Effects of sulfate on microcystin production, photosynthesis, and oxidative stress in Microcystis aeruginosa

Aptameric photonic structure-based optical biosensor for the detection of microcystin

An optical photonic biosensor for the detection of microcystin (MC) has been developed using an aptamer-immobilized interpenetrating polymeric network (IPNaptamer) intertwined with solid-state cholesteric liquid crystals (CLCsolids). The IPN was constructed with a polyacrylic acid hydrogel (PAA). Aptamer immobilization enhances polarity while blocking hydrogen bonding between the carboxylic groups of PAA-IPN hydrogel, thereby increasing the swelling ratio of the PAA-IPN hydrogel. This leads to an expansion in the helical pitch of the corresponding IPNaptamer-CLCsolid biosensor chip and results in a red-shift in the reflected color. Upon exposure to an aqueous MC solution, the IPNaptamer-CLCsolid biosensor chip exhibits aptamer-mediated engulfment of MC, resulting in reduced polarity of the IPNaptamer complex and a consequential blue-shift in the biosensor chip color occurred. The wavelength shift of the IPNaptamer-CLCsolid biosensor chip demonstrates a linear change with an increase in MC concentration from 3.8 to 150 nM, with a limit of detection of 0.88 nM. This novel optical biosensor is characterized by its low cost, simplicity, selectivity, and sensitivity, offering a promising strategy for designing similar toxin biosensors through the modification of biological receptors.

Differential protein analysis of saline-alkali promoting the oil accumulation in Nitzschia palea

BACKGROUND

The increasingly severe salinization of the aquatic environment has led to serious damage to the habitats of aquatic organisms. Benthic diatoms are commonly employed as indicator species for assessing water quality and serve as a reflection of the overall health of the aquatic ecosystem. Nitzschia palea is a common diatom found in freshwater, with high oil content, rapid reproductive rate, and it is a commonly dominant species in various rivers.

RESULTS

The results showed that after 4 days (d) of saline-alkali stress, the cell density and chlorophyll a content of Nitzschia palea reached their maximum values. Therefore, we selected Nitzschia palea under 4 d stress for Tandem Mass Tag (TMT) quantitative proteomic analysis to explore the molecular adaptation mechanism of freshwater diatoms under saline-alkali stress. Totally, 854 proteins were enriched, of which 439 differentially expressed proteins were identified. Gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and subcellular fractionation analysis revealed that these proteins were mainly enriched in the photosynthesis pathway, citric acid cycle (TCA cycle), fatty acid synthesis, and glutathione cycle.

CONCLUSIONS

This study aims to reveal the physiological, biochemical and proteomic mechanisms of salt and alkali tolerance and molecular adaptation of Nitzschia palea under different saline-alkali concentrations. This study showed that Nitzschia palea is one candidate of the environmental friendly, renewable bioenergy microalgae. Meantime, Nitzschia palea reveals for the proteome of the freshwater and provides the basis, it became a model algal species for freshwater diatoms.

Machine learning-assisted discovery of growth decision elements by relating bacterial population dynamics to environmental diversity

Microorganisms growing in their habitat constitute a complex system. How the individual constituents of the environment contribute to microbial growth remains largely unknown. The present study focused on the contribution of environmental constituents to population dynamics via a high-throughput assay and data-driven analysis of a wild-type Escherichia coli strain. A large dataset constituting a total of 12,828 bacterial growth curves with 966 medium combinations, which were composed of 44 pure chemical compounds, was acquired. Machine learning analysis of the big data relating the growth parameters to the medium combinations revealed that the decision-making components for bacterial growth were distinct among various growth phases, e.g., glucose, sulfate, and serine for maximum growth, growth rate, and growth delay, respectively. Further analyses and simulations indicated that branched-chain amino acids functioned as global coordinators for population dynamics, as well as a survival strategy of risk diversification to prevent the bacterial population from undergoing extinction.

Application of a Mechanistic Model for the Prediction of Microcystin Production by Microcystis in Lab Cultures and Tropical Lake

Microcystin is an algal toxin that is commonly found in eutrophic freshwaters throughout the world. Many studies have been conducted to elucidate the factors affecting its production, but few studies have attempted mechanistic models of its production to aid water managers in predicting its occurrence. Here, a mechanistic model was developed based on microcystin production by Microcystis spp. under laboratory culture and ambient field conditions. The model was built on STELLA, a dynamic modelling software, and is based on constitutive cell quota that varies with nitrogen, phosphorus, and temperature. In addition to these factors, varying the decay rate of microcystin according to its proportion in the intracellular and extracellular phase was important for the model’s performance. With all these effects, the model predicted most of the observations with a model efficiency that was >0.72 and >0.45 for the lab and field conditions respectively. However, some large discrepancies were observed. These may have arisen from the non-constitutive microcystin production that appear to have a precondition of nitrogen abundance. Another reason for the large root mean square error is that cell quota is affected by factors differently between strains.

The allelopathy and underlying mechanism of Skeletonema costatum on Karenia mikimotoi integrating transcriptomics profiling

The roles of allelopathy for succession of marine phytoplankton communities remain controversial, especially for the development of blooms. Physiological parameters measurement (Fv/Fm value, MDA content, SOD activity, Na⁺/K⁺, Ca²⁺/ Mg²⁺-ATPase activity, cell size, chlorophyll content, apoptosis and cell cycle) and whole transcriptome profiling analysis were used to investigate allelopathy effect of Skeletonema costatum on Karenia mikimotoi. Filtrate and extracts from S. costatum culture inhibited the growth of K. mikimotoi. Allelopathic effects were dose-dependent for filtrate culture and extract culture. K. mikimotoi scavenged excessive ROS and adapted to the stress fastly and easily, so oxidative damage was not the main cause of the growth inhibition. Allelochemicals of S. costatum were found to influence the structure and function of cell membrane of K. mikimotoi by damaging membrane structure till to cell necrosis, which caused high mortality. Coupled with the sensitivity of algal cells to environmental stress and restricted cell cycle, allelopathy was suggested to be deeply detrimental to the development of competition algal population.

Hormones induce the metabolic growth and cytotoxin production of Microcystis aeruginosa under terpinolene stress

Several organic compounds released into the aquatic environment have a detrimental impact on humans and other organisms. There is a lack of knowledge about natural hormones and herbicides on non-target organisms, including cyanobacteria. In this study, the response of Microcystis aeruginosa to four phytohormones, indole-3-acetic acid (IAA; 10^-5), zeatin (ZT; 10^-5), abscisic acid (ABA; 10^-7), and brassinolide (BRL; 10^-9 mol/L), exposed to terpinolene (TPN; (0.44, 0.88, 1.17, or 1.62 mmol/L) at the cellular and genetic levels were investigated. The results showed that TPN could inhibit the growth and photosynthetic activities and stimulate microcystins (MCs) of M. aeruginosa at various levels through the co-occurrence of oxidative stress, antioxidant defense activities, and an imbalance of the antioxidative system. Hormones played critical roles in the growth promotion and photosynthetic activity by enhancing the antioxidant defense mechanisms and MCs production of M. aeruginosa under TPN stress in both hormone and TPN dose-dependent manner. The growth performance and photosynthetic activities of M. aeruginosa were significant with IAA (p < 0.01) and BSL (p < 0.05) compared to ZT and ABA, as TPN concentrations increased. Hormones stimulated the MCs production significantly BSL (p < 0.05) at various levels and protected the cells against TPN-induced oxidative stress and expression of mcyB and mcyD genes involve in MCs synthesis. Our results indicated that hormone contamination in eutrophic lakes might increase the risk of Microcystis aeruginosa bloom and microcystin production with the TPN association.

Size-dependent toxic effects of polystyrene microplastic exposure on Microcystis aeruginosa growth and microcystin production

Due to increasingly severe microplastic pollution in freshwaters, the interaction between these contaminants and cyanobacteria warrants study. In this study, we expose the freshwater cyanobacterium Microcystis aeruginosa to different sizes (1 μm and 100 nm) of polystyrene (PS) microplastics of 5 mg/L. Results indicate 1 μm microplastics promote algal growth (12.42% ± 0.94%) at 96 h, and have greater potential to aggregate on algal cell surfaces and inhibit photosynthesis. But no significance was observed in 100 nm microplastics treatment on algal growth and photosynthetic activity after 96 h exposure. Especially, 1 μm microplastics increased the content of intracellular microcystins (MCs) (18.42% ±0.33%) after 72 h and inhibit MCs release (23.87% ±8.79%) at 72 h, while 100 nm PS microplastics promote MCs production only at 48 h (14.83% ± 7.07%). Results indicate that smaller size does not necessarily mean greater toxicity, 1 μm microplastics showing more adverse effects than 100 nm microplastics to M. aeruginosa, improving understanding of the toxicity of microplastics in freshwater ecosystems, and challenging the conventionally held belief that smaller microplastics are more toxic.

Selenoprotein V protects against endoplasmic reticulum stress and oxidative injury induced by pro-oxidants

Selenoprotein V (SELENOV) contains a thioredoxin-like fold and a conserved CxxU motif with a potential redox function. This study was to assess its in vivo and in vitro roles and mechanisms in coping with different oxidant insults. In Experiment (Expt.)1, SELENOV knockout (KO) and wild type (WT) mice (male, 8-wk old) were given an ip injection of saline, diquat (DQ, 12.5 mg/kg), or N-acetyl-para-aminophenol (APAP, 300 mg/kg) (n = 10), and killed 5 h after the injection. In Expt. 2, primary hepatocytes of WT and KO were treated with DQ (0-0.75 mM) or APAP (0-6 mM) for 12 h. In Expt. 3, 293 T cells overexpressing Selenov gene (OE) were treated with APAP (0-4 mM) for 24 h or H₂O₂ (0-0.4 mM) for 12 h. Compared with the WT, the DQ- and APAP-injected KO mice had higher (P < 0.05) serum alanine aminotransferase activities and hepatic malondialdehyde (MDA), protein carbonyl, endoplasmic reticulum (ER) stress-related proteins (BIP and CHOP), apoptosis-related proteins (FAK and caspase-9), and 3-nitrotyrosine, along with lower total anti-oxidizing-capability (T-AOC) and severer hepatic necrosis. Likewise, the DQ and APAP-treated KO hepatocytes had elevated (P < 0.05) cell death (10-40%), decreased (P < 0.05) T-AOC (63-83%), glutathione (26-87%), superoxide dismutase (SOD) activity (28-36%), mRNA levels of redox enzymes (Cat, Gcs, Gpx3, and Sod) and (or) sharper declines (P < 0.05) in cellular respiration and ATP production than that of the WT cells. In contrast, the OE cells had greater viability and T-AOC and lower MDA, and carbonyl contents after the APAP and H₂O₂ exposures (all at P < 0.05) than the controls. Moreover, the OE cells had greater (P < 0.05) redox enzyme activities (GPX, TrxR, and SOD), and lower (P < 0.05) expressions of ER stress-related genes (Atf4, Atf6, Bip, Xbp1t, Xbp1s, and Chop) and proteins (BIP, CHOP, FAK, and caspase-9) than the control cells after the treatment of H₂O₂ (0.4 mM). In conclusion, SELENOV conferred protections in vivo and in vitro against the reactive oxygen and nitrogen species-mediated ER stress-related signaling and oxidative injuries.

Effects of environmental factors on the growth and microcystin production of Microcystis aeruginosa under TiO2 nanoparticles stress

Due to the growing use and release of nanomaterials, their toxic impacts on aquatic ecosystems have drawn widespread attention in recent years. In this study, we exposed Microcystis aeruginosa to 5 mg/L titanium dioxide nanoparticles (nTiO₂) under different culture conditions (pH 6, 7, 8, 9; 20 °C, 25 °C, 30 °C). The results showed that algae had the worst growth status with lowest biomass, lowest photosynthetic activity and highest reactive oxygen species (ROS) generation under 5 mg/L nTiO₂ at pH 6 and 20 °C. Images by scanning electron microscopy (SEM) revealed that nTiO₂ hindered light absorption by algal cells by wrapping the algal surface, which led to obvious cell surface deformation at pH 6 or 20 °C. In addition, microcystin-LR (MC-LR) production increased as temperature or pH decreased when exposed to nTiO₂ at 5 mg/L, demonstrating that falling pH or temperature enhanced the adverse effects toward algal cells under nTiO₂ stress and the potential risk of algae to the environment.

Effects of high concentrations of sulfate on dissolved organic matter in paddy soils revealed by excitation-emission matrix analyzing

The problem of sulfate pollution is becoming increasingly serious in freshwater and wetlands. Since paddy fields are the largest constructed wetland in Earth's surface, the increased sulfate input may have great effect on dissolved organic matter (DOM) in paddy soils. To understand these effects, a 24-day anaerobic incubation experiment was conducted with four Chinese paddy soils amended with high concentrations (0, 10, 25, 50, and 100 mmol L^-1) of Na₂SO₄. Dissolved organic carbon (DOC) and chlorophyll a (Chl a) concentrations were determined after incubation. Parallel factor analysis (PARAFAC) of the excitation-emission matrix (EEM) spectra was used to analyze the DOM composition. In all four soils, DOC concentrations generally increased with increasing sulfate concentration, while the Chl a concentrations decreased. The EEM spectra of DOM were resolved into four components by PARAFAC. With increasing sulfate concentration, the proportion of the ultraviolet C humic acid-like compound decreased and the tyrosine-like compound increased in two algae-rich soils (Sichuan and Tianjin). No obvious variation was observed in the humification index (HIX) or the ratio of peak β to peak α (β:α) in any soils with added sulfate. Specific ultra-violet absorbance at 254 nm (SUVA₂₅₄) decreased with increasing sulfate concentration in Jilin, Tianjin, and Ningxia soils, and the fluorescence index (FI) decreased in two algae-rich soils. In conclusion, although sulfate addition increased the DOC concentration, the DOM composition depended more strongly on soil type and physicochemical properties than sulfate. Sulfate addition only affected soil DOM origin and composition by inhibiting algal growth in algae-rich paddy soils.

Salt Shock Responses of Microcystis Revealed through Physiological, Transcript, and Metabolomic Analyses

The transfer of Microcystis aeruginosa from freshwater to estuaries has been described worldwide and salinity is reported as the main factor controlling the expansion of M. aeruginosa to coastal environments. Analyzing the expression levels of targeted genes and employing both targeted and non-targeted metabolomic approaches, this study investigated the effect of a sudden salt increase on the physiological and metabolic responses of two toxic M. aeruginosa strains separately isolated from fresh and brackish waters, respectively, PCC 7820 and 7806. Supported by differences in gene expressions and metabolic profiles, salt tolerance was found to be strain specific. An increase in salinity decreased the growth of M. aeruginosa with a lesser impact on the brackish strain. The production of intracellular microcystin variants in response to salt stress correlated well to the growth rate for both strains. Furthermore, the release of microcystins into the surrounding medium only occurred at the highest salinity treatment when cell lysis occurred. This study suggests that the physiological responses of M. aeruginosa involve the accumulation of common metabolites but that the intraspecific salt tolerance is based on the accumulation of specific metabolites. While one of these was determined to be sucrose, many others remain to be identified. Taken together, these results provide evidence that M. aeruginosa is relatively salt tolerant in the mesohaline zone and microcystin (MC) release only occurs when the capacity of the cells to deal with salt increase is exceeded.

Simultaneous Microcystis algicidal and microcystin synthesis inhibition by a red pigment prodigiosin

Microcystis blooms and their secondary metabolites microcystins (MCs) occurred all over the world, which have damaged aquatic ecosystems and threatened public health. Techniques to reduce the Microcystis blooms and MCs are urgently needed. This study aimed to investigate the algicidal and inhibitory mechanisms of a red pigment prodigiosin (PG) against the growth and MC-producing abilities of Microcystis aeruginosa (M. aeruginosa). The numbers of Microcystis cells were counted under microscope. The expression of microcystin synthase B gene (mcyB) and concentrations of MCs were determined by quantitative real-time polymerase chain reaction (qRT-PCR) and enzyme linked immunosorbent assay (ELISA) methods, respectively. The inhibitory effects of PG against M. aeruginosa strain FACHB 905 with 50% algicidal concentration (LC₅₀) at 120 h was 0.12 μg/mL. When M. aeruginosa cells exposed to 0.08 μg/mL, 0.16 μg/mL, 0.32 μg/mL PG, the expression of mcyB of M. aeruginosa was down-regulated 4.36, 8.16 and 18.51 times lower than that of the control at 120 h. The concentrations of total MC (TMC) also were 1.66, 1.72 and 5.75 times lower than that of the control at 120 h. PG had high algicidal effects against M. aeruginosa, with the activities of superoxide dismutase (SOD) initially increased and then decreased after 72 h, the contents of malondialdehyde (MDA) increase, the expression of mcyB gene down-regulation, and MCs synthesis inhibition. This study was first to report the PG can simultaneously lyse Microcystis cells, down-regulate of mcyB expression and inhibit MCs production effectively probably due to oxidative stress, which indicated PG poses a great potential for regulating Microcystis blooms and MCs pollution in the environment.

Growth response of the ichthyotoxic haptophyte, Prymnesium parvum Carter, to changes in sulfate and fluoride concentrations

Golden alga Prymnesium parvum Carter is a euryhaline, ichthyotoxic haptophyte (Chromista). Because of its presumed coastal/marine origin where SO₄^2- levels are high, the relatively high SO₄^2- concentration of its brackish inland habitats, and the sensitivity of marine chromists to sulfur deficiency, this study examined whether golden alga growth is sensitive to SO₄^2- concentration. Fluoride is a ubiquitous ion that has been reported at higher levels in golden alga habitat; thus, the influence of F^- on growth also was examined. In low-salinity (5 psu) artificial seawater medium, overall growth was SO₄^2—dependent up to 1000 mg l^-1 using MgSO₄ or Na₂SO₄ as source; the influence on growth rate, however, was more evident with MgSO₄. Transfer from 5 to 30 psu inhibited growth when salinity was raised with NaCl but in the presence of seawater levels of SO₄^2-, these effects were fully reversed with MgSO₄ as source and only partially reversed with Na₂SO₄. Growth inhibition was not observed after acute transfer to 30 psu in a commercial sea salt mixture. In 5-psu medium, F^- inhibited growth at all concentrations tested. These observations support the hypothesis that spatial differences in SO₄^2- –but not F^-–concentration help drive the inland distribution and growth of golden alga and also provide physiological relevance to reports of relatively high Mg²⁺ concentrations in golden alga habitat. At high salinity, however, the ability of sulfate to maintain growth under osmotic stress was weak and overshadowed by the importance of Mg²⁺. A mechanistic understanding of growth responses of golden alga to SO₄^2-, Mg²⁺ and other ions at environmentally relevant levels and under different salinity scenarios will be necessary to clarify their ecophysiological and evolutionary relevance.

Effects of titanium dioxide nanoparticles on Microcystis aeruginosa and microcystins production and release

Due to growing production and use, release of nanoparticles (NPs) into the aquatic environment may pose a hazard to ecosystem. In this study, Microcystis aeruginosa was exposed to different concentrations (0.1, 1, 10, 50, 100 mg/L) of titanium dioxide (TiO₂) NPs to assess their impact on algae. Meanwhile, the production and release of microcystins (MCs) was determined. Results showed that TiO₂ NPs significantly decreased the maximal photochemical efficiency of photosystem II, and thus inhibited the photosynthetic activity of M. aeruginosa. They also increased the content of reactive oxygen species (ROS) and malondialdehyde (MDA), indicating their oxidative damage on algae. Besides, TiO₂ NPs at high concentrations (50 and 100 mg/L) aggregated on the algal surface and block the light, herein inhibited algae growth (16.03%±2.50% and 54.13%±0.93%) but induced the production (25.02%±1.23% and 114.43%±2.96%) and release (20.96%±13.30% and 12.10%±8.80%) of MCs. These results indicated that high concentrations of TiO2 NPs increased MCs concentration in water system, which may be harmful to aquatic ecosystem.

Fouling Microbial Communities on Plastics Compared with Wood and Steel: Are They Substrate- or Location-Specific?

Although marine biofouling has been widely studied on different substrates, information on biofouling on plastics in the Arabian Gulf is limited. Substrate- and location-specific effects were investigated by comparing the microbial communities developed on polyethylene terephthalate (PET) and polyethylene (PE) with those on steel and wood, at two locations in the Sea of Oman. Total biomass was lower on PET and PE than on steel and wood. PET had the highest bacterial abundance at both locations, whereas chlorophyll a concentrations did not vary between substrates. MiSeq 16S ribosomal RNA sequencing revealed comparable operational taxonomic unit (OTU) richness on all substrates at one location but lower numbers on PET and PE at the other location. Non-metric multidimensional scaling (NMDS) showed distinct clusters of the bacterial communities based on substrate (analysis of similarity (ANOSIM), R = 0.45-0.97, p < 0.03) and location (ANOSIM, R = 0.56, p < 0.0001). The bacterial genera Microcystis and Hydrogenophaga and the diatoms Licmophora and Mastogloia were specifically detected on plastics. Desulfovibrio and Pseudomonas spp. exhibited their highest abundance on steel and Corynebacterium spp. on wood. Scanning electron microscopy (SEM) revealed fissure formation on PET and PE, indicating physical degradation. The presence of free radicals on PET and carbonyl bonds (C=O) on PE, as revealed by Fourier transform infrared (FTIR) spectroscopy, indicated abiotic degradation while hydroxyl groups and spectral peaks for proteins and polysaccharides on PE indicated biotic degradation. We conclude that fouling microbial communities are not only substrate-specific but also location-specific and microbes developing on plastics could potentially contribute to their degradation in the marine environment.

Distinct Bloom Dynamics of Toxic and Non-toxic Microcystis (Cyanobacteria) Subpopulations in Hoedong Reservoir (Korea)

Despite the importance of understanding the bloom mechanisms that influence cyanobacterial toxin production, the dynamics of toxic Microcystis subpopulations are largely unknown. Here, we quantified both toxic and entire (i.e., toxic and non-toxic) Microcystis populations based on the microcystin synthetase E (mcyE) and 16S ribosomal RNA genes. Samples were collected from pelagic water and sediments twice per week from October to December 2011, and we investigated the effects of physicochemical factors (pH, water temperature, dissolved oxygen, nutrients, etc.) and biological factors (ciliates and zooplankton) on the abundance of toxic and non-toxic Microcystis. During the study period, Microcystis blooms were composed of toxic and non-toxic subpopulations. Resting stage Microcystis in sediment may be closely linked to Microcystis populations in pelagic water and may contribute to the toxic subpopulation composition in surface Microcystis blooms. In pelagic water, the toxic and entire Microcystis population had a significant positive correlation with the pH and water temperature (p < 0.05). However, their responses to changes in environmental factors were thought to be distinct. The ratio of the toxic to non-toxic Microcystis subpopulations was significantly (p < 0.05) enhanced by a lower pH and water temperature and an increase in protozoan grazers, reflecting environmental stresses. These results suggest that the toxic and non-toxic subpopulations of Microcystis have distinct tolerance levels against these stressors. The intracellular microcystin (MC) concentration was positively associated with the abundance of the mcyE-positive Microcystis. By comparison, the MC concentration in pelagic water body (extracellular) increased when Microcystis was lysed due to environmental stresses.

The Impacts of Potassium Permanganate and Powdered Activated Carbon on Cyanotoxin Release

Bench-scale trials were performed to: (1) Expose Microcystis aeruginosa cells to potassium permanganate doses of 1, 3 and 5 mg/L, at contact times of 15, 30 and 90 minutes, pH levels of 7 and 9, and turbidities of 0.1, 5 and 20 NTU; (2) Compare the impacts of oxidation alone and oxidation plus powdered activated carbon for the final 60 minutes of contact time and (3) evaluate the impact of these treatment conditions on extracellular microcystins, extra- plus intracellular (combined) microcystins, cell membrane integrity and chlorophyll-a concentrations. Toxin releases from the cells were observed at both pH levels. The greatest toxin releases were observed at the lowest KMnO₄ doses. The toxin releases were accompanied by relatively stable total cell counts, increases in membrane compromised cells and decreases in chlorophyll-a. The application of 10 mg/L PAC resulted in extracellular toxin concentrations that were markedly lower than those observed in oxidant-only situations.

Changes and relations of photosynthesis and iron cycling in anoxic paddy soil amended with high concentrations of sulfate

Sulfate contamination is an increasingly serious environmental problem related to microbial reduction processes in anoxic paddy soil. This study revealed the changes and interrelations of ferric iron [Fe(III)] reduction, ferrous iron [Fe(II)] oxidation, and oxygenic photosynthesis in an anoxic paddy soil (Fe-accumuli-Stagnic Anthrosols) amended with a range of high sulfate concentrations. Soil slurries mixed with 0 (control), 50, 100, 200, and 400 mmol kg^-1 Na₂SO₄ were incubated anaerobically under dark and light conditions. The changes in chlorophyll a (Chl a), Fe(II), pH levels, and the chlorophyll absorption spectrum were determined over a 42-day period. Fe(II) concentrations increased with the addition of sulfate under dark conditions, while Fe(III) reduction potential was enhanced by increasing sulfate addition. The effect of light on Fe(II) concentration was observed after 16 days of incubation, when Fe(II) started to decrease markedly in the control. The decrease in Fe(II) slowed with increasing sulfate addition. The concentrations of Chl a increased in all treatments after 16 days of incubation under light conditions. There was a reduction in Chl a accumulation with increasing sulfate at the same incubation time. The absorption peaks of chlorophyll remained shorter than the 700-nm wavelength throughout the incubation period. The pH of all treatments decreased in the first week and then increased thereafter. The pH increased with sulfate addition and light conditions. In conclusion, contamination with high concentrations of sulfate could accelerate Fe(III) reduction while inhibiting oxygenic photosynthesis, which correspondingly slows chemical Fe(II) oxidation in an anoxic paddy soil.

Species-dependent variation in sensitivity of Microcystis species to copper sulfate: implication in algal toxicity of copper and controls of blooms

Copper sulfate is a frequently used reagent for Microcystis blooms control but almost all the previous works have used Microcystis aeruginosa as the target organism to determine dosages. The aim of this study was to evaluate interspecific differences in the responses of various Microcystis species to varying Cu²⁺ concentrations (0, 0.05, 0.10, 0.25, and 0.50 mg L⁻¹). The half maximal effective concentration values for M. aeruginosa, M. wesenbergii, M. flos-aquae, and M. viridis were 0.16, 0.09, 0.49, and 0.45 mg L⁻¹ Cu²⁺, respectively. This showed a species-dependent variation in the sensitivity of Microcystis species to copper sulfate. Malonaldehyde content did not decrease with increasing superoxide dismutase content induced by increasing Cu²⁺, suggesting that superoxide dismutase failed to reduce Cu²⁺ damage in Microcystis. Considering the risk of microcystin release when Microcystis membranes are destroyed as a result of Cu²⁺ treatment and the stimulation effects of a low level of Cu²⁺ on growth in various species, our results suggest that copper sulfate treatment for Microcystis control could be applied before midsummer when M. aeruginosa and M. viridis are not the dominant species and actual amount of Cu²⁺ used to control M. wesenbergii should be much greater than 0.10 mg L⁻¹.