Research On Bacterioplankton Diversity And The Algicidal Bacteria In Lake Taihu

Lake Taihu, which is the second largest freshwater lake in China, isa very typical shallow freshwater lake located in Eastern China. Due toindustrial and agricultural pollution over the last two decades, LakeTaihu has experienced severe ecological and environmental problems.Therefore, to restore the water body, it is necessary to gather basicinformation regarding the microbial diversity of Lake Taihu. In general,it is also important to evaluate changes in the microbial diversity andcommunity composition in aquatic systems because these factors are thefoundation of biogeochemical cycles. Additionally, in order to developnew biological methods to control the cyanobacterial blooms occurredevery summer in Lake Taihu, it would be very useful to study algicidalbacteria in the waterbody of Lake Taihu.To describe the variation in bacterioplankton diversity in LakeTaihu, as well as changes in the diversity that occurred with time,PCR-DGGE was utilized to study water samples collected from LakeTaihu in China. To accomplish this, water samples were collected fromthree different locations (Wuxi, Qidu and Huzhou) and during differentmonths. The trophic status of these sampling sites ranged from eutrophic to hypertrophic. Cluster and MDS analyses revealed that the temporaltransition in the diversity of the bacterioplankton occurred primarily inresponse to a cyanobacterial bloom, and that all samples could bedivided into normal-bloom, peak-bloom and winter period groups.Spatial differences in the bacterial diversity were also detected amongthe three sampling sites, with diversity being found to be stronglycorrelated with the gradient of the trophic status of the three samplingsites (from eutrophic to hypereutrophic at Qidu, Huzhou and Wuxi,respectively). The gradual transition of bacterioplankton diversity wasconsistent with the gradual trophic status of the three sites. In addition,these temporal and spatial changes could be characterized by severalspecific DGGE bands. The results were further analyzed by canonicalcorrespondence analysis (CCA), which revealed that thebacterioplankton diversity of Lake Taihu was mainly associated withtemperature, pH, TN, TP and DO. Of these factors, TN and TP wereonly shown to be significant influencing factors at Wuxi, which had thehighest trophic level.In an effort to identify a bio-agent capable of controllingcyanobacterial blooms, we isolated ten algicidal bacterial strains (belongto Archaeon, Exiguobacteria, Aeromonas and Bacillus) including strainA27, which exhibited strong algicidal activity against the dominantbloom-forming species of Microcystis aeruginosa in Lake Taihu. Based on16s rRNA gene sequence analysis, this strain belongs to the genusExiguobacterium and of the Firmicutes. Strain A27exhibited algicidalactivity against a broad range of cyanobacteria, but elicited no to a lowresponse against the two green algal strains tested. This result suggeststhat Exiguobacterium sp. A27has great potential in the control ofcyanobacterial blooms of Lake Taihu. The algicidal activity of strainA27was shown to be dependent on the density of the bacteria and tohave a threshold density of about4×108CFU/mL. Our data also showedthat the algicidal activity of strain A27depended on different growthstages of Microcystis aeruginosa (exponential≈lag phase> earlystationary) rather than the growth stage of the bacterium itself.Our results also suggested the algicidal activity of strain A27occurred via the production of extracellular algicidal compounds.Three different algicidal compounds were found in the bacterial cultureof strain A27. Two of these three compounds were successfullyextracted and purified. One was identified as a compound with themolecular weight of1188.5Da and the other was identified as thymine.Investigation of these algicidal compounds revealed that thymine wasfound to inhibit the growth of Microcystis aeruginosa at a lowconcentration (100mg/L). This finding adds a possible mechanism tothe interactions between planktonbacteria and bloom-formingcyanobacteria. At the terminal stage of a cyanobacterial bloom, the death of cyanobacteria, planktonbacteria, aquatic animals and plantsmay release large amount of thymine into the water body causing itsconcentration to rise rapidly. At last, the concentration of thymine mightreach an effective level and the growth of many species of cyanobacteriawould be inhibited, which could cause the bloom to terminate. Of coursethis hypothesis still need to be verified by further study.