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.     

Changes in the bacterial community and extracellular compounds associated with the disaggregation of Microcystis colonies

Microcystis is a well-studied type of bloom-forming genus cyanobacteria that occurs as colonies in lakes. However, whenever Microcystis colonies are transferred to the laboratory, they always disaggregate into a unicellular form. The mechanism underlying this disaggregation of Microcystis colonies remains uncharacterized. Here, we report on the changes in morphology and the changes in the composition of the associated bacterial community of Microcystis wesenbergii colonies. Denaturing gradient gel electrophoresis analysis (DGGE) showed that the diversity of the associated bacterial community decreased during the disaggregation of Microcystis colonies. Two γ-Proteobacteria and one Bacteroidetes species from the mucilage of Microcystis colonies were not detected following colony disaggregation, suggesting that these species may influence Microcystis colony morphology. Solid phase microextraction and gas chromatography–mass spectrometry (SPME GC/MS) analysis revealed that seven of the forty-one extracellular compounds detected were exclusively present in the media of the Microcystis colony extracts; these compounds may be secreted by bacteria and may be a beneficial role in Microcystis colony maintenance. The results of this study indicate that changes in the composition of the bacterial community associated with Microcystis colonies are likely responsible for the disaggregation of these colonies in the laboratory.     

Seasonal variations of Microcystis populations in sediments of Lake Biwa, Japan

Seasonal variations of colony numbers of Microcystis aeruginosa(Kütz.) Kütz. and M. wesenbergii(Komárek) Komárek in N. V. Kondrat. in sediments of Lake Biwa were investigated over a period of 1 year. At two stations located in the shallow South Basin of Lake Biwa (ca. 4 m water depth), the colony number of Microcystisfluctuated seasonally. The number had a tendency to gradually decrease from winter to early summer, while it increased through mid-summer and autumn. Since the Microcystispopulation in sediment was rather small, intensive growth and accumulation in the water column should be important for the formation of Microcystisblooms in Lake Biwa. Microcystiscolonies in the sediment samples after June were observed to be floating in a counting chamber under a microscope. The observation suggests that the recruitment of Microcystis colonies into the water column mostly occurs in early summer. The number of Microcystiscolonies in the deep North Basin of Lake Biwa (70 – 90 m water depth) was larger than in the South Basin. Because the seasonal variation of colony numbers was not observed in the North Basin, and Microcystiscells do not have gas vesicles, these colonies will not return into the water column. The colonies isolated from the sediment of the North Basin were able to grow in cultured conditions, in the same way as those from the sediment of the South Basin. Therefore, Microcystiscolonies may survive for a long time under stable conditions of low temperature (ca. 8 °C) and darkness, in the sediment of the deep North Basin, accumulating gradually each year.     

Determination of Oligopeptide Diversity within a Natural Population of Microcystis spp. (Cyanobacteria) by Typing Single Colonies by Matrix-Assisted Laser Desorption Ionization–Time of Flight Mass Spectrometry

Besides the most prominent peptide toxin, microcystin, the cyanobacteria Microcystis spp. have been shown to produce a large variety of other bioactive oligopeptides. We investigated for the first time the oligopeptide diversity within a natural Microcystis population by analyzing single colonies directly with matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS). The results demonstrate a high diversity of known cyanobacterial peptides such as microcystins, anabaenopeptins, microginins, aeruginosins, and cyanopeptolins, but also many unknown substances in the Microcystis colonies. Oligopeptide patterns were mostly related to specific Microcystis taxa. Microcystis aeruginosa (Kütz.) Kütz. colonies contained mainly microcystins, occasionally accompanied by aeruginosins. In contrast, microcystins were not detected in Microcystis ichthyoblabe Kütz.; instead, colonies of this species contained anabaenopeptins and/or microginins or unknown peptides. Within a third group, Microcystis wesenbergii (Kom.) Kom. in Kondr., chiefly a cyanopeptolin and an unknown peptide were found. Similar patterns, however, were also found in colonies which could not be identified to species level. The significance of oligopeptides as a chemotaxonomic tool within the genus Microcystis is discussed. It could be demonstrated that the typing of single colonies by MALDI-TOF MS may be a valuable tool for ecological studies of the genus Microcystis as well as in early warning of toxic cyanobacterial blooms.     

RECRUITMENT OF BENTHIC MICROCYSTIS (CYANOPHYCEAE) TO THE WATER COLUMN: INTERNAL BUOYANCY CHANGES OR RESUSPENSION?1

In some lakes, large amounts of the potentially toxic cyanobacterium Microcystis overwinter in the sediment. This overwintering population might inoculate the water column in spring and promote the development of dense surface blooms of Microcystis during summer. In the Dutch Lake Volkerak, we found photochemically active Microcystis colonies in the sediment throughout the year. The most vital colonies originated from shallow sediments within the euphotic zone. We investigated whether recruitment of Microcystis colonies from the sediment to the water column was an active process, through production of gas vesicles or respiration of carbohydrate ballast. We calculated net buoyancy, as an indication of relative density, using the amounts and densities of the major cell constituents (carbohydrates, proteins, and gas vesicles). Carbohydrate content of benthic Microcystis cells was very low throughout the year. Buoyancy changes of benthic Microcystis were mostly a result of changes in gas vesicle volume. Before the summer bloom, net buoyancy and the amount of buoyant colonies in the sediment did not change. Therefore, recruitment of Microcystis from the sediment does not seem to be an active process regulated by internal buoyancy changes. Instead, our observations indicate that attachment of sediment particles to colonies plays an important part in the buoyancy state of benthic colonies. Therefore, we suggest that recruitment of Microcystis is more likely a passive process resulting from resuspension by wind‐induced mixing or bioturbation. Consequently, shallow areas of the lake probably play a more important role in recruitment of benthic Microcystis than deep areas.     

Genetic diversity of <Emphasis Type="Italic">Microcystis</Emphasis> blooms (Cyanobacteria) in recently constructed reservoirs in Tigray (Northern Ethiopia) assessed by rDNA ITS

The cyanobacterium Microcystis is notorious for forming extensive and potentially toxic blooms in nutrient-rich freshwater bodies worldwide. However, little is known about the factors underlying the genetic diversity and structure of natural Microcystis populations, despite the fact that this knowledge is essential to understand the build-up of blooms. Microcystis blooms are common and occur year-round in Africa, but are underinvestigated in this continent. We studied the genetic diversity and structure of Microcystis populations in 30 man-made reservoirs in Tigray (Northern Ethiopia) using Denaturing Gradient Gel Electrophoresis of the 16S–23S rDNA internal transcribed spacer (ITS) region and assessed the importance of local environmental conditions and geographic position of the reservoirs for the observed patterns. The analyses showed that both regional and local Microcystis ITS diversity in these recently constructed reservoirs was relatively low, with several dense blooms containing only a single ITS type. Especially one non-toxic ITS type dominated a considerable fraction of Microcystis blooms, but appeared restricted in its geographic distribution. The relationship between Microcystis ITS population structure and abiotic variables (water clarity, pH) and with zooplankton (Daphnia biomass) indicates a (limited) influence of environmental conditions on Microcystis population structure in the reservoirs of Tigray.     

Characterization of Microcystis morphotypes: Implications for colony formation and intraspecific variation

Groundworks on Microcystis colony formation and morphological variation are critical to understanding the whole eco-cycle of Microcystis blooms. In this study, we tested the cell adhesion effect, an important pathway for colony formation, among Microcystis colonies of different morphotypes, and examined the potential linkage between cell properties and morphological plasticity. Results showed that cell adhesion significantly contributed to the aggregation of Microcystis colonies, but such adhesion only occurred in colonies belonging to the same morphotype. This suggests that Microcystis cannot form large colonies through a direct adhesion effect among different morphotypes, possibly due to substantial differences in the chemical structures and compositions of their extracellular polymeric substances (EPS). Cell functional features also varied substantially with morphotypes, implying high intraspecific variation in competitive and defensive strategies of Microcystis. Our results offer new insights into colony formation of Microcystis and substantiate the importance of fundamental chemical characteristics of EPS in determining the morphological plasticity.     

Microbial activity and phosphorus dynamics in eutrophic lake sediments enriched with Microcystis colonies

1. Sediments from hypereutrophic Lake Vallentunasjön were enriched with Microcystis colonies from the lake water, thereby simulating the conditions after the autumn sedimentation. Release of phosphorus to the overlying lake water was followed during 2–3 weeks in the laboratory. X-ray microanalysis of individual Microcystis and bacterial cells, and chemical phosphorus fractionation, were used to assess the phosphorus pool size in different fractions of the sediment. 2. Benthic Microcystis colonies, most of these having survived within the sediment for 1 year or more, were less susceptible to decomposition, and the specific growth rate of bacteria in their mucilage was lower than for other sediment bacteria. 3. Pelagic Microcystis colonies from late August were resistant to decomposition, when placed on the sediments. When Microcystis colonies from a declining pelagic population in October were added to the sediments, however, a substantial fraction of these colonies was decomposed. The specific growth rate of mucilage bacteria was five times higher than for other sediment bacteria. 4. Release of molybdate-reactive phosphorus to the overlying lake water was larger from sediment cores enriched with Microcystis colonies than from control cores. Chemical phosphorus fractionation showed a decrease in organic-bound phosphorus (residual P). 5. X-ray microanalysis showed that the phosphorus bound in Microcystis cells decreased by -0.300 mg g^?1 DW in the October experiment, due both to a decrease in biomass (i.e. mineralization) and to a decrease in phosphorus content in the remaining cells. Heterotrophic bacteria increased their cellular concentration of phosphorus. The net release of phosphorus from the Microcystis and bacterial pools corresponded to 74% of the decrease of organic-bound phosphorus in the chemical phosphorus fractionation, and to 65% of the decrease of total phosphorus in the upper 0–1 cm of the sediment. 6. Benthic bacteria and cyanobacteria may thus contribute significantly to changes in phosphorus content and turnover of the sediment by changes in their biomass, turnover rate and cellular phosphorus content.     

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.     

Environmental factors controlling colony formation in blooms of the cyanobacteria Microcystis spp. in Lake Taihu,China

Nitrogen (N) and phosphorus (P) over-enrichment has accelerated eutrophication and promoted cyanobacterial blooms worldwide. The colonial bloom-forming cyanobacterial genus Microcystis is covered by sheaths which can protect cells from zooplankton grazing, viral or bacterial attack and other potential negative environmental factors. This provides a competitive advantage over other phytoplankton species. However, the mechanism of Microcystis colony formation is not clear. Here we report the influence of N, P and pH on Microcystis growth and colony formation in field simulation experiments in Lake Taihu (China). N addition to lake water maintained Microcystis colony size, promoted growth of total phytoplankton, and increased Microcystis proportion as part of total phytoplankton biomass. Increases in P did not promote growth but led to smaller colonies, and had no significant impact on the proportion of Microcystis in the community. N and P addition together promoted phytoplankton growth much more than only adding N. TN and TP concentrations lower than about TN 7.75–13.95 mg L⁻¹ and TP 0.41–0.74 mg L⁻¹ mainly promoted the growth of large Microcystis colonies, but higher concentrations than this promoted the formation of single cells. There was a strong inverse relationship between pH and colony size in the N&P treatments suggesting CO₂ limitation may have induced colonies to become smaller. It appears that Microcystis colony formation is an adaptation to provide the organisms adverse conditions such as nutrient deficiencies or CO₂ limitation induced by increased pH level associated with rapidly proliferating blooms.     

INVOLVEMENT OF MICROCYSTINS AND COLONY SIZE IN THE BENTHIC RECRUITMENT OF THE CYANOBACTERIUM MICROCYSTIS (CYANOPHYCEAE)1

The benthic recruitment of Microcystis was simulated in vitro in order to characterize the colonies of Microcystis recruited and to study the impact of intracellular and extracellular microcystins (MCs), and the influence of colony size on the recruitment process. We observed recruitment dynamics consisting of a lag phase followed by a peak and then a return to low recruitment rates, mainly controlled by passive resuspension throughout the experiment, and by physiological processes during the recruitment peak. Ninety‐seven percent of the Microcystis colonies recruited were <160 μm in maximum length, and their cells contained much greater amounts of MCs (0.26 ± 0.14 pg eq microcystin leucine‐arginine variant [MC‐LR] · cell^?1) than those in benthic colonies (0.021 ± 0.004 pg eq MC‐LR · cell^?1). The MC content of recruited Microcystis varied significantly over time and was not related to changes in the proportion of potentially toxic genotypes, determined using real‐time PCR. On the other hand, the changes in MC content in the potentially toxic Microcystis recruited were closely and negatively correlated with recruitment dynamics; the lowest MC contents corresponded to high recruitment rates, and the highest MC contents corresponded to low recruitment rates. Thus, depending on temperature and light conditions, these variations are thought to result from the selection of various subpopulations from among the smallest and the most toxic of the initial benthic population. Adding purified MC‐LR to experimental treatments led to a decreased recruitment of Microcystis and more specifically of mcyB genotypes.     

Individual Microcystis colonies harbour distinct bacterial communities that differ by Microcystis oligotype and with time

Interactions between bacteria and phytoplankton in the phycosphere have impacts at the scale of whole ecosystems, including the development of harmful algal blooms. The cyanobacterium Microcystis causes toxic blooms that threaten freshwater ecosystems and human health globally. Microcystis grows in colonies that harbour dense assemblages of other bacteria, yet the taxonomic composition of these phycosphere communities and the nature of their interactions with Microcystis are not well characterized. To identify the taxa and compositional variance within Microcystis phycosphere communities, we performed 16S rRNA V4 region amplicon sequencing on individual Microcystis colonies collected biweekly via high-throughput droplet encapsulation during a western Lake Erie cyanobacterial bloom. The Microcystis phycosphere communities were distinct from microbial communities in whole water and bulk phytoplankton seston in western Lake Erie but lacked ‘core’ taxa found across all colonies. However, dissimilarity in phycosphere community composition correlated with sampling date and the Microcystis 16S rRNA oligotype. Several taxa in the phycosphere were specific to and conserved with Microcystis of a single oligotype or sampling date. Together, this suggests that physiological differences between Microcystis strains, temporal changes in strain phenotypes, and the composition of seeding communities may impact community composition of the Microcystis phycosphere.     

Isolation of viable cell mass from frozen <Emphasis Type="Italic">Microcystis viridis</Emphasis> bloom containing microcystin-RR

Cyanobacterial species commonly occur in the phytoplankton of freshwater lakes and sometimes develop as toxin-producing blooms. Microcystis is one of the most common genera of freshwater cyanobacteria and is often the dominating phytoplankton of eutrophic lakes all over the world. In eutrophic lakes, large amounts of Microcystis may overwinter in the sediment and re-inoculate the water column in spring. In most cases, the overwintering pelagic population—if it exists—is small, and its role in re-inoculation has not been clear yet. In December 2005, we found large amounts of Microcystis on the surface, frozen in the ice cover in a eutrophic pond (Pond Hármashegy, Hungary). We identified the Microcystis species and investigated the viability and the toxicity of the frozen cells. The dominant species in the bloom samples was Microcystis viridis. Viability tests showed that the colonies isolated from the ice cover were composed of living cells. The isolated strain was found toxic, we analyzed the microcystin composition in the frozen planktonic Microcystis mass; in the investigated samples microcystin-RR was the main cyanotoxin.     

Changes in extracellular polysaccharide content and morphology of Microcystis aeruginosa at different specific growth rates

The cyanobacterium Microcystis mainly exists in colonies under natural conditions but as single cells in typical laboratory cultures. Understanding the mechanism by which single cells form small and large colonies can provide a deeper insight into the life history of Microcystis and the mechanisms of Microcystis bloom formation. In this paper, Microcystis aeruginosa cultured under varying light intensities and temperatures exhibited different specific growth rates. Correlations were found between the specific growth rate, extracellular polysaccharide (EPS) content, and morphology of M. aeruginosa. Under low light intensities and temperatures, M. aeruginosa formed small colonies (maximum colony size approximately 100 μm) and exhibited low specific growth rates. By contrast, standard culture conditions yielded single or paired cells with high specific growth rates. Moreover, the EPS content decreased dramatically with increasing specific growth rate. A significant positive linear relationship was observed between the EPS content per cell and colony size. High EPS content and colony formation were associated with low specific growth rates. The specific growth rate in laboratory cultures was higher than the in situ growth rate under natural conditions. This result may explain why Microcystis normally exists as single cells or (more rarely) as paired cells in axenic laboratory cultures after long-term cultivation, but forms colonies under natural conditions.     

RECRUITMENT OF MICROCYSTIS (CYANOPHYCEAE) FROM LAKE SEDIMENTS: THE IMPORTANCE OF LITTORAL INOCULA1

Recruitment of Microcystis from sediments to the water column was investigated in shallow (1–2 m) and deep (6–7 m) areas of Lake Limmaren, central Sweden. Recruitment traps attached to the bottom were sampled on a weekly basis throughout the summer season ( June–September). A comparison between the two sites showed that the recruitment from the shallow bay was significantly higher over the entire season for all three Microcystis species present in the lake. Maximum rates of recruitment were found in August, when 2.3 × 10⁵ colonies m ^{? 2}·day ^{? 1} 1 Received 18 April 2002. Accepted 29 October 2002. left the sediments of the shallow area. Calculated over the entire summer, Microcystis colonies corresponding to 50% of the initial abundance in the surface sediments were recruited in the shallow bay, whereas recruitment from the deep area was only 8% of the sediment colonies. From these results we conclude that shallow areas, which to a large extent have been overlooked in studies of recruitment of phytoplankton, may be crucial to the dynamics of these organisms by playing an important role as inoculation sites for pelagic populations.     

Size‐based interactions across trophic levels in food webs of shallow Mediterranean lakes

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Considerations for monitoring population trends of colonial waterbirds using the effective number of breeders and census estimates

Detecting trends in population size fluctuations is a major focus in ecology, evolution, and conservation biology. Populations of colonial waterbirds have been monitored using demographic approaches to determine annual census size (N_a). We propose the addition of genetic estimates of the effective number of breeders (N_b) as indirect measures of the risk of loss of genetic diversity to improve the evaluation of demographics and increase the accuracy of trend estimates in breeding colonies. Here, we investigated which methods of the estimation of N_b are more precise under conditions of moderate genetic diversity, limited sample sizes and few microsatellite loci, as often occurs with natural populations. We used the wood stork as a model species and we offered a workflow that researchers can follow for monitoring bird breeding colonies. Our approach started with simulations using five estimators of N_b and the theoretical results were validated with empirical data collected from breeding colonies settled in the Brazilian Pantanal wetland. In parallel, we estimated census size using a corrected method based on counting active nests. Both in simulations and in natural populations, the approximate Bayesian computation (ABC) and sibship assignment (SA) methods yielded more precise estimates than the linkage disequilibrium, heterozygosity excess, and molecular coancestry methods. In particular, the ABC method performed best with few loci and small sample sizes, while the other estimators required larger sample sizes and at least 13 loci to not underestimate N_b. Moreover, according to our N_b/N_a estimates (values were often ≤0.1), the wood stork colonies evaluated could be facing the loss of genetic diversity. We demonstrate that the combination of genetic and census estimates is a useful approach for monitoring natural breeding bird populations. This methodology has been recommended for populations of rare species or with a known history of population decline to support conservation efforts.     

Population-Genetic Structure of Beaver (Castor fiber L., 1758) Communities and Estimation of Effective Reproductive Size N e of an Elementary Population

The absence of panmixia at all hierarchical levels of the European beaver communities down to individual families implies a complex organization of the population-genetic structures of the species, in particular, a large intergroup component of gene diversity in the populations. Testing this assumption by analysis of 39 allozyme loci in the communities of reintroduced beaver from the Vyatka river basin (Kirov oblast) has shown that only the beaver colonies exhibit high intergroup gene diversity (G _st = 0.32) whereas this parameter is much lower when estimated among beaver groups from individual Vyatka River tributaries and among localities of one of the tributaries (0.07 and 0.11, respectively). The data suggesting genetic heterogeneity among individual settles within colonies have been obtained. The factors affecting the structure of the beaver communities of the lower hierarchical ranks are considered: the common origin, founder effect, selection, gene drift, assortative mating, and social and behavior features of the species. The conclusion is drawn that the founder effect could be the primary factor of population differentiation only at the time of their formation. The heterogeneity among colonies and among settles is maintained largely by isolation of colonies from one another. The strong interspecific competition for food resources, which is behaviorally implemented in the species at the level of minimal structural units (individual settles) creates a profound and unique population-genetic subdivision of the species. These results substantiate the suggestion that an elementary population (micropopulation) of European beaver is a colony, i.e., a set of related settles of different types. Based on ecological and genetic parameters, the effective reproductive size N _e of the minimum beaver population was estimated to be equal to three animals. This extremely low value of effective reproductive population size largely explains the high tolerance of European beaver to inbreeding and striking viability of the species, which from the early 19th century has been for more than hundred years on the brink of survival in the condition which would made any other mammalian species vanish from the Earth.     

Application of Bayesian network including Microcystis morphospecies for microcystin risk assessment in three cyanobacterial bloom-plagued lakes,China

Microcystis spp., which occur as colonies of different sizes under natural conditions, have expanded in temperate and tropical freshwater ecosystems and caused seriously environmental and ecological problems. In the current study, a Bayesian network (BN) framework was developed to access the probability of microcystins (MCs) risk in large shallow eutrophic lakes in China, namely, Taihu Lake, Chaohu Lake, and Dianchi Lake. By means of a knowledge-supported way, physicochemical factors, Microcystis morphospecies, and MCs were integrated into different network structures. The sensitive analysis illustrated that Microcystis aeruginosa biomass was overall the best predictor of MCs risk, and its high biomass relied on the combined condition that water temperature exceeded 24 °C and total phosphorus was above 0.2 mg/L. Simulated scenarios suggested that the probability of hazardous MCs (≥1.0 μg/L) was higher under interactive effect of temperature increase and nutrients (nitrogen and phosphorus) imbalance than that of warming alone. Likewise, data-driven model development using a naïve Bayes classifier and equal frequency discretization resulted in a substantial technical performance (CCI = 0.83, K = 0.60), but the performance significantly decreased when model excluded species-specific biomasses from input variables (CCI = 0.76, K = 0.40). The BN framework provided a useful screening tool to evaluate cyanotoxin in three studied lakes in China, and it can also be used in other lakes suffering from cyanobacterial blooms dominated by Microcystis.     

Colony size affects collective decision-making in the ant Temnothorax albipennis

Social insects are well-known for their ability to achieve robust collective behaviours even when individuals have limited information. It is often assumed that such behaviours rely on very large group sizes, but many insect colonies start out with only a few workers. Here we investigate the influence of colony size on collective decision-making in the house-hunting of the ant Temnothorax albipennis. In experiments where colony size was manipulated by splitting colonies, we show that worker number has an influence on the speed with which colonies discover new nest sites, but not on the time needed to make a decision (achieve a quorum threshold) or total emigration time. This occurred because split colonies adopted a lower quorum threshold, in fact they adopted the same threshold in proportion to their size as full-size colonies. This indicates that ants may be measuring relative quorum, i.e. population in the new nest relative to that of the old nest, rather than the absolute number. Experimentally reduced colonies also seemed to gain more from experience through repeated emigrations, as they could then reduce nest discovery times to those of larger colonies. In colonies of different sizes collected from the field, total emigration time was also not correlated with colony size. However, quorum threshold was not correlated with colony size, meaning that individuals in larger colonies adopted relatively lower quorum thresholds. Since this is a different result to that from size-manipulated colonies, it strongly suggests that the differences between natural small and large colonies were not caused by worker number alone. Individual ants may have adjusted their behaviour to their colony’s size, or other factors may correlate with colony size in the field. Our study thus shows the importance of experimentally manipulating colony size if the effect of worker number on the emergence of collective behaviour is to be studied. Received 13 December 2005; revised 9 May 2006; accepted 15 May 2006.