A freshwater bacterial strain,Shewanella sp. Lzh-2, isolated from Lake Taihu and its two algicidal active substances,hexahydropyrrolo[1,2-a]pyrazine-1,4-dione and 2, 3-indolinedione

Cyanobacterial blooms have become a serious problem in Lake Taihu during the last 20 years, and Microcystis aeruginosa and Synechococcus sp. are the two dominant species in cyanobacterial blooms of Lake Taihu. A freshwater bacterial strain, Shewanella sp. Lzh-2, with strong algicidal properties against harmful cyanobacteria was isolated from Lake Taihu. Two substances with algicidal activity secreted extracellularly by Shewanella sp. Lzh-2, S-2A and S-2B, were purified from the bacterial culture of strain Lzh-2 using ethyl acetate extraction, column chromatography, and high performance liquid chromatography (HPLC) in turn. The substances S-2A and S-2B were identified as hexahydropyrrolo[1,2-a]pyrazine-1,4-dione and 2, 3-indolinedione (isatin), respectively, based on liquid chromatography-mass spectrometry (LC-MS), gas chromatography-mass spectrometry (GC-MS), and hydrogen-nuclear magnetic resonance (H-NMR) analyses, making this the first report of their algicidal activity toward cyanobacteria. S-2A (hexahydropyrrolo[1,2-a]pyrazine-1,4-dione) had no algicidal effects against Synechococcus sp. BN60, but had a high level of algicidal activity against M. aeruginosa 9110. The LD50 value of S-2A against M. aeruginosa 9110 was 5.7 μg/ml. S-2B (2, 3-indolinedione) showed a potent algicidal effect against both M. aeruginosa 9110 and Synechococcus sp. BN60, and the LD50 value of S-2B against M. aeruginosa 9110 and Synechococcus sp. BN60 was 12.5 and 34.2 μg/ml, respectively. Obvious morphological changes in M. aeruginosa 9110 and Synechococcus sp. BN60 were observed after they were exposed to S-2A (or S-2B) for 24 h. Approximately, the algicidal activity, the concentration of S-2A and S-2B, and the cell density of Lzh-2 were positively related to each other during the cocultivation process. Overall, these findings increase our knowledge about algicidal substances secreted by algicidal bacteria and indicate that strain Lzh-2 and its two algicidal substances have the potential for use as a bio-agent in controlling cyanobacterial blooms in Lake Taihu.     

Targeted deep sequencing reveals high diversity and variable dominance of bloom-forming cyanobacteria in eutrophic lakes

Cyanobacterial blooms in eutrophic lakes are severe environmental problems worldwide. To characterize the spatiotemporal heterogeneity of cyanobacterial blooms, a high-throughput method is necessary for the specific detection of cyanobacteria. In this study, the cyanobacterial composition of three eutrophic waters in China (Taihu Lake, Dongqian Lake, and Dongzhen Reservoir) was determined by pyrosequencing the cpcBA intergenic spacer (cpcBA-IGS) of cyanobacteria. A total of 2585 OTUs were obtained from the normalized cpcBA-IGS sequence dataset at a distance of 0.05. The 238 most abundant OTUs contained 92% of the total sequences and were classified into six cyanobacterial groups. The water samples of Taihu Lake were dominated by Microcystis, mixed Nostocales species, Synechococcus, and unclassified cyanobacteria. Besides, all the samples from Taihu Lake were clustered together in the dendrogram based on shared abundant OTUs. The cyanobacterial diversity in Dongqian Lake was dramatically decreased after sediment dredging and Synechococcus became exclusively dominant in this lake. The genus Synechococcus was also dominant in the surface water of Dongzhen Reservoir, while phylogenetically diverse cyanobacteria coexisted at a depth of 10 m in this reservoir. In summary, targeted deep sequencing based on cpcBA-IGS revealed a large diversity of bloom-forming cyanobacteria in eutrophic lakes and spatiotemporal changes in the composition of cyanobacterial communities. The genus Microcystis was the most abundant bloom-forming cyanobacteria in eutrophic lakes, while Synechococcus could be exclusively dominant under appropriate environmental conditions.     

Novel algicidal evidence of a bacterium Bacillus sp. LP-10 killing Phaeocystis globosa,a harmful algal bloom causing species

While searching for effective bio-agents to control harmful algal blooms (HABs), the bacterial strain LP-10, which has strong algicidal activity against Phaeocystis globosa (Prymnesiophyceae), was isolated from surface seawater samples taken from the East China Sea. 16S rDNA sequence analysis and morphological characteristics revealed the strain LP-10 belonged to the genus Bacillus. The lytic effect of Bacillus sp. LP-10 against P. globosa was both concentration- and time-dependent. Algicidal activities of different growth stages of the bacterial culture varied significantly. The lytic effect of different parts of the bacterial cultures indicated that the algal cells were lysed by algicidal active compounds in the cell-free filtrate. Analysis of the properties of the active compounds showed that they had a molecular weight of less than 1000 Da and that the active compounds were stable between −80 and 121 °C. The algicidal range assay indicated that five other algal species were also suppressed by strain LP-10, including: Alexandrium catenella, A. tamarense, A. minutum, Prorocentrum micans and Asterionella japonica. Our results suggested that the algicidal bacterium Bacillus sp. LP-10 could be a potential bio-agent to control the blooms of harmful algal species.     

Effects of freshwater bacterial siderophore on Microcystis and Anabaena

Six siderophore-producing bacterial strains were isolated from the freshwater, in which five strains belonged to Pseudomona genus, and the other belonged to Stenotrophomonas genus. The strain, Stenotrophomonas maltophilia 15, which produced hydroxamate-type siderophore, was selected for siderophore preparation. Its siderophore production was inhibited by FeCl₃, especially when FeCl₃ concentration was higher than 20 μM. Effects of siderophore on cyanobacteria Microcystis aeruginosa FACHB-905 and Anabaena flos-aquae FACHB-245 were studied. Compared to the control, almost all the treated groups showed decrease in growth rate and chlorophyll a, carotenoids, phycocyanin, soluble protein, microcystin content, which was attributed the low iron bioavailability in the culture medium. In the study, S. maltophilia 15 showed algicidal activities by secreting siderophore and could inhibit cyanobacterial growth, especially when iron bioavailabity is very low. The two cyanobacterial strains showed distinct demand for iron. It was deduced that in the freshwater the competition between bacteria and cyanobacteria existed for the low-bioavailable iron, which may relate to the replacement of dominant cyanobacteria.     

Potential use of environmental cyanobacterial species in bioremediation of lindane-contaminated effluents

This study investigated the potential degradation of lindane [γ-hexachlorocyclohexane (γ-HCH)], resulting from agricultural runoff, by environmental species of cyanobacteria. Cyanobacterial species isolated from the Egyptian Lakes Qaroun and Mariut were exposed, either individually or as mixtures, to 5 and 10 ppm lindane for 7 days. Growth inhibition or stimulation percentage, as well as the percentage of lindane removal efficiency (RE), were calculated, and factors controlling both were discussed. Lindane exhibited different degrees of toxicity or stimulation for the selected cyanobacteria. Stimulation of growth ranged between 0.0- and 13.16-fold higher than controls, while inhibition ranged between 0.0% and 100%. Results also proved that Mariut species were more resistant to lindane toxicity than were Qaroun species. Resistance to lindane among Qaroun spp. was in the order Oscillatoria sp. 12>Oscillatoria sp. 13>Synechococcus sp.>Nodularia sp.>Nostoc sp.>Cyanothece sp.>Synechococcus sp. Among Mariut spp., it was Microcystis aeruginosa MA1>Anabaena cylindrica>Microcystis aeruginosa MA15>A. spiroides>A. flos-aquae. Mixed cultures showed varying sensitivity. Lindane was removed by all the species, either as individuals or mixtures, at both concentrations. The lindane RE percentage of Qaroun species ranged between 71.6% and 99.6% with a maximum of 98.0–99.6% at 5 ppm, 83.9% and 99.7% at 10 ppm, and maximum between 95.5% and 99.7%. Mariut species showed an RE percentage of 45.23–100.0% with maximum between 99.23% and 100.0% at 5 ppm and 43.15% and 100.0% at 10 ppm with maximal RE percentage between 99.67% and 100.0%. Mixed culture RE percentages ranged between 91.6% and 100% at 5 ppm with a maximum range of 99.3–100%, while at 10 ppm, the RE percentage ranged between 90.4% and 100%, with a maximum range of 96.0–100%. Results indicate the potential of natural resources as efficient agents for pollution control.     

Growth-promoting effects of a bacterium on raphidophytes and other phytoplankton

On 29 April 2003, a Heterosigma akashiwo bloom (9.5 × 10⁴ cells mL⁻¹) associated with a fish kill (>10⁴ dead fishes estimated from aerial surveys) was observed offshore of Bulls Bay, McLellanville, South Carolina, USA. To assess a potential cause of this bloom event, we investigated the bacterial diversity and algal/bacterial interactions in the bloom microbial community. Thirty-five bacterial strains were isolated and screened for algicidal or algal growth-promoting activities. One strain (BBB25) had significant growth-promoting effects on all 7 algal species tested: three raphidophytes (Heterosigma akashiwo, Chattonella subsalsa, Fibrocapsa japonica), two diatoms (Chaetoceros neogracile, Nitzschia sp.), a cryptophyte (Cryptomonas sp.), and a chlorophyte, Ankistrodesmus sp. This strain (BBB25) is a Gram-positive, rod-shaped spore-forming bacterium. Partial 16S rDNA gene sequence and morphological characters indicated that BBB25 is related closely to the genus Bacillus. The general nature of the algal response indicates that the growth-promoting effects of BBB25 are not specific to H. akashiwo, and suggests potentially widespread effects. Since the presence or relative abundance of the other algal species was not assessed during the bloom initiation period, the selective stimulatory effect on H. akashiwo bloom formation in Bulls Bay is unknown. These results demonstrate, however, the potential for bacterial species to play a regulatory role in bloom formation.     

Newly discovered role of the heterotrophic nanoflagellate Katablepharis japonica,a predator of toxic or harmful dinoflagellates and raphidophytes

Heterotrophic nanoflagellates are ubiquitous and known to be major predators of bacteria. The feeding of free-living heterotrophic nanoflagellates on phytoplankton is poorly understood, although these two components usually co-exist. To investigate the feeding and ecological roles of major heterotrophic nanoflagellates Katablepharis spp., the feeding ability of Katablepharis japonica on bacteria and phytoplankton species and the type of the prey that K. japonica can feed on were explored. Furthermore, the growth and ingestion rates of K. japonica on the dinoflagellate Akashiwo sanguinea—a suitable algal prey item—heterotrophic bacteria, and the cyanobacteria Synechococcus sp., as a function of prey concentration were determined. Among the prey tested, K. japonica ingested heterotrophic bacteria, Synechococcus sp., the prasinophyte Pyramimonas sp., the cryptophytes Rhodomonas salina and Teleaulax sp., the raphidophytes Heterosigma akashiwo and Chattonella ovata, the dinoflagellates Heterocapsa rotundata, Amphidinium carterae, Prorocentrum donghaiense, Alexandrium minutum, Cochlodinium polykrikoides, Gymnodinium catenatum, A. sanguinea, Coolia malayensis, and the ciliate Mesodinium rubrum, however, it did not feed on the dinoflagellates Alexandrium catenella, Gambierdiscus caribaeus, Heterocapsa triquetra, Lingulodinium polyedra, Prorocentrum cordatum, P. micans, and Scrippsiella acuminata and the diatom Skeletonema costatum. Many K. japonica cells attacked and ingested a prey cell together after pecking and rupturing the surface of the prey cell and then uptaking the materials that emerged from the ruptured cell surface. Cells of A. sanguinea supported positive growth of K. japonica, but neither heterotrophic bacteria nor Synechococcus sp. supported growth. The maximum specific growth rate of K. japonica on A. sanguinea was 1.01 d⁻¹. In addition, the maximum ingestion rate of K. japonica for A. sanguinea was 0.13 ng C predator⁻¹d⁻¹ (0.06 cells predator⁻¹d⁻¹). The maximum ingestion rate of K. japonica for heterotrophic bacteria was 0.019 ng C predator⁻¹d⁻¹ (266 bacteria predator⁻¹d⁻¹), and the highest ingestion rate of K. japonica for Synechococcus sp. at the given prey concentrations of up to ca. 10⁷ cells ml⁻¹ was 0.01 ng C predator⁻¹d⁻¹ (48 Synechococcus predator⁻¹d⁻¹). The maximum daily carbon acquisition from A. sanguinea, heterotrophic bacteria, and Synechococcus sp. were 307, 43, and 22%, respectively, of the body carbon of the predator. Thus, low ingestion rates of K. japonica on heterotrophic bacteria and Synechococcus sp. may be responsible for the lack of growth. The results of the present study clearly show that K. japonica is a predator of diverse phytoplankton, including toxic or harmful algae, and may also affect the dynamics of red tides caused by these prey species.     

On the control of HAB species using low biosurfactant concentrations

Biosurfactants have been suggested as a method to control harmful algal blooms (HABs), but warrant further and more in-depth investigation. Here we have investigated the algicidal effect of a biosurfactant produced by the bacterium Pseudomonas aeruginosa on five diverse marine and freshwater HAB species that have not been tested previously. These include Alexandrium minutum (Dinophycaee), Karenia brevis (Dinophyceae), Pseudonitzschia sp. (Bacillariophyceae), in marine ecosystems, and Gonyostomum semen (Raphidophyceae) and Microcystis aeruginosa (Cyanophyecae) in freshwater. We examined not only lethal but also sub-lethal effects of the biosurfactant. In addition, the effect of the biosurfactant on Daphnia was tested. Our conclusions were that very low biosurfactant concentrations (5 μg mL⁻¹) decreased both the photosynthesis efficiency and the cell viability and that higher concentrations (50 μg mL⁻¹) had lethal effects in four of the five HAB species tested. The low concentrations employed in this study and the diversity of HAB genera tested suggest that biosurfactants may be used to either control initial algal blooms without causing negative side effect to the ecosystem, or to provoke lethal effects when necessary.     

Effects of nitrogen on interspecific competition between two cell-size cyanobacteria: Microcystis aeruginosa and Synechococcus sp.

Micro-cyanobacteria and pico-cyanobacteria coexist in many lakes throughout the world. Their distinct cell sizes and nutrient utilization strategies may lead to dominance of one over the other at varying nutrient levels. In this study, Microcystis aeruginosa and Synechococcus sp. were chosen as representative organisms of micro- and pico-cyanobacteria, respectively. A series of nitrate and ammonia conditions (0.02, 0.1, 0.5, and 2.5 mg N L⁻¹) were designed in mono- or co-cultured systems, respectively. Growth rates of the two species were calculated and fitted by the Monod and Logistic equations. Furthermore, the interspecific competition was analyzed using the Lotka–Volterra model. In mono-cultures, the two cyanobacteria displayed faster growth rates in ammonia than in nitrate. Meanwhile, Synechococcus sp. showed faster growth rates compared to M. aeruginosa in lower N groups (≤ 0.5 mg N L⁻¹). However, in the highest nitrate treatment (2.5 mg N L⁻¹), M. aeruginosa achieved much higher biomass and faster growth rates than Synechococcus sp.. In co-cultures, Synechococcus sp. dominated in the lowest N treatment (0.02 mg N L⁻¹), but M. aeruginosa dominated under the highest nitrate condition (2.5 mg N L⁻¹). Based on the analysis of Raman spectra of living cells in mono-cultures, nitrate (2.5 mg N L⁻¹) upgraded the pigmentary contents of M. aeruginosa better than ammonia (2.5 mg N L⁻¹), but nitrogen in different forms showed little effects on the pigments of Synechococcus sp.. Findings from this study can provide valuable information to predict cyanobacterial community succession and aquatic ecosystem stability.     

Allelopathic effects of Microcystis aeruginosa on green algae and a diatom: Evidence from exudates addition and co-culturing

Allelopathic interactions among phytoplankton species are regarded as one of the important factors contributing to phytoplankton species competition and succession. The role and extent of allelopathic effects of blooming freshwater cyanobacteria on other phytoplankton species in eutrophied waters, however, are still unknown. We examined the allelopathic effect of Microcystis aeruginosa on two common green algae (Scenedesmus quadricauda, Chlorella pyrenoidosa) and a diatom (Cyclotella meneghiniana) by adding exudates from different growth phases and in co-culture tests. Exudates of M. aeruginosa from the exponential growth phase and the stationary phase significantly inhibited the growth of S. quadricauda, C. pyrenoidosa and C. meneghiniana, whereas those from the decline phase increased their growth. The presence of M. aeruginosa extremely inhibited the growth of all tested species in co-cultures within 24 h. Our results indicate that under the tested environmental conditions (25 °C, light 80 μmol quanta m⁻² s⁻¹, manual shaking twice a day), allelopathic effects of M. aeruginosa on other phytoplankton species can significantly contribute to their competitive success.     

Toxic and non-toxic strains of Microcystis aeruginosa induce temperature dependent allelopathy toward growth and photosynthesis of Chlorella vulgaris

Global warming was believed to accelerate the expansion of cyanobacterial blooms. However, the impact of changes due to the allelopathic effects of cyanobacterial blooms with or without algal toxin production on the ecophysiology of its coexisting phytoplankton species arising from global warming were unknown until recently. In this study, the allelopathic effects of toxic and non-toxic Microcystis aeruginosa strains on the growth of green alga Chlorella vulgaris and photosynthesis of the co-cultivations of C. vulgaris and toxic M. aeruginosa FACHB-905 or non-toxic M. aeruginosa FACHB-469 were investigated at different temperatures. The growth of C. vulgaris, co-cultured with the toxic or non-toxic M. aeruginosa strains, was promoted at 20 °C but inhibited at temperatures ≥25 °C. The inhibitory effects of the toxic and non-toxic M. aeruginosa strains on of the co-cultivations (C. vulgaris and non-toxic M. aeruginosa FACHB-469 or toxic M. aeruginosa FACHB-905) also linearly increased with elevated temperatures. Furthermore, toxic M. aeruginosa FACHB-905 induced more inhibition toward growth of C. vulgaris or P_max and R_d of the mixtures than non-toxic M. aeruginosa FACHB-469. C. vulgaris dominated over non-toxic M. aeruginosa FACHB-469 but toxic M. aeruginosa FACHB-905 overcame C. vulgaris when they were co-cultured in mesocosms in water temperatures from 20 to 25 °C. The results indicate that allelopathic effects of M. aeruginosa strains on C. vulgaris are both temperature- and species-dependent: it was stimulative for C. vulgaris at low temperatures such as 20 °C, but inhibitory at high temperatures (≥25 °C); the toxic strain was determined to be more harmful to C. vulgaris than the non-toxic one. This suggests that global warming may aggravate the ecological risk of cyanobacteria blooms, especially those with toxic species as the main contributors.     

Growth stimulation of Microcystis aeruginosa by a bacterium from hyper-eutrophic water (Taihu Lake, China)

Cyanobacteria are the causative organisms of the algal blooms that occur in Taihu Lake. Dissolved organic nitrogen (DON) comprises a significant composition of nitrogen (N) pool in the water and may increase the nutrient source of microalgae. In the present study, we investigated the relationship between Microcystis aeruginosa, Pseudomonas sp. A3CT isolated from Taihu, and DON compounds. Co-incubation (3 days) of the bacterium with six DON compounds (four free amino acids and two combined amino acids) was collected as six decomposed DON solutions. The decomposed DON solutions of six compounds were used to test the stimulatory effect of nutrient regeneration by the bacterium. The growth of M. aeruginosa was significantly enhanced by the six decomposed DON solutions. M. aeruginosa grew much better under the six decomposed DON solutions than the corresponding undigested DON forms. Especially, the decomposed l-lysine solution, not only avoided the inhibiting effect of lysine on M. aeruginosa, but significantly promoted the cyanobacterial growth. Further chemical tests indicated that A3CT transformed DON into NH₄ ⁺, which was utilized by M. aeruginosa. These results demonstrate that the bacterium plays an important role in decomposing unavailable DON forms into available NH₄ ⁺, which suggests that the bacterium contributes to the fast growth of M. aeruginosa. Moreover, this phenomenon, in conjunction with previous studies, indicates that the responsible and effective way of harmful blooms is reducing the N and P inputs (including DON and DOP).     

Toxicity studies of Trichodesmium erythraeum (Ehrenberg, 1830) bloom extracts,from Phoenix Bay,Port Blair,Andamans

Occurrence of toxic cyanobacterial blooms has become a worldwide problem, increasing the risk of human poisoning due to consumption of seafood contaminated with cyanotoxins. Though no such cases of human intoxication due to toxic blooms have been reported so far from India, most of the studies related to blooms have been restricted to reporting of a bloom and/or antimicrobial activity of its extract. Detailed toxicity study of cyanobacterial blooms are lacking. A study on the toxicity of a dense bloom (14.56 × 10⁶ trichomes L⁻¹) of the marine diazotrophic cyanobacteria, Trichodesmium erythraeum, observed in the coastal waters of Phoenix Bay, Port Blair, Andamans was undertaken. The significance of this bloom is that it was a single species and had conspicuously inhibited the growth of other phytoplankton and complete exclusion of zooplankton from the bloom region, intimating the involvement of toxins in the bloom. The cyanobacterial extracts showed prominent antimicrobial activity against certain human pathogenic bacteria and fungi. Studies on the toxicity of the cyanobacterial extracts was carried out using brine shrimp bioassay, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and comet assay. The cyanobacterial extract exhibited toxic effect to Artemia salina causing mortality of up to 40% after 48 h at a concentration of 1 mg mL⁻¹, while it induced cytotoxicity in cell lines (HepG2 and HaCat) and caused DNA damage in human lymphocytes in vitro.     

Wheat Bran Enhances the Cytotoxicity of Immobilized Alcaligenes aquatilis F8 against Microcystis aeruginosa

Algicidal bacteria offer a promising option for killing cyanobacteria. Therefore, a new Alcaligenes aquatilis strain F8 was isolated to control Microcystis aeruginosa in this study. The algicidal activity of strain F8 was dependent on the cell density of M. aeruginosa, and the maximal algicidal rate of the free bacterium reached 88.45% within 72 h. With a view to its application to the control of M. aeruginosa in the natural environment, strain F8 was immobilized in sodium alginate beads, but immobilization of the strain decreased its algicidal rate compared to that of the free bacterium. However, addition of wheat bran to the sodium alginate matrix used to immobilize strain F8 not only eliminated the adverse effects of immobilization on the bacteria but also resulted in an 8.83% higher algicidal rate of the immobilized than free bacteria. Exclusion and recovery methods were used to identify key ingredients of wheat bran and gain insight into the mechanism underlying the observed enhancement of algicidal activity. This analysis indicated that certain factors in wheat bran, including vitamins B₁, B₂, B₉, and E were responsible for promoting bacterial growth and thereby improving the algicidal rate of immobilized strain F8. Our findings indicate that wheat bran is able to improve the algicidal efficiency of A. aquatilis strain F8 for killing M. aeruginosa and is a good source of not only carbon and nitrogen but also vitamins for bacteria.     

Emergence of toxic cyanobacterial species in the Ganga River,India, due to excessive nutrient loading

Despite of several efforts by the Government of India, pollution in National River Ganga is rising. The aim of the present study is to investigate the pollution in Ganga River in relation to appearance of toxic cyanobacterial strains. Jajmau area of Kanpur city is the industrial hub of Uttar Pradesh and is the main source of adding unwanted discharge into Ganga River. Water samples were randomly collected from the most polluted stretch of Ganga River (Kanpur, Uttar Pradesh, India). Samples were also collected from other major water of Uttar Pradesh to compare their water chemistry with Ganga River. Physico-chemical parameters of water bodies were estimated periodically for three years 2013–2015. Pearson product-mean correlation showed strong correlation between water parameters of sampling sites. Regression analysis showed seasonal variation in water parameters of Ganga River. Cyanobacteria prevalence in Ganga River was highest in May while lowest in August month. Fourteen cultivable cyanobacteria were isolated from Ganga River. Two new isolates, Oscillatoria sp. RBD01 and Leptolyngbya sp. RBD05 were found to be toxic and showed the presence of algal toxin (microcystin). Phylogenetic relatedness of toxic cyanobacterial isolates with their close homologues was established using 16S rRNA sequence analysis. Microcystin content in water samples (extracellular release) and in cyanobacterial isolates (intracellular content) was estimated by enzyme linked immunosorbent assay. Ganga River was found to be positive for microcystin with concentration ≥2 ppb which is above the permissible limit of WHO. Toxic cyanobacteria Oscillatoria sp. RBD01 and Leptolyngbya sp. RBD05 showed the presence of 23 and 17 ppb of microcystin in cells. Growth of the toxic cyanobacteria Oscillatoria sp. RBD01 showed very strong correlation with phosphate (0.834) and nitrate (0.761) content of water. Toxic Oscillatoria sp. RBD01 growing in moderate combination of nitrate (16x) and phosphate (4x) showed optimum growth and protein content. Periodic assessment of water quality and monitoring of toxic cyanobacteria would be helpful in identification and regulation of toxins which are responsible for destroying its sanctity and making it unsafe for human consumption.     

Mixotrophy in the marine red-tide cryptophyte Teleaulax amphioxeia and ingestion and grazing impact of cryptophytes on natural populations of bacteria in Korean coastal waters

Cryptophytes are ubiquitous and one of the major phototrophic components in marine plankton communities. They often cause red tides in the waters of many countries. Understanding the bloom dynamics of cryptophytes is, therefore, of great importance. A critical step in this understanding is unveiling their trophic modes. Prior to this study, several freshwater cryptophyte species and marine Cryptomonas sp. and Geminifera cryophila were revealed to be mixotrophic. The trophic mode of the common marine cryptophyte species, Teleaulax amphioxeia has not been investigated yet. Thus, to explore the mixotrophic ability of T. amphioxeia by assessing the types of prey species that this species is able to feed on, the protoplasms of T. amphioxeia cells were carefully examined under an epifluorescence microscope and a transmission electron microscope after adding each of the diverse prey species. Furthermore, T. amphioxeia ingestion rates heterotrophic bacteria and the cyanobacterium Synechococcus sp. were measured as a function of prey concentration. Moreover, the feeding of natural populations of cryptophytes on natural populations of heterotrophic bacteria was assessed in Masan Bay in April 2006. This study reported for the first time, to our knowledge, that T. amphioxeia is a mixotrophic species. Among the prey organisms offered, T. amphioxeia fed only on heterotrophic bacteria and Synechococcus sp. The ingestion rates of T. amphioxeia on heterotrophic bacteria or Synechococcus sp. rapidly increased with increasing prey concentrations up to 8.6 × 10⁶ cells ml⁻¹, but slowly at higher prey concentrations. The maximum ingestion rates of T. amphioxeia on heterotrophic bacteria and Synechococcus sp. reached 0.7 and 0.3 cells predator⁻¹ h⁻¹, respectively. During the field experiments, the ingestion rates and grazing coefficients of cryptophytes on natural populations of heterotrophic bacteria were 0.3–8.3 cells predator⁻¹ h⁻¹ and 0.012–0.033 d⁻¹, respectively. Marine cryptophytes, including T. amphioxeia, are known to be favorite prey species for many mixotrophic and heterotrophic dinoflagellates and ciliates. Cryptophytes, therefore, likely play important roles in marine food webs and may exert a considerable potential grazing impact on the populations of marine bacteria.     

Investigation of the algicidal exudate produced by Shewanella sp. IRI-160 and its effect on dinoflagellates

The bacterium, Shewanella sp. IRI-160, was previously shown to have negative effects on the growth of dinoflagellates, while having no negative effects on other classes of phytoplankton tested (Hare et al., 2005). In this study, we investigated the mode of algicidal activity for Shewanella sp. IRI-160 and found that the bacterium secretes a bioactive compound. The optimum temperature for production of the algicidal compound by this bacterium was at 30 °C. Bacteria-free filtrate of medium containing the algicide (designated IRI-160AA) was stable at temperatures ranging from −80 °C to 121 °C, and could be stored at room temperature for at least three weeks with no loss in activity. Algicidal activity was eluted in the aqueous portion after C18 extraction, suggesting that the active compound is likely polar and water-soluble. The activity of IRI-160AA was examined on a broad range of dinoflagellates (Karlodinium veneficum, Karenia brevis, Gyrodinium instriatum, Cochlodinium polykrikoides, Heterocapsa triquetra, Prorocentrum minimum, Alexandrium tamarense and Oxyrrhis marina) and three species from other classes of algae as controls (Dunaliella tertiolecta, Rhodomonas sp. and Thalassiosira pseudonana). Algicidal activity was observed for each dinoflagellate and little to no negative effect was observed on chlorophyte and cryptophyte cultures, while a slight (non-significant) stimulatory effect was observed on the diatom culture exposed to the algicide. Finally, the effect of the algicide at different growth stages was investigated for K. veneficum and G. instriatum. IRI-160AA exhibited a significantly greater effect during logarithmic growth compared to stationary phase, suggesting a potential application of the algicide for prevention and control of harmful dinoflagellate blooms in the future.     

The potential use of bacterium strain R219 for controlling of the bloom-forming cyanobacteria in freshwater lake

Cyanobacterial blooms become a serious environmental threat to the freshwater ecosystem, and several physical and chemical methods have been developed for controlling the blooms. In order to develop a biocontrol agent for controlling the blooms, we isolated a bacterial strain R219 that exhibited strong algicidal activity against the dominant bloom-forming species of Microcystis aeruginosa from Lake Tai in China. Based on 16S rDNA sequence analysis we determined the strain R219 to be Pseudomonas aeruginosa by the virtue of its sharing about 99.8% similarity with reference strains in the DNA databases. Biochemical and morphological tests were used to support the accurate identification as that of the bacterium P. aeruginosa. We also tested culture filtrate and ethyl acetate extract of strain R219 and showed both of them exhibited strong algicidal effect on the growth of M. aeruginosa at mid-exponential phase when the R219 filtrate and ethyl acetate extract were applied at various cell densities. Moreover, the P. aeruginosa filtrate showed high potency in removal of the mixed species bloom-forming cyanobacteria collected directly from the Lake Tai. When adding the filtrate of the strain R219 to the mixed-species cyanobacteria, the content of chlorophyll-a of the algae were reduced by as much as 80–90%. Oral acute toxicity assessment for strain R219 demonstrated that all the mice that received the broth or filtrate in doses of 0.5 or 2.0 g kg^?1 were alive without any immediate behavioral changes within 14 days of administration of either broth or filtrate. These results indicate that the strain R219 may have potential for a use in controlling the bloom-forming cyanobacteria in freshwater ecosystems.     

3-Aryl-4-acyloxyethoxyfuran-2(5H)-ones as inhibitors of tyrosyl-tRNA synthetase: Synthesis,molecular docking and antibacterial evaluation

Thirty-eight 3-aryl-4-acyloxyethoxyfuran-2(5H)-ones were designed, prepared and tested for antibacterial activities. Some of them showed significant antibacterial activity against Gram-positive organism, Gram-negative organism and fungus. Out of these compounds, 4-(2-(3-chlorophenylformyloxy)ethoxy)-3-(4-chlorophenyl)furan-2(5H)-one (d40) showed the widest spectrum of activity with MIC₅₀ of 2.0 μg/mL against Staphylococcus aureus, 4.3 μg/mL against Escherichia coli, 1.5 μg/mL against Pseudomonas aeruginosa and 1.2 μg/mL against Candida albicans. Our data disclosed that MIC₅₀ values against whole cell bacteria are positive correlation with MIC₅₀ values against tyrosyl-tRNA synthetase. Meanwhile, molecular docking of d40 into S. aureus tyrosyl-tRNA synthetase active site was also performed, and the inhibitor tightly fitting the active site might be an important reason why it has high antimicrobial activity.     

Allelopathic inhibition of phytoplankton by exudates from Stratiotes aloides

The allelopathic potential of exudates from the aquatic macrophyte Stratiotes aloides on the growth of phytoplankton was investigated. A selection of phytoplankton species, occurring in habitats similar to that of Stratiotes, was used: two cyanobacterial strains (toxic and non-toxic Microcystis aeruginosa), one green alga (Scenedesmus obliquus) and one eustigmatophyte (Nannochloropsis limnetica). The results indicate allelopathic effects of Stratiotes on phytoplankton in six of the eight cases, expressed in an extended duration of the initial biovolume doubling time. The overall inhibitory effect (8–51%) was strain-specific for the two cyanobacteria. We also studied the effect of irradiance on the allelopathic potential of exudates from Stratiotes. Irradiance influenced the response of Scenedesmus only. The inhibitory effect of Stratiotes exudates on the growth of this green alga was stronger at 35 μmol m⁻² s⁻¹ than at 105 μmol m⁻² s⁻¹. We conclude that Stratiotes has allelopathic effects on phytoplankton, and that irradiance can, but does not always determine the extent of the allelopathic inhibition. In our experiments, the sensitivity of cyanobacteria to Stratiotes exudates was not higher than for other phytoplankton strains, but within cyanobacteria, the toxic strain was more sensitive than the non-toxic one.