Effects of arsenate on the growth and microcystin production of Microcystis aeruginosa isolated from Taiwan as influenced by extracellular phosphate

Arsenic pollution and eutrophication are both prominent issues in the aquaculture ponds of Taiwan. It is important to study the effects of arsenic on algal growth and toxin production in order to assess the ecological risk of arsenic pollution, or at least to understand naturally occurring ponds. The sensitivity of algae to arsenate has often been linked to the structural similarities between arsenate and phosphate. Thus, in this study we examined the effects of arsenate (10(-8) to 10(-4) M) on Microcystis aeruginosa TY-1 isolated from Taiwan, under two phosphate regimes. The present study showed that M. aeruginosa TY-1 was arsenate tolerant up to 10(-4) M, and that this tolerance was not affected by extracellular phosphate. However, it seems that extracellular phosphate contributed to microcystin production and leakage by M. aeruginosa in response to arsenate. Under normal phosphate conditions, total toxin yields after arsenate treatment followed a typical inverted U-shape hormesis, with a peak value of 2.25 +/- 0.06 mg L-1 in the presence of 10(-7) M arsenate, whereas 10(-8) to 10(-6) M arsenate increased leakage of similar to 75% microcystin. Under phosphate starvation, total toxin yields were not affected by arsenate, while 10(-6) and 10(-5) M arsenate stimulated microcystin leakage. It is suggested that arsenate may play a role in the process of microcystin biosynthesis and excretion. Given the arsenic concentrations in aquaculture ponds in Taiwan, arsenate favors survival of toxic M. aeruginosa in such ponds, and arsenate-stimulated microcystin production and leakage may have an impact on the food chain.

Seasonal changes, life cycle, and production of a psychrophilic chironomid (Propsilocerus akamusi) in a Chinese Lake

Seasonal changes, life cycle, and production of a psychrophilic Chironomidae species, Propsilacerus akamusi (Tokunage), were studied in eutrophic Lake Donghu. The P akamusi population was characterized by a single annual reproduction period during late November to December, and the larval growth mainly occurred in winter. Most of P akamusi were univoltine, while some of them came to emergence in two years or more. The average density and biomass were 318.9 ind./m(2) and 0.57 g dry weight /m(2) during January 1998 to June 2000, respectively, but these values did not include any summer measurement,; since the larvae aestivated in the deep sediment layer and could not be sampled routinely in summer. The annual production of P akamusi was 2.73g dry weight/m(2), and the corresponding production/biomass ratio was 4.60.

Simultaneous production of low- and high-frequency sounds by neonatal finless porpoises

Phocoenids are generally considered to be nonwhistling species that produce only high-frequency pulsed sounds. Here our results show that neonatal finless porpoises (Neophocaena phocaenoides) frequently produce clear low-frequency (2-3 kHz) pulsed signals, without distinct high-frequency energy, just after birth and can produce both low- (2-3 kHz) and high-frequency (>100 kHz) pulsed signals simultaneously until about 20 days postnatal. The results indicate that low-frequency signals of neonatal finless porpoises are not an early form of high-frequency signals and suggest that low- and high-frequency signals may be produced by different sound production mechanisms. (C) 2008 Acoustical Society of America.

Effects of iron on growth, pigment content, photosystem II efficiency, and siderophores production of Microcystis aeruginosa and Microcystis wesenbergii

Changes in growth, photosynthetic pigments, and photosystem II (PS II) photochemical efficiency as well as production of siderophores of Microcystis aeruginosa and Microcystis wesenbergii were determined in this experiment. Results showed growths of M. aeruginosa and M. wesenbergii, measured by means of optical density at 665 nm, were severely inhibited under an iron-limited condition, whereas they thrived under an iron-replete condition. The contents of chlorophyll-a, carotenoid, phycocyanin, and allophycocyanin under an iron-limited condition were lower than those under an iron-replete condition, and they all reached maximal contents on day 4 under the iron-limited condition. PS II photochemical efficiencies (maximal PS II quantum yield), saturating light levels (I-k ) and maximal electron transport rates (ETRmax) of M. aeruginosa and M. wesenbergii declined sharply under the iron-limited condition. The PS II photochemical efficiency and ETRmax of M. aeruginosa rose , whereas in the strain of M. wesenbergii, they declined gradually under the iron-replete condition. In addition, I-k of M. aeruginosa and M. wesenbergii under the iron-replete condition did not change obviously. Siderophore production of M. aeruginosa was higher than that of M. wesenbergii under the iron-limited condition. It was concluded that M. aeruginosa requires higher iron concentration for physiological and biochemical processes compared with M. wesenbergii, but its tolerance against too high a concentration of iron is weaker than M. wesenbergii.

Effects of nonylphenol on the growth and microcystin production of Microcystis strains

Both organic pollution and eutrophication are prominent environmental issues concerning water pollution in the world. It is important to reveal the effects of organic pollutants on algal growth and toxin production for assessing ecological risk of organic pollution. Since nonylphenol (NP) is a kind of persistent organic pollutant with endocrine disruptive effect which exists ubiquitously in environments, NP was selected as test compound in our study to study the relationship between NP stress and Microcystis growth and microcystin production. Our study showed that responses of toxic and nontoxic Microcystis aeruginosa to NP stress were obviously different. The growth inhibition test with NP on M. aeruginosa yielded effect concentrations EbC50 values within this range of 0.67-2.96 mg/L. The nontoxic M. aeruginosa strains were more resistant to NP than toxic strains at concentration above 1 mg/L. Cell growth was enhanced by 0.02-0.2 mg/L NP for both toxic and nontoxic strains, suggesting a hormesis effect of NP on M. aeruginosa. Both toxic and nontoxic strains tended to be smaller with increasing NP. But with the increased duration of the experiment, both the cell size and the growth rate began to resume, suggesting a quick adaptation of M. aeruginosa to adverse stress. NP of 0.05-0.5 mg/L significantly promoted microcystin production of toxic strain PCC7820, suggesting that NP might affect microcystin production of some toxic M. aeruginosa in the field. Our study showed that microcystin excretion was species specific that up to 75% of microcystins in PCC7820 were released into solution, whereas > 99% of microcystins in 562 remained in algal cells after 12 days' incubation. NP also significantly influenced microcystin release into cultural media. The fact that NP enhanced growth and toxin production of M. aeruginosa at low concentrations of 0.02-0.5 mg/L that might be possibly found in natural freshwaters implies that low concentration of NP may favor survival of M. aeruginosa in the field and may play a subtle role in affecting cyanobacterial blooms and microcystin production in natural waters. (c) 2006 Elsevier Inc. All rights reserved.

Improvement in growth and toxin production of Alexandrium tamarense by two-step culture method

The growth and toxin content of the dinoflagellate Alexandrium tamarense ATHK was markedly affected by culture methods. In early growth phase at lower cell density static or mild agitation methods were beneficial to growth, but continuous agitation or aeration, to some extent, had an adverse effect on cell growth. Static culture in 2 L Erlenmeyer flasks had the highest growth rate (0.38 d(-1)) but smaller cell size compared with other culture conditions. Cells grown under aerated conditions possessed low nitrogen and phosphorus cell yields, namely high N and P cell-quota. At day 18, cells grown in continuous agitated and 1 h aerated culture entered the late stationary phase and their cellular toxin contents were higher (0.67 and 0.54 pg cell(-1)) compared with cells grown by other culture methods (0.27-0.49 pg cell(-1)). The highest cell density and cellular toxin content were 17190 cells mL(-1) and 1.26 pg cell(-1) respectively in an airlift photobioreactor with two-step culture. The results indicate that A. tamarense could be grown successfully in airlift photobioreactor by a two-step culture method, which involved cultivating the cells statically for 4 days and then aerating the medium. This provides an efficient way to enhance cell and toxin yield of A. tamarense.

Effects of solar ultraviolet radiation on the photochemical efficiency, photosynthetic pigments and biomass production of Spirulina platensis

Effects of solar ultraviolet radiation (UVR) on Spirulina platensis were studied by investigating its photochemical efficiency, photosynthetic pigments and biomass production while exposed to full spectrum solar radiation or depleted of UVR for understanding how and to what extent UVR influences its photosynthetic physiology and production. It was found that UVR brought about an extra inhibition of photochemical efficiency by 26%-30%. The greatest inhibition of photochemical efficiency in S. platensis was observed at noontime, and then recovered to some extent in late afternoon no matter which treatment they were exposed to. The contents of chlorophyll a, phycocyanin and carotenoids increased during initial stage of the exposure, but decreased with elongated exposure. UVR decreased the biomass yield by about 6%. It indicated that filtering out UVR of solar radiation would raise the productivity of S. platensis, which is an important factor that should be considered in the production.

Life history and production of Sphaerium lacustre (Mollusca : Sphaeriidae) in a shallow subtropical Chinese lake

A fingernail clam, Sphaerium lacustre, was studied in subtropical Lake Donghu from June 1999 to May 2000. The S. lacustre population was characterized by a single annual reproduction period starting in March and ending in October; the population comprised three size groups, of which the 1999 cohort was dominant. The annual average density and biomass were 100.2 ind./m(2) and 12.11 g/m(2), respectively The annual production was 43.02 wet weight g/m(2), and the corresponding annual production/biomass ratio was 3.55.

Effects of nitrogen forms on the production of cyanobacterial toxin microcystin-LR by an isolated Microcystis aeruginosa

A cyanobacterial strain, which produced high content of microcystin-LR (MC-LR) but no rnicrocystin-RR (MC-RR), was isolated from the hypertrophic Dianchi Lake in China and identified as Microcystis aeruginosa DC-1. Effects of nitrogen containing chemicals and trace elements on the growth and the production of MC-LR by this strain were Studied. In the presence of bicine, compared with urea and ammonium, nitrate greatly promoted the growth and the production of MC-LR. However, leucine and arginine, which were the constitutional components in the molecular structure of MC-LR or RR, inhibited the production of MC-LR. Iron and silicon up to 10mg/L had little effects on the growth of M. aeruginosa DC-1, but the production of MC-LR was apparently enhanced. Under all conditions studied here, only MC-LR but no RR was detected within the cells of M. aeruginosa DC-1. Thus, chemical forms of nitrogen, rather than the usually concerned the total nitrogen, Lind trace elements played important roles in the production of MC toxins during cyanobacterial blooms.

Growth and production of free-living heterotrophic nanoflagellates in a eutrophic lake - Lake Donghu, Wuhan, China

In situ growth of heterotrophic nanoflagellates (HNF) in Lake Donghu, a eutrophic shallow lake in mainland China, was studied from January 1999 to March 2000 using a modified Weisse protocol. The study results indicated that the growth rates of HNF showed pronounced seasonal variation (-0.37-1.25 d(-1)), reaching the maximum during spring to early summer. When the water temperature was higher than 25.5 degreesC, HNF growth was inversely proportional to water temperature. There was an effect by bacterial abundance and autotrophic picoplankton on HNF growth that depended on location. HNF biomass was the highest in late spring, and the HNF production ranged from -2.25 to 35.45 mg l(-1) d(-1) with mean of 3.17 mg l(-1) d(-1). When considered in the context of biomass and production data for zooplankton in Lake Donghu, it was evident that HNF contributed significantly to the total zooplankton production in Lake Donghu. These in situ studies indicate that temperature and food supply are the major determinants of HNF abundance and productivity.

Singlet-triplet transition in quantum dots confined by triangular and bowl-like potentials: the effect of electric fields

We theoretically investigate the energy spectra of two-electron two-dimensional (2e 2D) quantum dots (QDs) confined by triangular potentials and bowl-like potentials in a magnetic field by exact diagonalization in the framework of effective mass theory. An in-plane electric field is,found to contribute to the singlet-triplet transition of the ground state of the 2e 2D QDs confined by triangular or bowl-like potentials in a perpendicular magnetic field. The stronger the in-plane electric field, the smaller the magnetic field for the total spin of the ground states in the dot systems to change from S = 0 to S = 1. However, the influence of an in-plane electric field on the singlet-triplet transition of the ground state of two electrons in a triangular QD modulated by a perpendicular magnetic field is quite small because the triangular potential just deviates from the harmonic potential well slightly. We End that the strength of the perpendicular magnetic field needed for the spin singlet-triplet transition of the ground state of the QD confined by a bowl-like potential is reduced drastically by applying an in-plane electric field.

Observation of electric current induced by optically injected spin current

Linearly polarized light at normal incidence injects a spin current into a strip of two-dimensional electron gas with Rashba spin-orbit coupling. The authors report observation of an electric current when such light is shed on the vincinity of the junction in a crossbar-shaped InGaAs/InAlAs quantum well Rashba system. The polarization dependence of this electric current was experimentally observed to be the same as that of the spin current. The authors attribute the observed electric current to the scattering of the optically injected spin current at the crossing. (c) 2007 American Institute of Physics.

Effects of electric field on the electronic structure and optical properties of quantum rods with wurtzite structure

The Hamiltonian of wurtzite quantum rods with an ellipsoidal boundary under electric field is given after a coordinate transformation. The electronic structure and optical properties are studied in the framework of the effective-mass envelope-function theory. The quantum-confined Stark effect is illustrated by studying the change of the electronic structures under electric field. The transition probabilities between the electron and hole states decrease sharply with the increase of the electric field. The polarization factor increases with the increase of the electric field. Effects of the electric field and the shape of the rods on the exciton effect are also investigated. The exciton binding energy decreases with the increase of both the electric field and the aspect ratio. In the end, considering the exciton binding energy, we calculated the band gap variation of size- and shape-controlled colloidal CdSe quantum rods, which is in good agreement with experimental results.

Formation mechanism of electric field domains

Hydrostatic pressure measurements are used to investigate the formation mechanism of electric field domains in doped weakly-coupled GaAs/AlAs superlattices. For the first plateau-like region in the I-V curve, two kinds of sequential resonant tunnelling are observed. For P<2 kbar the high-field domain is formed by the Gamma-Gamma process, while for P>2 kbar the high-field domain is formed by the T-X process. For the second plateau-libe region, the high-field domain is attributed to Gamma-X sequential resonant tunnelling. (C) 1998 Elsevier Science B.V. All rights reserved.

The effects of electric field on the electronic structure of a semiconductor quantum dot

The effect of electric field on the electronic structure of a spherical quantum dot is studied in the framework of the effective-mass envelope-function theory. The dependence of the energy of electron states and hole states on the applied electric field and on the quantum dot size is investigated; the mixing of heavy holes and light holes is taken into account. The selection rule for the optical transition between the conduction band and valence band states is obtained. The exciton binding energies are calculated as functions of the quantum dot radius and the strength of the electric field. (C) 1998 American Institute of Physics.