Amino acid availability determines the ratio of microcystin variants in the cyanobacterium Planktothrix agardhii

Cyanobacteria are capable of producing multiple microcystin variants simultaneously. The mechanisms that determine the composition of microcystin variants in cyanobacteria are still debated. [Asp³]microcystin-RR contains arginine at the position where the more toxic [Asp³]microcystin-LR incorporates leucine. We cultured the filamentous cyanobacterium Planktothrix agardhii strain 126/3 with and without external addition of leucine and arginine. Addition of leucine to the growth medium resulted in a strong increase of the [Asp³]microcystin LR/RR ratio, while addition of arginine resulted in a decrease. This demonstrates that amino acid availability plays a role in the synthesis of different microcystin variants. Environmental changes affecting cell metabolism may cause differences in the intracellular availability of leucine and arginine, which can thus affect the production of microcystin variants. Because leucine contains one nitrogen atom while arginine contains four nitrogen atoms, we hypothesized that low nitrogen availability might shift the amino acid composition in favor of leucine, which might explain seasonal increases in the [Asp³]microcystin LR/RR ratio in natural populations. However, when a continuous culture of P. agardhii was shifted from nitrogen-saturated to a nitrogen-limited mineral medium, leucine and arginine concentrations decreased, but the leucine/arginine ratio did not change. Accordingly, while the total microcystin concentration of the cells decreased, we did not observe changes in the [Asp³]microcystin LR/RR ratio in response to nitrogen limitation.     

Effects of light on the microcystin content of Microcystis strain PCC 7806

Many cyanobacteria produce microcystins, hepatotoxic cyclic heptapeptides that can affect animals and humans. The effects of photosynthetically active radiation (PAR) on microcystin production by Microcystis strain PCC 7806 were studied in continuous cultures. Microcystis strain PCC 7806 was grown under PAR intensities between 10 and 403 micro mol of photons m(-2) s(-1) on a light-dark rhythm of 12 h -12 h. The microcystin concentration per cell, per unit biovolume and protein, was estimated under steady-state and transient-state conditions and on a diurnal timescale. The cellular microcystin content varied between 34.5 and 81.4 fg cell(-1) and was significantly positively correlated with growth rate under PAR-limited growth but not under PAR-saturated growth. Microcystin production and PAR showed a significant positive correlation under PAR-limited growth and a significant negative correlation under PAR-saturated growth. The microcystin concentration, as a ratio with respect to biovolume and protein, correlated neither with growth rate nor with PAR. Adaptation of microcystin production to a higher irradiance during transient states lasted for 5 days. During the period of illumination at a PAR of 10 and 40 micro mol of photons m(-2) s(-1), the intracellular microcystin content increased to values 10 to 20% higher than those at the end of the dark period. Extracellular (dissolved) microcystin concentrations were 20 times higher at 40 micro mol of photons m(-2) s(-1) than at 10 micro mol of photons m(-2) s(-1) and did not change significantly during the light-dark cycles at both irradiances. In summary, our results showed a positive effect of PAR on microcystin production and content of Microcystis strain PCC 7806 up to the point where the maximum growth rate is reached, while at higher irradiances the microcystin production is inhibited.     

Collapse of a Planktothrix agardhii perennial bloom and microcystin dynamics in response to reduced phosphate concentrations in a temperate lake

Planktothrix agardhii dynamics, microcystin concentration and limnological variables were monitored every 2 weeks for 2 years (2004-2006) in a shallow hypereutrophic artificial lake (BNV, Viry-Chatillon, France). Time-series analysis identified two components in the P. agardhii biomass dynamics: (1) a significant decreasing trend in P. agardhii biomass (65% of the overall variance) and (2) a residual component without significant seasonal periodicity. A path-analysis model was built to determine the main factors controlling the P. agardhii dynamics over the period studied. The model explained 66% of P. agardhii biomass changes. The decreasing trend in P. agardhii biomass was significantly related to a decrease in the PO4(3-) concentration resulting from an improved treatment of the incoming watershed surface water. The residual component was related to zooplankton dynamics (cyclopoid abundances), supporting the hypothesis of a top-down control of P. agardhii, but only when the biomass was low. Forty-nine percent of the variability in the microcystin (MC) concentration (min:<0.1 microg equivalent MC-LR L(-1); max: 7.4 microg equivalent MC-LR L(-1)) could be explained by changes in the P. agardhii biomass. The highest toxin content was observed when P. agardhii biomass was the lowest, which suggests changes in the proportion of microcystin-producing and -nonproducing subpopulations and/or the physiological status of cells.     

Stoichiometry of the photosynthetic apparatus and phycobilisome structure of the cyanobacterium Plectonema boryanum UTEX 485 are regulated by both light and temperature

The role of growth temperature and growth irradiance on the regulation of the stoichiometry and function of the photosynthetic apparatus was examined in the cyanobacterium Plectonema boryanum UTEX 485 by comparing mid-log phase cultures grown at either 29 degrees C/150 micromol m(-2) s(-1), 29 degrees C/750 micromol m(-2) s(-1), 15 degrees C/150 micromol m(-2) s(-1), or 15 degrees C/10 micromol m(-2) s(-1). Cultures grown at 29 degrees C/750 micromol m(-2) s(-1) were structurally and functionally similar to those grown at 15 degrees C/150 micromol m(-2) s(-1), whereas cultures grown at 29 degrees C/150 micromol m(-2) s(-1) were structurally and functionally similar to those grown at 15 degrees C/10 micromol m(-2) s(-1). The stoichiometry of specific components of the photosynthetic apparatus, such as the ratio of photosystem (PS) I to PSII, phycobilisome size and the relative abundance of the cytochrome b(6)/f complex, the plastoquinone pool size, and the NAD(P)H dehydrogenase complex were regulated by both growth temperature and growth irradiance in a similar manner. This indicates that temperature and irradiance may share a common sensing/signaling pathway to regulate the stoichiometry and function of the photosynthetic apparatus in P. boryanum. In contrast, the accumulation of neither the D1 polypeptide of PSII, the large subunit of Rubisco, nor the CF(1) alpha-subunit appeared to be regulated by the same mechanism. Measurements of P700 photooxidation in vivo in the presence and absence of inhibitors of photosynthetic electron transport coupled with immunoblots of the NAD(P)H dehydrogenase complex in cells grown at either 29 degrees C/750 micromol m(-2) s(-1) or 15 degrees C/150 micromol m(-2) s(-1) are consistent with an increased flow of respiratory electrons into the photosynthetic intersystem electron transport chain maintaining P700 in a reduced state relative to cells grown at either 29 degrees C/150 micromol m(-2) s(-1) or 15 degrees C/10 micromol m(-2) s(-1). These results are discussed in terms of acclimation to excitation pressure imposed by either low growth temperature or high growth irradiance.     

Influence of environmental and nutritional factors in the production of astaxanthin from Haematococcus pluvialis

Astaxanthin extracted from green algae is desirable in the food and pharmaceutical industries due to its antioxidant properties. The green unicellular clear water microalga Haematococcus pluvialis has a high production rate of astaxanthin; indeed, it contains more than 80% astaxanthin content in its cells. This remarkable astaxanthin production is commonly obtained under stress conditions such as nutrient deficiency (N or P), high NaCl concentrations, variations of temperature, and other factors. In this vein, a great research effort has been oriented to determine optimal conditions for astaxanthin production by H. pluvialis.The objective of the present study was the analysis of environmental factors, such as light intensity, aeration and nutrients on the growth and astaxanthin production of H. pluvialis. Maximum growth of H. pluvialis obtained was 3.5x10(5) cells/ml in BBM medium at 28 degrees C under continuous illumination (177 micromol photon m(-2)s(-1)) of white fluorescent light, with continuous aeration (1.5 v.v.m.). Meanwhile, maximal astaxanthin production was 98 mg/g biomass in BAR medium with continuous illumination (345 micromol photon m(-2)s(-1)), with 1 g/l of sodium acetate and without aeration.     

Abundance of active and inactive microcystin genotypes in populations of the toxic cyanobacterium Planktothrix spp

To investigate the abundance of active and inactive microcystin genotypes in populations of the filamentous cyanobacterium Planktothrix spp., individual filaments were grown as clonal strains in the laboratory and analysed for microcystin synthetase (mcy) genes and microcystin. Twenty-three green-pigmented strains of P. agardhii originating mostly from shallow water bodies fell into two groups, those possessing mcyA and those lacking mcyA. In contrast, all of the 49 strains that were assigned to the red-pigmented P. rubescens contained mcyA. One strain of P. agardhii and eight strains of P. rubescens contained the total microcystin synthetase gene cluster but were found inactive in microcystin synthesis. To investigate the natural abundance of inactive mcy genotypes in P. rubescens individual filaments sampled from Lake Irrsee and Lake Mondsee (Austria) were analysed directly for the presence of mcyA and microcystin by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry. All filaments assigned to P. rubescens contained mcyA. The proportion of inactive microcystin genotypes in populations with a low (Irrsee) or high density (Mondsee) of P. rubescens was 5% and 21%, each. The results of this study demonstrate that P. rubescens typically contain mcy genes whereas P. agardhii have a patchy distribution of mcy genes. In both species microcystin producers co-occur with non-microcystin producers due to the absence/inactivation of mcy genes.     

The responses of photosynthesis and oxygen consumption to short-term changes in temperature and irradiance in a cyanobacterial mat (Ebro Delta, Spain)

We have evaluated the effects of short-term changes in incident irradiance and temperature on oxygenic photosynthesis and oxygen consumption in a hypersaline cyanobacterial mat from the Ebro Delta, Spain, in which Microcoleus chthonoplastes was the dominant phototrophic organism. The mat was incubated in the laboratory at 15, 20, 25 and 30 degrees C at incident irradiances ranging from 0 to 1,000 micromol photons m(-2) s(-1). Oxygen microsensors were used to measure steady-state oxygen profiles and the rates of gross photosynthesis, which allowed the calculation of areal gross photosynthesis, areal net oxygen production, and oxygen consumption in the aphotic layer of the mat. The lowest surface irradiance that resulted in detectable rates of gross photosynthesis increased with increasing temperature from 50 micromol photons m(-2) s(-1) at 15 degrees C to 500 micromol photons m(-2) s(-1) at 30 degrees C. These threshold irradiances were also apparent from the areal rates of net oxygen production and point to the shift of M. chthonoplastes from anoxygenic to oxygenic photosynthesis and stimulation of sulphide production and oxidation rates at elevated temperatures. The rate of net oxygen production per unit area of mat at maximum irradiance, J0, did not change with temperature, whereas, JZphot, the flux of oxygen across the lower boundary of the euphotic zone increased linearly with temperature. The rate of oxygen consumption per volume of aphotic mat increased with temperature. This increase occurred in darkness, but was strongly enhanced at high irradiances, probably as a consequence of increased rates of photosynthate exudation, stimulating respiratory processes in the mat. The compensation irradiance (Ec) marking the change of the mat from a heterotrophic to an autotrophic community, increased exponentially in this range of temperatures.     

THE TOXIC CYANOBACTERIUM NOSTOC SP. STRAIN 152 PRODUCES HIGHEST AMOUNTS OF MICROCYSTIN AND NOSTOPHYCIN UNDER STRESS CONDITIONS

The understanding of how environmental factors regulate toxic secondary metabolite production in cyanobacteria is important to guarantee water quality. Very little is known on the regulation of toxic secondary metabolite production in benthic cyanobacteria. In this study the physiological regulation of the production of the toxic heptapeptide microcystin (MC) and the non-toxic related peptide nostophycin (NP) in the benthic cyanobacterium Nostoc sp. strain 152 was studied under contrasting environmental conditions. I used a 2(k) levels factorial design, where k is the number of four factors that have been tested: Reduction in temperature (20 vs. 12°C), irradiance (50 vs. 1 μmol · m(-2) · s(-1)), P-PO(4) (144 vs. 0.14 μM P-PO(4)), N-NO(3) (5.88 mM vs. N-NO(3) free). While the growth rate was reduced more than hundred fold under most severe conditions of temperature, irradiance, and phosphate reduction the production of MC and NP never ceased. The MC and NP contents per cell varied at maximum 5- and 10.6-fold each, however the physiological variation did not outweigh the highly significant linear relationship between the daily cell division rate and the MC and NP net production rates. Surprisingly the MC and NP contents per cell showed a maximum under P-PO(4) reduced and irradiance reduced conditions. Both intra- and extracellular MC and NP concentrations were negatively related to P-PO(4) and irradiance. It is concluded that the proximate factor behind maximal cellular MC and NP contents is physiological stress.     

Photoautotrophic culture of Coffea arabusta somatic embryos: photosynthetic ability and growth of different stage embryos

Coffea arabusta somatic embryos were cultured and development of stomata, rate of CO2 fixation or production, chlorophyll content and chlorophyll fluorescence were studied in embryos at different stages of development. Cotyledonary and germinated embryos have photosynthetic capacity, although pretreatment at a high photosynthetic photon flux (PPF) (100 micromol m(-2) s(-1)) for 14 d increased photosynthetic ability. Except in a very small number of cases, stomata did not develop fully in precotyledonary stage embryos and were absent in torpedo stage embryos. Low chlorophyll content (90-130 microg g(-1) fresh mass) was noted in torpedo and precotyledonary stage embryos compared with cotyledonary and germinated embryos (300-500 microg g(-1) fresh mass). Due to the absence of stomata and low chlorophyll content in the torpedo and precotyledonary stage embryos, the photosynthetic rate was low and, in some cases, CO2 production was observed. These data suggest that the cotyledonary stage is the earliest stage that can be cultured photoautotrophically to ensure plantlet development. When grown photoautotrophically (in a sugar-free medium with CO2 enrichment in the culture headspace and high photosynthetic photon flux), torpedo and precotyledonary stage embryos lost 20-25% of their initial dry mass after 60 d of culture. However, in cotyledonary and germinated embryos, the dry mass of each embryo increased by 10 and 50%, respectively. By using a porous supporting material, growth (especially root growth) was increased in cotyledonary stage embryos. In addition, photoautotrophic conditions, high PPF (100-150 micromol m(-2) s(-1)) and increased CO2 concentration (1100 micromol mol(-1)) were found to be necessary for the development of plantlets from cotyledonary stage embryos.     

Enhancement of photosynthetic O2 evolution in Chlorella vulgaris under high light and increased CO2 concentration as a sign of acclimation to phosphate deficiency.

The photosynthetic oxygen evolution of Chlorella vulgaris (Beijer.) cells taken from phosphate-deficient (-P) and control cultures was measured during 8 days of culture growth. Under inorganic carbon concentration (50 microM) in the measuring cell suspension and irradiance (150 micromol m(-2) s(-1)), the same as during culture growth, there were no marked differences in the photosynthetic O2 evolution rate between the -P cells and the controls. The much slower growth of -P cultures indicated that the utilization of absorbed photosynthetically active radiation (PAR) in the CO2 assimilation and biomass production were in -P cells less efficient than in the controls. Alga cells under the phosphorus stress utilized more of the absorbed PAR in the nitrate reduction than the control cells. However, under conditions of more efficient CO2 supply (inorganic carbon concentration 150 microM, introducing of exogenous carbonic anhydrase to the measuring cell suspension) and under increased irradiance (500 micromol m(-2) s(-1)), the photosynthetic O2 evolution in -P cells reached a higher rate than in the controls. The results suggest that in -P cells the restricted CO2 availability limits the total photosynthetic process. But under conditions more favorable for the CO2 uptake and under high irradiance, the -P cells may reveal a higher photosynthetic oxygen evolution rate than the controls. It is concluded that an increased potential activity of the photosynthetic light energy absorption and conversion in the C. vulgaris cells from -P cultures is a sign of acclimation to phosphorus stress by a sun-type like adaptation response of the photosynthetic apparatus.     

Regulation of photosynthesis and oxygen consumption in a hypersaline cyanobacterial mat (Camargue, France) by irradiance, temperature and salinity

Short-term effects of irradiance (0-1560 micromol photons m(-2) s(-1)), temperature (10-25 degrees C), and salinity (40-160) on oxygenic photosynthesis and oxygen consumption in a hypersaline mat (Salin-de-Giraud, France) were investigated with microsensors under controlled laboratory conditions. Dark O(2) consumption rates were mainly regulated by the mass transfer limitations imposed by the diffusive boundary layer. Areal rates of net photosynthesis increased with irradiance and saturated at irradiances >400 micromol photons m(-2) s(-1). At low irradiances, oxygen consumption increased more strongly with temperature than photosynthesis, whereas the opposite was observed at saturating irradiances. Net photosynthesis vs. irradiance curves were almost unaffected by decreasing salinity (100 to 40), whereas increasing salinities (100 to 160) led to a decrease of net photosynthesis at each irradiance. Dark O(2) consumption rates, maximal gross and net photosynthesis at light saturation were relatively constant over a broad salinity range (60-100) and decreased at salinities above the in situ salinity of 100. Within the range of natural variation, temperature was more important than salinity in regulating photosynthesis and oxygen consumption. At higher salinities the inhibitory impact of salinity on these processes and therefore the importance of salinity as a regulating environmental parameter increased, indicating that in more hypersaline systems, salinity has a stronger limiting effect on microbial activity.     

Effect of photon flux density and temperature on the production of halogenated monoterpenes by Plocamium cartilagineum (Plocamiaceae, Rhodophyta)

The effect of different photon flux densities (PFD) and temperatures on the relative growth rate (RGR) and the concentration of three halogenated monoterpenes in samples of Plocamium cartilagineum L.( Dixon), a marine alga (Rhodophyceae), were studied. The highest RGR (22.8 +/- 0.04 d(-1)) was obtained at 15 degreebC and 41 ,mol m(-2) s)(-1) of PFD and the lowest (18.0 +/- 0.12 d(-1)) was obtained at 18 degrees C and 120 micromol m(-2) s(-1). The different temperatures and light used in assays did not affect significantly the production of organic compounds. The production of mertensene and violacene was not affected significantly. However, compound 1 reached the highest concentration at 15 degrees C and 65 micromol m(-2) s(-1)). The relationship between growth and production of monoterpenes of P. cartilagineum and the effect of temperature and the PFD were analyzed.     

Effects of light, temperature, nitrate, orthophosphate, and bacteria on growth of and hepatotoxin production by Oscillatoria agardhii strains

The effects of bacteria, temperature, light, nitrate, and orthophosphate on growth of and hepatotoxin (desmethyl-3-microcystin-RR) production by Oscillatoria agardhii strains were studied under laboratory conditions. Strains were cultivated in Z8 medium under continuous illumination. Growth was determined by measuring dry weight and chlorophyll a, while toxin was analyzed by high-performance liquid chromatography. Two of the three toxic cultures studied produced more toxins in axenic than in nonaxenic cultures. High toxin production correlated with high nitrogen concentrations (test range, 0.42 to 84 mg of N per liter) and low light intensity (test range, 12 to 95 microeinsteins/m2 per s). Toxin production depended on phosphorus concentration at low levels of phosphorus (0.1 to 0.4 mg of P per liter) and higher concentrations had no additional effect. The optimum temperature for toxin production and growth of green O. agardhii was 25 degrees C. Red O. agardhii produced almost similar amounts of toxin at temperatures of 15 to 25 degrees C. The lowest toxin production by both strains was at 30 degrees C.     

Morphological and stomatal responses of Norway spruce foliage to irradiance within a canopy depending on shoot age

Morphological and stomatal responses of Norway spruce (Picea abies) foliage to light availability were studied in respect to shoot age. Needle minor diameter (D(1), anatomical width), major diameter (D(2), anatomical thickness), dry weight (M), and tissue density index (I(D)) increased, and needle flatness (Fl) and specific leaf area (SLA) decreased with foliage age, while shade foliage demonstrated higher morphological plasticity as compared to sun foliage. Needle minor diameter, dry weight, and the ratio of total to projected leaf area increased, and needle flatness and specific leaf area decreased with daily average photosynthetic photon flux density (Q(D)). The current-year foliage exhibited the highest variation with irradiance, while the morphological plasticity decreased with needle ageing. The morphological characteristics of needles were independent of irradiance if Q(D) was above 300 μmol m(-2) s(-1). D(1) was the only linear needle characteristic which significantly changed with light availability within a canopy, and thus determined needle flatness, SLA, as well as the ratio of total to projected leaf area (TLA/PLA). Needle flatness was a characteristic responding most sensitively to the photosynthetic photon flux density, R(2) was 0.68, 0.44, and 0.49 for the current-year, 1-year-old, and 2-year-old foliage, respectively. TLA/PLA ranged from 2.2 to 4.0 depending on D(1). Variation in SLA in response to light availability can be attributed to changes both in needle shape and tissue density. Stomatal responses to photosynthetic photon flux density (Q(P)) depended on foliage type (sun or shade) and age. Sun needles demonstrated higher daily maximum leaf conductances to water vapour compared to shade needles. The shade needles responded more sensitively to changes in Q(P) at dawn and sunset than the sun needles, while older needles of both foliage types exhibited faster stomatal responses. The light-saturation of leaf conductance (g(L)) was achieved by 20 μmol m(-2) s(-1) for shade foliage, and approximately by 50 μmol m(-2) s(-1) for sun foliage. As a rule, g(L) changed in response to irradiance faster in the evening, i.e. at decreasing irradiance. Stomata were not usually completely closed in the dark before sunrise and after sunset, the phenomenon being more pronounced in older shoots and sun needles. Nightly water losses from spruce foliage are attributable primarily to older shoots, and are related to age-dependent changes in stomatal responsiveness.     

青藏高原药用植物唐古特山莨菪和唐古特大黄光合作用对强光的响应

与唐古特大黄相比,唐古特山莨菪的表观光合量子效率(AQY)较高,但最大净光合速率(Pmax)较低。在光强小于1200μmolm-2s-1时,后者用于碳同化的电子传递占总光合电子传递的比例(JC/JF)比前者高,而分配于光呼吸的电子传递(JO/JF)及Rubisco氧化和羧化速率的比值(VO/VC)则相反;光强大于1200μmolm-2s-1以后两种植物的这些参数都趋向稳定。随光强增加,后者叶片吸收光能分配于热耗散(D)的增加斜率较前者高,表明两高山植物对强辐射的适应方式略有不同。加强光呼吸途径的耗能代谢和PSII天线热耗散份额是唐古特山莨菪适应高原强辐射的主要方式,而提高叶片光合能力则是唐古特大黄的一种适应方式。     

热带季节雨林冠层树种绒毛番龙眼的光合生理生态特性

采用Li-6400便携式光合作用测定仪,对西双版纳热带季节雨林冠层树种绒毛番龙眼成树树冠上、中、下3层叶片进行了测定,分析西双版纳热带季节雨林冠层树木的光合作用.结果表明,绒毛番龙眼成树具有喜光的光合特性,光饱和点较高(1 000～1 500 μmol·m^-2·s^-1),而光补偿点较低(7.7～15.3 μmol·m^-2·s^-1),对光环境有较强的适应和调节能力,光合有效辐射是影响绒毛番龙眼光合日进程的关键因子;12月,叶片处于成熟期,生长良好,光合能力较强,树冠上层净光合速率（P_n）日变化为单峰型,最大净光合速率（A_max）约为8.9 μmol CO₂·m^-2·s^-1;4月处于新老树叶更替期,光合能力下降,树冠上层P_n日变化为双峰型,中午出现“午休”现象,树冠上层A_max约为4.3 μmol CO₂·m^-2·s^-1;7月上、中层叶片P_n为单峰型,下层出现“午休”.如人为使CO₂浓度在短期内迅速升高,则绒毛番龙眼的P_n会增加,而气孔导度和蒸腾速率降低;CO₂浓度从400 μmol·mol^-1升高到800 μmol·mol^-1时,干季水分利用效率（WUE）提高约50%～100％,雨季WUE较低.     

Microcystins in natural blooms and laboratory cultured Microcystis aeruginosa from Laguna de Bay,Philippines

Laguna de Bay, the largest freshwater lake in the Philippines, experiences periodic blooms of the cyanobacteria Microcystis aeruginosa. Blooms of these cyanobacteria in 1996, 1998 and 1999 were sampled. HPLC and MALDI-TOF mass spectrometry were used to analyze for microcystins. A total of 16 structural variants of the toxin were isolated from the samples with microcystin LR (MC-LR) as the most abundant variant in the samples from 1996 and 1999 making up 77 to 85% of the total, respectively. MC-RR was the dominant variant in the 1998 bloom making up 38%. The samples from 1996 had the highest total toxin concentration (4049 microg g(-1)) followed by those from 1998 (1577 microg g(-1)) and 1999 (649 microg g(-1)). A strain of M. aeruginosa previously isolated from the lake was also cultured in the laboratory under different nitrogen concentrations (1, 3 and 6 mg L(-1)) and elevated phosphorus concentration (0.5 mg L(-1)) to determine the influence of these factors on toxin production. A total of 9 different structural variants of microcystin were isolated from the laboratory cultures with MC-LR consisting more than 75% of the total in all treatments. No significant differences in the total toxin concentration as well as the % distribution of the different variants among treatments were observed. However, the strain of M. aeruginosa cultured in the laboratory had from 3 to 20 times higher total microcystin than those harvested from the lake.     

Effects of light, temperature, nitrate, orthophosphate, and bacteria on growth of and hepatotoxin production by Oscillatoria agardhii strains.

The effects of bacteria, temperature, light, nitrate, and orthophosphate on growth of and hepatotoxin (desmethyl-3-microcystin-RR) production by Oscillatoria agardhii strains were studied under laboratory conditions. Strains were cultivated in Z8 medium under continuous illumination. Growth was determined by measuring dry weight and chlorophyll a, while toxin was analyzed by high-performance liquid chromatography. Two of the three toxic cultures studied produced more toxins in axenic than in nonaxenic cultures. High toxin production correlated with high nitrogen concentrations (test range, 0.42 to 84 mg of N per liter) and low light intensity (test range, 12 to 95 microeinsteins/m2 per s). Toxin production depended on phosphorus concentration at low levels of phosphorus (0.1 to 0.4 mg of P per liter) and higher concentrations had no additional effect. The optimum temperature for toxin production and growth of green O. agardhii was 25 degrees C. Red O. agardhii produced almost similar amounts of toxin at temperatures of 15 to 25 degrees C. The lowest toxin production by both strains was at 30 degrees C.     

Allelopathic activity of Spirogyra sp.: stimulating bloom formation and toxin production by Oscillatoria agardhii in some irrigation canals, Egypt

It has often been observed that irrigation canals in Egypt thatcontain Spirogyra are covered with blooms of Oscillatoria duringthe summer season. These blooms do not occur in the canals notcontaining Spirogyra. Thus, two irrigation canals, one containingSpirogyra and another not containing Spirogyra, were chosento study the possible allelopathic activity of Spirogyra onthe growth of Oscillatoria agardhii. The growth of O. agardhiiand other associated cyanobacterial species was followed inthese two canals during the period May–September 2000.The results revealed that O. agardhii was dominant and formedblooms in the canals containing Spirogyra, while it had a moderate/rareoccurrence in those not containing Spirogyra. To confirm thestimulatory allelopathic activity of this green alga on thegrowth of and microcystin production by O. agardhii, a laboratoryexperiment was run by growing O. agardhii with different concentrationsof Spirogyra aqueous extract. The results showed that the growthof and microcystin production by O. agardhii increased withincreasing concentrations of Spirogyra aqueous extract. Thisfinding demonstrates the allelopathic activity of the greenalga Spirogyra stimulating the growth of and toxin productionby O. agardhii. Such a biotic factor should be taken into considerationwhen cyanobacterial blooms are monitored in freshwater bodies.