Evaluation of Microcystis as food for zooplankton in a eutrophic lake

Four experiments were conducted to evaluate Microcystis as food for zooplankton in Lake Kasumigaura, and the following results were obtained. (1) Moina micrura (Cladocera) showed little growth and no reproduction when the animal was reared with Microcystis cultured in the laboratory. The animal did not grow nor reproduce well when Chlorella was mixed with Microcystis as food. (2) Moina micrura assimilated Microcystis much less than Chlorella when the animal fed on single species of Microcystis or a mixture with Chlorella. (3) Microcystis collected from Lake Kasumigaura could not be utilized by Moina micrura even though the colonies were broken up into edible sizes. However, the alga turned into utilizable food when it was decomposed. (4) No inhibitors of Moina micrura population growth could be found in the non-filtered water of Lake Kasumigaura where Microcystis was blooming heavily. Decomposed Microcystis seemed to be utilized by zooplankton as an important food source in Lake Kasumigaura.     

Acute inhibition of protease and suppression of growth in zooplankter,Moina macrocopa,by Microcystis blooms collected in Central India

Cyanobacterial blooms consisting of Microcystis spp., collected from 14 water-bodies in Central India, and an adapted culture, were studied for likely impact on zooplankton community. When fed with single cells of Microcystis from several locations, in mixtures with Chlorella, population growth of the cladoceran Moina macrocopa was suppressed. Microcystis alone was unsuitable as food. In three cases, bloom extracts enhanced mortality of starved zooplankton. Extracts from several sources inhibited protease activity when trypsin or a crude extract from zooplankton served as enzyme source. Upon fractionation by solid-phase extraction, the C-18 passed extract contained the anti-protease and toxic substances for zooplankton, whereas a methanol eluted fraction retained the trypsin inhibitory substance. The study suggests that production of protease inhibitors by cyanobacteria is a factor responsible for feeding inhibition and mortality in zooplankton, which in turn could regulate the community structure of grazers.     

Anatoxin-a concentration inAnabaena andAphanizomenon under different environmental conditions and comparison of growth by toxic and non-toxicAnabaena-strains — a laboratory study

Anatoxin-a-concentration in cells ofAnabaena- andAphanizomenon-strains and in their growth media were studied in the laboratory in batch cultures at different temperatures, light fluxes, orthophosphate and nitrate concentrations and with different nitrogen sources for growth. Toxin concentrations were detected by HPLC. Also, the growth of the toxicAnabaena-strains was compared to that of a non-toxic one. The non-toxicAnabaena was never found to produce anatoxin-a. The amount of toxin in the cells of the toxic strains was high, often exceeding 1% of their dry weight. High temperature decreased the amount of the toxin regardless of growth. Growth limiting low and growth inhibiting high light decreased the amount of the toxin in the cells ofAnabaena-strains. The highest light flux studied did not limit the growth or decrease the level of the toxin in the cells ofAphanizomenon. Growth in N-free medium (i.e. N₂ fixation) showed that the cells contained more toxin than growth in N-rich medium. Orthophosphate concentration had no effect on toxin levels, although the lowest concentrations limited the growth of all strains studied. The toxic strains tolerated higher temperatures than the non-toxic one, but the non-toxic strain seemed to be more adjustable to high irradiance than the toxic ones. The yields (dry weight) of non-toxic and toxic strains differed significantly in different phosphate concentrations.     

Contaminated sediments in the Elbe estuary: Ecological and economic problems for the Port of Hamburg

The effects of food density on competition between Bosmina longirostris and Bosmina fatalis from Lake Kasumigaura were examined in the laboratory. When the animals were reared with a high concentration of Chlorella, B. fatalis, which is slightly larger than B. longirostris in body length, had larger brood sizes and a higher rate of population growth than B. longirostris. When the species were reared together at high food density, B. fatalis overcame B. longirostris. But B. longirostris won the competition at low food density.The effects of Microcystis on the competition were also examined. When the two species of Bosmina were reared in a mixture of Chlorella and Microcystis, the populations of both species hardly increased, although individual B fatalis showed slightly better individual growth and reproduction than B. longirostris. Decomposed Microcystis, however, was utilized well by both species.In Lake Kasumigaura, the decomposition of abundant Microcystis seemed to raise the food level, favouring B. fatalis over B. longirostris.     

Potassium-induced inhibition of nitrogen and phosphorus metabolism as a strategy of controlling <Emphasis Type="Italic">Microcystis</Emphasis> blooms

To explore potassium toxicity in Microcystis sp., growth, chlorophyll a, carotenoid and phycocyanin content, uptake of nitrate, phosphate and ammonium and activities of the assimilatory enzymes nitrate reductase, alkaline phosphatase and glutamine synthetase (GS) were studied. Nitrate, phosphate, ammonium uptakes and chlorophyll a and phycocyanin contents decreased with increase in the concentration of potassium, but carotenoid content registered an increase at increasing potassium concentration. Alkaline phosphatase and GS activities followed the trend of inhibition of their respective nutrients, whereas nitrate and nitrate reductase showed negative correlation (p < 0.01). Potassium was found to inhibit the activities of all the assimilatory enzymes in a non-competitive manner. Inhibitions of these parameters support the view that potassium has the potential to regulate Microcystis blooms in an eco-friendly manner.     

Comparative studies on physiological responses to phosphorus in two phenotypes of bloom-forming <Emphasis Type="Italic">Microcystis</Emphasis>

Toxic Microcystis blooms frequently occur in eutrophic water bodies and exist in the form of colonial and unicellular cells. In order to understand the mechanism of Microcystis dominance in freshwater bodies, the physiological and biochemical responses of unicellular (4 strains) and colonial (4 strains) Microcystis strains to phosphorus (P) were comparatively studied. The two phenotype strains exhibit physiological differences mainly in terms of their response to low P concentrations. The growth of four unicellular and one small colonial Microcystis strain was significantly inhibited at a P concentration of 0.2 mg l⁻¹; however, that of the large colonial Microcystis strains was not inhibited. The results of phosphate uptake experiments conducted using P-starved cells indicated that the colonial strains had a higher affinity for low levels of P. The unicellular strains consumed more P than the colonial strains. Alkaline phosphatase activity in the unicellular strains was significantly induced by low P concentrations. Under P-limited conditions, the oxygen evolution rate, F _v/F _m, and ETR _max were lower in unicellular strains than in colonial strains. These findings may shed light on the mechanism by which colonial Microcystis strains have an advantage with regard to dominance and persistence in fluctuating P conditions. Handling editor: L. Naselli-Flores     

In vitro evaluation of Trichoderma and Gliocladium antagonism against the symbiotic fungus of the leaf-cutting ant Atta cephalotes

The antagonistic activity of Trichoderma and Gliocladium isolates against Attamyces sp., a symbiotic fungus of the leaf-cutting ant Atta cephalotes, was investigated. A. cephalotes cultures this fungus as the primary food source. Most of the Trichodema and Gliocladium isolates tested in vitro (82.6%) inhibited the Attamyces sp. mycelial growth, which was probably due to their colonization ability and competition for nutrients, both of them known mechanisms of some species of these genera. T. lignorum strain T-26 was the strongest inhibitor achieving a colonization of 23%. Microscopical observations indicate that the inhibitory effect was caused by an interaction that took place in close contact with the host hypha, causing wall deformation that led to the collapse of the turgor pressure. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of mixed diets (cyanobacteria and green algae) on the population growth of the cladocerans <Emphasis Type="Italic">Ceriodaphnia dubia</Emphasis> and <Emphasis Type="Italic">Moina macrocopa</Emphasis>

Microcystis aeruginosa, a cosmopolitan form, is a colonial cyanobacterium, which is also common in many freshwater bodies in Mexico. In eutrophic water bodies cyanobacteria are often the main phytoplankton that co-exist with cladocerans. We evaluated the effect of mixed diets, comprising 0, 25, 50, 75, and 100% on dry weight basis of M. aeruginosa, and the rest of one of two green algal species (Chlorella vulgaris and Scenedesmus acutus), on the population growth of the cladocerans Ceriodaphnia dubia and Moina macrocopa. Regardless of the share of M. aeruginosa in the mixed diet, C. dubia fed Chlorella had a longer initial lag phase. However, in mixed diet with S. acutus, the lag phase of C. dubia increased with increasing proportion of M. aeruginosa. When raised on 100% M. aeruginosa, the population growth of C. dubia was lowered compared with 100% S. acutus or 100% C. vulgaris. Increased proportion of M. aeruginosa in the mixed diet also resulted in decreased abundance of M. macrocopa. Irrespective of diet type, M. macrocopa had a shorter lag phase than C. dubia. Depending on the diet type, the rate of population increase (r) of C. dubia varied from 0.07 to 0.26 d⁻¹ while that of M. macrocopa was higher (0.14–0.61 d⁻¹). For both cladoceran species, the lower r values were obtained when fed Microcystis. Our study showed that the strain of M. aeruginosa was not highly toxic to cause total elimination of either C. dubia or M. macrocopa. Addition of a green algal component to the diet improved the population growth rates of both cladoceran species.     

The effects of initial population density on the competition for limiting nutrients in two freshwater algae

Eighteen long-term competition experiments were performed on two freshwater algae, a blue-green alga, Anabaena flos-aquae, and a diatom, Cyclotella sp., under controlled light and temperature conditions and various nutrient limitations. As predicted, Anabaena displaced Cyclotella when nitrate was in short supply to both species, whereas Cyclotella became dominant when both species were phosphate-limited. The two species stably coexisted when phosphate and silicate were limited. Anabaena either displaced or coexisted with Cyclotella when nitrate and phosphate or nitrate, phosphate and silicate were limited, depending on their initial density ratios. This study revealed strong effects of initial population densities on the outcomes of algal competition for limiting nutrients.     

Effects of temperature, and availability of nitrogen and phosphorus on the abundance of Anabaena and Microcystis in Lake Biwa, Japan: an experimental approach

Under optimal nutrient conditions, both Microcystis sp. and Anabaena sp. isolated from Lake Biwa grew optimally at 28–32°C but differed in maximal growth rates, phosphate uptake kinetics, maximal phosphorus quotas, and growth responses to nitrogen and phosphorus limitation. The maximal growth rates of Microcystis and Anabaena were 1.6 and 1.25 divisions day⁻¹, respectively. With phosphate and nitrate in the growth-limiting range, the growth of Microcystis was optimal at an N : P ratio of 100 : 1 (by weight) and declined at lower (nitrogen limitation) and higher (phosphorus limitation) ratios. In contrast, Anabaena growth rates did not change at N : P ratios from 1000 : 1 to 10 : 1. Starting with cells containing the maximal phosphorus quota, Microcystis growth in minus-phosphorus medium ceased in 7–9 days, compared with 12–13 days for Anabaena. The phosphate turnover time in cultures starved to their minimum cell quotas was 7.9 min for Microcystis and 0.6 min for Anabaena. Microcystis had a higher K _s (0.12 μg P l⁻¹ 10⁻⁶ cells) and lower V _max (9.63 μg P l⁻¹ h⁻¹ 10⁻⁶ cells), than Anabaena (K _s 0.02 μg P l⁻¹ h⁻¹ 10⁻⁶ cells; V _max 46.25 63 μg P l⁻¹ h⁻¹ 10⁻⁶ cells), suggesting that Microcystis would not be able to grow well in phosphorus-limited waters. We conclude that in spite of the higher growth rate under ideal conditions, Microcystis does not usually bloom in the North Basin because of low availability of phosphorus and nitrogen. Although Anabaena has an efficient phosphorus-uptake system, its main strategy for growth in low-phosphorus environments may depend on storage of phosphorus during periods of abundant phosphorus supply, which are rare in the North Basin. Received: July 31, 2000 / Accepted: October 18, 2000     

Spatial and temporal dynamics of phytoplankton communities along a regulated river system, the Nakdong River, Korea

The longitudinal distribution and seasonal fluctuation of phytoplankton communities was studied along the middle to lower part of a regulated river system (Nakdong River, Korea). Phytoplankton biomass decreased sharply in the middle part of the river (182 km upward the estuary dam), and then increased downstream reaching a maximum at the last sampling station (27 km upward the estuary dam). In contrast, there was little downstream fluctuation in species composition, irrespective of pronounced differences in nutrient concentrations (TN, TP, NO₃, NH₄, PO₄) as well as in algal biomass. In the main river channel, small centric diatoms (Stephanodiscus hantzschii, Cyclotella meneghiniana) and pennate diatoms (Synedra, Fragilaria, Nitzschia) were dominant from winter to early spring (November–April). A mixed community of cryptomonads, centric and pennate diatoms, and coenobial greens (Pediastrum, Scenedesmus) was dominant in late spring (May–June). Blue-green algae (Anabaena, Microcystis, Oscillatoria) were dominant in the summer (July–September). A mid-summer Microcystis bloom occurred at all study sites during the dry season, when discharge was low, though the nutrient concentration varied in each study site. Nutrients appeared everywhere to be in excess of algal requirement and apparently did not influence markedly the downstream and seasonal phytoplankton compositional differences in this river.     

Factors controlling the seasonal incidence of Pandorina morum (Mull.) Bory (Chlorophyta,Volvocales) in a small pond

Pandorina morum normally forms large summer populations in Abbot's Pond, a shallow (3.5 m maximum) but sheltered and thermally stratified pond. Restriction of growth until summer is shown not to be due to secretion of inhibitory substances by other organisms in winter and spring, nor to the prior production of stimulatory substances. Growth in February is prevented by low temperature, and between February and May 1972 was restricted by phosphate limitation. Development of summer populations was associated with direct thermal stratification and the decline of non-motile algal competitors for nutrients, and with the availability of phosphate from the anaerobic hypolimnion through diurnal migration of the coenobia.     

PCR-based identification of microcystin-producing genotypes of different cyanobacterial genera

Microcystins are harmful hepatotoxins produced by many, but not all strains of the cyanobacterial genera Anabaena, Microcystis, Anabaena, Planktothrix, and Nostoc. Waterbodies have to be monitored for the mass development of toxic cyanobacteria; however, because of the close genetic relationship of microcystin-producing and non-producing strains within a genus, identification of microcystin-producers by morphological criteria is not possible. The genomes of microcystin-producing cells contain mcy genes coding for the microcystin synthetase complex. Based on the sequence information of mcy genes from Microcystis and Planktothrix, a primer pair for PCR amplification of a mcyA gene fragment was designed. PCR with this primer pair is a powerful means to identify microcystin-producing strains of the genera Anabaena, Microcystis, and Planktothrix. Moreover, subsequent RFLP analysis of the PCR products generated genus-specific fragments and allowed the genus of the toxin producer to be identified. The assay can be used with DNA from field samples.Abbreviations RFLP Restriction fragment length polymorphism - MALDI-TOF Matrix-assisted laser desorption/ionization-time of flight spectrometry - HPLC High performance liquid chromatography     

The effect of temperature on growth characteristics and competitions of <Emphasis Type="Italic">Microcystis aeruginosa</Emphasis> and <Emphasis Type="Italic">Oscillatoria mougeotii</Emphasis> in a shallow,eutrophic lake simulator system

Blue-green algal blooms formed by Microcystis and Oscillatoria often occur in shallow eutrophic lakes, such as Lake Taihu (China) and Lake Kasumigaura (Japan). Growth characteristics and competitions between Microcystis aeruginosa and Oscillatoria mougeotii were investigated using lake simulator systems (microcosms) at various temperatures. Oscillatoria was the superior competitor, which suppressed Microcystis, when temperature was <20°C, whereas the opposite phenomenon occurred at 30°C. Oscillatoria had a long exponential phase (20 day) and a low growth rate of 0.22 day⁻¹ and 0.20 day⁻¹ at 15°C and 20°C, respectively, whereas Microcystis had a shorter exponential phase (2–3 days) at 30°C and a higher growth rate (0.86 day⁻¹). Interactions between the algae were stronger and more complex in the lake simulator system than flask systems. Algal growth in the lake simulator system was susceptible to light attenuation and pH change, and algae biomasses were lower than those in flasks. The outcome of competition between Microcystis and Oscillatoria at different temperatures agrees with field observations of algal communities in Lake Taihu, indicating that temperature is a significant factor affecting competition between Microcystis and Oscillatoria in shallow, eutrophic lakes.     

Inhibition of Chlorella growth by the lipids of cyanobacterium Microcystis aeruginosa

The total lipids of the cyanobacterium Microcystis aeruginosa have been isolated and fractionated into its components. Of these lipid components, only the fatty acid-containing fraction inhibited the growth of the green alga Chlorella pyrenoidosa. The inhibitory activity appears to be due to linoleic and linolenic acids, which are both present in significants quantities. These acids may be the substances responsible for the reported toxicity of Microcystis aeruginosa to Chlorella.     

Spatial and temporal diversity of microcystins and microcystin-producing genotypes in Oneida Lake, NY

Oneida Lake is a shallow, eutrophic lake with a well-established cyanobacterial population with reported toxic blooms containing hepatotoxic microcystins (MC). Peak bloom events from the summers of 2002 and 2003 were analyzed to determine the principal cyanobacterial genera containing microcystin synthetase (mcy) genes. Sequence analysis of a partial mcyA amplicon targeting Microcystis, Anabaena and Planktothrix sp. indicated that Microcystis sp. was the dominant mcy genotype. This Microcystis clade was split into two distinct sub-clades. Bloom events contained members of both sub-clades with the higher MC concentrations found when both sub-clades were present in near equal proportions. The proportion of Microcystis containing the mcyD gene ranged from 0 to 37% of the total Microcystis population as determined by quantitative PCR (qPCR). The total concentration of Microcystis containing mcyD genes was linearly related to the concentration of MCs (r² = 0.63). The relationship between mcy genotype and physiochemical variables was examined to determine the factor(s) controlling the periodicity in MC production in Oneida Lake. Multivariate statistical analyses, used to correlate the continuous-response variables, revealed a strong relationship between chlorophyll a, MCs and total Microcystis.     

Antibiotic treatment enhances C2 toxin production by Alexandrium tamarense in batch cultures

The effects of a mixture of penicillin G and streptomycin on the growth and C2 toxin production of a marine dinoflagellate, Alexandrium tamarense CI01, were investigated to determine if antibiotic treatment would increase the toxin yield of the cultured algae in batch cultures. Algal growth and toxin production were both enhanced markedly when the culture was supplemented with the antibiotics, each at an initial concentration of 100 unit ml⁻¹ in medium,² but were severely inhibited when the concentration was 500 unit ml⁻¹ or higher. Short-term pretreatment of algal inocula with the antibiotics at 100, 500, and 1000 unit ml⁻¹ all produced the enhancing effects on the algal cultures in an autoclaved medium. A prolonged antibiotic pretreatment of the algal culture followed by repeated sterile cultivation resulted in an algal culture free of cultivable bacteria. This “drug-treated” culture became more resistant to the toxicity and more responsive to the enhancing effects of the antibiotics. Our results indicated that the antibiotics can enhance growth and C2 toxin productivity not only through their inhibition of the growth of bacteria that compete for nutrients with the coexisting algae, but also through their direct effects on the physiology of the algae. Supplementation of the two antibiotics therefore is an efficient way to increase the yield of C2 toxin in the production cultures of A. tamarense CI01.     

Allelopathic effects of the submerged macrophyte <Emphasis Type="Italic">Potamogeton malaianus</Emphasis> on <Emphasis Type="Italic">Scenedesmus obliquus</Emphasis>

Allelopathic effects of the submerged macrophyte Potamogeton malaianus on Scenedesmus obliquus were assessed using a two-phase approach under controlled laboratory conditions. In the co-culture experiment (phase І), the growth of S. obliquus at two different initial cell densities was significantly inhibited by P. malaianus. Moreover, the growth inhibition was dependent on the biomass density of P. malaianus. Antioxidant enzymes (SOD, CAT and POD), MDA, APA, total soluble protein, protein electrophoretic pattern and morphology of S. obliquus were determined after the co-culture experiment was terminated. The activities of SOD, CAT, POD and APA at the low initial cell density were stimulated, the contents of MDA and total soluble protein were increased, and some special protein bands disappeared in P. malaianus treatments. The macrophyte had no effect on the activities of SOD and APA at the high initial cell density, but significantly influenced other physiological parameters of S. obliquus with the increase of biomass density. The morphology of S. obliquus showed no difference in the macrophyte treatments and the controls, and the cultures were dominated by 4-celled coenobia. The results indicated P. malaianus had significant allelopathic effects on the growth and physiological processes of S. obliquus. Moreover, the allelopathic effects depended on initial algal cell density, biomass density of the macrophyte, and their interaction. In the experiment of P. malaianus culture filtrates (phase II), filtrates from combined culture of plant and S. obliquus at the low initial cell density exhibited no apparent growth inhibitory effect on S. obliquus. The result showed that initial addition of growth-inhibiting plant filtrates had no allelopathic effect on S. obliquus. We concluded that the allelopathic effects on S. obliquus were found in the presence of P. malaianus, but not in P. malaianus filtrates. However, the absence of allelopathic effect on S. obliquus might be due to the very low concentrations of allelochemicals in the filtrates. Handling editor: S. M. Thomas     

Gastropod grazers and nutrients,but not light,interact in determining periphytic algal diversity

The potential interactions of grazing, nutrients and light in influencing autotroph species diversity have not previously been considered. Earlier studies have shown that grazing and nutrients interact in determining autotroph species diversity, since grazing decreases species diversity when nutrients (i.e. N or P) limit autotroph growth, but increases it when nutrients are replete. We hypothesized that increased light intensities would intensify the interactions between grazing and nutrients on algal species diversity, resulting in even stronger reductions in algal species diversity through grazing under nutrient–poor conditions, and to even stronger increases of algal species diversity through grazing under nutrient-rich conditions. We studied the effects of grazing (absent, present), nutrients (ambient, N + P enriched) and light (low light, high light) on benthic algal diversity and periphyton C:nutrient ratios (which can indicate algal nutrient limitation) in a factorial laboratory experiment, using the gastropod grazer Viviparus viviparus. Grazing decreased algal biomass and algal diversity, but increased C:P and N:P ratios of periphyton. Grazing also affected periphyton species composition, by decreasing the proportion of Spirogyra sp. and increasing the proportion of species in the Chaetophorales. Grazing effects on diversity as well as on periphyton N:P ratios were weakened when nutrients were added (interaction between grazing and nutrients). Chlorophyll a (Chl a) per area increased with nutrient addition and decreased with high light intensities. Light did not increase the strength of the interaction between grazing and nutrients on periphytic algal diversity. This study shows that nutrient addition substantially reduced the negative effects of grazing on periphytic algal diversity, whereas light did not interact with grazing or nutrient enrichment in determining periphytic algal diversity.     

Investigations on the relationships between algal blooms and bacterial populations in the Schlei Fjord (western Baltic Sea)

Summary 1. Investigations were carried out on monthly voyages to the Schlei, a fjord of the western Baltic Sea, from 1972 to 1974.2. The Schlei is characterized by two successive water blooms — one in the spring caused byChlorella sp., and a Cyanophyceae bloom in late summer withMicrocystis aeruginosa as the dominant species.3. The blooms are accompanied by different bacteria populations. It was noted, e. g., that the proportion of red pigmented bacteria is greatest during the spring, whereas in late summer the numbers of pigmented bacteria decrease and white or colorless forms dominate.4. In order to determine the relationship of the bacteria to the phytoplankton blooms, the predominant algae and bacteria species in the Schlei were isolated, and laboratory experiments with these microorganisms were carried out.5. The results of these experiments showed that the growth of the red bacteria was clearly enhanced when cultivated together withChlorella sp.6. The enhancement takes place immediately with actively growingChlorella cultures. Apparently these algae secrete organic substances into the culture medium during their exponential phase of growth, which are utilized by the red bacteria. It is likely that a similar process occurs in the Schlei. The rapid increase in red bacteria has been shown to take place parallel to the development of theChlorella water bloom in the spring. The proportion of pigmented bacteria decreases in the late summer with the breakdown of theChlorella bloom, and a new bacterial population becomes dominant.7. Other experiments withChlorella sp. showed thatEscherichia coli is strongly inhibited by the presence of these algae. The inhibition takes place only afterChlorella has reached the end of the exponential phase of growth, however. The inhibitory substances are probably autolysis or degradation products which accumulate in the culture medium during the stationary phase of algal growth.8. None of the bacteria tested had any influence on the growth rate ofChlorella sp., nor were the algae cells colonized by bacteria at any time.9. With the development of theMicrocystis bloom in late summer, white or colorless bacteria are more frequently found in the waters of the Schlei. At the same time, there is generally a decrease in the saprophyte counts including pigmented bacteria.10. Two strains of white bacteria commonly associated withMicrocystis were able to suppress the growth of many other bacteria on agar plates, especially red, yellow and brown pigmented species.11. Sterile filtrates prepared from the culture solutions of the white bacteria had no effect on the strains tested, however. The suppression on agar plates is therefore more likely due to competition for nutrients. There is no evidence that antibiotic substances are involved.12. Filtrates prepared from non-axenic cultures ofMicrocystis aeruginosa during the exponential as well as stationary phase of growth also did not inhibit the growth of the bacteria tested.13. The decrease in the numbers of saprophytic bacteria in the Schlei during the summer may be due in part to the presence of the large white colonies on agar plates which prevent other bacteria from developing. These white bacteria are apparently associated withMicrocystis water blooms; possibly a symbiotic or mutualistic relationship exists between these microorganisms.

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Untersuchungen über die Beziehungen zwischen Algenblüten und Bakterienpopulationen in der Schlei (westliche Ostsee)

Kurzfassung Die Wechselbeziehungen zwischen Algenblüten und Bakterien wurden in der Schlei, einer Förde der westlichen Ostsee, untersucht. Die Wasserblüten, im Frühjahr vonChlorella spec. und im Spätsommer vonMicrocystis aeruginosa gebildet, werden von Änderungen in den Bakterienpopulationen begleitet. Laborversuche wurden durchgeführt mit den hauptblütenbildenden Algenarten und den dominanten Bakterienarten. Die Ergebnisse zeigen, daß das Wachstum von einigen rotpigmentierten Bakterien(Flavobacterium), die die Wasserblüte vonChlorella begleiten, durch Stoffe, die von den Algen in der logarithmischen Wachstumsphase in das Medium abgegeben werden, gefördert wird.Microcystis aeruginosa wird von mindestens zwei chitinabbauenden Bakterienarten (darunterPseudomonas spec.) begleitet. Die ökologischen Zusammenhänge werden diskutiert.