Comparative structure of the gas-vacuoles of blue-green algae

Summary An investigation was made of 5 species of blue-green algae reported to contain gas-vacuoles. All organisms were grown and harvested under standard conditions. Gas-vacuoles were characterised as reddish structures which are destroyed by applying pressure. Using a simple direct preparation technique gascylinders were observed with the transmission electron microscope in gas-vacuolate cells. Gas-vacuoles were present in the strains of Anabaena flos-aquae, Gloeotrichia echinulata and Oscillatoria agardhii studied and absent from Microcystis aeruginosa and Nostoc linckia. The reddish, refractile central area of N. linckia and M. aeruginosa cells was tentatively identified as nucleoplasm. Gas-vacuoles are collections of gas-cylinders 70 m wide, which in A. flos-aquae and G. echinulata are clearly bounded by photosynthetic lamellae and associated with -granules. The presence of bounding photosynthetic lamellae in these species is suggested as a causal factor of the unusual optical properties of their gas-vacuoles. The range of lengths of gas-cylinders in G. echinulata and O. agardhii is from 100 m to 500 m and in A. flos-aquae it is from 100 m to 1300 m. The percentage of cell volume occupied by gas-vacuoles was estimated by direct measurement. In A. flos-aquae and G. echinulata it was 22%. In O. agardhii gas-cylinders were not clearly associated with photosynthetic lamellae and -granules and occupied 39% of cell volume. Gascylinder membranes showed reasonable preservation in KMnO₄ and excellent preservation in OsO₄. The widths of membranes after treatment with these two fixatives was 3 m and 2 m respectively.     

Functional response of Anodonta anatina feeding on a green alga and four strains of cyanobacteria, differing in shape, size and toxicity

We studied the functional response of the freshwater unionid bivalve Anodonta anatina, feeding on five phytoplankton strains differing in food quality: the small green alga Scenedesmus obliquus, a toxic and a non-toxic strain of the filamentous cyanobacterium Planktothrix agardhii and a toxic and a non-toxic strain of the coccoid cyanobacterium Microcystis aeruginosa. On S. obliquus, A. anatina had a type II functional response with a maximum mass-specific ingestion rate (IR_max) of 5.24 mg C g DW⁻¹ h⁻¹ and a maximum mass-specific clearance rate (CR_max) of 492 (±38) ml g DW⁻¹ h⁻¹, the highest values for all the phytoplankton strains that were investigated. On toxic and non-toxic P. agardhii filaments, A. anatina also had a type II functional response, but IR_max and CR_max were considerably lower (IR_max 1.90 and 1.56 mg C g DW⁻¹ h⁻¹; CR_max 387 (±97) and 429 (±71) ml g DW⁻¹ h⁻¹, respectively) than on S. obliquus. Toxicity of P. agardhii had no effect on the filtration rate of the mussels. On the non-toxic M. aeruginosa (small coccoid cells), we also observed a type II functional response, although a type I functional response fitted almost as good to these data. For the colonial and toxic M. aeruginosa, a type I functional response fitted best to the data: IR increased linearly with food concentration and CR remained constant. CR_max and IR_max values for the (colonial) toxic M. aeruginosa (383 (±40) ml g DW⁻¹ h⁻¹; 3.7 mg C g DW⁻¹ h⁻¹) demonstrated that A. anatina filtered and ingested this cyanobacterium as good as the other cyanobacterial strains. However, on the non-toxic M. aeruginosa we observed the lowest CR_max of all phytoplankters (246 (±23) ml g DW⁻¹ h⁻¹, whereas IR_max was similar to that on toxic M. aeruginosa. The high maximum ingestion rates on S. obliquus and M. aeruginosa indicate a short handling time of these phytoplankton species. The high clearance rates on S. obliquus, toxic M. aeruginosa and P. agardhii reflect a high effort of the mussels to filter these particles out of the water column at low concentrations. The low clearance rates on non-toxic M. aeruginosa may be explained by the small size and coccoid form of this cyanobacterium, which may have impaired A. anatina to efficiently capture the cells. Although A. anatina had relatively high maximum clearance rates on non-toxic and toxic P. agardhii, this cyanobacterium does not seem to be a good food source, because of the observed high rates of pseudofaeces production and hence low ingestion rates.     

Growth characteristics and growth modeling of <Emphasis Type="Italic">Microcystis aeruginosa</Emphasis> and <Emphasis Type="Italic">Planktothrix agardhii</Emphasis> under iron limitation

Although iron is a key nutrient for algal growth just as are nitrogen and phosphorus in aquatic systems, the effects of iron on algal growth are not well understood. The growth characteristics of two species of cyanobacteria, Microcystis aeruginosa and Planktothrix agardhii, in iron-limited continuous cultures were investigated. The relationships between dissolved iron concentration, cell quota of iron, and population growth rate were determined applying two equations, Monod’s and Droop’s equations. Both species produced hydroxamate-type siderophores, but neither species produced catechol-type siderophores. The cell quota of nitrogen for both M. aeruginosa and P. agardhii decreased with decreasing cell quota of iron. The cell quota of phosphorus for M. aeruginosa decreased with decreasing cell quota of iron, whereas those for P. agardhii did not decrease. Iron uptake rate was measured in ironlimited batch cultures under different degrees of iron starvation. The results of the iron uptake experiments suggest that iron uptake rates are independent of the cell quota of iron for M. aeruginosa and highly dependent on the cell quota for P. agardhii. A kinetic model under iron limitation was developed based on the growth characteristics determined in our study, and this model predicted accurately the algal population growth and iron consumption. The model simulation suggested that M. aeruginosa is a superior competitor under iron limitation. The differences in growth characteristics between the species would be important determinants of the dominance of these algal species.     

Phytoplankton succession in a Eutrophic lake with special reference to blue-green algal blooms

Summary An investigation of phytoplankton in Astotin Lake was made between mid-May of 1966 and September of 1967 with particular attention to the ice-free seasons. Astotin Lake is a typical, small eutrophic, kettle lake with shallow, landlocked, hard water in the Canadian prairies. High concentrations of nutrients supported heavy blooms of blue-green algae throughout the summer. The spring communities were dominated by Asterionella formosa in 1966 and by Cyclotella meneghiniana in 1967. Oxygen depletion under ice-cover probably explains the failure of an Asterionella formosa population to appear in 1967. Deficiency of silica and a rise in water temperature apparently caused the decline of the spring pulses of diatoms. Relatively high summer water temperature favoured the blue-green algal blooms and resulted in a high concentration of organic matter. The decomposition of dead Anabaena cells played an important part in the development of subsequent waterblooms, i.e., Microcystis aeruginosa and Aphanizomenon flos-aquae. The sequence of waterblooms of those species was closely related to the change in water temperature. A flos-aquae became incompatible with M. aeruginosa when the temperature fluctuated in a wide range. Most of the non-blue green algae apparently were inhibited by these cyanophyte blooms. Great species diversity appeared intermittently between blooms and a few species of the Scenedesmaceae and the Oocystaceae were relatively compatible to these blooms.Part of a thesis submitted to the Department of Botany, University of Alberta, Edmondton, Alberta, in partial fulfillment of the requirements for the M. Sc. degree.     

Viability of dried filaments, survivability and reproduction under water stress, and survivability following heat and UV exposure in Lyngbya martensiana, Oscillatoria agardhii, Nostoc calcicola, Hormidium fluitans, Spirogyra sp. and Vaucheria geminata

Dried vegetative filaments ofSpirogyra sp.,Vaucheria geminata andNostoc calcicola died within 1/2, 1 and 4 h, respectively; those ofHormidium fluitans, Oscillatoria agardhii andLyngbya martensiana retained under similar storage conditions viability for 3, 5 and 10 d, respectively. The viability of dried vegetative filaments ofL. martensiana, O. agardhii andH. fluitans decreased on storage at 20°C in the dark.L. martensiana andO. agardhii tolerated 0.8 mol/L NaCl. The resistance to desiccation inL. martensiana andO. agardhii exhibited similar dependence as that to frost, to heat and UV light.O. agardhii filaments became slightly broader and their cells developed large number of gas vacuoles when grown in 0.8 mol/L NaCl-containing medium. The water stress imposed on growing algae either on high-agar solid media or in NaCl-containing liquid media reduced hormogonium formation inL. martensiana andO. agardhii, heterocyst and akinete formation inN. calcicola and fragmentation inH. fluitans; it did not induce conjugation inSpirogyra sp. and formation of reproductive organs inV. geminata. In all studied algae the stress reduced at various levels the survival of vegetative parts. Generally, algal body form and composition rather than habitats seem to decide primarily the level of resistance against various stress conditions.     

The effects of the cyanobacterium Microcystis aeruginosa, the cyanobacterial hepatotoxin microcystin–LR, and ammonia on growth rate and ionic regulation of brown trout

Brown trout were exposed for 63 days to five treatments: a control; the purified cyanobacterial hepatotoxin microcystin—LR (MC—LR) (41—57 μg MC—LR 1^?1); lysed toxic Microcystis aeruginosa cells (41–68 μg MC—LR 1^?1 and 288 μg chlorophyll a 1^?1); lysed non—toxic M. aeruginosa cells (non—MC—LR containing and 288 μg chlorophyll a 1^?1); ammonia (65–325 μg NH₃ 1^?1). All treatments produced significantly reduced growth compared to controls (P<0·05, Fisher test). Exposure to ammonia resulted weight loss over the first 7 days followed by weight increase, though at a significantly lower level than in the other treatments. First exposed to lysed toxic M. aeruginosa cells grew less than those exposed to lysed non—toxic cyanobacteria or purified MC—LR. Sodium influx rates after 63 days exposure to purified MC—LR, lysed toxic M. aeruginosa cells, or ammonia showed a significant increase compared to control fish or those exposed to lysed non—toxic M. aeruginosa cells. There were no significant differences in Na⁺ efflux or net Na⁺ uptake rates between treatments. Significant increases in body Na⁺ and Cl^— were seen in fish exposed to lysed toxic M. aeruginosa cells or ammonia. Only fish exposed to ammonia showed a significant increase in body ammonia. Short—term exposure, over 4 h, to lysed toxic cells, non—toxic cells or purified MC—LR resulted in insignificant changes in Na⁺ flux rates compared to controls although there was a significant net Na⁺ loss in fish exposed to ammonia. Chronic exposure of fish to toxic cyanobacterial blooms may result in ionic imbalance and reduced growth.     

Effects of unicellular and colonial forms of toxic Microcystis aeruginosa from laboratory cultures and natural populations on tropical cladocerans

Three life-table experiments, two growth experiments and one feedinginhibition experiment, were performed to study the effects of the toxiccyanobacterium Microcystis aeruginosa on the cladoceransofa tropical lagoon (Jacarepaguá Lagoon, Rio de Janeiro, Brazil).Differentexperimental designs were used to estimate toxic effects of both field samplesand laboratory cultures of Microcystis aeruginosa oncladoceran life history parameters and juvenile growth rates. Effects ofnutritional deficiency could be distinguished from toxic effects in experimentswhere green algae in high carbon concentration were mixed withMicrocystis. Our results show that natural assemblages ofMicrocystis caused much less pronounced toxic effects thanlaboratory cultures and that unicellular forms were more toxic than colonialforms, even though both contained high concentrations of toxins. One possibleexplanation is that colonies were too large to be ingested by the smallMoina micrura and Ceriodaphniacornuta. Feeding inhibition by single cells and small colonies seemsto be another mechanism that contributes to the harmful effects ofMicrocystis on cladocerans, both in the laboratory and inthe field. Thus, caution is needed in extrapolating results from the laboratoryto the field. We did find, however, that toxic algae in natural seston caninhibit growth and reproduction of native cladocerans populations.     

Plasmid distribution among unicellular and filamentous toxic cyanobacteria

Plasmid content of 5 hepatotoxin and 2 neurotoxin producing cyanobacterial strains were analyzed. Among the hepatotoxin-producing strains, Microcystis aeruginosa PCC7820, M. aeruginosa M228 and M. aeruginosa UV027 were found to carry plasmids, whereas other hepatotoxin and neurotoxin producing strains did not harbor any plasmids. Correlations were sought between toxicity and the presence of plasmids in toxic cyanobacteria as a function of age. Aged cultures of M. aeruginosa PCC7820 exhibited toxicity and harbored plasmids. In other cyanobacterial strains, plasmids were not detected. The data add to and support the current understanding that plasmids are probably not involved in toxin production in cyanobacteria.Author for Correspondence     

Comparative study of immunobiological activities ofPseudomonas aeruginosa andBrucella melitensis lipopolysaccharides

The toxic activity ofBrucella melitensis andPseudomonas aeruginosa lipopolysaccharides as well as their behavior as immunogens, mitogens, and interferon inducers have been studied. Although their toxicities were very similar, the former molecule was incapable of eliciting a primary immune response in mice. Rabbit hyperimmunization gave titers half of those obtained withP. aeruginosa lipopolysaccharide. Optimal mitogenic responses of spleen cell cultures were obtained using 10–50 μg/ml and 50–100 μg/ml ofPseudomonas andBrucella lipopolysaccharide, respectively, giving the latter a lower stimulation of³H-thymidine uptake. Interferon titers induced in chickens byBrucella lipopolysaccharide were three times lower than those obtained withPseudomonas lipopolysaccharide.     

N2-fixing cyanobacteria: Why they do not become dominant in shallow hypertrophic lakes

Summary Phytoplankton species shifts and succession phenomenona in lakes of increasing trophic state were considered, using the basic information on the growth kinetics of the species involved. One of the most obvious signs of advanced eutrophication is the dominance of cyanobacteria (blue-green algae). Striking examples are the shallow, hypertrophic Dutch lakes The Veluwerandmeren (e.g., Wolderwijd and Veluwemeer), whereOscillatoria agardhii, a non-N₂-fixing cyanobacterium, has become dominant over the green algae, diatoms and N₂-fixing cyanobacteria (BERGER, 1975).We have studied the natural population ofO.agardhii during the growing season, by using physiological indicators, and could adduce that the natural population was successively growing under phosphorus, light, or nitrogen limitation (ZEVENBOOM and MUR, 1978a,b; ZEVENBOOMet al., 1982). One might expect that during the period of nitrogen limitation the N₂-fixing speciesAphanizomenon flos-aquae would be favoured and would be able to outgrow the nitrogen-limitedO.agardhii. However, in these lakes,A. flos-aquae was present only in few numbers and a succession fromO. agardhii toA. flos-aquae did not occur. Although field observations may give some indication, they cannot give decisive answers to the question which factor is triggering the observed species shifts and species dominance in natural waters. Such answers can only be obtained from growth kinetic and physiological data of the species involved. In our opinion, the most important factor to consider is the availability of light energy, which decreases with increasing eutrophication.The hypothesis was proposed by Mur and coworkers (MURet al., 1978) that in hypertrophic lakes the prevailing light conditions (low light irradiance) are more favourable forO.agardhii, since this species has a much lower requirement of light energy for growth than green algae as a consequence of its lower specific maintenance rate constant, _e (VAN LIERE, 1979; GONS, 1977). Competition experiments, performed withO. agardhii andScenedesmus protuberans under lightlimiting conditions, confirmed the hypothesis (MURet al., 1978), Continuous culture experiments withA. flos-aquae showed that also this species had a higher energy requirement thanO. agardhii (ZEVENBOOM, 1980). The differences were not found in the value of _e, but in the growth efficiency. The higher energy requirement ofA.flos-aquae was expected, since energy is needed for heterocyst production and N₂ fixation. Under light-limiting conditions and nutrient sufficiency (including nitrogen-nitrate) it can thus be expected that the N₂-fixer will be outcompeted by the non-N₂-fixing cyanobacterium. This was indeed observed (ZEVENBOOM et al., 1981).We further investigated the competitive interactions betweenA.flos-aquae, O. agardhii andS. protuberans under different sets of irradiance values and nitrate concentrations. We used the growth kinetic data of the species involved, which were obtained by means of continuous culture experiments (GONS, 1977; VAN LIERE. 1979; VAN LIERE and MUR, 1979; GONS and MUR, 1980; ZEVENBOOM and MUR, 1980; ZEVENBOOMet al., 1980; ZEVENBOOMet al., 1981). The competing species could be placed along the gradients of light irradiance values and nitrate concentrations, their positions being defined by their energy requirements and half-saturation constants for nitrate-limited growth, respectively. Distinct niches for the three species were found with respect to light and nitrate. Under conditions of low irradiance values and low (realistic) nitrate concentrations, nitrogen-limitedO.agardhii was able to outgrowA. flos-aquae andS. protuberans as a consequence of its low energy requirement and its high affinity for nitrate. The growth rates of the last two species were restricted by the limited availability of light. However, at high irradiance values,O.agardhii was inhibited in its growth rate and therefore failed to outgrow the other two species. The competition was then restricted to nitrogen-limitedS.protuberans and light-limitedA.flos-aquae; the latter could dominate at low nitrate concentrations. The results of competition experiments withO.agardhii andA.flos-aquae under different sets of irradiance values and nitrate concentrations agreed well with the niche-model described above (Zevenboom, unpubl. results).In conclusion, kinetic data of growth, obtained with continuous culture experiments, can provide basic information to explain species shifts and dominance in lakes with increasing eutrophication. Nitrogen-limiting conditions favour N₂-fixing cyanobacteria only when sufficient light is available for their growth (in less hypertrophic waters). The trophic state is thus of major importance and decisive with regard to which species will dominate.     

ACUTE TOXICITY OF SOME BLUEGREEN ALGAE TO THE PROTOZOAN PARAMECIUM CAUDATUM1

Four species of bluegreen algae were tested for possible effect on the protozoan Paramecium caudatum Ehrenberg. Toxicity was demonstrated using lyophilized cells of Fischerella epiphytica Ghose and Gloeotrichia echinulata (Smith) Richter. Nostoc linckia (Roth) Bornet & Thuret failed to show any effects when lyophilized but became toxic when sonified. Anabaena flos-aquae (Lyngb.) Bréb. was nontoxic in all tests. G. echinulata was lethal at 0.1 mg·ml^?1 which is comparable to the toxic concentration of Aphanizomenon flos-aquae (L.) Ralfs reported for microcrustaceans.     

Characterization of type II protein secretion (xcp) genes in the plant growth-stimulatingPseudomonas putida, strain WCS358

InPseudomonas aeruginosa, the products of thexcp genes are required for the secretion of exoproteins across the outer membrane. Despite structural conservation of the Xcp components, secretion of exoproteins via the Xcp pathway is generally not found in heterologous organisms. To study the specificity of this protein secretion pathway, thexcp genes of another fluorescent pseudomonad, the plant growth-promotingPseudomonas putida strain WCS358, were cloned and characterized. Nucleotide sequence analysis revealed the presence of at least five genes, i.e.,xcpP, Q, R, S, andT, with homology toxcp genes ofP. aeruginosa. Unlike the genetic organization inP. aeruginosa, where thexcp cluster consists of two divergently transcribed operons, thexcp genes inP. putida are all oriented in the same direction, and probably comprise a single operon. Upstream ofxcpP inP. putida, an additional open reading frame, with no homolog inP. aeruginosa, was identified, which possibly encodes a lipoprotein. Mutational inactivation ofxcp genes inP. putida did not affect secretion, indicating that no proteins are secreted via the Xcp system under the growth conditions tested, and that an alternative secretion system is operative. To obtain some insight into the secretory pathway involved, the amino acid sequence of the N-terminus of the major extracellular protein was determined. The protein could be identified as flagellin. Mutations in thexcpQ andR genes ofP. aeruginosa could not be complemented by introduction of the correspondingxcp genes ofP. putida. However, expression of a hybrid XcpR protein, composed of the N-terminal one-third ofP. aeruginosa XcpR and the C-terminal two-thirds ofP. putida XcpR, did restore protein secretion in aP. aeruginosa xcpR mutant.     

Contribution of benthic blue-green algal recruitment to lake populations and phosphorus translocation

1. A two-season investigation was undertaken to determine the contribution of benthic recruitment to the population development of several species of blue-green algae. 2. Gloevtrichia echinulata populations were shown to be heavily subsidized by benthic recruitment, deriving on average 40% of their planktonic colonies from the benthos. 3. Benthic recruitment of Aphanizomenon flos-aquae, Anabaena flos-aquae and a second Anabaena species contributed less than 2% to planktonic increases, while Microcystis aeruginosa recruitment was negligible. 4. Phosphorus translocation via migrating G. echinulata accounted for most of the phosphorus (P) in the planktonic colonies, and constituted a significant portion of the internal loading of the lake. 5. Estimated P translocation via Aph. flos-aquae was relatively minor, although there was evidence of luxury uptake in the benthos.     

Horizontal distribution of Daphnia pulex in response to toxic and non-toxic algal extracts

We studied the horizontal distribution of Daphnia pulex as aresponse to gradients of toxic extracts of two cyanobacteria,Microcystis aeruginosa and Oscillatoria agardhii and non-toxicextracts of the green alga Scenedesmus obtusiusculus. The experimentwas performed in the dark using a flow through system with fivechambers. Daphnia pulex aggregated at the inlet of Scenedesmusextract, but showed similar responses to extracts of toxic cyanobactenaand food-free medium. Chemical cues seem to affect food search,but do not effect Daphnia pulex's avoidance of toxic algae.     

Grazing on toxic and non-toxic Microcystis aeruginosa PCC7820 by Unio douglasiae and Corbicula fluminea

To explore the potential grazing effects of mussels on Microcystis aeruginosa, a common bloom-forming phytoplankton, Unio douglasiae and Corbicula fluminea were fed with Scenedesmus obliquus, toxic and non-toxic strains of Microcystis aeruginosa as single food and as mixtures in the laboratory. When fed with single foods, U. douglasiae has similar clearance rates on the three algae populations, while C. fluminea has significantly lower clearance rate on toxic M. aeruginosa than those on the other two algae populations. When fed with mixture foods, both the mussels show significantly higher clearance rates than on single foods. The clearance rates of U. douglasiae on the different food mixtures are not significantly different, and C. fluminea has a significantly lower clearance rate on the toxic food mixtures than that on non-toxic food mixtures. Although the relative lower clearance rates of C. fluminea on toxic food, we may still deduce that both the mussels can exert grazing pressure on phytoplankton. The deduction is supported by the composition of the excretion products. The excretion products (faeces and pseudofaeces) of both mussels contained mainly S. obliquus. In both mixed-food treatments, the ratios of S. obliquus to M. aeruginosa in the excrete products are significantly higher than those in the foods. Therefore, it can be concluded that both mussels prefer M. aeruginosa to S. obliquus, and can cause grazing pressure on M. aeruginosa.     

A new rust species ofRoestelia onSorbus collected in China

A rust species ofRoestelia onSorbus collected in China is newly described asR. echinulata. This species is morphologically different from the otherRoestelia spp. and aecial stages ofGymnosporangium species in surface structures of aeciospores and peridial cells. Contribution No. 141, Laboratory of Plant Pathology and Mycology, Institute of Agriculture and Forestry, University of Tsukuba.     

Sensitivity of Two Disjunct Bacterioplankton Communities to Exudates from the Cyanobacterium Microcystis aeruginosa Kützing

Abstract Microcystis aeruginosa Kützing releases a variety of bioactive compounds during growth. This study determined whether bacteria from communities co-occurring (M+) or not (M-) with this cosmopolitan cyanobacterium respond similarly to its products. Fifty M+ bacteria from a M. aeruginosa bloom site (Western Basin of Lake Erie) and 50 M- bacteria from a Microcystis-free site (East Twin Lake, Portage Co., OH) were isolated and grown on Standard Methods Agar. Three levels of testing were performed: chemotaxis, antibiotic response, and 48-h cell abundance. Chemotaxis was compared using capillary tubes placed in contact with bacterial, Standard Methods Broth (SMB) suspensions. The capillary choices were conditioned SMB, M. aeruginosa exudate, and BG-11. M+ bacteria showed significantly greater (Tukey's test, p < 0.005) positive chemotaxis to M. aeruginosa exudate compared to control conditions and to M-strains. The latter showed a negative chemotactic response to M. aeruginosa exudate compared to control conditions. Antibiotic response was tested by sensitivity disk assays, first using M. aeruginosa exudates, whole cells, and homogenized cells, and then placing the disks on bacterial lawns of each strain. M+ bacteria were significantly more resistant to inhibition than M- bacteria (chi-square test, p < 0.01). M. aeruginosa exudate, BG-11 algal medium, SMB, and distilled water effects on 48-h abundance of the strains were compared. The M- community bacteria exhibited significantly lower growth yields (Tukey's comparison of means test, p < 0.005) in M. aeruginosa exudate than did the M+ strains. It is evident that those bacteria co-occurring with M. aeruginosa are more likely to be attracted to it, able to withstand exposure to it, and able to utilize its products without inhibition than are bacteria from communities without previous exposure to this cyanobacterium. Received: 6 December 1999; Accepted: 3 April 2000; Online Publication: 18 July 2000     

Strain-specific Influence of Microcystis Aeruginosa on Food Ingestion and Assimilation of some Cladocerans and Copepods

Ingestion rates where estimated for daphnids, Cyclops spp. and Bosmina (Eubosmina) coregoni thersites fed hepatotoxic and non-toxic M. aeruginosa either separate or mixed with the readily available food alga Ankistrodesmus falcatus. The ingestion rates of hepatotoxic strains of M. aeruginosa are very low compared with those of A. falcatus or non-toxic M. aeruginosa HUB 5-3 fed to Daphnia magna or D. longispina. However, a close relationship between ingestion rate of different M. aeruginosa strains and their toxicity could not be observed. Addition of the toxic strain M. aeruginosa HUB 5-2-4 reduces the ingestion rates of A. falcatus progressively due to increased food rejection by D. magna. Additionally, the assimilation efficiency of M. aeruginosa HUB 5-2-4 is two times lower compared with A. falcatus and M. aeruginosa HUB 5-3 leading to strong starvation.     

Molecular characterization of Cylindrospermopsis raciborskii strains isolated from Portuguese freshwaters

Cylindrospermopsis raciborskii is a toxic bloom forming cyanobacteria that is a common component of the phytoplankton assemblage in temperate freshwaters, as well as in temperate climates. This species is of major concern in public health, due to its known ability to produce toxins, including cylindrospermopsin and paralytic shellfish poisoning toxin (PSP).In this study, M13 PCR fingerprinting, ERIC PCR fingerprinting and amplification of the internal transcribed spacer (ITS) region were used to characterize nine cultured strains of C. raciborskii, sourced from several freshwater lakes and rivers in Portugal, and two other Australian. Strains belonging to other taxa including Microcystis aeruginosa, Aphanizomenon spp., Planktothrix agardhii and Oscillatoria neglecta were also analysed to evaluate the taxonomical potential of the fingerprinting methods.Data obtained from genomic fingerprinting were used to perform hierarchical cluster analysis and demonstrated ability to differentiate strains at intra-specific level. However, the high level of variability prevents their use as an identification tool. ITS amplification displayed intra-specific polymorphism both in number and length of the obtained amplicons, but revealed itself as a good method for strain clustering. The unsuccessful amplification of peptide synthetase (PS) and polyketide synthase (PKS) genes pointed to the inability of Portuguese C. raciborskii strains to produce cylindrospermopsin. HPLC analysis further confirmed this lack of toxicity, since negative results were obtained for cylindrospermopsin and PSP toxins.     

Adaptation of the toxic freshwater cyanobacterium Microcystis aeruginosa to salinity is achieved by the selection of spontaneous mutants

The cyanobacterium Microcystis aeruginosa causes most of the harmful toxic blooms in freshwater ecosystems. Some strains of M. aeruginosa tolerate low‐medium levels of salinity, and because salinization of freshwater aquatic systems is increasing worldwide it is relevant to know what adaptive mechanisms allow tolerance to salinity. The mechanisms involved in the adaptation of M. aeruginosa to salinity (acclimation vs. genetic adaptation) were tested by a fluctuation analysis design, and then the maximum capacity of adaptation to salinity was studied by a ratchet protocol experiment. Whereas a dose of 10 g NaCl L^?1 completely inhibited the growth of M. aeruginosa, salinity‐resistant genetic variants, capable of tolerating up to 14 g NaCl L^?1, were isolated in the fluctuation analysis experiment. The salinity‐resistant cells arose by spontaneous mutations at a rate of 7.3 × 10^?7 mutants per cell division. We observed with the ratchet protocol that three independent culture populations of M. aeruginosa were able to adapt to up to 15.1 g L^?1 of NaCl, suggesting that successive mutation‐selection processes can enhance the highest salinity level to which M. aeruginosa cells can initially adapt. We propose that increasing salinity in water reservoirs could lead to the selection of salinity‐resistant mutants of M. aeruginosa.