Microcystins Induce Morphological and Physiological Changes in Selected Representative Phytoplanktons

Dissolved microcystins (MC) are regularly present in water dominated by microcystin-producing, bloom-forming cyanobacteria. In vitro experiments with environmentally feasible concentrations (5 × 10⁻⁷ M) of the three most common microcystins, MC-LR, -RR, and -YR, revealed that they influence the metabolism of different representative phytoplanktons. At light intensities close to the cyanobacterial bloom environment (50 μmol m⁻² s⁻¹), they produce morphological and physiological changes in both microcystin-producing and nonproducing Microcystis aeruginosa strains, and also have similar effects on the green alga Scenedesmus quadricauda that is frequently present in cyanobacterial blooms. All three microcystin variants tested induce cell aggregation, increase in cell volume, and overproduction of photosynthetic pigments. All three effects appear to be related to each other, but are not necessarily caused by the same mechanism. The biological activity of microcystins toward the light-harvesting complex of photobionts can be interpreted as a signal announcing the worsening of light conditions due to the massive proliferation of cyanobacteria. Although the function of microcystins is still unknown, it is evident that they have numerous effects on phytoplankton organisms in nature. These effects depend on the individual organism as well as on the various intracellular and extracellular signaling pathways. The fact that dissolved microcystins also influence the physiology of microcystin-producing cyanobacteria leads us to the conclusion that the role of microcystins in the producing cells differs from their role in the water environment.     

Extract of <Emphasis Type="Italic">Microcystis</Emphasis> water bloom affects cellular differentiation in filamentous cyanobacterium <Emphasis Type="Italic">Trichormus variabilis</Emphasis> (Nostocales,Cyanobacteria)

Cyanobacteria produce a wide spectrum of biologically active compounds such as microcystins, whose effects on photoautotrophic organisms and role in the aquatic ecosystems have been most intensively discussed and studied. In our study, we examined effects of semipurified Microcystis extract containing microcystins (0.2–20 nM corresponding to 0.2–20 μg L⁻¹) on age-induced cell differentiation of filamentous cyanobacterium Trichormus variabilis. The heterocyst and akinete formation was significantly decreased after exposure to extract containing 2 or 20 nM of microcystins within 10 days of exposure. To our knowledge, this is the first information about effects of metabolites produced by planktonic cyanobacteria on cell differentiation in filamentous cyanobacteria. Since the effects were induced by environmentally relevant concentrations of microcystins, our observations may indicate not only that microcystins or other bioactive peptides could affect conservation, overwintering, and nitrogen-fixation in filamentous cyanobacteria under environmental conditions but also contribute to the understanding of possible signaling function of cyanobacterial metabolites.     

Heme oxygenase 2 of the cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 is induced under a microaerobic atmosphere and is required for microaerobic growth at high light intensity

Cyanobacteria, red algae, and cryptomonad algae utilize phycobilin chromophores that are attached to phycobiliproteins to harvest solar energy. Heme oxygenase (HO) in these organisms catalyzes the first step in phycobilin formation through the conversion of heme to biliverdin IXα, CO, and iron. The Synechocystis sp. PCC 6803 genome contains two open reading frames, ho1 (sll1184) and ho2 (sll1875), whose products have in vitro HO activity. We report that HO2, the protein encoded by ho2, was induced in the cells growing under a microaerobic atmosphere [0.2% (v/v) O₂], whereas HO1 was constitutively expressed under both aerobic and microaerobic atmospheres. Light intensity did not have an effect on the expression of both the HOs. Cells, in which ho2 was disrupted, were unable to grow microaerobically at a light intensity of 40 μmol m⁻² s⁻¹, but did grow microaerobically at 10 μmol m⁻² s⁻¹ light intensity. These cells grew normally aerobically at both light intensities. Comparative analysis of complete cyanobacterial genomes revealed that possession of two HOs is common in cyanobacteria. In phylogenetic analysis of their amino acid sequences, cyanobacterial HO1 and HO2 homologs formed distinct clades. HO sequences of cyanobacteria that have only one isoform were most similar to HO1 sequences. We propose that HO2 might be the more ancient HO homolog that functioned under low O₂ tension, whereas the derived HO1 can better accommodate increased O₂ tension in the environment.     

Useful Carriers for Cyanobacteria: Their Response to Cyanobacterial Growth, Acetylene-Reductase Activity, Cyanobacterial Grazers and Paddy Yield in Calcareous Soil

Summary Snails and nematodes, the potential cyanobacterial grazers, differ in their choice for cyanobacterial diet. Snails prefer non-mucilaginous forms while nematodes prefer mucilaginous forms. Such differences in feeding choice between the cyanobacteria suggests that it may not be possible to select strains of diazotrophic cyanobacteria that are resistant to all grazers. The potential consumption of cyanobacteria at an average field density of 20,000 snails ha⁻¹ was estimated to be about 50 kg (fresh weight) ha⁻¹ day⁻¹. Dorylamus sp. was most dominant nematode associated with cyanobacterial consumption. Phytoextracts of neem (Azadirachta indica), bel (Aegle marmelos) and tobacco (Nicotiana tabacum) were effective in controlling these cyanobacterial grazers. The minimum concentration of neem, bel and tobacco phytoextract in water for 100 % mortality of snails were 0.1, 2.0 and 0.05%, respectively. However, trepellent level was only 0.01% for neem and tobacco phytoextract. Complete mortality of nematode (Dorylamus spp.) required a higher concentration level (2%) even in the most effective tobacco phytoextract. Lower levels of phytoextract (0.1%) were found to stimulate growth and nitrogen fixation of cyanobacteria. Application of these plant biomasses resulted in significant increase in cyanobacterial acetylene-reducing activity (ARA) and rice yield and a significant decrease in snail and nematode population. Augmentation of cyanobacterial acetylene-reducing activity was two to three times higher in comparison to the control in both the years of experimentation. Rice yield also increased between 3.8 and 58.5% over the control, depending on the quantity and nature of plant biomass. Tobacco waste was significantly superior in comparison to neem and bel biomass as carrier of cyanobacterial culture.     

Eco-physiological responses of nitrogen-fixing cyanobacteria to light

The eco-physiological responses of three nitrogen-fixing cyanobacteria (N-fixing cyanobacteria), Aphanizomenon gracile, Anabaena minderi, and Ana. torques-reginae, to light were assessed under nutrient saturation. The N-fixing cyanobacteria were isolated into monocultures from a natural bloom in a shallow colored lake and their growth irradiance parameters and pigment composition were assessed. The different ecological traits related to light use (μ_max, α, I _k) suggest that these N-fixing cyanobacteria are well adapted to low light conditions at sufficient nutrients, yet interspecific differences were observed. Aphanizomenon gracile and Anabaena minderi had high relative growth rates at low irradiances (ca. 70% of those in high light), low half saturation constant for light-limited growth (I _k < 9.09 μmol photon m⁻² s⁻¹) and high efficiency (α < 0.11 day⁻¹ μmol photon⁻¹ m² s). Conversely, Ana. torques-reginae showed poorer light competitiveness: low relative growth rates at low irradiances (ca. 40% of those in high light), low α (0.009 day⁻¹ μmol photon⁻¹ m² s) and higher I _k (35.5 μmol photon m⁻² s⁻¹). Final densities in Aphanizomenon gracile and Anabaena minderi reached bloom densities at irradiances above 30 μmol photon m⁻² s⁻¹ with different hierarchy depending on irradiance, whereas Ana. torques-reginae never achieved bloom densities. All species had very low densities at irradiances ≤17 μmol photon m⁻² s⁻¹, thus no N-fixing blooms would be expected at these irradiances. Also, under prolonged darkness and at lowest irradiance (0 and 3 μmol photon m⁻² s⁻¹) akinetes were degraded, suggesting that in ecosystems with permanently dark sediments, the prevalence of N-fixing cyanobacteria should not be favored. All species displayed peaks of phycocyanin, but no phycoeritrin, probably due to the prevailing red light in the ecosystem from which they were isolated.     

Incidence of microcystin‐producing cyanobacteria in Lake Tana,the largest waterbody in Ethiopia

The occurrence of toxic cyanobacterial blooms is a serious problem for fast‐developing countries in Africa, such as Ethiopia, that are struggling with significant degradation of the natural environment and limited access to water of good quality. Research undertaken on Lake Tana in Ethiopia between 2009 and 2011 was intended to assess the seasonal threat from cyanobacteria and to select methods for tracking of this threat in the future. The cyanobacterial genus Microcystis was found to be present throughout the monitoring period, and M. aeruginosa was determined as the dominant species. Moreover, in all samples, toxigenic cyanobacteria with the potential to produce microcystins were detected. High levels of microcystins, ranging from 0.58 to 2.65 μg L^?1, were detected each November, which indicates that in the postrainy season, water usage should be limited. The correlation between concentrations of chlorophyll‐a and microcystins suggested that chlorophyll‐a could be used as an indicator of the potential presence of cyanobacterial‐derived hepatotoxins in Lake Tana in the future. Furthermore, for quick quantitative confirmation of the presence of microcystins, a simple and rapid ELISA test was recommended.     

Variation of Microcystin Content of Cyanobacterial Blooms and Isolated Strains in Lake Grand-Lieu (France)

Abstract Cyanobacterial blooms were sampled at five locations in Lake Grand-Lieu on seven different occasions during May–October 1994. Strains of Microcystis aeruginosa and Anabaena circinalis were isolated from the samples. Microcystins were detected in freeze-dried field samples and the isolated strains by HPLC. The toxins were present in the blooms sampled between June and October. The microcystin content in the blooms varied with site and time, from undetectable concentrations to 0.23 mg g⁻¹. The highest concentrations of microcystin were found in blooms sampled in September. Microcystin-LR and microcystins with retention times close to the retention time of [Dha⁷]microcystin-RR (probably varieties of microcystin-RR) were found in the field samples. Sixteen of the 98 isolated M. aeruginosa strains and 2 of the 24 A. circinalis strains produced microcystins. The total amount of microcystins varied from undetectable concentrations to 5.06 mg g⁻¹ in the M. aeruginosa isolates, and from undetectable concentrations to 1.86 mg g⁻¹ in the A. circinalis strains. Microcystin-LR was the main toxin found in strains of M. aeruginosa, but was not present in strains of A. circinalis. Both microcystin-producing strains and strains that did not produce microcystin coexisted in the bloom samples. Received: 23 January 1997; Accepted: 25 March 1997     

Modulation of biocidal activity of <Emphasis Type="Italic">Calothrix</Emphasis> sp. and <Emphasis Type="Italic">Anabaena</Emphasis> sp. by environmental factors

An investigation was undertaken to evaluate a set of cyanobacterial strains in terms of production of biocidal compounds exhibiting allelochemical and fungicidal properties. Two cyanobacterial strains — Anabaena sp. and Calothrix sp. were selected for further investigation, on the basis of their larger inhibition zones on the lawn of Synechocystis and Synechococcus sp. and two phytopathogenic fungi — Rhizoctonia bataticola and Pythium debaryanum. The diameter of the inhibition zone was largest when extracellular filtrates of the two cultures incubated at high light intensity (90–100 μmol photons m⁻² s⁻¹) and temperature (40 ± 2 °C) or grown in medium containing two-folds higher P (1.4 mg/L, as compared to 0.7 mg/L in BG 11 medium) were taken. A pH of 8 was the most optimal for both strains, in terms of growth and biocidal activity. Partial purification of ethyl acetate extract using TLC, followed by GLC revealed a single peak. This study highlights the importance of environmental factors in aggravating or reducing the toxic effects of these harmful cyanobacteria and their potential as a biocontrol agent.     

Efficacy of New Inexpensive Cyanobacterial Biofertilizer Including its Shelf-life

Summary Four cyanobacterial inoculants all significantly increased grain and straw yield of rice either alone or in combination with chemical fertilizer. A saving of 25 kg N ha⁻¹ can be attained through cyanobacterial fertilization. Tobacco waste-based cyanobacterial biofertilizer was best in performance. Cyanobacterial acetylene reducing activity in vivo varied from 144 to 255 μmol C₂H₄ m⁻² h⁻¹ in different treatments, being highest for tobacco-based cyanobacterial biofertilizer integrated with 50% chemical N. The nutrient balance for total N, available N, total P and available P was found positive in biofertilizer- and chemical fertilizer-treated plots. The total and available K showed negative balance in all the treatments. The shelf-life of cyanobacterial biofertilizer can be augmented by selecting translucent packing material, dry mixing and paddy straw as a carrier. Dry mixing and a mixing ratio of 50:50 (carrier:cyanobacteria) gave better inoculum loading and shelf-life. Decrease in cyanobacterial population was least in dried cyanobacterial flacks, indicating a possibility of developing cyanobacterial biofertilizer without carrier mixing at the time of production.     

Effects of the inoculation of cyanobacteria on the microstructure and the structural stability of a tropical soil

Cyanobacteria are widespread photosynthetic microorganisms among which some are able to fix atmospheric nitrogen. We investigated the impact of indigenous cyanobacteria strains (Nostoc) inoculation on physical characteristics of poorly aggregated soils from Guquka (Eastern Cape, South Africa). The soil aggregates (3–5 mm) were arranged into a layer of 10–20 mm thick, and sprayed with cyanobacteria solution. Subsequently the inoculated and un-inoculated samples were incubated (30°C, 80% humidity, continuous illumination at 100 μmol m⁻² s⁻¹). Their micromorphological characteristics and aggregate stability were investigated, after 1, 2, 3, 4 and 6 weeks of incubation, by using high resolution Cryo-SEM and aggregate breakdown tests. Micromorphological investigations revealed that the surface of un-inoculated samples remained uncovered, while the inoculated samples were partially covered by cyanobacteria material after one week of incubation. A dense superficial network of cyanobacterial filaments and extracellular polymer secretions (EPS) covered their surface after 4 and 6 weeks of incubation. Organo-mineral aggregates comprising cyanobacterial filaments and EPS were observed after 6 weeks of incubation. The results of aggregate breakdown tests showed no significant difference between un-inoculated samples after 1, 2, 3, 4 or 6 weeks, while they revealed improvement of aggregate stability for inoculated samples. The improvement of aggregate stability appeared in a short while following inoculation and increased gradually with time and cyanobacteria growth. The increase in aggregate stability is likely related to the changes induced in micromorphological characteristics by cyanobacterial filaments and EPS. It reflects the effect of coating, enmeshment, binding and gluing of aggregates and isolated mineral particles by cyanobacteria material. Our study presents new data demonstrating the improvement of soil physical quality in a few weeks after cyanobacteria inoculation. The interaction of the inocula and other biotic components is worthy of study before field application of cyanobacteria.     

Effects of bacillamide and newly synthesized derivatives on the growth of cyanobacteria and microalgae cultures

The antialgal activity of newly synthesized bacillamides against several cyanobacteria and microalgae isolates was screened using a rapid 96-well microplate bioassay. Cultures were exposed to serial dilutions of each bacillamide derivative (0–160 μg mL⁻¹) in the microplate wells and daily optical measurements were used to estimate growth over a 216 h period. Inhibition values (%) were calculated from the estimated growth curves and inhibitory concentrations (IC₅₀-216 h) were obtained from the sigmoidal inhibition curves fitted by regression analysis. The effects of bacillamides on cell morphology and ultrastructure were also analysed by light and transmission electron microscopy. In general, the toxic cyanobacteria Microcystis aeruginosa, Aphanizomenon gracile, Anabaena circinalis and Anabaenopsis circularis were much more sensitive to bacillamides then the chlorophytes Ankistrodesmus falcatus and Scenedesmus obliquus. However, clear signs of morphological and ultrastructural changes induced by bacillamide were observed on both cyanobacteria and chlorophytes. Other cyanobacteria, namely the nostocalean Nodularia spumigena and the oscillatorialeans Leptolyngbya sp. and Planktothrix rubescens, exhibit higher tolerances to bacillamides, similar to that shown by different eukaryotic microalgae. Diatoms, on the other hand, proved to be quite as sensitive to most bacillamides as the most affected cyanobacteria. The properties of 5-iodo-Bacillamide (algicide or algistatic) were further investigated. This compound acted as an algistactic agent against eukaryotic algae and, depending on its concentration, acted as either an algicide or algistactic agent against most of the cyanobacteria tested. Although bacillamides cannot be considered as broad spectrum cyanobacterial algicides, different bacillamides might be of use in selectively controlling the growth of particular species of cyanobacteria.     

Dinitrogen-Fixing Cyanobacteria in Microbial Mats of Two Shallow Coral Reef Ecosystems

Dinitrogen-fixing organisms in cyanobacterial mats were studied in two shallow coral reef ecosystems: La Reunion Island, southwestern Indian Ocean, Sesoko (Okinawa) Island, and northwestern Pacific Ocean. Rapidly expanding benthic miniblooms, frequently dominated by a single cyanobacterial taxon, were identified by microscopy and molecular tools. In addition, nitrogenase activity by these blooms was measured in situ. Dinitrogen fixation and its contribution to mat primary production were calculated using ¹⁵N₂ and ¹³C methods. Dinitrogen-fixing cyanobacteria from mats in La Reunion and Sesoko showed few differences in taxonomic composition. Anabaena sp. among heterocystous and Hydrocoleum majus and Symploca hydnoides among nonheterocystous cyanobacteria occurred in microbial mats of both sites. Oscillatoria bonnemaisonii and Leptolyngbya spp. occurred only in La Reunion, whereas Hydrocoleum coccineum dominated in Sesoko. Other mats dominated by Hydrocoleum lyngbyaceum, Phormidium laysanense, and Trichocoleus tenerrimus occurred at lower frequencies. The 24-h nitrogenase activity, as measured by acetylene reduction, varied between 11 and 324 nmoles C₂H₂ reduced μg⁻¹ Chl a. The highest values were achieved by heterocystous Anabaena sp. performed mostly during the day. Highest values for nonheterocystous cyanobacteria were achieved by H. coccineum mostly during the night. Daily nitrogen fixation varied from nine (Leptolyngbya) to 238 nmoles N₂ μg⁻¹ Chl day⁻¹ (H. coccineum). Primary production rates ranged from 1,321 (S. hydnoides) to 9,933 nmoles C μg⁻¹ Chl day⁻¹ (H. coccineum). Dinitrogen fixation satisfied between 5% and 21% of the nitrogen required for primary production.     

To bloom or not to bloom: contrasting responses of cyanobacteria to recent heat waves explained by critical thresholds of abiotic drivers

Past heat waves are considered harbingers of future climate change. In this study, we have evaluated the effects of two recent Central European summer heat waves (2003 and 2006) on cyanobacterial blooms in a eutrophic, shallow lake. While a bloom of cyanobacteria developed in 2006, consistent with our expectations, cyanobacterial biomass surprisingly remained at a record-low during the entire summer of 2003. Critical thresholds of abiotic drivers extracted from the long-term (1993–2007) data set of the studied lake using classification tree analysis (CTA) proved suitable to explain these observations. We found that cyanobacterial blooms were especially favoured in 2006 because thermal stratification was critically intense (Schmidt stability >44 g cm cm⁻²) and long-lasting (>3 weeks). Our results also suggest that some cyanobacterial species (Anabaena sp.) benefitted directly from the stable water column, whereas other species (Planktothrix sp.) took advantage of stratification-induced internal nutrient loading. In 2003, conditions were less favourable for cyanobacteria due to a spell of lower temperatures and stronger winds in mid-summer; as a result, the identified thresholds of thermal stratification were hardly ever reached. Overall, our study shows that extracting critical thresholds of environmental drivers from long-term records is a promising avenue for predicting ecosystem responses to future climate warming. Specifically, our results emphasize that not average temperature increase but changes in short-term meteorological variability will determine whether cyanobacteria will bloom more often in a warmer world.     

The influence of environmental conditions on the seasonal variation of <Emphasis Type="Italic">Microcystis</Emphasis> cell density and microcystins concentration in San Francisco Estuary

A bloom of the cyanobacteria Microcystis aeruginosa was sampled over the summer and fall in order to determine if the spatial and temporal patterns in cell density, chlorophyll a (chl a) concentration, total microcystins concentration, and percent microcystins composition varied with environmental conditions in San Francisco Estuary. It was hypothesized that the seasonal variation in Microcystis cell density and microcystin concentration was ecologically important because it could influence the transfer of toxic microcystins into the aquatic food web. Sampling for Microcystis cell density, chl a concentration, total microcystins concentration and a suite of environmental conditions was conducted biweekly at nine stations throughout the freshwater tidal and brackish water regions of the estuary between July and November 2004. Total microcystins in zooplankton and clam tissue was also sampled in August and October. Microcystis cell density, chl a concentration and total microcystins concentration varied by an order of magnitude and peaked during August and September when and α^B were high. Low streamflow and high water temperature were strongly correlated with the seasonal variation of Microcystis cell density, total microcystins concentration (cell)⁻¹ and total microcystins concentration (chl a)⁻¹ in canonical correlation analyses. Nutrient concentrations and ratios were of secondary importance in the analysis and may be of lesser importance to seasonal variation of the bloom in this nutrient rich estuary. The seasonal variation of Microcystis density and biomass was potentially important for the structure and function of the estuarine aquatic food web, because total microcystins concentration was high at the base of the food web in mesozooplankton, amphipod, clam, and worm tissue during the peak of the bloom. Handling editor: D. Hamilton     

Response of Cyanobacteria and Algae from Antarctic Wetland Habitats to Freezing and Desiccation Stress

Antarctic wetlands are characterized by the presence of liquid water during short austral summer. Filamentous cyanobacteria are often dominant there and are exposed to severe conditions, of which the changes in the desiccation–rehydration and freeze–thaw cycles are two of the most stressful. Vigor, after freezing and desiccation, was laboratory tested in cyanobacterial and algal strains from wetland habitats collected in maritime and continental Antarctica. Whereas minor sub-zero temperatures (−4°C), demonstrating summer diurnal freeze–thaws did not cause significant damage on either cyanobacteria or algae, low sub-zero temperatures (−40, −100, −196°C), demonstrating annual winter freeze, caused little harm to cyanobacteria, but was fatal for more than 50% of the population of algae. Freezing and desiccation tolerance of these strains was compared using multiregression methods: cyanobacteria from continental Antarctica were significantly more tolerant to low sub-zero temperatures than similar strains from maritime Antarctica (P = 0.026; F = 3.66); and cyanobacteria from seepages habitat were less tolerant to freezing and desiccation than cyanobacteria from other wetlands (P = 0.002; F = 5.69).     

The Effect of Exogenous β-<Emphasis Type="Italic">N-</Emphasis>Methylamino-<Emphasis Type="SmallCaps">l</Emphasis>-alanine on the Growth of <Emphasis Type="Italic">Synechocystis</Emphasis> PCC6803

β-N-Methylamino-l-alanine (BMAA), a non-proteinogenic amino acid, has been detected in a range of cyanobacteria, including terrestrial, aquatic, free living and endosymbiotic species. The widespread occurrence of cyanobacteria in the environment raises concerns regarding the ecological and toxicological impact of BMAA, and consequently, studies have focussed extensively on the toxicity and environmental impact of BMAA, while no research has addressed the ecophysiological or metabolic role of the compound in cyanobacteria. In this study, both the uptake of exogenous BMAA by and the effect of exogenous BMAA on the growth of Synechocystis PCC6803 were investigated. BMAA was rapidly taken up by the non-diazotrophic cyanobacterium Synechocystis PCC6803 in a concentration dependent manner. The presence of exogenous BMAA resulted in a substantial and concentration-dependent decrease in cell growth and the substantial loss of photosynthetic pigmentation. Similar effects were seen in the presence of the non-proteinogenic amino acid, 2,4-diaminobutyric acid but to a lesser degree than that of BMAA. The effects were reversed when light was decreased from 16 to 10 μmol m⁻² s⁻¹. Control cultures grown in the presence of l-arginine, l-asparagine, l-glutamate and glycine showed normal or slightly increased growth with no change in pigmentation. The decrease in growth rate coupled to bleaching indicates that BMAA may induce chlorosis in the presence of adequate photosynthetic radiation suggesting a connection between BMAA and the induction of conditions, such as nitrogen or sulphur depletion, that result in growth arrest and the induction of chlorosis.     

Response of cyanobacteria to arsenic toxicity

Arsenic content of cyanobacterial biomass, soil and water samples from arsenic-contaminated area of eastern India were estimated. It was found that arsenic content in cyanobacterial biomass (276.9 μg g⁻¹) was more than soil (19.01 μg g⁻¹) or water sample (244.13 μg L⁻¹). Shallow tube well water showed more arsenic (244.13 μg L⁻¹) than deep tube well water (146.13 μg L⁻¹). Arsenic resistant genera recorded from the contaminated area were Oscillatoria princeps, Oscillatoria limosa, Anabaena sp. and Phormidium laminosum. Among these, P. laminosum was isolated and exposed to different concentration of Arsenic in vitro (0.1–100 ppm) to study the toxicity level of arsenic. Modulation in stress enzymes and stress-related compounds were studied in relation to lipid peroxidase, catalase, super oxide dismutase (SOD), ascorbate peroxidase (APX), reduced glutathione and carotenoids in arsenic exposed biomass to understand the resistance mechanism of the genus both in laboratory condition as well as in natural condition. Arsenic content of cyanobacterial biomass from contaminated area was more (276.9 μg g⁻¹) than laboratory exposed sample (37.17 μg g⁻¹), indicating bioconcentration of arsenic in long-term-exposed natural biomass. Overall, more activity of catalase was recorded in cyanobacterial biomass of natural condition whereas SOD and APX were at higher level in laboratory culture.     

Bio-optical Characteristics and the Vertical Distribution of Photosynthetic Pigments and Photosynthesis in an Artificial Cyanobacterial Mat

Abstract Zonations of photosynthesis and photopigments in artificial cyanobacterial mats were studied with (i) oxygen and pH microsensors, (ii) fiber-optic microprobes for field radiance, scalar irradiance, and PSII fluorescence, and (iii) a light microscope equipped with a spectrometer for spectral absorbance and fluorescence measurements. Our analysis revealed the presence of several distinct 1–2 mm thick cyanobacterial layers mixed with patches of anoxygenic photosynthetic bacteria. Strong attenuation of visible light confined the euphotic zone to the uppermost 3 mm of the mat, where oxygen levels of 3–4 times air saturation and a pH peak of up to pH 8.8 were observed under saturating irradiance (413 μmol photon m⁻² s⁻¹). Oxygen penetration was 5 mm in light and decreased to 1 mm in darkness. Volumetric oxygen consumption in the photic and aphotic zones of illuminated mat was 5.5 and 2.9 times higher, respectively, than oxygen consumption in dark incubated mats. Scalar irradiance reached 100–150% of incident irradiance in the upper 0.5 mm of the mat due to intense scattering in the matrix of cells, exopolymers, and carbonate precipitates. In deeper mat layers scalar irradiance decreased nearly exponentially, and highest attenuation coefficients of 6–7 mm⁻¹ were found in cyanobacterial layers, where photosynthesis and photopigment fluorescence also peaked. Visible light was attenuated >100 times more strongly than near infrared light. Microscope spectrometry on thin sections of mats allowed detailed spectral absorbance and fluorescence measurements at defined positions relative to the mat surface. Besides strong spectral signals of cyanobacterial photopigments (Chl a and phycobiliproteins), the presence of both green and purple photosynthetic bacteria was evident from spectral signals of Bchl a and Bchl c. Microprofiles of photopigment absorbance correlated well with microdistributions of phototrophs determined in an accompanying study. Received: 20 December 1999; Accepted: 10 June 2000; Online Publication: 28 August 2000     

Anti-cyanobacterial activity of <Emphasis Type="Italic">Moringa oleifera</Emphasis> seeds

Filtrates from crushed Moringa oleifera seeds were tested for their effects on growth and Photosystem II efficiency of the common bloom-forming cyanobacterium Microcystis aeruginosa. M. aeruginosa populations exhibited good growth in controls and treatments with 4- and 8-mg crushed Moringa seeds per liter, having similar growth rates of 0.50 (±0.01) per day. In exposures of 20- to 160-mg crushed Moringa seeds L⁻¹, growth rates were negative and on average −0.23 (±0.05) .day⁻¹. Presumably, in the higher doses of 20- to 160-mg crushed seeds per liter, the cyanobacteria died, which was supported by a rapid drop in the Photosystem II efficiency (Φ_PSII), while the Φ_PSII was high and unaffected in 0, 4, and 8 mg L⁻¹. High-density populations of M. aeruginosa (chlorophyll-a concentrations of ∼270 μg L⁻¹) were reduced to very low levels within 2 weeks of exposure to ≥80-mg crushed seeds per liter. At the highest dosage of 160 mg L⁻¹, the Φ_PSII dropped to zero rapidly and remained nil during the course of the experiment (14 days). Hence, under laboratory conditions, a complete wipeout of the bloom could be achieved. This is the first study that yielded evidence for cyanobactericidal activity of filtrate from crushed Moringa seeds, suggesting that Moringa seed extracts might have a potential as an effect-oriented measure lessening cyanobacterial nuisance.     

First attempt to apply whole-lake food-web manipulation on a large scale in The Netherlands

Lake Breukeleveen (180 ha, mean depth 1.45 m), a compartment of the eutrophic Loosdrecht lakes system, was selected to study the effects of whole-lake foodweb manipulation on a large scale. In Lake Loosdrecht (dominated by filamentous cyanobacteria), due to water management measures taken from 1970–1984 (sewerage systems, dephosphorization) the external P load has been reduced from 1.2 g m⁻² y⁻¹ to 0.35 g m⁻² y⁻¹. The water transparency (Secchi-depthca. 30 cm), however, has not improved. The aim of the food-web manipulation in Lake Breukeleveen was not only to improve the light climate of the lake, but also to study if the successfull effects observed in small lakes (a few ha) can be upscaled. In March 1989 the standing crop of planktivorous and bentivorous fish populations was reduced by intensive fishery, fromca. 150 kg ha⁻¹ toca. 57 kg ha⁻¹. The lake was made unaccessible to fish migrating from the other lakes and it was stocked with large-sized daphnids and 0⁺ pike. However, water transparency did not increase in the following summer and autumn 1989, which is in contrast with great improvement in the light conditions previously observed in smaller lakes. The main explanations for the negative outcome in Lake Breukeleveen are: 1) the rapid increase of the planktivorous fish biomass and carnivorous cladocerans, predating on the zooplankton community; 2) suppression of the large daphnids by the high concentrations of filamentous cyanobacteria; 3) high turbidity of the lake due to resuspension of bottom material induced by wind, unlike in smaller lakes, and thus inability of submerged macrophytes to develop and to stabilize the ecosystem.