Biodegradation of the cyanobacterial toxin microcystin LR in natural water and biologically active slow sand filters

A bacterium (MJ-PV) previously demonstrated to degrade the cyanobacterial toxin microcystin LR, was investigated for bioremediation applications in natural water microcosms and biologically active slow sand filters. Enhanced degradation of microcystin LR was observed with inoculated (1 x 10(6) cell/mL) treatments of river water dosed with microcystin LR (>80% degradation within 2 days) compared to uninoculated controls. Inoculation of MJ-PV at lower concentrations (1 x 10(2)-1 x 10(5) cells/mL) also demonstrated enhanced microcystin LR degradation over control treatments. Polymerase chain reactions (PCR) specifically targeting amplification of 16S rDNA of MJ-PV and the gene responsible for initial degradation of microcystin LR (mlrA) were successfully applied to monitor the presence of the bacterium in experimental trials. No amplified products indicative of an endemic MJ-PV population were observed in uninoculated treatments indicating other bacterial strains were active in degradation of microcystin LR. Pilot scale biologically active slow sand filters demonstrated degradation of microcystin LR irrespective of MJ-PV bacterial inoculation. PCR analysis detected the MJ-PV population at all locations within the sand filters where microcystin degradation was measured. Despite not observing enhanced degradation of microcystin LR in inoculated columns compared to uninoculated column, these studies demonstrate the effectiveness of a low-technology water treatment system like biologically active slow sand filters for removal of microcystins from reticulated water supplies.     

Effects of low or medium-pressure UV irradiation on the release of intracellular microcystin

UV irradiation could be an alternative growth inhibition treatment against toxic Microcystis blooms in lakes. This study examined the effect of UV irradiation on the release of toxic intracellular microcystin. Conventional algicidal treatment (e.g. copper sulfate) was known to cause rapid release of intracellular microcystin and additional problems, but no studies have examined the case of UV treatment. A pure culture of Microcystis aeruginosa PCC 7806 was exposed to monochromatic low-pressure or polychromatic medium-pressure UV lamps. Irradiated pure culture suspension was subsequently incubated for 14 days under white light fluorescent lamps. During incubation, profiles of the number of cells, intracellular and extracellular microcystin concentration were determined. After the UV exposure, the number of cells gradually diminished; the net log cell reduction after 6 days of incubation was 1.6 log or 2.0 log for 600 mJ/cm2 of LP or MP UV irradiation, respectively. There were three findings for UV irradiation effects on the release of intracellular microcystin. First, because UV exposure can inhibit increases in the number of cells for about 6 days, it inhibited the increase of microcystin concentrations in water. Second, intracellular microcystin was gradually released into the surrounding water through a gradual reduction in the number of cells, preventing a rapid increase of microcystin concentration in water. Microcystin concentration in water was not significantly higher in UV-irradiated samples than non-UV-irradiated sample. Third, UV irradiation was able to degrade intracellular microcystin; it was reduced from 24.6 to 7.06 or 7.16 fg/cell by 600 mJ/cm2 of LP or MP UV irradiation, respectively. This contributed to reduce increases in microcystin concentrations in water. UV treatment can inhibit Microcystis growth and reduce intracellular microcystin content without rapid release of intracellular microcystin.     

Removal and fate of Cryptosporidium parvum, Clostridium perfringens and small-sized centric diatoms (Stephanodiscus hantzschii) in slow sand filters

The decimal elimination capacity (DEC) of slow sand filtration (SSF) for Cryptosporidium parvum was assessed to enable quantitative microbial risk analysis of a drinking water production plant. A mature pilot plant filter of 2.56m(2) was loaded with C. parvum oocysts and two other persistent organisms as potential surrogates; spores of Clostridium perfringens (SCP) and the small-sized (4-7microm) centric diatom (SSCD) Stephanodiscus hantzschii. Highly persistent micro-organisms that are retained in slow sand filters are expected to accumulate and eventually break through the filter bed. To investigate this phenomenon, a dosing period of 100 days was applied with an extended filtrate monitoring period of 150 days using large-volume sampling. Based on the breakthrough curves the DEC of the filter bed for oocysts was high and calculated to be 4.7log. During the extended filtrate monitoring period the spatial distribution of the retained organisms in the filter bed was determined. These data showed little risk of accumulation of oocysts in mature filters most likely due to predation by zooplankton. The DEC for the two surrogates, SCP and SSCD, was 3.6 and 1.8log, respectively. On basis of differences in transport behaviour, but mainly because of the high persistence compared to the persistence of oocysts, it was concluded that both spores of sulphite-reducing clostridia (incl. SCP) and SSCD are unsuited for use as surrogates for oocyst removal by slow sand filters. Further research is necessary to elucidate the role of predation in Cryptosporidium removal and the fate of consumed oocysts.     

利用富营养化水体生物膜降解微囊藻毒素研究

采用一种自行设计的简便生物膜采集装置，自然培养和收集富营养化湖泊——武汉东湖水体中不同深度（20cm和100cm）处的生物膜，研究不同水深、相同培养时间以及同一水深、不同培养时间所得生物膜在有氧条件下对微囊藻毒素（MC）的降解过程。结果表明，东湖水体生物膜对MC的降解经历了2～3d的延滞期，约11d后可使之降解完全；20cm水深处培养的生物膜对MC的降解速率快于100cm水深处培养的生物膜；随着附着生物膜载玻片数量的增多，对MC的降解速率加快；对MC的降解效果与生物膜的培养时间密切相关，按对MC降解速率的快慢排序为：14d〉21d〉7d；在降解MC的过程中，溶液的pH值持续上升。经分析，生物降解是去除东湖水体中MC的主要途径之一。     

Assessment of nitrification in groundwater filters for drinking water production by qPCR and activity measurement

In groundwater treatment for drinking water production, the causes of nitrification problems and the effectiveness of process optimization in rapid sand filters are often not clear. To assess both issues, the performance of a full-scale groundwater filter with nitrification problems and another filter with complete nitrification and pretreatment by subsurface aeration was monitored over nine months. Quantitative real-time polymerase chain reaction (qPCR) targeting the amoA gene of bacteria and archaea and activity measurements of ammonia oxidation were used to regularly evaluate water and filter sand samples. Results demonstrated that subsurface aeration stimulated the growth of ammonia-oxidizing prokaryotes (AOP) in the aquifer. Cell balances, using qPCR counts of AOP for each filter, showed that the inoculated AOP numbers from the aquifer were marginal compared with AOP numbers detected in the filter. Excessive washout of AOP was not observed and did not cause the nitrification problems. Ammonia-oxidizing archaea grew in both filters, but only in low numbers compared to bacteria. The cell-specific nitrification rate in the sand and backwash water samples was high for the subsurface aerated filter, but systematically much lower for the filter with nitrification problems. From this, we conclude that incomplete nitrification was caused by nutrient limitation.     

Isolation and identification of a novel microcystin-degrading bacterium from a biological sand filter

A novel bacterium capable of degrading two microcystin analogues, microcystin-LR and -LA (MCLR and MCLA), was isolated from a biological sand filter which was previously shown to effectively remove these toxins from source waters. Based on phylogenetic analysis of the 16S rRNA gene sequence, the isolated organism, LH21, most likely belonged to the genus Sphingopyxis and of the previously cultured species clustered with Sphingopyxis witflariensis. Using polymerase chain reaction (PCR), isolate LH21 was shown to contain homologues to each of the four genes, mlrA, mlrB, mlrC and mlrD previously associated with the degradation of MCLR by Sphingomonas sp. ACM-3962. Isolate LH21 was able to effectively degrade MCLR and MCLA in batch experiments under environmentally relevant conditions, with complete removal observed within 5 h after re-exposure of the toxins.     

Degradation of microcystin in sediments at oxic and anoxic, denitrifying conditions

The potent toxin microcystin is frequently released during cyanobacterial blooms in eutrophic waters and may impose a risk to human health, when surface water is used for drinking water. For removal of microcystin in surface waters, infiltration through sediment is commonly used. In the present study, mineralization of 14C-labelled microcystin (accumulation of 14CO(2)) and concentration changes (protein phosphatase inhibition assay) demonstrated that indigenous microorganisms in the sediment of a water recharge facility were capable of degrading microcystin. At oxic or microaerophilic (<2% O(2)) conditions, microcystin added to sediment slurries at 70 microg l(-1) was reduced to <20 microg l(-1) in 1-2 weeks, and less than 3 microg l(-1) after 7 weeks. At anoxic conditions (<0.3% O(2)) and with addition of nitrate, the degradation was significantly stimulated, reducing microcystin from 100 to <20 microg l(-1) within 1 day. The simultaneous production of N(2)O in the samples suggests that the microcystin degradation was coupled to dissimilative nitrate reduction (denitrification). Since aquifers and sediments beneath drinking water reservoirs often are anoxic, nitrate respiration may be an important process in removal and detoxification of microcystin.     

Detection of enteric viruses, Giardia and Cryptosporidium in two different types of drinking water treatment facilities

In this study, two types of drinking water treatment facilities (two conventional drinking water treatment plants (DWTPs) and two compact units (Cus)) were compared referring to their production capacity. Water samples were collected from three main points: (a) different water treatment steps (b) washings of sand filters and (c) distribution system at different distances from the water treatment plants. Both viruses and protozoa were concentrated from each water sample by adsorption and accumulation on the same nitrocellulose membrane filters (0.45 microm pore size). Enteroviruses were detected by plaque infectivity assay in BGM cells and HAV, HEV and Norovirus were detected by RT-PCR. Giardia and Cryptosporidium were detected by conventional staining methods and PCR. The results revealed that enterovirus load at the intake ranged between 10-15 PFU/L for the two compact units and between 4.5 and 75 PFU/L for the two conventional DWTPs. The virus load in distribution system of the first type DWTPs at 1 km from the plant was the same as that of the intake. Viruses in the other type of treatment plants CUs at 1, 5 and 7 km, were much reduced. Investigation of raw water sediments of the two DWTPs showed enterovirus counts between 12 and 17.5 PFU/L. Virus count was reduced in sand of filters after washing. Giardia cysts were equally detected by microscopy and PCR in only intake samples of EL-Hawamdia CU (33.3%) and Meet Fares DWTP (50%). Cryptosporidium oocysts were equally detected by microscopy and PCR in intake samples of Abo EL-Nomros CU (100%), EL-Hawamdia CU (66.7%) and Fowa DWTP (50%). At Meet Fares DWTP three positive intake samples for Cryptosporidium were detected by PCR, compared with only two positive samples by microscopy. Giardia cysts and Cryptosporidium oocysts were detected in raw water sediment and sand of filters before washing. Only one sample from Meet Fares DWTP sand of filters after washing was positive for both Giardia and Cryptosporidium. It can be concluded that the poor microbial quality of the water may be due to improper operational skills and management of the various water treatment plants (especially at the two high capacity treatment plants).     

Reduction in microcystin concentrations in large and shallow lakes: water and sediment-interface contributions

Blooms of cyanobacteria, or blue-greens, are known to produce chemicals, such as microcystins, which can be toxic to aquatic and terrestrial organisms. Although previous studies have examined the fate of microcystins in freshwater lakes, primary elimination pathways and factors affecting degradation and loss have not been fully explained. The goal of the present study was to explore sources of algal toxins and investigate the distribution and biodegradation of microcystins in water and sediment through laboratory and field analyses. Water and sediment samples were collected monthly from several locations in Lake Taihu from February 2005 to January 2006. Samples were analyzed for the presence of microcystin. Water and sediment were also used in laboratory studies to determine microcystin degradation rates by spiking environmental samples with known concentrations of the chemical and observing concentration changes over time. Some water samples were found to efficiently degrade microcystins. Microcystin concentrations dropped faster in water collected immediately above lake sediment (overlying water). Degradation in sediments was higher than in water. Based on spatial distribution analyses of microcystin in Lake Taihu, higher concentrations (relative to water concentrations) of the chemical were found in lake sediments. These data suggest that sediments play a critical role in microcystin degradation in aquatic systems. The relatively low levels of microcystins found in the environment are most likely due to bacterial biodegradation. Sediments play a crucial role as a source (to the water column) of bio-degrading bacteria and as a carbon-rich environment for bacteria to proliferate and metabolize microcystin and other biogenic toxins produced by cyanobacteria. These, and other, data provide important information that may be applied to management strategies for improvement of water quality in lakes, reservoirs and other water bodies.     

Assessing granular media filtration for the removal of chemical contaminants from wastewater

Granular media filtration was evaluated for the removal of a suite of chemical contaminants that can be found in wastewater. Laboratory- and pilot-scale sand and granular activated carbon (GAC) filters were trialled for their ability to remove atrazine, estrone (E1), 17α-ethynylestradiol (EE2), N-nitrosodimethylamine (NDMA), N-nitrosomorpholine (NMOR) and N-nitrosodiethylamine (NDEA). In general, sand filtration was ineffective in removing the contaminants from a tertiary treated wastewater, with the exception of E1 and EE2, where efficient removals were observed after approximately 150 d. Batch degradation experiments confirmed that the removal of E1 was through biological activity, with a pseudo-first-order degradation rate constant of 7.4 × 10⁻³ h⁻¹. GAC filtration was initially able to effectively remove all contaminants; although removals decreased over time due to competition with other organics present in the water. The only exception was atrazine where removal remained consistently high throughout the experiment. Previously unreported differences were observed in the adsorption of the three nitrosamines, with the ease of removal following the trend, NDEA > NMOR > NDMA, consistent with their hydrophobic character. In most instances the removals from the pilot-scale filters were generally in agreement with the laboratory-scale filter, suggesting that there is potential in using laboratory-scale filters as monitoring tools to evaluate the performance of pilot- and possibly full-scale sand and GAC filters at wastewater treatment plants.     

Discriminating and assessing adsorption and biodegradation removal mechanisms during granular activated carbon filtration of microcystin toxins

Microcystins are cyanobacterial toxins that are problematic for water authorities due to their resistance to conventional water treatment. Granular activated carbon (GAC) filtration has been shown to be effective in removing microcystin from water using both adsorption and biodegradation removal mechanisms; however, little is known regarding which removal mechanism predominates and to what extent. In this study, microcystin removal due to adsorption and biodegradation in GAC filtration were discriminated and assessed by commissioning three parallel laboratory columns, including a sterile GAC column, a conventional GAC column and a sand column. The results demonstrate that biodegradation is an efficient removal mechanism once it commences and that the rate of biodegradation was dependent upon temperature and initial bacterial concentration. Adsorption of microcystins was prevalent during the initial stages of the GAC columns and was modelled using the homogeneous surface diffusion model (HSDM). The HSDM provided evidence that an active biofilm present on the surface of the conventional GAC hindered adsorption of microcystin compared with the sterile GAC with no active biofilm. Up to 70% removal of microcystin-LR was still observed after 6 months of operation of the sterile GAC column, indicating that adsorption still played a vital role in the removal of this toxin.     

Slow sand filters effectively reduce Phytophthora after a pathogen switch from Fusarium and a simulated pump failure

Slow sand filtration has been shown to effectively reduce Phytophthora zoospores in irrigation water. This experiment tested the reduction of Phytophthora colony forming units (CFUs) by slow sand filtration systems after switching the pathogen contaminating plant leachate from Fusarium to Phytophthora and the resilience of the system to a short period without water, as might be caused by a pump failure. The slow sand filtration system greatly reduced Phytophthora CFUs and transmission after switching the pathogens. In addition, Phytophthora reduction by the slow sand filter was equally effective before and after the simulated pump failure. Reduction of Fusarium was not seen by the SSFs, before or after the simulated pump failure. The results suggest that slow sand filters are effective at reducing larger organisms, such as Phytophthora zoospores, even after a pump failure or a change in pathogens.     

Transport and fate of Cryptosporidium parvum oocysts in intermittent sand filters.

The transport potential of Cryptosporidium parvum (C. parvum) through intermittent, unsaturated, sand filters used for water and wastewater treatment was investigated using a duplicated, 2³ factorial design experiment performed in bench-scale, sand columns. Sixteen columns (dia=15 cm, L=60 cm) were dosed eight times daily for up to 61 days with 65,000 C. parvum oocysts per liter at 15°C. The effects of water quality, media grain size, and hydraulic loading rates were examined. Effluent samples were tested for pH, turbidity, and oocyst content. C. parvum effluent concentrations were determined by staining oocysts on polycarbonate filters and enumerating using epifluorescent microscopy. At completion, the columns were dismantled and sand samples were taken at discrete depths within the columns. These samples were washed in a surfactant solution and the oocysts were enumerated using immunomagnetic separation techniques.

The fine-grained sand columns (d₅₀=0.31 mm) effectively removed oocysts under the variety of conditions examined with low concentrations of oocysts infrequently detected in the effluent. Coarse-grained media columns (d₅₀=1.40 mm) yielded larger numbers of oocysts which were commonly observed in the effluent regardless of operating conditions. Factorial design analysis indicated that grain size was the variable which most affected the oocyst effluent concentrations in these intermittent filters. Loading rate had a significant effect when coarse-grained media was used and lesser effect with fine-grained media while the effect of feed composition was inconclusive. No correlations between turbidity, pH, and effluent oocyst concentrations were found. Pore-size calculations indicated that adequate space for oocyst transport existed in the filters. It was therefore concluded that processes other than physical straining mechanisms are mainly responsible for the removal of C. parvum oocysts from aqueous fluids in intermittent sand filters used under the conditions studied in this research.     

Dynamics of microcystin production and quantification of potentially toxigenic Microcystis sp. using real-time PCR

Cyanobacterial blooms in eutrophied water body are generally composed of various genotypes with or without microcystin-producing genes (mcy gene cluster). Thus there is a need for quantification of potent toxin producing strains. The present study aimed at identifying microcystin variants and its producer strains in Durgakund pond, Varanasi, India, based on quantification of cpcBA-IGS and mcyA (condensation domain) genes using real-time PCR and LC-MS. Increase in microcystin concentrations was correlated with increase in mcyA copy number and the level of pigments (chlorophyll a, phycocyanin and carotenoids). Also, selected environmental factors (water temperature, light irradiance, rainfall, pH, N and P) and the concentration of microcystin variants (MC-LR, -RR and -YR) were also assessed in samples during May 2010 to April 2011 to establish the possible correlation among these parameters. Nutrients favored cyanobacterial bloom but it could not be correlated with the levels of microcystin variants and seemed to be geographically specific. Microcystis sp. dominant in the pond comprised potentially toxigenic cells. The ratio of potentially toxigenic Microcystis sp. to that of total Microcystis sp. ranged from 0% to 14%. Such studies paved the way to identify and quantify the most potent microcystin producer in the tropical aquatic body.     

Are meiofauna transient or resident in sand filters of marine aquariums?

A paradoxical situation was found in the sand filters of a cold marine mesocosm: meiofaunal masses which were large enough to inhibit the mineralization and nitrification processes coexisted with nitrogen cycling bacteria. To test whether the copepod-dominated meiofauna were resident and actively feeding or transient and carried passively through the sand filters, residence times (RTs) were measured for various meiofaunal groups in a newly started filter and in a long established one. Most meiofauna colonized the newly started filter in less than 6 h, but their RTs were less than 24 h. In contrast, RTs were 147d for halacarids, 291 d for harpacticoid copepods and 1228d for nematodes in the long established filter. Mesocosm periphyton. which occupied a large fraction of the mesocosm surface area and was characterized by high meiofaunal densities, was probably the main source of meiofauna in the sand filters. Pool sediments, consisting of gravel or sand, were second to periphyton and contributed hydrozoans and mesopsammic species to the filters. The small copepod Pseudonychocamptus proximus progressively replaced the large Tisbe furcata in sand filters during the fall of 1995 and was responsible for the large increase in meiofaunal biomass observed after spring 1996. This replacement was presumably facilitated by the copepod size selection process operated by the filters. Large copepods were retained by the surface layer of sand or brought up by the backwash water and then exit the mesocosm through the drain. High meiofaunal populations did not significantly affect nitrogen cycling bacteria in sand filters probably because meiofauna also fed on other abundant food sources which were carried in by the water flow.     

Modeling antimicrobial contaminant removal in slow sand filtration

Slow sand filters are used in rural regions where source water may be subjected to antimicrobial contaminant loads from waste discharges and diffuse pollution. A numerical model (LETA) was derived to calculate aqueous antimicrobial concentrations through time and depth of a slow sand filter and estimate accumulating contaminant mass in the schmutzdecke. Input parameters include water quality variables easily quantified by water system personnel and published adsorption, partitioning, and degradation coefficients. Simulation results for the tetracycline, quinolone, and macrolide classes of antimicrobials suggested greater than 3-log removal from 1 microg/L influent concentrations within the top 40 cm of the sand column, with schmutzdecke antimicrobial concentrations comparable to other land-applied waste biosolids. A 60-day challenge experiment injecting 1 microg/L tylosin to a pilot slow sand filter showed an average 0.1mg/kg of the antimicrobial remaining in the schmutzdecke layer normally removed during filter maintenance, and this value was the same order of magnitude as the sorbed concentration predicted by the LETA model.     

原水预臭氧化对常规处理工艺的影响

杭州南星水厂的原水经预臭氧处理后,砂滤池除铁、锰作用得以加强并发挥了生物除氨氮作用,但反冲时滤砂难以洗净,池壁还滋生了青苔;混凝效果得到强化,矾耗降低.通过生产性试验分析了原水预臭氧化对常规工艺的不利影响,认为原水由预氯化变为预臭氧化后,生物砂滤池宜改为气水反冲洗方式,斜管沉淀池和滤池宜采用遮阳方法防止池内滋生藻类,另外从成本方面考虑,用投加臭氧来降低矾耗是不经济的.     

Transport and fate of Cryptosporidium parvum oocysts in intermittent sand filters

The transport potential of Cryptosporidium parvum (C. parvum) through intermittent, unsaturated, sand filters used for water and wastewater treatment was investigated using a duplicated, 2³ factorial design experiment performed in bench-scale, sand columns. Sixteen columns (dia=15 cm, L=60 cm) were dosed eight times daily for up to 61 days with 65,000 C. parvum oocysts per liter at 15°C. The effects of water quality, media grain size, and hydraulic loading rates were examined. Effluent samples were tested for pH, turbidity, and oocyst content. C. parvum effluent concentrations were determined by staining oocysts on polycarbonate filters and enumerating using epifluorescent microscopy. At completion, the columns were dismantled and sand samples were taken at discrete depths within the columns. These samples were washed in a surfactant solution and the oocysts were enumerated using immunomagnetic separation techniques.The fine-grained sand columns (d₅₀=0.31 mm) effectively removed oocysts under the variety of conditions examined with low concentrations of oocysts infrequently detected in the effluent. Coarse-grained media columns (d₅₀=1.40 mm) yielded larger numbers of oocysts which were commonly observed in the effluent regardless of operating conditions. Factorial design analysis indicated that grain size was the variable which most affected the oocyst effluent concentrations in these intermittent filters. Loading rate had a significant effect when coarse-grained media was used and lesser effect with fine-grained media while the effect of feed composition was inconclusive. No correlations between turbidity, pH, and effluent oocyst concentrations were found. Pore-size calculations indicated that adequate space for oocyst transport existed in the filters. It was therefore concluded that processes other than physical straining mechanisms are mainly responsible for the removal of C. parvum oocysts from aqueous fluids in intermittent sand filters used under the conditions studied in this research.     

Effect of plants and filter materials on bacteria removal in pilot-scale constructed wetlands

Due to the lack of testing units or appropriate experimental approaches, only little is known about the removal of bacteria in constructed wetlands. However, improved performance in terms of water sanitation requires a detailed understanding of the ongoing processes. Therefore, we analyzed the microbial diversity and the survival of Enterobacteriaceae in six pilot-scale constructed wetland systems treating domestic wastewater: two vertical sand filters, two vertical expanded clay filters and two horizontal sand filters (each planted and unplanted). Samples were taken from the in- and outflow, from the rhizosphere, and from the bulk soil at various depths. Colony-forming units of heterotrophic bacteria and coliforms were analyzed and the removal of bacteria between the in- and outflow was determined to within 1.5-2.5 orders of magnitude. To access the taxon-specific biodiversity of potential pathogens in the filters and to reduce the complexity of the analysis, specific primers for Enterobacteriaceae were developed. While performing PCR-SSCP analyses, a pronounced decrease in diversity from the inflow to the outflow of treated wastewater was observed. No differences were observed between the bulk soil of planted and unplanted vertical filters. Some bands appeared in the rhizosphere that were not present in the bulk soil, indicating the development of specific communities stimulated by the plants. The fingerprinting of the rhizosphere of plants grown on sand or expanded clay exhibited many differences, which show that different microbial communities exist depending on the soil type of the filters. The use of the taxon-specific primers enabled us to evaluate the fate of the Enterobacteriaceae entering the wetlands and to localize harboring in the rhizosphere. The most abundant bands of the profiles were sequenced: Pantoea agglomerans was found in nearly all samples from the soil but not in the effluent, whereas Citrobacter sp. could not be removed by the horizontal unplanted sand and vertical planted expanded clay filters. These results show that the community in wetland system is strongly influenced by the filtration process, the filter material and the plants.     

An effective pathway for the removal of microcystin LR via anoxic biodegradation in lake sediments

Aerobic biodegradation has been considered to be the main attenuation mechanism for microcystins, but the role of anoxic biodegradation remains unclear. We investigated the potential for anoxic biodegradation of microcystin and the effects of environmental factors on the process through a series of well-controlled microcosm experiments using lake sediments as inocula. Microcystin LR could be degraded anoxically from 5 mg L⁻¹ to below the detection limit at 25 °C within 2 days after a lag phase of 2 days. The rate was highly dependent on temperature, with a favorable temperature range of 20-30 °C. The addition of glucose or low levels of NH₄-N had no effect on the anoxic biodegradation of microcystin, whereas the addition of NO₃-N significantly inhibited the biodegradation at all experimental concentrations, and the inhibition increased with increasing amount of NO₃-N-amended. Adda (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-deca-4,6-dienoic acid), a previously reported nontoxic product of aerobic degradation of microcystin, was identified as the anoxic biodegradation product. This is the first report of Adda as a degradation product of microcystin under anoxic conditions. No other product containing Adda residue was detected during the anoxic degradation of microcystin. These results strongly indicated that anoxic biodegradation is an effective removal pathway of microcystin in lake sediments, and represents a significant bioremediation potential.