Spatiotemporal Variation In Microcystis Genotype Composition And Related Environmental Factors During Cyanobacterial Blooms

Microcystis (cyanobacteria) can produce toxic microcystin (MC), cyanobacterial blooms can cause massive problems for industry, recreation, tourism and local drinking water supplies. During summer and autumn, Lake Taihu is heavily polluted by MC, and poses serious health threats to local people.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. The study of temporal variation in Microcystis genotype composition showed that the number of dominant cpcBA genotype per sample increased from spring to autumn. Principal component analysis ordination plots of denaturing gradient gel electrophoresis (DGGE) profiles showed that 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, nitrate, pH, and chemical oxygen demand (COD) 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 variables affecting succession of Microcystis community composition might also influence the growth of Microcystis and MC production.Lake Taihu is a large shallow freshwater lake with an area of 2250 m2. During cyanobacterial bloom period, phytoplankton communities of the northern part, western part and open area of Lake Taihu are dominated by cyanobacteria. Study on the distribution pattern of Microcystis blooms among these water areas and related factors would contribute to the prevention and control of blooms. The study of spatial variation in Microcystis cpcBA genotype composition among northern part, western part and open area of Lake Taihu showed that on the same day, there was no spatial difference in Microcystis cpcBA genotype composition. This result indicated that the transportation of cyanobacterial blooms could happen among the water areas where frequent occurrences of cyanobacterial blooms are observed. Redundancy analyses of the gel patterns showed that there was no significant correlation between spatial distribution of cpcBA genotypes and spatial heterogenity of envieomental variables. Unlike cpcBA genotype composition, mcyJ/cpcBA ratio showed spatial variation, but the spatial variation of mcyJ/cpcBA ratio was not significantly correlated with any environmental variable. Chl a showed abviously spatial difference and the spatial distribution of chl a was positively and significantly correlated with nitrogen (N), phosphorus (P) and COD on June 9 and August 22 but not on October 15. When combining samples from different sampling dates together, nitrate, total dissolved nitrogen (TDN), COD and tolal phosphorus (TP) were found to be significantly correlated with the temporal variation of Microcystis cpcBA genotype composition. The temporal variation of mcyJ/cpcBA ratio was negatively and significantly correlated with TDN and nitrate, and positively and significantly correlated with TP and total dissolved phosphorus (TDP). Chl a was also positively and significantly coreelated with P and COD, but showed negative and significant correlation with N. These results suggested that the influences of eutrophication level on Microcystis cpcBA genotype composition and mcyJ/cpcBA ratio could be a long term process while the biomass of cyanbacterial bloom could be altered quickly by eutrophication level, and that the biomass of cyanbacterial bloom of specific water area was not always be mainly determined by in situ growth of cyanobacteria.Cyanobacterial blooms can migrate between different water areas, the migration of cyanobacterial blooms in Lake Taihu are driven by wind-induced surface drift. Since the buoyancy of Microcystis colony can be correlated with its size, the spatial distribution of Microcystis colonies with different sizes can be different. We classified Microcystis community 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 and gave some suggestions on the spatial distribution of MC. The comparison of MC production and genetic structure among different Microcystis colony size classes showed that colonies with size of <50 μm,50～100 μm,100～270 μm and >270 μm produced 12.19±11.17%,19.48±7.85%, 61.29±12.57%, and 7.03±9.56% of total MC, respectively. The proportion of cell density of colonies with size of 50～100 μm, 100～270 μm 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 slightly 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 conger 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-DGGE 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 conger 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. Large colonies are mianly concentrated on water surface and can be easily transported by wind. Therefore, during blooms the authorities of water quality monitoring and drinking water supply service in Lake Taihu should keep an eye on the occurrence and transportation of large colonies, and the abundance of large colonies could be a reference indicator of bloom toxicity.Considering that Microcystis occurs mainly as colonial aggregates in nature, we compared the algicidal effects of bacterium Pseudomonas aeruginosa ACB3 on unicellular and colonial Microcystis strains. Results showed that P. aeruginosa effectively inhibited the growth (by 82.62% to 97.54%) and photosynthetic activities of unicellular Microcystis strains, but hardly inhibited the growth or photosynthetic activities of colonial Microcystis strains. We suggest that it is necessary for the control of Microcystis blooms to assess the algicidal activities of algicidal bacteria on both unicellular and colonial Microcystis strains. In addition, the colonial Microcystis strains treated with P. aeruginosa were found to exhibit much smaller colony size (3.18% to 49.67% of the control). This study could provide us some information about influence of algicidal bacteria on selective advantages of colonial Microcystis over unicellular Microcystis and the growth of Microcystis in nature.