Metribuzin impairs the unicell-colony transformation in the green alga Scenedesmus obliquus

Active growth is a prerequisite for the formation of grazing-protective, mostly eight-celled colonies by the ubiquitous green alga Scenedesmus in response to chemical cues from zooplankton. Colonies can also be evoked by chemically quite similar manmade anionic surfactants, such as FFD-6. In this study, it was hypothesized that growth-inhibiting concentrations of the herbicide metribuzin impair the ability of Scenedesmus obliquus to form colonies in response to the surfactant morphogen FFD-6. The results confirmed that the formation of colonies in S. obliquus was hampered by metribuzin. EC₅₀ values of metribuzin for colony inhibition (approximately 11 μg L⁻¹) were similar to those for growth and photosynthesis inhibition (12-25 μg metribuzin L⁻¹). In the absence of the colony-inducing surfactant FFD-6, S. obliquus populations were comprised of 92% unicells, having on average 1.2 cells per colony at all tested metribuzin concentrations (0-100 μg L⁻¹). In contrast, in the presence of FFD-6 and at low metribuzin concentrations (0 and 5 μg L⁻¹), S. obliquus had more than five cells per colony with a high portion of eight-celled colonies. However, increasing concentrations of metribuzin decreased the number of colonies in the FFD-6-exposed populations and caused them to remain mostly unicellular at the highest concentrations (50 and 100 μg L⁻¹). This study revealed that metribuzin impeded growth and by doing so, also obstructed the possibility for unicellular Scenedesmus to form colonies. Consequently, an increase in mortality of Scenedesmus from grazing is expected.     

Comparing aquatic risk assessment methods for the photosynthesis-inhibiting herbicides metribuzin and metamitron

Three different risk assessment procedures are described that aim to protect freshwater habitats from risks of the photosynthesis-inhibiting herbicides metribuzin and metamitron. These procedures are (1) the first-tier approach, based on standard toxicity tests and the application of an assessment factor, (2) the Species Sensitivity Distribution (SSD) approach, based on laboratory tests with a wider array of species and the application of a statistical model to calculate the HCx (the Hazardous Concentration for x% of the species), and (3) the model ecosystem approach, based on the evaluation of treatment-related effects in field enclosures. A comparison of the risk assessment procedures reveals that the first-tier approach is the most conservative for metamitron and metribuzin, and that HC5 values (and even HC10 values) based on acute EC50 values of algae and aquatic vascular plants may be used to derive maximum permissible concentrations for single applications. For both compounds these HC5 values were very similar to the ecological threshold concentrations in the enclosure studies. In contrast to model ecosystem experiments, however, HCx values based on lab toxicity tests do not provide information on the recovery potential of sensitive endpoints and on indirect effects, which may be important for regulatory decision-making. In the enclosure study, indirect effects of metribuzin on invertebrate populations were observed at an exposure concentration that was approximately 20 times lower than the corresponding HC5 value based on lab toxicity data for aquatic invertebrates.     

Relationship between chromium(VI) resistance and extracellular polymeric substances (EPS) concentration by some cyanobacterial isolates

BACKGROUND, AIM, AND SCOPE: Chromium(VI) resistance and its association with extracellular polymeric substance (EPS) concentration in cyanobacteria was investigated. Increased EPS concentration was associated with Cr(VI) resistance. The most resistant isolate, Chroococcus sp. H(4), secreted the most EPS (427 mg/L). MATERIALS AND METHODS: EPS concentration of the two most resistant isolates (Chroococcus sp. H(4) and Synechocystis sp. S(63)) was investigated following exposure to 15 and 35 ppm Cr(VI). The composition of EPS produced by Chroococcus sp. H(4) following exposure to 10 ppm Cr(VI) was analyzed using high-performance liquid chromatography. Control EPS was composed of glucose (99%) and galactronic acid (1%); in the presence of 10 ppm Cr(VI), EPS composition changed to glucose (9%), xylose (75%), rhamnose (14%), and galacturonic acid (2%). RESULTS AND DISCUSSION: Results indicated that (1) exposure to elevated concentrations of Cr(VI) affected the composition of EPS produced by Chroococcus sp. H(4), and (2) there was a correlation between Cr(VI) resistance and EPS concentration in some cyanobacteria.     

Environmental impact of two successive chemical treatments in a small shallow eutrophied lake: Part II. Case of copper sulfate

The appearance of cyanobacteria ( > 10 colony per ml) was not prevented after alum treatment. In order to prevent cyanobacteria efflorescences in a small shallow polymictic lake (Courtille, France), copper sulfate was applied. Treatment level was 63 microg 1(-1) as Cu2+ from CUSO4, 5 H2O. Cyanobacteria were kept under control during the summer. Microcystis sp. completely disappeared, which allowed swimming in the lake throughout the tourist season. Microcystis only reappeared 2 months after the treatment. Copper content in the water column only returned to its background level 2 months after copper addition. This high residence time of copper in the water might have been caused by complexation and adsorption of copper on natural organic matter, whose level was high in the ecosystem studied. A mechanism of transfer of 'truly' dissolved copper towards particulate copper has been underlined and explains the disappearance of this fraction of copper in the water column.     

Removal of Microcystins by Phototrophic Biofilms. A Microcosm Study (6 pp)

Background, Aims and Scope Microcystins (MCs) are a family of natural toxins produced by cyanobacteria (blue-green algae). As a result of eutrophication, massive cyanobacterial blooms occur more frequently and MCs represent important contaminants of freshwater ecosystems. Bacterial biodegradation is considered a main mechanism for MC breakdown in environmental conditions. While existing studies were mostly focused on MC biodegradation by planktonic bacteria, our experiments examined the fate and kinetics of MC degradation in river-originated phototrophic biofilms and investigated factors influencing the rate of MC removal. Methods The fate of dissolved MCs was studied in laboratory microcosms with different composition (containing water only, water with phytoplankton and/or phototrophic biofilms). Biofilms originated from river ecosystem were pre-incubated under various conditions (with/without presence of cyanobacterial biomass or model organic substrates: glucose and protein - casein). Changes in MC concentration (0-14 days) in water columns were measured by HPLC DAD after external additions of purified MCs (160 μg L-1, MC-LR and MC YR), and halftimes (t1/2) of MC removal were estimated. Results and Discussion The slow degradation of MC was revealed in tap water (t1/2 ~ 14 days) and river water without cyanobacteria (t1/2 ~ 8 days). Enhanced removal occurred in the presence of natural planktonic cyanobacteria (t1/2 ~ 44 h), most probably due to microorganisms associated with the biomass of cyanobacterial bloom. More rapid MC elimination occurred in the variants containing phototrophic biofilms, and was particularly pronounced at those biofilms pre-cultivated in the presence of cyanobacterial blooms (t1/2 ~ 20 h). Much slower removal was observed in the variants simulating possible substrate-dependent induction of microorganism metabolism (biofilms pre-incubated with glucose: t1/2 ~ 35 h, and casein: t1/2 ~ 80 h). After termination of experiments, total amounts of MCs accumulated in the biofilms were below 5% of the initial toxin level revealing significant biodegradation processes. Conclusion The microcosm studies contributed to understanding of the environmental fate of MCs and revealed a rapid biodegradation by phototrophic biofilms. The rate of MC elimination depends on history of biofilm community, previous contact with cyanobacteria seems to be a selective factor improving the biodegradation potential. Recommendation and Outlook Our results experimentally showed a positive role of biofilms in MC elimination during water treatment processes such as bank filtration or slow sand filtration, and could eventually serve for further research of biofilm-based technological applications for MCs removal in small-scale drinking water treatment facilities.     

无机碳对浮游藻类生长和群落结构的影响

采用水族箱生态模拟方法,利用碳酸钠和碳酸氢钠作为碱度调节剂设置不同碱度水平,研究了氮磷营养相对充足的试验水体中藻类各相关指标对无机碳的响应,并对试验过程中所测得的理化指标和生物指标进行统计分析.研究表明:富营养水体无机碳含量与藻类叶绿素水平存在着正相关关系;适当增加水体中的无机碳浓度能够改变蓝、绿藻门种类比例;当碱度水平达到一定程度时,铜绿微囊藻可以逐渐成为优势种,并从体系中上升,演替成为水华.     

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

Environmental Science and Pollution Research - Microcystis aeruginosa (M. aeruginosa) is one of the most common genera of cyanobacteria in algal blooms. In the present work, the impact of the...     

Non-microcystin producing Microcystis wesenbergii (Komárek) Komárek (Cyanobacteria) representing a main waterbloom-forming species in Chinese waters

It is well known that several morphospecies of Microcystis, such as Microcystis aeruginosa (Kützing) Lemmermann and Microcystis viridis (A. Brown) Lemmermann can produce hepatotoxic microcystins. However, previous studies gave contradictory conclusions about microcystin production of Microcystis wesenbergii (Komárek) Komárek. In the present study, ten Microcystis morphospecies were identified in waterblooms of seven Chinese waterbodies, and Microcystis wesenbergii was shown as the dominant species in these waters. More than 250 single colonies of M. wesenbergii were chosen, under morphological identification, to examine whether M. wesenbergii produce hepatotoxic microcystin by using multiplex PCR for molecular detection of a region (mcyA) of microcystin synthesis genes, and chemical analyses of microcystin content by ELISA and HPLC for 21 isolated strains of M. wesenbergii from these waters were also performed. Both molecular and chemical methods demonstrated that M. wesenbergii from Chinese waters did not produce microcystin.     

Effects of a surfactant (FFD-6) on Scenedesmus morphology and growth under different nutrient conditions

Surfactants are man-made compounds that are meanwhile omnipresent in the environment, but environmental concentrations of surfactants are such that they are thought to have little risk for aquatic systems. The major anionic surfactants currently on the global market are linear alkylbenzene sulfonates (LAS), a class where the commercially available FFD-6 belongs to. The hypothesis was tested that sublethal effects of FFD-6, i.e. the morphological effect of colony formation in the common test alga Scenedesmus obliquus, occurs at a concentration lower than the no-observed-effect concentrations for endpoints commonly used in regulatory toxicity testing with algae. The surfactant FFD-6 induced colonies in Scenedesmus at concentrations a few orders of magnitude lower (i.e. between 0.001 and 0.01 g l-1) than at which growth inhibition was observed (i.e. between 1 and 10g l -1). Growth rates were lowest for Scenedesmus grown in P-limited medium, intermediate for algae reared in N-limited medium and highest for algae cultured in non-limited standard medium. Growth inhibition due to FFD-6 was similar for non-limited and nutrient-limited Scenedesmus, but colony formation was stronger in non-limited Scenedesmus than in nutrient limited cultures. The colony inducing effect of the surfactant FFD-6 on Scenedesmus occurs at much lower concentrations than growth inhibition and might affect species interactions, the survival of species and the energy flow along the food chain.     

Dynamics of microcystins-LR and -RR in the phytoplanktivorous silver carp in a sub-chronic toxicity experiment

A sub-chronic toxicity experiment was conducted to examine tissue distribution and depuration of two microcystins (microcystin-LR and microcystin -RR) in the phytoplanktivorous filter-feeding silver carp during a course of 80 days. Two large tanks (A, B) were used, and in Tank A, the fish were fed naturally with fresh Microcystis viridis cells (collected from a eutrophic pond) throughout the experiment, while in Tank B, the food of the fish were M. viridis cells for the first 40 days and then changed to artificial carp feed. High Performance Liquid Chromatography (HPLC) was used to measure MC-LR and MC-RR in the M. viridis cells, the seston, and the intestine, blood, liver and muscle tissue of silver carp at an interval of 20 days. MC-RR and MC-LR in the collected Microcystis cells varied between 268-580 and 110-292 microg g(-1) DW, respectively. In Tank A, MC-RR and MC-LR varied between 41.5-99.5 and 6.9-15.8 microg g(-1) DW in the seston, respectively. The maximum MC-RR in the blood, liver and muscle of the fish was 49.7, 17.8 and 1.77 microg g(-1) DW, respectively. No MC-LR was detectable in the muscle and blood samples of the silver carp in spite of the abundant presence of this toxin in the intestines (for the liver, there was only one case when a relatively minor quantity was detected). These findings contrast with previous experimental results on rainbow trout. Perhaps silver carp has a mechanism to degrade MC-LR actively and to inhibit MC-LR transportation across the intestines. The depuration of MC-RR concentrations occurred slowly than uptakes in blood, liver and muscle, and the depuration rate was in the order of blood>liver>muscle. The grazing ability of silver carp on toxic cyanobacteria suggests an applicability of using phytoplanktivorous fish to counteract cyanotoxin contamination in eutrophic waters.     

Differential Sensitivity of a Coccal Green Algal and a Cyanobacterial Species to Dissolved Natural Organic Matter (NOM) (8 pp)

Background

and Aim. In non-eutrophicated freshwaters, humic substances (HS) pose chemical stresses on aquatic organisms and, hence, separating sensitive from less sensitive or even tolerant species. One of the stresses, identified so far, is the reduction of photosynthetic oxygen production and reduction in growth in freshwater macrophytes and algae. In a previous paper, it has been shown that even closely related coccal green algae responded differently upon identical stress by HS, which is consistent with the hypothesis above. Due to their much simpler cellular ultrastructure, cyanobacteria are supposed to be more sensitive to HS exposure than eukaryote should be. One coccal green algal species (Desmodesmus communis) and one cyanobacterium (Chroococcus minutus) were exposed to four natural organic matter (NOM) isolates. One NOM has been isolated from a brown water lake (Schwarzer See) in Brandenburg State; three were obtained from a comprehensive Scandinavian NOM research project and originated from Norway (Birkenes), Finland (Hietajärvi), and Sweden (Svartberget).

Methods

Cultures of D. communis and C. minutus were obtained from the Culture Collection of Algae, Göttingen, and maintained in a common medium. The cultures were non-axenic. The algae and cyanobacteria were exposed under identical conditions to environmentally realistic NOM concentrations. Cell numbers were counted microscopically in Neugebauer cuvettes in triplicates. To avoid limitation by nutrient depletion, the experiments were terminated after 14 to 15 days. Until culture day 12, no growth limitation has been observed in the controls.

Results

All NOM isolates modulated the growth of the algae and cyanobacteria. During the early culture days, there was a slight growth promotion with the coccal green alga and to a much lesser degree with the cyanobacterium. Yet, the major effect were significant reductions in cell yield in both primary producer cultures. C. minutus was much more affected than D. communis. This applies particularly to the three tested Scandinavian NOM isolates, which were effective at concentrations even below 1 mg L-1 DOC.

Discussion

The growth promoting effect may be due to an increase in bioavailability of some trace nutrients in the presence of NOM, the release of some growth promoting substances by microbial or photochemical processing of the NOM, and/or a hormetic effect. The growth reducing effect can be explained as a herbicide-like mode of action that affects the photosystem II most prevalent by blocking the electron transport chain, absorption of electrons, or production of an internal oxidative stress after processing the bioconcentrated HS. Furthermore, it may be postulated that also photo-toxicity of these HS in the algal cells contributes to the overall toxicity; however, experimental evidence is lacking so far.

Conclusions

Upon exposure to HS, cyanobacteria appear to be much more sensitive than coccal green algae and respond in growth reduction. This high sensitivity of cyanobacteria to HS may explain phytoplankton patterns in the field. Eutrophic, humic-rich lakes do not support the cyanobacterial blooms characteristic of eutrophic, but humic-poor lakes. In the humic-rich systems, raphidophytes or, less frequent, specific coccal greens are more common. Obviously, cyanobacteria appear to be unable to make advantage of their accessory pigments (phycocyanin) to exploit the reddish light prevailing in humic-rich lakes.

Recommendation

. At present, no effective structure can be figured out which may be responsible for the adverse effect on the cyanobacterial species. It is reserved to future research whether or not HS may be applied more specifically (for instance, with elevated moieties of the effective structures) as a natural geochemical to combat cyanobacterial blooms.

     

Modifying sorbents in controlled release formulations to prevent herbicides pollution

The herbicides chloridazon and metribuzin, identified as groundwater pollutants, were incorporated in alginate-based granules to obtain controlled release properties. In this research the effect of incorporation of sorbents such as bentonite, anthracite and activated carbon in alginate basic formulation were not only studied on encapsulation efficiency but also on the release rate of herbicides which was studied using water release kinetic tests. In addition, sorption studies of herbicides with bentonite, anthracite and activated carbon were made. The kinetic experiments of chloridazon and metribuzin release in water have shown that the release rate is higher in metribuzin systems than in those prepared with chloridazon, which has lower water solubility. Besides, it can be deduced that the use of sorbents reduces the release rate of the chloridazon and metribuzin in comparison to the technical product and to the alginate formulation without sorbents. The highest decrease in release rate corresponds to the formulations prepared with activated carbon as a sorbent. The water uptake, permeability, and time taken for 50% of the active ingredient to be released into water, T(50), were calculated to compare the formulations. On the basis of a parameter of an empirical equation used to fit the herbicide release data, the release of chloridazon and metribuzin from the various formulations into water is controlled by a diffusion mechanism. Sorption capacity of the sorbents for chloridazon and metribuzin, ranging from 0.53mgkg(-1) for the metribuzin sorption on bentonite to 2.03x10(5)mgkg(-1) for the sorption of chloridazon on the activated carbon, was the most important factor modulating the herbicide release.     

Effect of soil amendments on sorption and mobility of metribuzin in soils

Metribuzin (4-amino-6-tert-butyl-4,5-dihydro-3-methylthio-1,2,4-triazin-5-one), is weakly sorbed to soil therefore, leaches easily to lower soil profiles. Soil amendments play a significant role in the management of leaching losses of pesticides. Therefore, present study reports the effect of organic manure and fly ash amendments on metribuzin downward mobility in sandy loam soil columns. Application of animal manure [T-1(OM) and T-2(OM)] and fly ash [T-1(FA) and T-2(FA)] at 2.5% and 5.0% levels increased the metribuzin retention in the soil. Freundlich constant [K(f)(1/n)] values of metribuzin for treatments T-1(OM) and T-2(OM) were 0.70 and 1.11, respectively, which were significantly higher than the value (0.27) in natural soil (T-0). The respective values for treatments T-1(FA) and T-2(FA) were 1.80 and 4.61. Downward mobility of metribuzin was studied in packed soil columns [300 mm (l)x59 mm (i.d.)]. Both the amendments significantly reduced the downward mobility of metribuzin and affected breakthrough time and maximum concentration of metribuzin in the leachate. Leaching losses of metribuzin were decreased from 97% in natural soil (T-0) column to 64% [T-1(OM)] and 42% [T-2(OM)] for animal manure-amended columns and 26% [T-1(FA)] to 100% [T-2(FA)] for fly ash-amended columns, as metribuzin did not leach out of 5% fly ash-amended column. Study indicates that both animal manure and fly ash were quite effective in reducing the downward mobility of metribuzin in packed soil columns of a sandy loam soil.     

Leaching of trifluralin,metolachlor, and metribuzin in a clay loam soil of Louisiana

Trifluralin[2,6-dinitro-N,N-dipropyl-4-(trifluormethyl)benzenamine], metolachlor[2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], and metribuzin[4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)one] were applied in field plots located on a Commerce clay loam soil near Baton Rouge, Louisiana at the rate of 1683 g/ha, 2759 g/ha and 609 g/ha, respectively. The half-lives of trifluralin, metolachlor, and metribuzin in the top 0-15 cm soil depth were found to be 54.7 days, 35.8 days and 29.8 days, respectively. The proportion of trifluralin, metolachlor, and metribuzin in the top 0-15 cm soil depth was 94.7%, 86.6%, and 75.4%, respectively of that found in the top 0-60 cm soil depth 30 days after application. Trifluralin concentrations were within a range of 0.026 ng/mL to 0.058 ng/mL in 1 m deep well water, and between 0.007 ng/mL and 0.039 ng/mL in 2 m deep well water over a 62 day period after application. Metolachlor concentrations in the 1 m and 2 m wells ranged from 3.62 ng/mL to 82.32 ng/mL and 8.44 ng/mL to 15.53 ng/mL, respectively. Whereas metribuzin concentrations in the 1 m and 2 m wells ranged from 0.70 ng/mL to 27.75 ng/mL and 1.71 ng/mL to 3.83 ng/mL, respectively. Accordingly, trifluralin was found to be strongly adsorbed on the soil and showed negligible leaching. Although metolachlor and metribuzin were also both readily adsorbed on the soil, their leaching potential was high. As a result, in the clay loam soil studied, metribuzin concentration in groundwater with shallow aquifers is likely to exceed the 10 mg/L US Environmental Protection Agency (EPA) advisory level for drinking water early in the application season, whereas trifluralin and metolachlor concentrations are expected to remain substantially lower than their respective 2 ng/mL and 175 ng/mL EPA advisory levels.     

Lysis of cyanobacteria with volatile organic compounds

One of bacteria collected from Lake Sagami, Japan, Brevibacillus sp., was found to have a lytic activity of cyanobacteria, but did not produce active compounds. Instead, the co-culturing of Microcystis with the Brevibacillus sp. enhanced the production of two volatile compounds, beta-cyclocitral and 3-methyl-1-butanol, and the former had a characteristic lytic activity. It was confirmed that these volatile compounds were derived from the cyanobacteria themselves. beta-Ionone, geosmin and 2-methylisoborneol derived from cyanobacteria and similar volatile compounds, terpenoids, produced by plants also had a lytic activity. The minimum inhibitory concentration values of the cyanobacterial metabolites were estimated to be higher than those of compounds from plants except for a few compounds. Among them, beta-cyclocitral only produced a characteristic color change of culture broth from green to blue. This color change is similar to the phenomenon observed when a sudden decline in growth of cyanobacteria begins in a natural environment.     

Copper complexing properties of dissolved organic materials exuded by the freshwater microalgae Scenedesmus acuminatus (Chlorophyceae)

Dissolved organic materials released by the freshwater microalgae Scenedesmus acuminatus were fractionated into low- and high-molecular weight materials, which were investigated for their capacity to bind copper. The high-molecular weight material was also investigated for its monosaccharide composition and is further discussed in relation to the copper binding property. S. acuminatus was grown in batch cultures under laboratory controlled conditions and harvested at the beginning of stationary growth phase when exuded organic materials were obtained. Copper-complexing property of the total exuded organic materials and exopolysaccharides before and after freeze-drying was evaluated by complexometric titrations and Scatchard Plot Analysis of the titration data. The results revealed the presence of two copper-complexing ligands in the total exuded material, but only one in the exopolysaccharide. Stronger copper-complexing ligands are associated to low molecular weight compounds (LogK'1=7.3, LogCL1=-5.6; LogK'2=6.3, LogCL2=-5.1), whereas weaker ligands to the high molecular weight fraction (LogK'2=6.4, LogCL2=-5.6). Although freeze-drying the polymeric organic material (exopolysaccharide) may result in conformational changes of the molecule, no effect on copper-complexing properties was detected. Gas chromatography was used to evaluate the monosaccharide composition of the microalgal exopolysaccharide, which detected high content of mannose and 12% of acid monosaccharides.     

Identification of microcystins from three collection strains of Microcystis aeruginosa

Microcystins (MCs) are toxic cyclic heptapeptides produced by various cyanobacteria genera, especially Microcystis. We identified 10 out of 12 MCs produced by three Microcystis aeruginosa strains from cyanobacteria collections, UTEX 2666, UTEX 2670 and UAM 1303, by using two analytical methods: Matrix-assisted Laser Desorption Ionization Time of Flight Mass Spectrometry (MALDI-TOF/MS) and HPLC Photodiode Array Detector coupled to a hybrid Quadrupole Time of Flight Mass Spectrometry (HPLC-PDA-QTOF/MS). MALDI-TOF/MS failed to detect non-polar MCs, such as MC-LY and MC-LW. HPLC-QTOF/MS permitted the accurate identification of most MCs present in methanolic extracts. Besides, three new MCs, namely: [D-Glu(OCH₃)⁶, D-Asp³] MC-LAba, MC-YL and MC-YM were detected by HPLC-QTOF/MS.     

Biodegradation of triazine herbicide metribuzin by the strain Bacillus sp. N1

By enrichment culturing of soil contaminated with metribuzin, a highly efficient metribuzin degrading bacterium, Bacillus sp. N1, was isolated. This strain grows using metribuzin at 5.0% (v/v) as the sole nitrogen source in a liquid medium. Optimal metribuzin degradation occurred at a temperature of 30ºC and at pH 7.0. With an initial concentration of 20 mg L^?1, the degradation rate was 73.5% in 120 h. If the initial concentrations were higher than 50 mg L^?1, the biodegradation rates decreased as the metribuzin concentrations increased. When the concentration was 100 mg L^?1, the degradation rate was only 45%. Degradation followed the pesticide degradation kinetic equation at initial concentrations between 5 mg L^?1 and 50 mg L^?1. When the metribuzin contaminated soil was mixed with strain N1 (with the concentration of metribuzin being 20 mg L^?1 and the inoculation rate of 10¹¹ g^?1 dry soil), the degradation rate of the metribuzin was 66.4% in 30 days, while the degradation rate of metribuzin was only 19.4% in the control soil without the strain N1. These results indicate that the strain N1 can significantly increase the degradation rate of metribuzin in contaminated soil.     

Effects of cyanobacterium Fischerella ambigua isolates and cell free culture media on zebrafish (Danio rerio) embryo development

The toxic effects of several species of fresh water cyanobacteria, notably Microcystis species and associated toxins, the microcystins, Anabaena species (anatoxin), Nodularia sp. (nodularin), and Cylindrospermopsis raciborskii (cylindrospermopsin), are well known. Little, however, is known about the effects of secondary metabolites other than alkaloids. Early life stage tests with zebrafish (Danio rerio) were used to detect bioactive properties of compounds released by healthy cyanobacteria (Fischerella ambigua), particularly on the early developmental stages of fish. This approach, using F. ambigua is probably most valuable as it shows the toxicity of healthy growing cyanobacteria. The effects of cyanobacterial secondary metabolites on the embryonic stages of fish are of considerable interest as many aquatic creatures, particularly fish, are unable to avoid the potential toxins that may be released by undesirable algal blooms or as a result of allelopathic effects. In the current study, the zebrafish (D. rerio) was used as a model experimental system to investigate the effects of ambigols A and C, tjipanazole D and C, 2,4-dichlorobenzoic acid, cell free culture media, and media extracts of a terrestrial/fresh water strain of the cyanobacterium F. ambigua on embryo development. Fish embryo tests performed with the cell free culture medium showed that after 3h of exposure to undiluted culture medium all fish embryos died. At a tenfold dilution the process of epiboly (formation of the gastrula) was retarded in all embryos, lesions were observed, and their general development was significantly arrested, finally followed by death. The same tests performed with extracts (dichloromethane, n-butanol, and residual cell free culture medium) of the cell free culture medium, ambigol A, ambigol C, 2,4-dichlorobenzoic acid and tjipanazole D showed only ambigol A to have an influence on zebrafish development at concentrations>or=1 mg/l (2.06 microM). After 55 h all embryos showed pectoral oedema, irregularly shaped fin folds, bent tails, and unusual circular neoplasms in the dorsal tail fin fold. Due to the high concentration of ambigol A used in this assay these effects were considered to be of minor importance when compared to those of the culture medium.     

Enhancement of dissolved phosphorus release from sediment to lake water by Microcystis blooms--an enclosure experiment in a hyper-eutrophic,subtropical Chinese lake

To clarify the possible influence of Microcystis blooms on the exchange of phosphorus (P) between sediment and lake water, an enclosure experiment was conducted in the hypereutrophic subtropical Lake Donghu during July-September 2000. Eight enclosures were used: six received sediment while two were sediment-free. In mid-August, Microcystis blooms developed in all the enclosures. There was a persistent coincidence between the occurrence of Microcystis blooms and the increase of both total P (TP) and soluble reactive P (SRP) concentrations in the water of the enclosures with sediments. In sediment-free enclosures, TP and SRP concentrations remained rather stable throughout the experiment, in spite of the appearance of Microcystis blooms. The results indicate that Microcystis blooms induced massive release of P from the sediment, perhaps mediated by high pH caused by intense algal photosynthesis, and/or depressed concentrations of nitrate nitrogen (NO3-N).