng L-1 antibiotic mixture during UV-C treatment significantly reduced (p < 0.05) the treatment efficiency of 100-300 mJ cm-2 UV-C on microcystin concentration, while 80 and 160 ng L-1 antibiotic mixture significantly reduced (p < 0.05) the treatment efficiency of 100-500 mJ cm-2 UV-C on cell density and microcystin concentration. The antibiotic mixture alleviated the toxicity of UV-C on M. aeruginosa through a significant stimulation of photosynthetic activity (p < 0.05) and the upregulation of proteins involved in photosynthesis, biosynthesis, protein expression, and DNA repair. Microcystin release in UV-C-treated cyanobacterial cells was further stimulated by the antibiotic mixture through the upregulation of mcyH and four ATP-binding cassette transport proteins. The interference effects of antibiotic contaminants should be fully considered when UV-C is applied to control cyanobacterial bloom in antibiotic-polluted environments. In order to eliminate the interference effects of antibiotics, the concentration of each target antibiotic is suggested to be controlled below 5 ng L-1 before the application of UV-C irradiation.

Effects of high light and temperature on Microcystis aeruginosa cell growth and ß-cyclocitral emission.

Cyanobacteria always massively grow and even occur blooms in summer, with releasing amount of ß-cyclocitral. To uncover the effects of summer high irradiance and temperature on cyanobacterial growth and ß-cyclocitral emission, the cell growth, reactive oxygen species (ROS) levels, photosynthetic pigment content, chlorophyll fluorescence and ß-cyclocitral emission were investigated in Microcystis aeruginosa under high light and temperature. Compared to the control under 50 µmol m-2·s-1, the cell growth was promoted under 100 µmol m-2·s-1, but inhibited under 500 and 1000 µmol m-2·s-1. The inhibition was also detected under high temperature at 30 and 35 °C in contrast to the control at 25 °C. Under high light and high temperature, M. aeruginosa increased ROS levels and reduced photosynthetic pigment content and photosystem II (PSII) efficiency, which resulted in the inhibition on cell growth. With increasing the light intensity and temperature, 1O2 levels gradually increased, while ß-carotene content gradually decreased by quenching 1O2, with increasing ß-cyclocitral emission. In summer, high irradiance and temperature not benefited the growth of cyanobacteria, but the emission of ß-cyclocitral derived from ß-carotene quenching 1O2 may offset the disadvantages by poisoning other algae.

Effects of Temperature on The UV-B Sensitivity of Toxic Cyanobacteria Microcystis aeruginosa CS558 and Anabaena circinalis CS537.

Rising global temperatures have been suggested to favor cyanobacteria over eukaryotic algae, but UV-B fluxes are also predicted to remain high and may interact with temperature to affect algal growth. To understand the interactive effects of temperature and UV-B radiation, cultures of Microcystis aeruginosa and Anabaena circinalis were grown at either 25 or 30°C and then exposed to an acute irradiance of UV-B (1.4 W m-2 ). Both species showed differences in growth rates at both temperature regimes. The growth rates of M. aeruginosa (0.41 ± 0.02 day-1 ) and A. circinalis (0.38 ± 0.01 day-1 ) were higher at 25 and 30°C, respectively. Rates of damage (k) and repair (r) were calculated from the kinetics of change in effective quantum yield, Fv '/Fm '. Analysis of the estimates of r and k shows that M. aeruginosa exhibited relatively high values for both parameters, compared to A. circinalis, at both growth temperatures. In both species, repair rates were higher at 30°C than at 25°C but in A. circinalis damage was also greater at the higher temperature. In contrast, M. aeruginosa showed a lower damage rate at the higher temperature. For both species, the ratio of r:k was higher at the higher temperature. However, the percent inhibition of effective quantum yield by UV-B was greater in A. circinalis than in M. aeruginosa as the r:k was lower A. circinalis. Therefore, it could be concluded that temperature may influence growth and bloom formation of cyanobacteria and that different species may respond differently to UV-B and temperature interactions.

Effects of electron beam irradiation on proteins and exopolysaccharide production and changes in Microcystis aeruginosa.

Purpose:Microcystis aeruginosa often threaten human health and safety for the toxin and unpleasant odor and removal difficulties during water treatment process. In order to remove it, a novel method high energy was studied in this research.Materials and methods: The electron beam generated by an accelerator was applied to irradiate M. aeruginosa at various doses of 1, 2, 3, 4 and 5 kGy. The effects of irradiation on M. aeruginosa characteristics and mechanism have been researched through surveying the changes in pH and conductivity, changes of algae cell community structure and respiration rate, and changes of protein and exopolysaccharides production were also detected.Result: The data showed that exposure to 2-5 kGy radiation could make pH decrease. Microcystis aeruginosa increased through its own photosynthesis and physiological regulation. The increasing damage to algal cells led to the exosmosis of the contents, which increased the electrical conductivity of algae liquid and decreased the area of algae cells colony. 2-5 kGy irradiation decreased protein content and destroy the antioxidant system and thus reduced the secretion of extracellular polysaccharidesConclusions: 2-5 kGy radiation could control the algae growth and produced obvious effect. The respiration rate decreased obviously that made M. aeruginosa lose activity in a short time. The results proved that irradiation could change the algae growth and affect its life characteristic efficiently in a short time.

Antibiotics promoted the recovery of Microcystis aeruginosa after UV-B radiation at cellular and proteomic levels.

Elevated UV-B radiation due to ozone layer depletion may prevent the growth of bloom-forming cyanobacteria in aquatic environments, while antibiotic contaminants may cause effects opposite to that of UV-B due to hormesis. This study investigated the influence of a quaternary antibiotic mixture on Microcystis aeruginosa after UV-B radiation through a 15-day exposure test. UV-B radiation extended the lag phase of M. aeruginosa at doses of 600 and 900 mJ/cm2, and significantly (p < 0.05) reduced the growth rate and the Fv/Fm value at doses of 300-900 mJ/cm2. Although UV-B radiation significantly (p < 0.05) stimulated the microcystin production ability in each cyanobacterial cell, the total microcystin concentration still significantly (p < 0.05) decreased due to the reduction of cell density. Mixed antibiotics and UV-B regulated the proteomic expression profile of M. aeruginosa in different manners. UV-B radiation upregulated 19 proteins and downregulated 49 proteins in M. aeruginosa, while mixed antibiotics upregulated 45 proteins and downregulated 25 proteins in UV-B treated cells. Mixed antibiotics significantly (p < 0.05) stimulated growth and photosynthesis, increased cell density and microcystin concentration, and reduced oxidative stress in UV-B treated cells through the upregulation of proteins involved in photosynthesis, biosynthesis, cell division, oxidation-reduction, gene expression and microcystin synthesis. This study verified the hypothesis that antibiotics accelerated the recovery of M. aeruginosa from UV-B induced damage. A safe threshold of 20 ng/L was suggested for mixed antibiotics (5 ng/L for each antibiotic), in order to eliminate the stimulatory effects of antibiotics on bloom-forming cyanobacteria.

Direct Effects of Temperature on Growth of Different Tropical Phytoplankton Species.

Temperature increase may influence competition among phytoplankton species, potentially intensifying cyanobacteria blooms that can be favored by direct and indirect effects of temperature. In this study, we aimed to clarify how cyanobacteria can be favored by the direct effects of increased temperature compared to diatoms and chlorophytes. Strains of the most representative species of a eutrophic coastal lagoon (Microcystis aeruginosa, Planktothrix agardhii, Desmodesmus communis, and Cyclotella meneghiniana) were used to test the hypothesis that cyanobacteria would be favored by the direct effect of temperature increase. First, we evaluated the effect of temperature increase on growth in monocultures (batch and chemostats) at 25 and 30 °C and after in mixed cultures (chemostats). In batch monocultures, the cyanobacteria showed higher growth rates in 30 °C than in 25 °C. However, in continuous culture experiments (chemostats), growth rates of M. aeruginosa and P. agardhii were not affected by temperature, but the strains showed higher biovolume in steady-state with the temperature increase. In continuous mixed cultures, M. aeruginosa was always dominant and C. meneghiniana was excluded, regardless of temperature tested. D. communis was able to coexist with lower biomass. This study shows that rising temperatures can be detrimental to diatoms, even for a tropical strain. Although some studies indicate that the dominance of cyanobacteria in warmer climates may be due to the indirect effect of warming that will promote physical conditions in the environment more favorable to cyanobacteria, the outcomes of mixed cultures demonstrate that the direct effect of temperature can also favor the dominance of cyanobacteria.

Application of N-TiO2 for visible-light photocatalytic degradation of Cylindrospermopsis raciborskii - More difficult than that for photodegradation of Microcystis aeruginosa ?

Nowadays Cylindrospermopsis raciborskii (C. raciborskii) and the metabolites produced, such as cylindrospermopsin (CYN), pose a serious threat to the ecosystem. Advanced oxidation technologies have been verified as constituting a very promising means to eliminate the risk from harmful algae. But so far little research has focused on the visible-light photocatalytic destruction of C. raciborskii cells and the degradation of their metabolites. In our study, N-doped TiO2 (N-TiO2) was used to degrade C. raciborskii and the degradation was compared with that of the Microcystis aeruginosa (M. aeruginosa). Results showed that although the photodegradation of C. raciborskii was more difficult than that of M. aeruginosa, the treatment with N-TiO2 was still satisfactory. After adding 200 mg/L N-TiO2, C. raciborskii cells (5 × 106 cells/mL) were completely destroyed within 20 h under visible light irradiation, and nearly 90% of the organic matter and CYN in the suspensions were also degraded, thereby markedly improving the water quality. The photocatalytic process starts with damage to the cell membrane resulting in the leakage of internal components. Subsequently, the leaked metabolites were oxidised by the reactive oxidizing species produced by N-TiO2. Thus, the application of N-TiO2 is a promising method for the treatment of C. raciborskii.

Suppressing Cyanobacteria with Hydrogen Peroxide Is More Effective at High Light Intensities.

Hydrogen peroxide (H2O2) can be used as an emergency method to selectively suppress cyanobacterial blooms in lakes and drinking water reservoirs. However, it is largely unknown how environmental parameters alter the effectiveness of H2O2 treatments. In this study, the toxic cyanobacterial strain Microcystis aeruginosa PCC 7806 was treated with a range of H2O2 concentrations (0 to 10 mg/L), while being exposed to different light intensities and light colors. H2O2 treatments caused a stronger decline of the photosynthetic yield in high light than in low light or in the dark, and also a stronger decline in orange than in blue light. Our results are consistent with the hypothesis that H2O2 causes major damage at photosystem II (PSII) and interferes with PSII repair, which makes cells more sensitive to photoinhibition. Furthermore, H2O2 treatments caused a decrease in cell size and an increase in extracellular microcystin concentrations, indicative of leakage from disrupted cells. Our findings imply that even low H2O2 concentrations of 1-2 mg/L can be highly effective, if cyanobacteria are exposed to high light intensities. We therefore recommend performing lake treatments during sunny days, when a low H2O2 dosage is sufficient to suppress cyanobacteria, and may help to minimize impacts on non-target organisms.

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 ß-cyclocitral production show similar pattern of incremental rate with growth rate on population level shown as intracellular concentrations of MC-LR (CMC) and ß-cyclocitral (CBCC), but the cellular quota of MC-LR (QMC) and ß-cyclocitral (QBCC) 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 CMC consistently decreased by 10-41% with increase of UV-C dosage (50-200 mJ cm-2) while CBCC 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 CMC and CBCC for 3-14 days. The suppressing effects on QMC and QBCC 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 CMC and CBCC, indicating interruptions on biosynthesis processes partially contribute to suppression effects of CMC and CBCC. The suppression effect on either CBCC or QBCC, with higher decrement rates while lower recovery rates, was more severe than CMC and QMC.

Evaluation of disinfection by-product formation during chlor(am)ination from algal organic matter after UV irradiation.

This study evaluated the effect of low-pressure ultraviolet (UV) irradiation on the formation of disinfection by-products (DBPs) from algal organic matter of Microcystis aeruginosa during subsequent chlorination and chloramination. The algal organic matter includes extracellular organic matter (EOM) and intracellular organic matter (IOM). The fluorescence excitation-emission matrix spectra indicated that the humic/fulvic acid-like organics of EOM and the protein-like organics of IOM may be preferentially degraded by UV treatment. UV irradiation with low specific UV absorbance values was effective in reducing the formation of trihalomethanes and dichloroacetic acid from EOM and IOM during the subsequent chlorination. During the UV-chloramine process, higher UV dose (1000 mJ/cm2) led to the decrease of the formation of dichloroacetic acid, trichloroacetic acid, and haloketones from IOM by an average of 24%. Furthermore, UV irradiation can slightly increase the bromine substitution factors (BSFs) of haloacetic acids from EOM during chlorination, including dihaloacetic acids and trihaloacetic acids in the presence of bromide (50 µg/L). However, UV irradiation did not shift the formation of DBPs from IOM to more brominated species, since the BSFs of trihalomethanes, dihaloacetic acids, trihaloacetic acids, and dihaloacetonitriles almost kept unchanged during UV-chlorine process. As for UV-chloramine process, UV irradiation decreased the BSFs of trihalomethanes, while increased the BSFs of dihaloacetic acid for both EOM and IOM. Overall, the UV pretreatment process is a potential technology in treating algae-rich water.

Effects of solution chemistry on the sunlight inactivation of particles-associated viruses MS2.

The inactivation efficacy of bacteriophage MS2 by simulated sunlight irradiation was investigated to understand the effects of MS2 aggregation and adsorption to particles in solutions with different components. Kaolinite and Microcystis aeruginosa were used as model inorganic and organic particles, respectively. Lower pH and di-valent ions (Ca2+) were main factors on the aggregation and inactivation of MS2. In the presence of both particles, there was no significant impact on the MS2 inactivation efficacy by kaolinite (10-200mM) or Microcystis aeruginosa (102-105Cells/mL) in 1mM NaCl at pH 7. However at lower pH 3, MS2 aggregates formed in the particle-free and kaolinite-containing solutions, caused lower inactivation since the outer viruses of aggregation protect the inner viruses. In addition, more MS2 adsorbed on Microcystis aeruginosa at lower pH (3 and 4). Microcystis aeruginosa would act as a potential photosensitizer for ROS production to inactivate the adsorbed MS2, since extracellular organic matter (EOM) of Microcystis aeruginosa was detected in this study, which has been reported to produce ROS under solar irradiation. At pH 7, Na+ had no effect on the inactivation of MS2, because MS2 was stable and dispersed even at 200mM Na+. MS2 aggregated and adsorbed on particles even at 10mM Ca2+ and led to lower inactivation. Kaolinite cannot offer enough protection to adsorbed MS2 as aggregation and Microcystis aeruginosa acts as potential photosensitizer to produce ROS and inactivate the adsorbed MS2 at high concentration of Ca2+. In particle-free solution, SRNOM inhibited MS2 inactivation by shielding the sunlight and coating MS2 to increase its survival.

Physiological responses and toxin production of Microcystis aeruginosa in short-term exposure to solar UV radiation.

The aim of this study was to evaluate the effects of short-term (hours) exposure to solar UV radiation (UVR, 280-400 nm) on the physiology of Microcystis aeruginosa. Three solar radiation treatments were implemented: (i) PAR (PAR, 400-700 nm), (ii) TUVA (PAR + UVAR, 315-700 nm) and (iii) TUVR (PAR + UVAR + UVBR, 280-700 nm). Differential responses of antioxidant enzymes and the reactive oxygen species (ROS) production to UVR were observed. Antioxidant enzymes were more active at high UVR doses. However, different responses were observed depending on the exposure to UVAR or UVBR and the dose level. No effects were observed on the biomass, ROS production or increased activity of superoxide dismutase (SOD) and catalase (CAT) compared to the control when UVR + PAR doses were lower than 9875 kJ m-2. For intermediate doses, UVR + PAR doses between 9875 and 10 275 kJ m-2, oxidative stress increased while resistance was imparted through SOD and CAT in the cells exposed to UVAR. Despite the increased antioxidant activity, biomass decrease and photosynthesis inhibition were observed, but no effects were observed with added exposure to UVBR. At the highest doses (UVR + PAR higher than 10 275 kJ m-2), the solar UVR caused decreased photosynthesis and biomass with only activation of CAT by UVBR and SOD and CAT by UVAR. In addition, for such doses, a significant decrease of microcystins (MCs, measured as MC-LR equivalents) was observed as a consequence of UVAR. This study facilitates our understanding of the SOD and CAT protection according to UVAR and UVBR doses and cellular damage and reinforces the importance of UVR as an environmental stressor. In addition, our results support the hypothesized antioxidant function of MCs.

Role of illumination intensity in microcystin development using Microcystis aeruginosa as the model algae.

Microcystis aeruginosa (M. aeruginosa) is one of the most common genera of cyanobacteria in algal blooms. In the present work, the impact of the illumination intensity on the growth of M. aeruginosa has been studied and a grinding method for the extraction of intracellular microcystins (MCs) was developed. The variations of algal density, pH, total phosphorus (TP), and total nitrogen (TN) have been investigated during MCs' culturing period. Results showed that the extraction efficiency of MC-YR by the grinding method was 275% higher than the sonication method, and the extraction efficiencies of MC-RR and MC-LR by the grinding method were similar to the sonication method. The optimal illumination intensity for M. aeruginosa was found to be 19-38 µmol m-2 s-1 with suitable pH range of 7.5-10.5. Active release of extracellular MCs was not significantly observed when illumination intensities were ≤ 38 µmol m-2 s-1. Furthermore, the intracellular MC yields under different illumination intensities were found to be a relatively stable level. However, excess illumination intensity (≥ 47 µmol m-2 s-1) led to the lysis of algal cell and increased the concentrations of extracellular MCs, with MC-RR as the dominant compound. The calculated intracellular/extracellular MCs ratios for MC-RR, MC-LR, and MC-YR were 2.38 (N = 100, SD = 2.44), 2.68 (N = 64, SD = 3.48), and 1.25 (N = 30, SD = 1.64), respectively. Strong illumination intensity and cell lysis were found to be the two major factors influencing the release of extracellular MCs.

Cylindrospermopsin induced changes in growth, toxin production and antioxidant response of Acutodesmus acuminatus and Microcystis aeruginosa under differing light and nitrogen conditions.

Growing evidence suggests that some bioactive metabolites (e.g. cyanotoxins) produced by cyanobacteria have allelopathic potential, due to their inhibitory or stimulatory effects on competing species. Although a number of studies have shown that the cyanotoxin cylindrospermopsin (CYN) has variable effects on phytoplankton species, the impact of changing physicochemical conditions on its allelopathic potential is yet to be investigated. We investigated the physiological response of Microcystis aeruginosa (Cyanobacteria) and Acutodesmus acuminatus (Chlorophyta) to CYN under varying nitrogen and light conditions. At 24h, higher microcystins content of M. aeruginosa was recorded under limited light in the presence of CYN, while at 120h the lower levels of the toxins were observed in the presence of CYN under optimum light. Total MCs concentration was significantly (p<0.05) lowered by CYN after 120h of exposure under limited and optimum nitrogen conditions. On the other hand, there were no significant (p>0.05) changes in total MCs concentrations after exposure to CYN under high nitrogen conditions. As expected, limited light and limited nitrogen conditions resulted in lower cell density of both species, while CYN only significantly (p<0.05) inhibited the growth of M. aeruginosa. Regardless of the light or nitrogen condition, the presence of CYN increased internal H2O2 content of both species, which resulted in significant (p<0.05) changes in antioxidant enzyme (catalase, peroxidase, superoxide dismutase and glutathione S-transferase) activities. The oxidative stress caused by CYN was higher under limited light and limited nitrogen. These results showed that M. aeruginosa and A. acuminatus have variable response to CYN under changing light and nitrogen conditions, and demonstrate that need to consider changes in physicochemical conditions during ecotoxicological and ecophysiological investigations.

Different physiological responses of cyanobacteria to ultraviolet-B radiation under iron-replete and iron-deficient conditions: Implications for underestimating the negative effects of UV-B radiation.

Iron deficiency has been considered one of the main limiting factors of phytoplankton productivity in some aquatic systems including oceans and lakes. Concomitantly, solar ultraviolet-B radiation has been shown to have both deleterious and positive impacts on phytoplankton productivity. However, how iron-deficient cyanobacteria respond to UV-B radiation has been largely overlooked in aquatic systems. In this study, physiological responses of four cyanobacterial strains (Microcystis and Synechococcus), which are widely distributed in freshwater or marine systems, were investigated under different UV-B irradiances and iron conditions. The growth, photosynthetic pigment composition, photosynthetic activity, and nonphotochemical quenching of the different cyanobacterial strains were drastically altered by enhanced UV-B radiation under iron-deficient conditions, but were less affected under iron-replete conditions. Intracellular reactive oxygen species (ROS) and iron content increased and decreased, respectively, with increased UV-B radiation under iron-deficient conditions for both Microcystis aeruginosa FACHB 912 and Synechococcus sp. WH8102. On the contrary, intracellular ROS and iron content of these two strains remained constant and increased, respectively, with increased UV-B radiation under iron-replete conditions. These results indicate that iron-deficient cyanobacteria are more susceptible to enhanced UV-B radiation. Therefore, UV-B radiation probably plays an important role in influencing primary productivity in iron-deficient aquatic systems, suggesting that its effects on the phytoplankton productivity may be underestimated in iron-deficient regions around the world.

UV/persulfate preoxidation to improve coagulation efficiency of Microcystis aeruginosa.

The performance of UV-activated persulfate (UV/PS) technology as preoxidation process to enhance Microcystis aeruginosa removal by subsequent coagulation-sedimentation was firstly evaluated. The results demonstrate that UV/PS preoxidation could successfully promote coagulation of algae cells through the effective neutralization of zeta potential, which was caused by the changes of cell morphology, size distribution and surface properties after simultaneous UV irradiation and formed reactive species (i.e. SO4- and HO) oxidation. Since excessive oxidation would cause cell rupture along with the release of organics, which could deteriorate coagulation efficiency, optimal PS dose (60mg/L) and UV dose (375mJ/cm2) were proposed to exist in this study. The concentrations of extracellular algal organic matter (AOM) sharply increased by 48.2% during the preoxidation period, while gradually decreased in the following coagulation and sedimentation. Most of the concerned disinfection by-products (DBPs) monotonically decreased or followed fluctuant reduction with increasing PS doses, whereas the trichloromethane, trichloroacetic acid and dichloroacetonitrile persistently increased, which was inferred to be related to the variation of AOM. This study suggests that UV/PS might be a potential pretreatment process to assist coagulation on the removal of algae.

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.

UV-B-Heavy Metal Interaction: Comparative Evaluation on Photosynthetic Properties of Three Cyanobacteria.

Gradual increase in UV-B component reaching earth surface together with heavy metal contamination appears to be a serious environmental concern. Differential sensitivity in photosynthetic characters of Anabaena doliolum, Microcystis sp., and Nostoc muscorum were observed following exposure to UV-B radiation and heavy metals (Cd and Zn) which displayed reduced photosynthesis with maximum inhibition in Anabaena. PSII was the most sensitive component of the electron transport chain depicting 84, 80, and 70 % inhibition in A. doliolum, Microcystis sp., and N. muscorum, respectively. Cadmium and UV-B-induced inhibition of DCPIP photoreduction could not reversed by artificial electron donors in all the strains. However, they substantially reversed the inhibition caused by Zn as well as Zn + UV-B interactively in N. muscorum, not observed in other two strains. Absorption spectra of all the strains showed differential decrement in chl a peak in treated than the control cells (P < 0.25, r = -0.942). Significantly decreased peaks (P < 0.05) by different states of stresses to all the three cyanobacteria were observed in emission spectra. Excitation spectra of the test strains suggest disorganization or delinking of phycobilisomes from the PSII reaction center, also attested by strong negative correlation between the treatment of stress and phycocyanin (P < 0.025, r = -0.971). The present study qualified N. muscorum as most tolerant followed by Microcystis sp., whereas A. doliolum emerged as the most sensitive one and also endorses high toxicity of Cd as compared to Zn.