Insights into the relationship between colony formation and extracellular polymeric substances (EPS) composition of the cyanobacterium Microcystis spp

Extracellular polymeric substances (EPS) were considered as fundamental substances in colony formation; however, the understanding of EPS composition remains limited. This study analyzed the content and composition of EPS fractions (soluble EPS, loosely bound EPS, and tightly bound EPS) of four Microcystis species from laboratory cultures in both unicellular and colonial morphologies, as well as colonies collected during Microcystis blooms, using fluorescence excitation - emission matrix spectroscopy combined with parallel factor analysis (EEM-PARAFAC). This method enables to make insight into protein-like and humic acid-like components but cannot detect polysaccharides. The EPS was successfully categorized into three humic acid-like components (C1 – C3) and a protein-like component (C4). Component C1 was discovered to be involved in colony formation and colony size growth of Microcystis. EPS content varied among Microcystis morphospecies, such as M. aeruginosa, M. wesenbergii and M. ichthyoblabe, and this was significantly affected by the environmental constraints rather than the morphospecies. The proportion of C1 relating to larger colony size was negatively correlated to temperature and concentrations of TN and TP. The tightly bound EPS directly promoted colony formation, but the soluble EPS or loosely bound EPS alone did not induce colony formation in Microcystis. These results advanced the current knowledge on the chemical materials involved in the colony formation of Microcystis and provided new clues in unicellular-multicellular transformation as well as colonial morphology changes in Microcystis.     

Role of Cell Hydrophobicity on Colony Formation in Microcystis (Cyanobacteria)

Colony formation is highly import ant for the competitive advantage of the cyanobacterium Microcystis over other phytoplankton species. The laboratory‐grown colonial Microcystis strains isolated from Lake Taihu (China) maintained colonial forms under the low light condition (10 μE m^–2 s^–1). The cell surface hydrophobicities of the Microcystis colonies were measured by cyanobacterial adherence to xylene in comparison with unicellular Microcystis strains. The cells of the tested colonial strains were all hydrophobic, while the cells of the tested unicellular strains were all hydrophilic. Incubation under the higher light condition (75 μE m^–2 s^–1) leaded to the significant decrease in the cell hydrophobicities of the colonial Microcystis and the transition from colonial forms to unicellular forms. These findings indicated that the cell hydrophobicity of Microcystis may play a role in cell‐cell adherence and colony formation. Phosphate‐limitation, nitrate‐limitation and pH did not affect cell hydrophobicities of colonial Microcystis. Treatment with proteolytic enzymes had no effect on the cell hydrophobicity, indicating that cell surface proteins did not contribute to high cell hydrophobicity. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

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.     

Simulation of water-bloom formation in the cyanobacterium Microcystis aeruginosa

A mechanism for buoyancy increases in the cyanobacterium Microcystisaeruginosa and the associated formation of surface water-bloomsis presented. The mechanism is based on considering a responsetime in the rate of carbohydrate accumulation. When irradianceincreases, the Microcystis cells may require time to increasetheir rate of carbohydrate accumulation. If irradiance decreasesbefore adjustment, the maximum rate of carbohydrate accumulationis not reached. Colony buoyancy increases during mixing whenthe time scales of the light fluctuations are shorter than theresponse time. To examine the mechanism, a model of Microcystisbuoyancy that incorporates the response time has been coupledwith a hydrodynamics model that simulates mixing. The modelwas applied to a shallow lake to show that a prolonged episodeof intense mixing caused the simulated Microcystis coloniesto become excessively buoyant. Once the mixing subsided, thecolonies accumulated at the surface. Decreases in carbohydratewere reduced in large colonies as their size afforded buoyancyforces that could readily overcome the entraining forces ofthe mixing.     

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.     

FATE OF THE TOXIC CYCLIC HEPTAPEPTIDES,THE MICROCYSTINS,FROM BLOOMS OF MICROCYSTIS (CYANOBACTERIA) IN A HYPERTROPHIC LAKE1

The in situ fate of the toxic cyclic heptapeptides, the microcystins, produced by blooms of Microcystis was examined at two stations in a hypertrophic Japanese lake. Microcystins were detected in all samples of Microcystis with quantities varying seasonally and spatially (230–950 μg · g dry wt^?1 at St. 1 and 160–746 μg · g dry wt^?1 at St. 2) and composed of microcystin-LR, -RR, and-YR. Microcystin-RR was the dominant toxin in most samples. A large amount of microcystin (1.1 μg · L^?1) was detected in only one sample of filtered lake water. Accumulation of microcystin in zooplankton was indirectly estimated from a newly developed equation model. Large amounts of microcystin (75–1387 μg · g dry wt^?1) were accumulated in the zooplankton community, which consisted of two cladocerans, Bosmina fatalis Burckhardt and Diaphanosoma brachyurum Lieve, and a copepod, Cyclops vicinus Uljanin, that co-occurred with the toxic Microcystis blooms. The maximum percent of microcystin content in zooplankton to that in Microcystis was 202%. Among the three species of zooplankton, only B. fatalis seemed to be responsible for accumulation of the microcystins because C. vicinus appeared to avoid contact with Microcystis cells and D. brachyurum did not consume colonies of Microcystis. Microcystins may be transferred to higher trophic levels through B. fatalis.     

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.     

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.     

Morphological and physiological changes in Microcystis aeruginosa as a result of interactions with heterotrophic bacteria

1. To reveal the role of aquatic heterotrophic bacteria in the process of development of Microcystis blooms in natural waters, we cocultured unicellular Microcystis aeruginosa with a natural Microcystis‐associated heterotrophic bacterial community. 2. Unicellular M. aeruginosa at different initial cell densities aggregated into colonies in the presence of heterotrophic bacteria, while axenic Microcystis continued to grow as single cells. The specific growth rate, the chl a content, the maximum electron transport rate (ETR_max) and the synthesis and secretion of extracellular polysaccharide (EPS) were higher in non‐axenic M. aeruginosa than in axenic M. aeruginosa after cell aggregation, whereas axenic and non‐axenic M. aeruginosa displayed the same physiological characteristic before aggregation. 3. Heterotrophic bacterial community composition was analysed by PCR–denaturing gradient gel electrophoresis (PCR–DGGE) fingerprinting. The biomass of heterotrophic bacteria strongly increased in the coinoculated cultures, but the DGGE banding patterns in coinoculated cultures were distinctly dissimilar to those in control cultures with only heterotrophic bacteria. Sequencing of DGGE bands suggested that Porphyrobacter, Flavobacteriaceae and one uncultured bacterium could be specialist bacteria responsible for the aggregation of M. aeruginosa. 4. The production of EPS in non‐axenic M. aeruginosa created microenvironments that probably served to link both cyanobacterial cells and their associated bacterial cells into mutually beneficial colonies. Microcystis colony formation facilitates the maintenance of high biomass for a long time, and the growth of heterotrophic bacteria was enhanced by EPS secretion from M. aeruginosa. 5. The results from our study suggest that natural heterotrophic bacterial communities have a role in the development of Microcystis blooms in natural waters. The mechanisms behind the changes of the bacterial community and interaction between cyanobacteria and heterotrophic bacteria need further investigations.     

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     

LOCALIZATION OF MICROCYSTIN SYNTHETASE GENES IN COLONIES OF THE CYANOBACTERIUM MICROCYSTIS USING FLUORESCENCE IN SITU HYBRIDIZATION1

To better understand the production of microcystins (MCs) in Microcystis colonies, fluorescence in situ hybridization (FISH) methods were developed to detect DNA involved in the synthesis of these cyanobacterial hepatotoxins. Using colonies of Microcystis aeruginosa (Kütz.) Kütz. isolated from environmental blooms of cyanobacteria and from a colony‐forming, MC‐producing laboratory strain of Microcystis, amplified PCR products were observed, coincident with positive controls. The total MC content of individual colonies of Microcystis, determined by ELISA, showed a positive correlation with colony cross‐sectional area. FISH analysis of Microcystis colonies gave high fluorescence in comparison to negative controls, indicating the presence of MC synthetase DNA (mcyA) in situ. FISH analysis for MC synthetase genes has the potential to be developed into an effective early warning tool for drinking and recreational water management.     

Capsular polysaccharides facilitate enhanced iron acquisition by the colonial cyanobacterium Microcystis sp. isolated from a freshwater lake

Microcystis sp., especially in its colonial form, is a common dominant species during cyanobacterial blooms in many iron‐deficient water bodies. It is still not entirely clear, however, how the colonial forms of Microcystis acclimate to iron‐deficient habitats, and the responses of unicellular and colonial forms to iron‐replete and iron‐deficient conditions were examined here. Growth rates and levels of photosynthetic pigments declined to a greater extent in cultures of unicellular Microcystis than in cultures of the colonial form in response to decreasing iron concentrations, resulting in the impaired photosynthetic performance of unicellular Microcystis as compared to colonial forms as measured by variable fluorescence and photosynthetic oxygen evolution. These results indicate that the light‐harvesting ability and photosynthetic capacity of colonial Microcystis was less affected by iron deficiency than the unicellular form. The carotenoid contents and nonphotochemical quenching of colonial Microcystis were less reduced than those of the unicellular form under decreasing iron concentrations, indicating that the colonial morphology enhanced photoprotection and acclimation to iron‐deficient conditions. Furthermore, large amounts of iron were detected in the capsular polysaccharides (CPS) of the colonies, and more iron was found to be attached to the colonial Microcystis CPS under decreasing iron conditions as compared to unicellular cultures. These results demonstrated that colonial Microcystis can acclimate to iron deficiencies better than the unicellular form, and that CPS plays an important role in their acclimation advantage in iron‐deficient waters.     

Multicellular group formation in response to predators in the alga Chlorella vulgaris

A key step in the evolution of multicellular organisms is the formation of cooperative multicellular groups. It has been suggested that predation pressure may promote multicellular group formation in some algae and bacteria, with cells forming groups to lower their chance of being eaten. We use the green alga Chlorella vulgaris and the protist Tetrahymena thermophila to test whether predation pressure can initiate the formation of colonies. We found that: (1) either predators or just predator exoproducts promote colony formation; (2) higher predator densities cause more colonies to form; and (3) colony formation in this system is facultative, with populations returning to being unicellular when the predation pressure is removed. These results provide empirical support for the hypothesis that predation pressure promotes multicellular group formation. The speed of the reversion of populations to unicellularity suggests that this response is due to phenotypic plasticity and not evolutionary change.     

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.     

Does Formation of Multicellular Colonies by Choanoflagellates Affect Their Susceptibility to Capture by Passive Protozoan Predators?

Microbial eukaryotes, critical links in aquatic food webs, are unicellular, but some, such as choanoflagellates, form multicellular colonies. Are there consequences to predator avoidance of being unicellular vs. forming larger colonies? Choanoflagellates share a common ancestor with animals and are used as model organisms to study the evolution of multicellularity. Escape in size from protozoan predators is suggested as a selective factor favoring evolution of multicellularity. Heterotrophic protozoans are categorized as suspension feeders, motile raptors, or passive predators that eat swimming prey which bump into them. We focused on passive predation and measured the mechanisms responsible for the susceptibility of unicellular vs. multicellular choanoflagellates, Salpingoeca helianthica, to capture by passive heliozoan predators, Actinosphaerium nucleofilum, which trap prey on axopodia radiating from the cell body. Microvideography showed that unicellular and colonial choanoflagellates entered the predator's capture zone at similar frequencies, but a greater proportion of colonies contacted axopodia. However, more colonies than single cells were lost during transport by axopodia to the cell body. Thus, feeding efficiency (proportion of prey entering the capture zone that were engulfed in phagosomes) was the same for unicellular and multicellular prey, suggesting that colony formation is not an effective defense against such passive predators.     

Field and experimental studies on the combined impacts of cyanobacterial blooms and small algae on crustacean zooplankton in a large,eutrophic, subtropical,Chinese lake

Field and experimental studies were conducted to evaluate the combined impacts of cyanobacterial blooms and small algae on seasonal and long-term changes in the abundance and community structure of crustacean zooplankton in a large, eutrophic, Chinese lake, Lake Chaohu. Seasonal changes of the crustacean zooplankton from 22 sampling stations were investigated during September 2002 and August 2003, and 23 species belonging to 20 genera were recorded. Daphnia spp. dominated in spring but disappeared in mid-summer, while Bosmina coregoni and Ceriodaphnia cornuta dominated in summer and autumn. Both maximum cladoceran density (310 ind. l⁻¹) and biomass (5.2 mg l⁻¹) appeared in autumn. Limnoithona sinensis, Sinocalanus dorrii and Schmackeria inopinus were the main species of copepods. Microcystis spp. were the dominant phytoplankton species and formed dense blooms in the warm seasons. In the laboratory, inhibitory effects of small colonial Microcystis on growth and reproduction of Daphnia carinata were more remarkable than those of large ones, and population size of D. carinata was negatively correlated with density of fresh large colonial Microcystis within a density range of 0–100 mg l⁻¹ (r = −0.82, P< 0.05). Both field and experimental results suggested that seasonal and long-term changes in the community structure of crustacean zooplankton in the lake were shaped by cyanobacterial blooms and biomass of the small algae, respectively, i.e., colonial and filamentous cyanobacteria contributed to the summer replacement of dominant crustacean zooplankton from large Daphnia spp. to small B. coregoni and C. cornuta, while increased small algae might be responsible for the increased abundance of crustacean zooplankton during the past decades.     

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

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

Effect of cyanobacterial blooms on thermal stratification

Enclosure experiments were performed at Akanoi Bay, Lake Biwa, in 1995 to determine whether the blooms of cyanobacterial algae changed thermal stratification in the lake. We used four rectangular enclosures, each 10 m × 10 m, with a volume of 200 m³, which were open to the sediments. Two enclosures, A and B, were mixed artificially by aquatic pumps from 1000 to 1400 every day, and the other two enclosures, C and D, were controls with no mixing. The experiment was conducted during late summer from August 3 to September 27. Chlorophyll a concentrations were highest in enclosure D, followed by enclosure C, both of which were controls without mixing. Enclosure A had lower concentrations than enclosures C and D, and enclosure B had the lowest concentrations. No large cyanobacterial algae blooms of Anabaena sp. and Microcystis sp. were seen in the mixed enclosures A and B. In enclosures C and D, blooms of Anabaena sp. occurred in the middle of August, and Microcystis sp. later became dominant in enclosure D at the end of August. In enclosure D, the water temperature changed over the diel cycle before August 17, with thermal stratification during the day and complete mixing at night. After August 17, as Anabaena sp. and Microcystis sp. became dominant, the temperature at the bottom of the enclosure did not change clearly over the 24-h cycle. The APE (available potential energy) density (a measure of water column stability) in the enclosures increased by almost 100% when the biovolume of Anabaena sp. + Microcystis sp. exceeded 20 mm³ l⁻¹. These results indicate that blooms of Anabaena sp. and Microcystis sp. can increase the available potential energy in the water column and create more stable stratification for their growth. Received: September 25, 1999 / Accepted: January 6, 2000     

Structural Variations Increase the Upper Limit of Colony Size of Microcystis: Implications from Laboratory Cultures and Field Investigations

Wild Microcystis have highly diverse colonial structures and sizes, including variable colony geometry, cell arrangement, and diameter. These structural and dimensional variations may play an important role in continual, frequent Microcystis blooms during summer and autumn, the cause of which still remains unclear. Here, laboratory cultures and field investigations were applied to assess mechanisms that drive variation in structure and size, as well as factors that influence diversity. The results demonstrated that colonies grew to large sizes at the expense of their structure being loose and inhomogeneous. Furthermore, colonies may spontaneously change structure to relieve the constraints of size in return. Influencing factors (nutrient limits and turbulent shear) tended to promote these variations. Our work highlights that the diversity of Microcystis colonies may be a result of structural variations as survival strategies for gaining a higher upper size limit. Therefore, during seasonal successions, large colonies commonly have porous or loosely arranged structures, such as in M. aeruginosa. Additionally, this study hypothesized three possible transition routes for better understanding structural diversity and variations in Microcystis.