Environmental factors favouring the formation of Microcystis aeruginosa hyperscums in a hypertrophic lake

Hyperscums are crusted buoyant mats of densely packed cyanobacteria, often decimeters thick, that persist for periods of weeks to months at the same site. In Hartbeespoort Dam, a hypertrophic lake in South Africa, hyperscums of the cyanobacterium Microcystis aeruginosa that cover more than a hectare and contain up to 2 tons of chlorophyll a typically form in winter and persist for 2–3 months. This paper reports on the environmental conditions that favour hyperscum formation.Reynolds & Walsby (1975) postulated that cyanobacterial bloom formation depended on the coincidence of three preconditions: a pre-existing population, a significant proportion of the organisms having positive buoyancy, and turbulent mixing that is too weak to overcome the tendency of the cells to float. This model of bloom formation is evaluated in the context of hyperscums, based on a case-study from Hartbeespoort Dam. We examine the occurrence of hyperscums and the dynamics of their formation and breakdown in relation to diurnal and seasonal changes in the wind regime and in relation to the population dynamics and buoyancy of Microcystis. We conclude that Reynolds and Walsby's preconditions are essential but not sufficient to explain hyperscum formation. The additional preconditions are prolonged low speed wind regime, suitable lake morphometry, large cyanobacterial standing crops, and high insolation. The rare co-occurrence of these conditions make hyperscums an uncommon phenomenon, but with increasing eutrophication worldwide the frequency and distribution of hyperscum occurrence are likely to increase.     

Microcystin production by Microcystis aeruginosa in a phosphorus-limited chemostat

The production of microcystins (MC) from Microcystis aeruginosa UTEX 2388 was investigated in a P-limited continuous culture. MC (MC-LR, MC-RR, and MC-YR) from lyophilized M. aeruginosa were extracted with 5% acetic acid, purified by a Sep-Pak C(18) cartridge, and then analyzed by high-performance liquid chromatography with a UV detector and Nucleosil C(18) reverse-phase column. The specific growth rate (mu) of M. aeruginosa was within the range of 0.1 to 0.8/day and was a function of the cellular P content under a P limitation. The N/P atomic ratio of steady-state cells in a P-limited medium varied from 24 to 15 with an increasing mu. The MC-LR and MC-RR contents on a dry weight basis were highest at mu of 0.1/day at 339 and 774 microg g(-1), respectively, while MC-YR was not detected. The MC content of M. aeruginosa was higher at a lower mu, whereas the MC-producing rate was linearly proportional to mu. The C fixation rate at an ambient irradiance (160 microeinsteins m(-2) s(-1)) increased with mu. The ratios of the MC-producing rate to the C fixation rate were higher at a lower mu. Accordingly, the growth of M. aeruginosa was reduced under a P limitation due to a low C fixation rate, whereas the MC content was higher. Consequently, increases in the MC content per dry weight along with the production of the more toxic form, MC-LR, were observed under more P-limited conditions.     

Structural, physical and chemical characteristics of Microcystis aeruginosa hyperscums from a hypertrophic lake

SUMMARY. 1. This paper examines the structural, physical and chemical characteristics of cyanobacterial hyperscums (floating scums of densely packed eyanobacteria. measuring decimetres in thickness, that are covered by a dry crust of photo-oxidized cells) from hypertrophic Hartbeespoort Dam, South Africa. 2. The hyperscum community was a cyanobacterial-baclcrial association, in which the cyanobacterium Microcystis aeruginosa comprised 98% of the biovolumc, with cell concentrations exceeding 10⁹ ml^?1. 3. The buoyancy mechanism of M. aeruginosa and evaporation at the surface led to increasing compaction of the colonies with declining distance from the surface, and the formation of three distinct, but continuous layers: a 1–2 mm dry surface crust (water content: 14%, chlorophyll a concentration: 3 g l^?1), a 5–10 mm compact layer just below the crust (77% water, 1 g l^?1 chlorophyll a), and a less compact layer (96% water, 200–500 mg l^?1 chlorophyll a) from about 1 cm depth to the bottom, comprising the bulk of the hyperscum. 4. The crust attenuated all the incident light and reduced free gas exchange. Beneath it continuously dark anaerobic, highly reduced conditions prevailed. As the hyperscum aged over 3 months in 1984, at 10 cm depth the pH gradually declined from 6.6 to 5.9, interstitial water ammonia-N concentrations increased from 0.45 to 119 mg l^?1, soluble reactive phosphorus from 2.8 to 83.3 mg l^?1, and dissolved organic carbon reached a maximum of 460 mg l^?1. At any point in time these concentrations declined gradually with increasing depth within the hyperscum, and declined dramatically beneath the hyperscum. Similar patterns were recorded in another hyperscum in 1986. 5. The chemical and temperature depth profiles indicated that free water movement took place around and under the hyperscum, but within it water movement was restricted to diffusion. 6. Gas bubbles composed of 28% methane. 19% CO₂, 53% N₂, and traces of H₂ trapped within the hyperscum, and the presence of volatile fatty acids in the interstitial water were indicative of anaerobic decomposition processes mediated by fermenting and methanogenic bacteria, and N:P ratios below 1.5 in the interstitial water suggested that nitrogen was lost as gas, possibly through denitrification. 7. We hypothesize that the major sites of decay of M. aeruginosa were the crust and the compact layer beneath it, while deeper within the hyperscum this cyanobacterium could survive prolonged periods of dark anaerobic conditions. This hypothesis requires confirmation.     

Effect of temperature and light on the toxicity and growth of the blue-green alga Microcystis aeruginosa (UV-006)

The toxicity and growth of Microcystis aeruginosa (UV-006) from the Hartbeespoort Dam, South Africa were investigated at different temperatures and photon fluence rates under laboratory conditions. Cells harvested in late logarithmic growth phase were most toxic when grown at 20°C (LD₅₀) median lethal dose [IP, mouse]=25.4 mg kg^-1). Toxicity was markedly reduced at growth temperatures above 28° C. Fluence rate had a smaller effect on the toxicity of the cells, but toxicity tended to be less at the very low and high light fluences. Optimal conditions for growth did not coincide with those for toxin production. Well-aerated cultures of this isolate kept at pH 9.5 by CO₂ addition, a temperature of 20–24° C, a fluence rate of 145 mol photons m^-2 s^-1 and harvested in the late logarithmic growth phase yielded the maximum quantity of toxin.Abbreviation LD₅₀ median lethal dose An abstract of this work, presented as a poster at the IUBS symposium on toxins and lectins, held at the CSIR, Pretoria, South Africa during 1982 was published in S. Afr. J. Sci. 78, 375 (1982)     

The influence of experimentally generated turbulence on the Mash01 unicellular Microcystis aeruginosa strain

Phytoplankton experience a continuously changing fluid environment and the response to this is reflected at individual and community levels. The large-scale motions of winds, waves and artificial circulations are coupled by turbulence to the viscous small-scale environment of the phytoplankton cell. To investigate the significance of turbulence in the ecology of Microcystis aeruginosa, cultures were exposed to turbulent conditions using a vertically oscillating grid for a period of 7 days under controlled laboratory conditions. M. aeruginosa was exposed to a range of turbulent intensities, by adjusting the frequency of oscillation from 1 to 4 Hz. To improve the resolution of scale between turbulence phenomena and phytoplankton, flow cytometry and fluorescent probes were used to assess the response of M. aeruginosa. Metabolic activity and cell viability were monitored daily in both the turbulent cultures and quiescent control cultures using the FDA and Sytox green fluorescent probes, respectively. Initially, low turbulence levels generated by the grid at frequencies of 1 and 2 Hz stimulated metabolic activity, and did not affect cell viability compared to the control quiescent cultures. However, higher levels of turbulence generated by the grid at frequencies of 3 and 4 Hz were deleterious to metabolic activity and viability. Metabolic activity significantly decreased and over 85 % of cells were nonviable after 96 h at a grid oscillation of 4 Hz. It was concluded that due to the long lag time (>96 h) and high intensities needed to exert a deleterious effect, small-scale turbulence is unlikely to be a significant factor controlling M. aeruginosa compared to large scale motion which lead to changes in light and nutrient conditions.     

Evidence of the cost of the production of microcystins by Microcystis aeruginosa under differing light and nitrate environmental conditions

The cyanobacterium Microcystis aeruginosa is known to proliferate in freshwater ecosystems and to produce microcystins. It is now well established that much of the variability of bloom toxicity is due to differences in the relative proportions of microcystin-producing and non-microcystin-producing cells in cyanobacterial populations. In an attempt to elucidate changes in their relative proportions during cyanobacterial blooms, we compared the fitness of the microcystin-producing M. aeruginosa PCC 7806 strain (WT) to that of its non-microcystin-producing mutant (MT). We investigated the effects of two light intensities and of limiting and non-limiting nitrate concentrations on the growth of these strains in monoculture and co-culture experiments. We also monitored various physiological parameters, and microcystin production by the WT strain. In monoculture experiments, no significant difference was found between the growth rates or physiological characteristics of the two strains during the exponential growth phase. In contrast, the MT strain was found to dominate the WT strain in co-culture experiments under favorable growth conditions. Moreover, we also found an increase in the growth rate of the MT strain and in the cellular MC content of the WT strain. Our findings suggest that differences in the fitness of these two strains under optimum growth conditions were attributable to the cost to microcystin-producing cells of producing microcystins, and to the putative existence of cooperation processes involving direct interactions between these strains.     

Hyperscums of the cyanobacterium Microcystis aeruginosa in a hypertrophic lake (Hartbeespoort Dam, South Africa)

It is proposed that surface scums of densely packed planktoniccyanobacteria (blue-green algae) which exist for weeks to months,measure several decimeters in thickness and are covered by acrust of photo-oxidized cells, be called hyperscums. Hyperscumsof Microcystis aeruginosa formed during prolonged periods ofcalm weather in wind-protected sites in a hypertrophic lakesubject to low wind speeds (Hart beespoort Dam, South Africa).A hyperscum that extended over 1–2 hectares and persistedfor 103 days during winter 1983 was studied. Chlorophyll a concentrationsranged from 100 to 300 mg l^–2 Microcystis cell concentrationsreached 1.76x10⁹ cells ml^–1 or 116 cm³l^–1. The hyperscumenvironment was anoxic, aphotic, with a fluctuating temperatureregime and low pH values. The densely packed Microcystis cellssurvived these conditions for more than 2 months. This was shownby comparing the potential photosynthetic capacity of Microcystisfrom the hyperscum with that of Microcystis from the main basinof the lake. However, after 3 months the hyperscum algae losttheir photosynthetic capacity and decomposition processes prevailed.The hyperscum gradually shrank in size until a storm causedits complete disintegration.     

Steady-state assemblages in a Mediterranean hypertrophic reservoir. The role of Microcystis ecomorphological variability in maintaining an apparent equilibrium

Lake Arancio is a hypertrophic Mediterranean man-made lake, located on the southern coast of Sicily. Its artificial origin and the climate make it a very dynamic environment, strongly characterised by very wide water-level fluctuations. These vertical water movements interfere with the thermal stability of the water body often causing the breaking of the thermocline in mid-summer. In addition, the summer level-decrease influences the nutrient dynamics and modifies the z_mix/z_eu ratio. All these modifications were observed to support a high environmental variability, which was reflected by the richness of its phytoplankton composition and by its dynamics. Nevertheless, an investigation carried out from March 2001 to March 2002 showed that the assemblage was strongly dominated by a few species, one by one. In particular, two different Microcystis morphotypes dominated the assemblage from mid-April till the beginning of October. The prolonged dominance of these `species' should suggest that a steady state condition took place in Lake Arancio during spring and summer 2001. This is in contrast with previous investigations, which showed high diversity values especially occurring in the period of strong environmental instability when the continuous dewatering caused the breaking of the thermocline in the middle of summer. Nevertheless, this dominant species showed a very wide morphological variability and alternated among `more S', `S', and `R' (sensu Reynolds) ecotypes. The ever-changing morphological features suggest a different ecological behaviour of the species involved. They seem to confirm that the environmental variability of Mediterranean reservoirs sustains high diversity values, even though this diversity has to be sought in the amplitude of morphological plasticity of one or a few species, rather than in the coexistence of a variety of species.     

Photosynthesis and primary production of Microcystis aeruginosa Kutz. in Lake Kasumigaura

Seasonal changes in the photosynthesis and primary productionof Microcystis aeruginosa Kütz. were investigated in LakeKasumigaura during 1981–1982. Microcystis always showeda light-saturated photosynthesis-light curve. Both P_max andthe initial slope of the photosynthesis-light curve of Microcystisin early summer were very high, so it was concluded that Microcystisutilized both low and high light intensities efficiently. TheP_max of Microcystis was found to be a function of the watertemperature except in August and September. The linear regressionon the temperature-P_max relationship discontinued at 11°C,where the P_max value dropped; Microcystis did not photosynthesizebelow 4°C. The initial slope of the curve was also descendingbelow 11°C. It is suggested that Microcystis changes itsphysiological properties below 11°C. The highest value ofgross production calculated for M. aeruginosa was 5.4 gC m^–2d^–1 in July; the annual gross production was estimatedto be 300 gC m^–2year^–1 (i.e., 40% of the total primaryproduction in this lake).     

Effect of light intensity on the relative dominance of toxigenic and nontoxigenic strains of Microcystis aeruginosa

In aquatic ecosystems, the factors that regulate the dominance of toxin-producing cyanobacteria over non-toxin-producing strains of the same species are largely unknown. One possible hypothesis is that limiting resources lead to the dominance of the latter because of the metabolic costs associated with toxin production. In this study, we tested the effect of light intensity on the performance of a microcystin-producing strain of Microcystis aeruginosa (UTCC 300) when grown in mixed cultures with non-microcystin-producing strains with similar intrinsic growth rates (UTCC 632 and UTCC 633). The endpoints measured included culture growth rates, microcystin concentrations and composition, and mcyD gene copy numbers determined using quantitative PCR (Q-PCR). In contrast to the predicted results, under conditions of low light intensity (20 μmol·m(-2)·s(-1)), the toxigenic strain became dominant in both of the mixed cultures based on gene copy numbers and microcystin concentrations. When grown under conditions of high light intensity (80 μmol·m(-2)·s(-1)), the toxigenic strain still appeared to dominate over nontoxigenic strain UTCC 632 but less so over strain UTCC 633. Microcystins may not be so costly to produce that toxigenic cyanobacteria are at a disadvantage in competition for limiting resources.     

Dynamics and metabolic activity of the benthic cyanobacterium Microcystis aeruginosa in the Grangent reservoir (France)

The dynamics of benthic colonies of Microcystis aeruginosa (cyanobacteria)play an important part in the formation of the summer bloomunderlying many harmful effects. Because this benthic phaseremains somewhat unknown, we developed an approach using flowcytometry to follow the esterase activity of this species inthe Grangent reservoir. The esterase activity of benthic cyanobacteriaextracted from two layers of sediment (0–2 cm and 2–4cm deep) was measured weekly, as an indicator of viability,by flow cytometry with carboxyfluorescein diacetate. In parallel,the concentration of benthic cyanobacteria was estimated underepifluorescence microscopy and the water temperature and dissolvedoxygen concentration were measured in the hypolimnion. Esteraseactivity increased gradually as spring temperatures increased.This metabolic reactivation allowed one part of the benthiccyanobacteria to regain the water column, entailing a simultaneousdecrease of their number in the upper sediment (0–2 cm).Then, after a maximum in summer, esterase activity decreasedwhile the number of colonies increased. This corresponded withthe domination of the senescent planktonic form following thedecline of the bloom.     

The limit of resistance to salinity in the freshwater cyanobacterium Microcystis aeruginosa is modulated by the rate of salinity increase

The overall mean levels of different environmental variables are changing rapidly in the present Anthropocene, in some cases creating lethal conditions for organisms. Under this new scenario, it is crucial to know whether the adaptive potential of organisms allows their survival under different rates of environmental change. Here, we used an eco‐evolutionary approach, based on a ratchet protocol, to investigate the effect of environmental change rate on the limit of resistance to salinity of three strains of the toxic cyanobacterium Microcystis aeruginosa. Specifically, we performed two ratchet experiments in order to simulate two scenarios of environmental change. In the first scenario, the salinity increase rate was slow (1.5‐fold increase), while in the second scenario, the rate was faster (threefold increase). Salinity concentrations ranging 7–10 gL^‐1 NaCl (depending on the strain) inhibited growth completely. However, when performing the ratchet experiment, an increase in salinity resistance (9.1–13.6 gL^‐1 NaCl) was observed in certain populations. The results showed that the limit of resistance to salinity that M. aeruginosa strains were able to reach depended on the strain and on the rate of environmental change. In particular, a higher number of populations were able to grow under their initial lethal salinity levels when the rate of salinity increment was slow. In future scenarios of increased salinity in natural freshwater bodies, this could have toxicological implications due to the production of microcystin by this species.     

光照、温度对纵坑切梢小蠹起飞行为的影响

纵坑切梢小蠹有明显的趋光性。在光照为 1～ 4 0 0 lx范围内 ,趋光性随光照强度提高而增强。完全黑暗条件下 ,蠹虫起飞量极少。在光照 10 0 0 lx,温度 2 5℃下 ,蠹虫起飞率达 77.7%。研究认为 ,温度和光照是纵坑切梢小蠹起飞的重要环境因素     

Insights into the impact of increasing temperature,light intensity,and UV-B exposure on the circadian rhythm of microcystin production and release,and the expression of mcy genes in the cyanobacterium Microcystis aeruginosa

Journal of Applied Phycology - Increasing temperature and varying light quality can favor the formation of blooms of toxin-producing cyanobacteria. With respect to light quality, increasing type-B...     

从光合作用特性看铜绿微囊藻(Microcystis aeruginosa)的竞争优势

通过测定净光合放氧速率,研究了温度、光照和pH对铜绿微囊藻(M icrocystis aeruginosa)和玫瑰拟衣藻(Chlorom onas rosae)光合作用的影响。两种藻的光合放氧速率都随着温度的升高而加快,在10~35℃范围内,铜绿微囊藻净光合放氧速率随温度升高而直线上升,其最适温度高于35℃,而当温度高于30℃后玫瑰拟衣藻的净光合放氧速率迅速下降;两种微藻的光合放氧速率-光强变化曲线有所不同,铜绿微囊藻光饱和点在500μmol.m-2.s-1附近,光强达到900μmol.m-2.s-1时仍无光抑制现象发生,玫瑰拟衣藻光饱和点在630μmol.m-2.s-1附近,当光强进一步升高,光合放氧速率开始下降;铜绿微囊藻最适pH值是10.0,在pH值6.5~11.5范围内,光合放氧都很活跃,变化幅度不大,玫瑰拟衣藻最适pH值7.0,偏酸或偏碱光合放氧都迅速地下降,pH高于10.0出现了负值。比较两种藻的光合作用特性,铜绿微囊藻光合作用具有3个特点:(1)适应温度范围宽,对高温具有良好的适应性,并且光合作用随温度的升高显著提高;(2)光饱和点低,光合作用活性高,能在弱光环境中高效地进行光合作用,并且抗强光伤害;(3)对pH变化具有超强的适应能力,在中性和碱性环境中,都能进行活跃的光合作用。铜绿微囊藻在光能利用、温度和pH适应性方面的特点,可以使其快速生长繁殖,积累大量的生物量,在与其它藻类的竞争中占据显著的优势。     

Allelopathic effect of Microcystis aeruginosa on Microcystis wesenbergii: microcystin-LR as a potential allelochemical

Microcystis aeruginosa and Microcystis wesenbergii are two cyanobacteria commonly found in eutrophic shallow lakes. Previous studies reported that microcystin-producing M. aeruginosa could have an increased competitive potential on other algae and aquatic plants, and microcystin-LR (MC-LR) was regarded as an allelochemical. Based on this hypothesis, the allelopathic interaction between these two cyanobacteria was studied for the first time under laboratory conditions, and potential allelochemicals were screened. Cyanobacteria biomass and microcystin-LR (MC-LR) concentration were monitored under different culture conditions. The potential allelochemicals from M. aeruginosa were investigated by extract fractionation and GC(LC)/MS analysis. The growth of M. wesenbergii was inhibited by the addition of cell-free filtrates of M. aeruginosa whereas M. aeruginosa was promoted by the addition of cell-free filtrates of M. wesenbergii. The higher polarity the extract of M. aeruginosa is, the stronger the inhibition effect of the extract on M. wesenbergii will be. According to our results, M. aeruginosa has a significant allelopathic inhibition effect on M. wesenbergii. Allelopathic compounds from M. aeruginosa have synergistic effects on inhibition of M. wesenbergii. Besides microcystin, there may be other allelopathic compounds in M. aeruginosa.     

Impact of inorganic carbon availability on microcystin production by Microcystis aeruginosa PCC 7806

Batch culture experiments with the cyanobacterium Microcystis aeruginosa PCC 7806 were performed in order to test the hypothesis that microcystins (MCYSTs) are produced in response to a relative deficiency of intracellular inorganic carbon (C(i,i)). In the first experiment, MCYST production was studied under increased C(i,i) deficiency conditions, achieved by restricting sodium-dependent bicarbonate uptake through replacement of sodium bicarbonate in the medium with its potassium analog. The same experimental approach was used in a second experiment to compare the response of the wild-type strain M. aeruginosa PCC 7806 with its mcyB mutant, which lacks the ability to produce MCYSTs. In a third experiment, the impact of varying the C(i,i) status on MCYST production was examined without suppressing the sodium-dependent bicarbonate transporter; instead, a detailed investigation of a dark-light cycle was performed. In all experiments, a relative C(i,i) deficiency was indicated by an elevated variable fluorescence signal and led to enhanced phycocyanin cell quotas. Higher MCYST cell quotas (in the first and third experiments) and increased total (intracellular plus extracellular) MCYST production (in the first experiment) were detected with increased C(i,i) deficiency. Furthermore, the MCYST-producing wild-type strain and its mcyB mutant showed basically the same response to restrained inorganic carbon uptake, with elevated variable fluorescence and phycocyanin cell quotas with increased C(i,i) deficiency. The response of the wild type, however, was distinctly stronger and also included elevated chlorophyll a cell quotas. These differences indicate the limited ability of the mutant to adapt to low-C(i,i) conditions. We concluded that MCYSTs may be involved in enhancing the efficiency of the adaptation of the photosynthetic apparatus to fluctuating inorganic carbon conditions in cyanobacterial cells.