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.     

Photosynthetic adaptation to light availability shapes the ecological success of bloom-forming cyanobacterium Pseudanabaena to iron limitation

The poorly understood filamentous cyanobacterium Pseudanabaena is commonly epiphytic on Microcystis colonies and their abundances are often highly correlated during blooms. The response and adaptation of Microcystis to iron limitation have been extensively studied, but the strategies Pseudanabaena uses to respond to iron limitation are largely unknown. Here, physiological responses to iron limitation were compared between one Pseudanabaena and two Microcystis strains grown under different light intensities. The results showed that low-intensity light exacerbated, but high-intensity light alleviated, the negative effect of iron limitation on Pseudanabaena growth relative to two Microcystis strains. It was found that robust light-harvesting and photosynthetic efficiency allowed adaptation of Pseudanabaena to low light availability relative to two Microcystis strains only during iron sufficiency. The results also indicated that a larger investment in the photosynthetic antenna probably contributed to light/iron co-limitation of Pseudanabaena relative to two Microcystis strains under both light and iron limitation. Furthermore, the lower antenna pigments/chlorophyll a ratio and photosynthetic efficiency, and higher nonphotochemical quenching and saturation irradiance provided Pseudanabaena photoadaptation and photoprotection advantages over the two Microcystis strains under the high-light condition. The lower investment in antenna pigments of Pseudanabaena than the two Microcystis strains under high-light intensity is likely an efficient strategy for both saving iron quotas and decreasing photosensitivity. Therefore, when compared with Microcystis, the high plasticity of antenna pigments, along with the excellent photoadaptation and photoprotection ability of Pseudanabaena, probably ensures its ecological success under iron limitation when light is sufficient.     

Colony formation in the cyanobacterium Microcystis

Morphological evolution from a unicellular to multicellular state provides greater opportunities for organisms to attain larger and more complex living forms. As the most common freshwater cyanobacterial genus, Microcystis is a unicellular microorganism, with high phenotypic plasticity, which forms colonies and blooms in lakes and reservoirs worldwide. We conducted a systematic review of field studies from the 1990s to 2017 where Microcystis was dominant. Microcystis was detected as the dominant genus in waterbodies from temperate to subtropical and tropical zones. Unicellular Microcystis spp. can be induced to form colonies by adjusting biotic and abiotic factors in laboratory. Colony formation by cell division has been induced by zooplankton filtrate, high Pb²⁺ concentration, the presence of another cyanobacterium (Cylindrospermopsis raciborskii), heterotrophic bacteria, and by low temperature and light intensity. Colony formation by cell adhesion can be induced by zooplankton grazing, high Ca²⁺ concentration, and microcystins. We hypothesise that single cells of all Microcystis morphospecies initially form colonies with a similar morphology to those found in the early spring. These colonies gradually change their morphology to that of M. ichthyoblabe, M. wesenbergii and M. aeruginosa with changing environmental conditions. Colony formation provides Microcystis with many ecological advantages, including adaption to varying light, sustained growth under poor nutrient supply, protection from chemical stressors and protection from grazing. These benefits represent passive tactics responding to environmental stress. Microcystis colonies form at the cost of decreased specific growth rates compared with a unicellular habit. Large colony size allows Microcystis to attain rapid floating velocities (maximum recorded for a single colony, ∼ 10.08 m h⁻¹) that enable them to develop and maintain a large biomass near the surface of eutrophic lakes, where they may shade and inhibit the growth of less‐buoyant species in deeper layers. Over time, accompanying species may fail to maintain viable populations, allowing Microcystis to dominate. Microcystis blooms can be controlled by artificial mixing. Microcystis colonies and non‐buoyant phytoplankton will be exposed to identical light conditions if they are evenly distributed over the water column. In that case, green algae and diatoms, which generally have a higher growth rate than Microcystis, will be more successful. Under such mixing conditions, other phytoplankton taxa could recover and the dominance of Microcystis would be reduced. This review advances our understanding of the factors and mechanisms affecting Microcystis colony formation and size in the field and laboratory through synthesis of current knowledge. The main transition pathways of morphological changes in Microcystis provide an example of the phenotypic plasticity of organisms during morphological evolution from a unicellular to multicellular state. We emphasise that the mechanisms and factors influencing competition among various close morphospecies are sometimes paradoxical because these morphospecies are potentially a single species. Further work is required to clarify the colony‐forming process in different Microcystis morphospecies and the seasonal variation in this process. This will allow researchers to grow laboratory cultures that more closely reflect field morphologies and to optimise artificial mixing to manage blooms more effectively.     

In-situ growth rate of <Emphasis Type="Italic">Microcystis</Emphasis> spp. and their growth-limiting factors: use of cellular RNA content

In 2003 and 2004, we conducted field investigations of a canal during the summer algal bloom to estimate the in-situ growth rate of Microcystis spp. and its limiting factors. Cellular RNA content (RNA/cell), determined by the real-time PCR method with a primer specific for amplification of Microcystis rRNA, was used as an index of in-situ growth rate. A Microcystis bloom was found in the canal in summer 2004 but not in summer 2003, because of its coldness. Corresponding to this, the average value of RNA/cell in 2004 was significantly higher than that in 2003. Water temperature, light intensity, and NO₃ and PO₄ concentrations were regarded as the factors limiting the in-situ growth rate of Microcystis in the canal, and their effects were quantified on the basis of laboratory experimental data. Effects of temperature and light intensity (photoinhibition by excessive photon flux density) were found to be important in limiting the growth rate, and more severe limitation was suggested in 2003. We then estimated the in-situ growth rate from the combined effect of these limiting factors. The estimated in-situ growth rates correlated significantly with RNA/cell in each year and in the combined (2003 + 2004) data. This agreement between our two different methods for estimation of in-situ growth rate suggests the validity of our approaches. This study was first field application of cellular RNA content as an index of algal growth rate in natural water samples.     

Growth and photosynthetic response of the cyanobacterium Microcystis aeruginosa in relation to photoperiodicity and irradiance

Growth and photosynthetic characteristics, P _max (maximum light-saturated oxygen production rate) and (photosynthetic affinity), of Microcystis aeruginosa were studied in continuous cultures under a range of photoperiod lengths and growth irradiances. Microcystis showed a low specific maintenance rate constant and a high growth affinity for light (typical cyanobacterial features), but required a dark period to obtain maximum growth rate. P _max and per unit dry weight increased, as did pigment content, when less light became available. By regulation in and P _max (crucial in light-limiting and high-light conditions, respectively) this buoyant species can flourish in low light, but also in high-light environments which may arise when buoyancy is lost.The two different types of light conditions affected growth, and photosynthesis, in different ways. One needs thus to discriminate between photoperiod- and irradiance-limitation, which restricts the utility of simple algal growth models. It was emphasized that photosynthetic adaptation patterns of light-limited species may resemble short-term nutrient uptake kinetics of nutrient-limited organisms.With prior knowledge of the growth limitation, we were able to assess the growth rate of a natural population of Microcystis from its photosynthetic response and from data of laboratory cultures of a known physiological state.     

Efficacy of a dual fluorescence method in detecting the viability of overwintering cyanobacteria

Chill in the light is the major environmental stress that cyanobacteria encounter in winter. Cyanobacterial cells may acquire chill‐light tolerance upon exposure to low temperature in autumn and early winter. We sought to establish the efficacy of the dual fluorescence method in detecting the viability of overwintering cyanobacteria and to provide further evidence for the chill‐light tolerance of preconditioned cyanobacteria. Synechocystis sp. PCC 6803 and Microcystis aeruginosa PCC 7806 were exposed to chill (5°C)‐light stress with or without pretreatment at 15°C and stained with SYTO 9 and propidium iodide. Live and dead cells were observed under a fluorescence microscope, and the percentage of viable cells was quantified on a microplate reader. The dual fluorescence method showed consistent results with tests of the ability to reinitiate growth. Cell viability was quantitatively correlated with ratio of SYTO 9/propidium iodide fluorescence. Previously, Microcystis colonies in Lake Taihu had been found to accumulate RNA‐binding protein 1 in autumn and winter. Use of this method directly showed the viability of such Microcystis colonies throughout the winter.

Significance and Impact of the Study

This study established the efficacy of the dual fluorescence method in evaluating the viability of cyanobacteria under chill‐light stress. The results provided the direct evidence for acquired chill‐light tolerance and the viability of overwintering Microcystis colonies. Such information can be useful in prediction of cyanobacterial blooms.     

WAX AND WANE OF MICROCYSTIS (CYANOPHYCEAE) AND MICROCYSTINS IN LAKE SEDIMENTS: A CASE STUDY IN QUITZDORF RESERVOIR (GERMANY)1

Benthic stages of the annual life cycle of the meroplanktonic cyanobacterium Microcystis spp. in relation to microcystin (MCYST) dynamics in sediments of a shallow lake (Quitzdorf Reservoir, Germany) were investigated. Based on changes in the absolute abundance of benthic Microcystis, the annual life cycle was subdivided into four phenological stages: reinvasion, pelagic growth, sedimentation, and overwintering. Habitat‐coupling processes, such as reinvasion of the pelagic zone in spring as well as autumnal sedimentation, were particularly triggered by changes in water temperature. During reinvasion substantial losses of Microcystis were detected. Only a minor part of benthic Microcystis (about 3%) formed the inoculum for pelagic growth. Between 65% and 85% of the benthic Microcystis stock disappeared during the reinvasion phase. Because these colonies were neither detected within the sediments nor in the pelagic inoculum, it was concluded that they were subjected to decay. The occurrence of extracellular MCYSTs in the pelagic zone during this period, which cannot solely originate from the pelagic Microcystis population, supports this conclusion. Dynamics of benthic Microcystis and MCYSTs were characterized by almost identical successions with a decrease during reinvasion, an increase during sedimentation, and remarkable invariability throughout pelagic growth and overwintering. It can be deduced that MCYSTs are preserved within benthic resting stages of Microcystis because they could play a role during overwintering or reinvasion.     

Grazing on <Emphasis Type="Italic">Microcystis</Emphasis> (Cyanophyceae) by testate amoebae with special reference to cyanobacterial abundance and physiological state

We examined the growth of testate amoebae preying on Microcystis whose physiological states were different in laboratory experiments and a hypertrophic pond. We prepared three experimental systems using water samples dominated by Microcystis aeruginosa: light incubation (control), dark incubation (dark), and light incubation with addition of nitrogen and phosphorus (+NP). In all the systems, the colony density of M. aeruginosa decreased slightly during incubation. Physiological activity of phytoplankton as determined by chlorophyll fluorescence was high and almost constant in the control and +NP systems, whereas it decreased in the dark system. Cell densities of testate amoebae increased in the control and +NP systems, whereas in the dark system they remained low. Thus, growth of the amoebae was low in the systems where physiological activity of Microcystis was low. In a hypertrophic pond, cell density of testate amoebae increased and remained high when M. aeruginosa predominated. Cell density of testate amoebae increased remarkably, simultaneously with the increases in M. aeruginosa colony density and phytoplankton physiological activity. We also found a significant correlation between densities of M. aeruginosa colonies and testate amoebae. We suggested that the physiological activity of Microcystis is one important factor affecting the growth of testate amoebae grazing on Microcystis.     

Influence of N/P ratio on competitive abilities for nitrogen and phosphorus by <Emphasis Type="Italic">Microcystis aeruginosa</Emphasis> and <Emphasis Type="Italic">Aulacoseira distans</Emphasis>

Microcystis aeruginosa and Aulacoseira distans strains were grown in batch cultures to investigate the consequences of N/P ratio on the growth of these species and on their abilities to take up nitrogen and phosphorus. N/P ratio did not influence the growth rates, which were similar under all the experimental conditions. However, exponential growth lasted longer in Microcystis than in Aulacoseira, especially under low N/P ratio conditions. Distinct patterns of nutrient uptake for Aulacoseira and Microcystis were observed. N-uptake was higher in Microcystis, but not influenced by N/P ratio. However, the amount absorbed was proportional to the concentration in the culture medium for both strains studied. Although Microcystis showed lower uptake of N per biomass unit, a greater yield of Microcystis growth relative to the diatom was observed. This could have resulted from its ability to produce biomass using less nitrogen per unit of biomass. A variation of N/P ratio in the culture medium during the growth of both species was observed. This owed to the uptake of nutrients, with Microcystis showing greater potential than Aulacoseira to influence the N/P ratio. Thus, in contrast to what has been stated in the literature, our results indicated that a low N/P ratio could be a consequence of the capacities and rates of cyanobacterial uptake of nitrogen and phosphorus.     

Characterization of morphospecies and strains of the genus Microcystis (Cyanobacteria) for a reconsideration of species classification

For characterization of Microcystis species and strains, cell size, growth temperature optimum, salinity tolerance, dark chemoheterotrophy, photoheterotrophy, guanine + cytosine content in DNA, total fatty acid composition and restriction fragment length polymorphism of a polymerase chain reaction product (PCR-RFLP) of the cpcBA intergenic spacer and flanking region were examined using 24 strains of Microcystis isolated from various lakes and ponds in Japan. From the results obtained it was observed that Microcystis spp. displayed low phenotypic diversity. Cell diameters of these strains were overlapping and there was no clear correlation with morphospecies. Slight differences in growth temperature optimum and salinity tolerance were observed among all strains. No strains showed either chemoheterotrophy or photoheterotrophy. The fatty acids present were the same in different strains although the amounts were different. All the strains had a similar G + C content ranging from 39 to 43 mol%. The phonogram constructed from the PCR-RFLP analysis showed that the species assignment for Microcystis species by morphology did not correspond with the genetic background.     

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     

The effect of long wave ultraviolet radiation (UV-A) on the photosynthetic activity of natural population of freshwater phytoplankton

The effect of ultraviolet radiation on diel changes and depth profiles of phytoplankton photosynthesis was studied in four temperate freshwater lakes. Photosynthetic oxygen production was determined by incubating lake water in light and dark bottles under various weather conditions. Half the light bottles were wrapped with sheets of vinyl chloride film to exclude light with wavelengths shorter than 400 nm. The inhibition of photosynthesis due to UV-A (320–400 nm) was observed during most of the daytime and was very strong around noon on both sunny and cloudy days. On sunny days, when the surface waters of the highly eutrophic Lake Suwa and Senzoku Pond were dominated by denseMicrocystis populations, cumulative daily production at the surface, estimated from the incubation of bottles from which UV-A was excluded by the vinyl film, were about double the rates obtained from glass bottles in which UV-A was present. The UV-A inhibition was detected from the surface toca 20 cm depth in hypereutrophic lakes and at depths greater than 50 cm in mesotrophic lakes. Analysis of the photosynthesis-irradiance (P-I) relationship obtained in the present study shows β, a parameter that describes photo-inhibition, is higher in the presence of UV-A than in its absence. This indicates that UV-A is the major cause of photo-inhibition of phytoplankton photosynthesis.     

IMPROVED PROCEDURES FOR THE CLONING AND PURIFICATION OF MICROCYSTIS CULTURES (CYANOPHYTA)1

A cloned axenic culture of Microcystis Kützing was obtained by combining two procedures: a) the disaggregation of multicellular Microcystis colonies by dilution into deionized water, and b) the selective growth of Microcystis in agar media containing Na₂S, which inhibited or killed the associated contaminants. Microcystis growth was stimulated by 0.3–1 mM Na₂SO₃, but not by 0.1–33 mM Na₂SO₄. Although Microcystis cells survived temporary exposure to high Na₂S concentrations, their growth was not stimulated by 1 × 10^?5 to 1.0 M Na₂S. Possible metabolic roles of reduced sulfur compounds are considered. Microcystis colonies disaggregated to unicells at ionic concentrations below 1 mM for univalent cations, 10–100 μM for the divalent cations, and 3–10 μM for Fe³⁺. Higher cation concentrations prompted cell aggregation. With > 100 mM Fe³⁺, the Microcystis capsule appeared rust-colored. Neither nonionic solutes nor anions detectably influenced aggregation. These observations suggest cation interactions with the Microcystis capsule and are discussed with regard to: a) possible siderochrome activity, cation chelation or luxury uptake of cations, b) the questionability of using cell aggregation as a criterion for identifying Microcystis in samples of unknown ionic strength, c) the utility of low ionic strength media in releasing contaminating bacteria from the capsule and in obtaining algal unicells for cloning, and d) a model for cation interactions with the capsule.     

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.     

Diurnal buoyancy changes of Microcystis in an artificially mixed storage reservoir

In a storage reservoir, which is artificially mixed in order to reduce algal and especially cyanobacterial growth, the cyanobacterium Microcystis is still present. The aim of the research was to investigate why Microcystis was able to grow in the artificially mixed reservoir. From the results it could be concluded that the large shallow area in the reservoir allows this growth. The loss of buoyancy during the day was much higher in this shallow part than in the deep part. Assuming that the loss of buoyancy was the result of a higher carbohydrate content, a higher growth rate in the shallow part may be expected. A higher received light dose by the phytoplankton in the shallow mixed part of the reservoir than in the deep mixed part explains the difference in buoyancy loss. A significant correlation between the received light dose (calculated for homogeneously mixed phytoplankton) and the buoyancy loss was found. Apparently, the Microcystis colonies were entrained in the turbulent flow in both the shallow and the deep part of the reservoir. With a little higher stability on one sampling day, due to the late start of the artificial mixing, the loss of buoyancy at the deep site was higher than on the other days and almost comparable to the loss at the shallow site. Although the vertical biomass distribution and the temperature profiles showed homogeneous mixing, the colonies in the upper layers apparently received a higher light dose than those deeper in the water column. Determination of the buoyancy state of cyanobacteria appeared to be a valuable method to investigate the light history and hence their entrainment in the turbulent flow in the water column.     

Potassium-induced inhibition of photosynthesis and associated electron transport chain of Microcystis: Implication for controlling cyanobacterial blooms

Potassium toxicity to survival and growth of Microcystis has been investigated for the first time by taking photosynthetic parameters and change in internal pH of Microcystis. The concentration of potassium reducing 50% population of Microcystis was found to be 6 mM. At this concentration, the internal pH of cells increased from 7.2 to 9.8 in comparison to control. 6.0 mM concentration of potassium reduced protein content by 44% and generated Na⁺ efflux of 55% as compared to control. O₂ evolution, ATP content and CO₂ fixation were found to be very sensitive to above K⁺ concentration and registered a respective decline of 38, 32 and 36%. PS II was the primary site of action depicting about 35% inhibition at above K⁺ concentration. PS I and whole electron transport chain were also inhibited but the extent was less pronounced in comparison to PS II. A definite correlation between requirement of Na⁺ for growth and maintenance of cytoplasmic pH was observed. K⁺-induced loss of Na⁺ from cells of Microcystis could result in increase in internal pH, which in turn affects survival, growth, and other physiological parameters of Microcystis. Thus, K⁺ appears to hold excellent potential for the control of Microcystis blooms in fresh water ponds and lakes.     

The importance of morphological versus chemical defences for the bloom-forming cyanobacterium Microcystis against amoebae grazing

Amoebae grazing can be an important loss factor for blooms of the common cyanobacterium Microcystis. Some Microcystis strains seem to be protected against amoebae grazing, but it is unclear whether this is achieved by their colony morphology or biochemically. These factors were investigated in grazing experiments using two Microcystis-grazing amoebae (Korotnevella sp. and Vannella sp.) and two Microcystis strains with differing colony morphology (aeruginosa and viridis morphotype) and different sensitivity to amoebae grazing. Amoebae did not increase in density and failed to reduce the growth rate of cultures of the amoebae insensitive viridis strain, irrespective of whether the Microcystis strain was colonial or unicellular. This suggests that the extended mucilage matrix surrounding viridis colonies is not the main defence mechanism against amoebae grazing. At the same time, the growth rate of both unicellular and colonial cultures of the amoebae-sensitive aeruginosa strain was heavily reduced by the growing amoebae. The addition of filtered viridis-conditioned medium to aeruginosa cultures significantly decreased both amoebae growth and its effect on aeruginosa growth rates, which indicates that extracellular compounds constitutively produced by viridis are at least partially responsible for their insensitivity to amoebae grazing. These results demonstrate the potential importance of chemical interactions between lower trophic levels (protists) for Microcystis bloom dynamics.     

Increased algicidal activity of Aeromonas veronii in response to Microcystis aeruginosa: interspecies crosstalk and secondary metabolites synergism

Toxic Microcystis spp. blooms constitute a serious threat to water quality worldwide. Aeromonas veronii was isolated from Microcystis sp. colonies collected in Lake Kinneret. Spent Aeromonas media inhibits the growth of Microcystis aeruginosa MGK isolated from Lake Kinneret. The inhibition was much stronger when Aeromonas growth medium contained spent media from MGK suggesting that Aeromonas recognized its presence and produced secondary metabolites that inhibit Microcystis growth. Fractionations of the crude extract and analyses of the active fractions identified several secondary metabolites including lumichrome in Aeromonas media. Application of lumichrome at concentrations as low as 4 nM severely inhibited Microcystis growth. Inactivation of aviH in the lumichrome biosynthetic pathway altered the lumichrome level in Aeromonas and the extent of MGK growth inhibition. Conversely, the initial lag in Aeromonas growth was significantly longer when provided with Microcystis spent media but Aeromonas was able to resume normal growth. The longer was pre-exposure to Microcystis spent media the shorter was the lag phase in Aeromonas growth indicating the presence of, and acclimation to, secondary MGK metabolite(s) the nature of which was not revealed. Our study may help to control toxic Microcystis blooms taking advantage of chemical languages used in the interspecies communication.     

The effects of temperature and nutrients on the growth and dynamics of toxic and non-toxic strains of Microcystis during cyanobacteria blooms

In temperate latitudes, toxic cyanobacteria blooms often occur in eutrophied ecosystems during warm months. Many common bloom-forming cyanobacteria have toxic and non-toxic strains which co-occur and are visually indistinguishable but can be quantified molecularly. Toxic Microcystis cells possess a suite of microcystin synthesis genes (mcyA–mcyJ), while non-toxic strains do not. For this study, we assessed the temporal dynamics of toxic and non-toxic strains of Microcystis by quantifying the microcystin synthetase gene (mcyD) and the small subunit ribosomal RNA gene, 16S (an indicator of total Microcystis), from samples collected from four lakes across the Northeast US over a two-year period. Nutrient concentrations and water quality were measured and experiments were conducted which examined the effects of elevated levels of temperatures (+4 °C), nitrogen, and phosphorus on the growth rates of toxic and non-toxic strains of Microcystis. During the study, toxic Microcystis cells comprised between 12% and 100% of the total Microcystis population in Lake Ronkonkoma, NY, and between 0.01% and 6% in three other systems. In all lakes, molecular quantification of toxic (mcyD-possessing) Microcystis was a better predictor of in situ microcystin levels than total cyanobacteria, total Microcystis, chlorophyll a, or other factors, being significantly correlated with the toxin in every lake studied. Experimentally enhanced temperatures yielded significantly increased growth rates of toxic Microcystis in 83% of experiments conducted, but did so for non-toxic Microcystis in only 33% of experiments, suggesting that elevated temperatures yield more toxic Microcystis cells and/or cells with more mcyD copies per cell, with either scenario potentially yielding more toxic blooms. Furthermore, concurrent increases in temperature and P concentrations yielded the highest growth rates of toxic Microcystis cells in most experiments suggesting that future eutrophication and climatic warming may additively promote the growth of toxic, rather than non-toxic, populations of Microcystis, leading to blooms with higher microcystin content.     

Effect of nitrogen and light on nutrient concentrations and associated physiological responses in birch and fir seedlings

We grew seedlings of two co-occurring high elevation tree species in controlled light and nitrogen (N) environments to examine the effect on foliar N and P concentrations and the resulting correlation with photosynthesis and growth. Foliar N concentrations in both heart-leaf paper birch (Betula cordifolia) and balsam fir (Abies balsamea) seedlings were greater in low light treatments than in high light treatments. P concentrations, however, were lower in birch and fir foliage grown in low light than in high light. N-availability had no effect on foliar N in birch but tended to increase N concentration in fir needles at all but 100% ambient light. N-availability had no effect on P concentration in fir seedlings, but high N decreased foliar P in birch. There was a positive relationship between foliar N-concentration (mg g^–1) and mass-based maximum photosynthetic rate (Asat) in birch seedlings and a corresponding growth response to increased N-availability (suggesting N-limitation). Fir photosynthesis exhibited a positive correlation up to 22 mg g^–1 – N and a negative correlation above that point, suggesting that high N concentrations may be detrimental to photosynthesis in the fir seedlings. There was no significant effect of N-treatment on growth.