Fanxiang Kong

CHANGES IN THE MORPHOLOGY AND POLYSACCHARIDE CONTENT OF MICROCYSTIS AERUGINOSA (CYANOBACTERIA) DURING FLAGELLATE GRAZING

To investigate the changes in the morphology and polysaccharide content of Microcystis aeruginosa (Kütz.) Kütz. during flagellate grazing, cultures of M. aeruginosa were exposed to grazing Ochromonas sp. for a period of 9 d under controlled laboratory conditions. M. aeruginosa responded actively to flagellate grazing and formed colonies, most of which were made up of several or dozens of cells, suggesting that flagellate grazing may be one of the biotic factors responsible for colony formation in M. aeruginosa. When colonies were formed, the cell surface ultrastructure changed, and the polysaccharide layer on the surface of the cell wall became thicker. This change indicated that synthesis and secretion of extracellular polysaccharide (EPS) of M. aeruginosa cells increased under flagellate grazing pressure. The contents of soluble extracellular polysaccharide (sEPS), bound extracellular polysaccharide (bEPS), and total polysaccharide (TPS) in colonial cells of M. aeruginosa increased significantly compared with those in single cells. This finding suggested that the increased amount of EPS on the cell surface may play a role in keeping M. aeruginosa cells together to form colonies.

Morphological response of Microcystis aeruginosa to grazing by different sorts of zooplankton

In the experiment we investigated the effect of grazing by different sorts of zooplankton on the induction of defensive morphology in the cyanobacterium Microcystis aeruginosa. The results showed that protozoan flagellate Ochromonas sp. grazing could induce colony formation in M. aeruginosa, whereas M. aeruginosa populations in the control and the grazing treatments of copepod Eudiaptomus graciloides, cladoceran Daphnia magna, and rotifer Brachionus calyciflorus were still strongly dominated by unicells and paired cells and no colony forma occurred. In the protozoan grazing treatment, the proportion of unicells reduced from 83.2% to 15.7%, while the proportion of cells in colonial form increased from 0% to 68.7% of the population at the end of the experiment. The occurrence of a majority of colonial M. aeruginosa being in the treatment with flagellates, indicated that flagellate grazing on solitary cells could induce colony formation in M. aeruginosa. The colonies could effectively deter flagellate from further grazing and thus increase the survival of M. aeruginosa. The colony formation in M. aeruginosa may be considered as an inducible defense against flagellate grazing under the conditions that toxin cannot deter flagellate from grazing effectively.

Contributions of meteorology to the phenology of cyanobacterial blooms: Implications for future climate change

Cyanobacterial blooms are often a result of eutrophication. Recently, however, their expansion has also been found to be associated with changes in climate. To elucidate the effects of climatic variables on the expansion of cyanobacterial blooms in Taihu, China, we analyzed the relationships between climatic variables and bloom events which were retrieved by satellite images. We then assessed the contribution of each climate variable to the phenology of blooms using multiple regression models. Our study demonstrates that retrieving ecological information from satellite images is meritorious for large-scale and long-term ecological research in freshwater ecosystems. Our results show that the phenological changes of blooms at an inter-annual scale are strongly linked to climate in Taihu during the past 23 yr. Cyanobacterial blooms occur earlier and last longer with the increase of temperature, sunshine hours, and global radiation and the decrease of wind speed. Furthermore, the duration increases when the daily averages of maximum, mean, and minimum temperature each exceed 20.3 °C, 16.7 °C, and 13.7 °C, respectively. Among these factors, sunshine hours and wind speed are the primary contributors to the onset of the blooms, explaining 84.6% of their variability over the past 23 yr. These factors are also good predictors of the variability in the duration of annual blooms and determined 58.9% of the variability in this parameter. Our results indicate that when nutrients are in sufficiently high quantities to sustain the formation of cyanobacterial blooms, climatic variables become crucial in predicting cyanobacterial bloom events. Climate changes should be considered when we evaluate how much the amount of nutrients should be reduced in Taihu for lake management.

Effects of Wind and Wind-Induced Waves on Vertical Phytoplankton Distribution and Surface Blooms of Microcystis aeruginosa in Lake Taihu

ABSTRACT Effects of wind and wind-induced waves on vertical phytoplankton distribution and Microcystis aeruginosa bloom formation were studied in Meiliang Bay of Lake Taihu from September 5 to 9, 2004. Concentrations of chlorophyll a, b, and phycocyanin in the water column were used to represent the abundance of total phytoplankton, chlorophytes, and cyanobacteria, respectively. Water samples were taken under different wind and wind-induced wave conditions. Ca. 90% of Microcystis aeruginosa had densities less than that of lake water and were thereby positively buoyant. Vertical distributions of total phytoplankton, chlorophytes, and cyanobacteria were markedly affected by wind and waves, with coefficients of variation of pigment concentrations throughout the water column showing a negative correlation with increasing wind and waves. Surface proportions of M. aeruginosa versus total amounts in the water column also closely correlated with wind and waves, which demonstrated that wind and waves directly influenced surface M. aeruginosa blooms in Lake Taihu. A less significant correlation was found for total phytoplankton, but there was no correlation for chlorophytes in the water column.

Relationship between Temperature and Cyanobacterial Recruitment from Sediments in Laboratory and Field Studies

ABSTRACT Cyanobacterial recruitment from sediments and its driving factors were studied in both a laboratory simulation and a field study in Lake Taihu (China) during March through June 2004. Microcystis aeruginosa constituted 95% of the cyanobacteria in the laboratory and field samples. In the laboratory, senescent cyanobacteria renewed growth between 5 °C and 9 °C and started recruitment to the water column at 14°C, while in Lake Taihu they simultaneously grew and left the sediment surface at 9 °C. We found a close correlation between algal recruitment and cumulative temperatures, in terms of the sum of effective temperatures, both in the laboratory simulation and the field study. Because recruitment is the result of algal growth, we assume cumulative temperature only indirectly drove cyanobacterial recruitment but directly promoted cyanobacterial growth to reduce ballast.

Differences in microcystin production and genotype composition among Microcystis colonies of different sizes in Lake Taihu.

The cyanobacterium Microcystis, which occurs as colonies of different sizes under natural conditions, can produce toxic microcystins (MCs). To monitor the toxicity and assess the risk of Microcystis blooms in Lake Taihu, it is important to investigate the relationship between MC production and Microcystis colony size. In this study, we classified Microcystis collected from Zhushan Bay of Lake Taihu during blooms into four classes with size of <50 μm, 50-100 μm, 100-270 μm and >270 μm and studied their differences in MC production and genetic structure. The results showed that colonies with size of <50, 50-100, 100-270 and >270 μm produced 12.2 ± 11.2%, 19.5 ± 7.9%, 61.3 ± 12.6%, and 7.0 ± 9.6% of total MC, respectively. The proportion of cell density of colonies with size of 50-100, 100-270 and >270 μm was positively correlated with MC concentration during blooms, while that of colonies with size of <50 μm was negatively correlated. The MC cell quota tended to be higher during blooms in colonies with larger size except that of colonies with size of 100-270 μm was higher than that of colonies with size of >270 μm from June 11 to September 16. Colonies with size of <50 μm showed the highest proportion of the less toxic MC congener MC-RR, and colonies with size of >100 μm showed higher proportion of the most toxic MC congener MC-LR than colonies with size of <100 μm. Real-time PCR indicated that larger colonies had higher proportion of potential toxic genotype. Principal component analysis of PCR-denaturing gradient gel electrophoresis profile showed that cpcBA and mcyJ genotype compositions were different between colonies with size of <50 μm and colonies with size of >50 μm, and cpcBA genotype composition was also different among colonies with size of 50-100 μm, 100-270 μm and >270 μm. These results indicated that MC cell quota and congener composition were different in Microcystis colonies with different sizes in Lake Taihu during blooms, and the differences in MC production in colonies with different size resulted chiefly from the difference in their genotype composition. Therefore, the authorities of water quality monitoring and drinking water supply service in Lake Taihu should be alert that the toxicity of Microcystis colony with different size was different during blooms, and the high abundance of colonies larger than 50 μm could be an indicator of relatively high bloom toxicity.

THE ACCLIMATIVE CHANGES IN PHOTOCHEMISTRY AFTER COLONY FORMATION OF THE CYANOBACTERIA MICROCYSTIS AERUGINOSA(1).

Microcystis aeruginosa (Kütz.) Kütz. commonly occurs as single cells at early recruitment but forms large colonies in summer. Colony formation will induce many acclimative changes. In this study, we demonstrated the photochemical changes before and after colony formation. In the laboratory, light curves showed that colonies were more responsive to high light than single cells. The values of the maximal slope of electron transport rate (ETR)—light curve (α), relative maximal electron transport rate (rETRmax), and onset of light saturation (Ik) of colonies were significantly higher than those of single cells (P < 0.05), indicating that colonies have higher photosynthetic capability than single cells, especially in high light, where values of rETRmax and Ik of colonies were 2.32 and 2.41 times those of single cells. Moreover, the dark‐light experiments showed that colonial cells can more effectively resist darkness damage. In addition, pigments of colonial cells were higher than those of single cells (P < 0.05). The higher pigment contents probably contribute to higher photosynthetic capability. In the field, the inhibition rate of Fv/Fm in single cells increased significantly faster than that of colonies as light increased (P < 0.05), but nonphotochemical quenching (NPQ) value of colonies was higher (32.4%) than that of single cells at noon, which indicated colonial cells can more effectively resist high‐light inhibition than single cells (P < 0.05). Polysaccharides of colonies were significantly higher compared to those in unicellular cells (P < 0.05) based on their contents and ultrastructural characteristics. This finding implies that colonies could not effectively decrease photoinhibition by negative buoyancy regulation. In fact, NPQ may be an important mechanism for avoiding photodamage. All of these phenomena can help explain the ecological success of colonial M. aeruginosa in eutrophic water.

Phylogenetic diversity and specificity of bacteria associated with Microcystis aeruginosa and other cyanobacteria.

Interactions between bacteria and cyanobacteria have been suggested to have a potential to influence harmful algal bloom dynamics; however, little information on these interactions has been reported. In this study, the bacterial communities associated with five strains of Microcystis aeruginosa, three species of other Microcystis spp., and four representative species of non-Microcystis cyanobacteria were compared. Bacterial 16S rDNA fragments were amplified and separated by denaturing gradient gel electrophoresis (DGGE) followed by DNA sequence analysis. The similarities among bacterial communities associated with these cyanobacteria were compared to the digitized DGGE profiles using the cluster analyses. The bacterial community structure of all cyanobacterial cultures differed. Cluster analysis showed that the similarity values among M. aeruginosa cultures were higher than those of other cyanobacterial cultures. Sequence analysis of DGGE fragments indicated the presence of bacteria including, Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Bacteroidetes and Actinobacteria in the cyanobacterial cultures. Members of the Sphingomonadales were the prevalent group among the Microcystis-associated bacteria. The results provided further evidence for species-specific associations between cyanobacteria and heterotrophic bacteria, which are useful for understanding interactions between Microcystis and their associated bacteria.

Microcystis genotype succession and related environmental factors in Lake Taihu during cyanobacterial blooms.

From spring to autumn, heavy Microcystis blooms always occur in Lake Taihu, although environmental conditions vary markedly. We speculated that Microcystis genotype succession could play an important role in adaptation to environmental changes and long-term maintenance of the high Microcystis biomass. In this study, we investigated Microcystis genotype succession pattern and the related environmental variables in Lake Taihu during cyanobacterial blooms. Denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction -amplified the genus-specific cpcBA and mcyJ gene fragments was used to monitor the variations of Microcystis genotype and potential microcystin (MC)-producing Microcystis genotype compositions during blooms biweekly in three sites (Meiliang Bay, lake center, and Gonghu Bay) and CANOCO 4.5 for Windows were used for the multivariate statistical analysis of their relationships to environmental variables. DGGE patterns indicated that the number of dominant cpcBA genotype per sample increased from spring to autumn. Principal component analysis ordination plots of DGGE profiles showed clear temporal distribution pattern, but not spatial distribution pattern based on both cpcBA and mcyJ genotype compositions. These results indicated there were relatively gradual successions of Microcystis cpcBA and mcyJ genotype compositions in each site, and no distinct spatial difference among the three sites. Redundancy analyses of the gel patterns showed that, in all the three sites, three environmental factors (nitrate, pH, and chemical oxygen demand) were correlated significantly to successions of both cpcBA and mcyJ genotypes except for mcyJ genotype in the lake center. Spearman’s correlations indicated that the three environmental variables were also strongly correlated with chl a and MC concentrations. These results suggested that the environmental factors affecting succession of Microcystis community composition might also influence the growth of Microcystis and MC production.

Low concentrations of polycyclic aromatic hydrocarbons promote the growth of Microcystis aeruginosa.

There is an increasing need to describe the growth characteristics of cyanobacteria exposed to polycyclic aromatic hydrocarbons (PAHs) because the presence of PAHs in lakes is known to affect the growth of this kind of microorganisms. In this work, the effects of low concentrations of PAHs on Microcystis aeruginosa (M. aeruginosa) were investigated. M. aeruginosa were cultivated in the medium with a mixture of PAHs (0.486 mg L(-1) naphthalene, 0.049 mg L(-1) phenanthrene, and 0.0015 mg L(-1) pyrene) and different concentrations of nitrogen and phosphorus. During 31 d of incubation, profiles of cell number and chlorophyll-a content were determined. The results indicated that when the concentration of an individual PAH was below its no observed effect concentration (NOEC), the exposure of M. aeruginosa to a mixture of PAHs markedly promoted cell density after 7d of culture. Low concentrations of nutrients in the medium improved the growth of M. aeruginosa in the presence of PAHs. When concentrations of both phosphorus and nitrogen were 50% lower than those of the control, the specific growth rate of M. aeruginosa increased by 100% when exposed to PAHs, and the generation time decreased from 10.5 to 5.3d. The chlorophyll-a content in medium also increased from 2.23 to 3.18 μg mL(-1), which was attributed to an increase in cell numbers.