The distribution and role of four nitrogen cycle bacteria, including ammonifying bacteria, denitrifying bacteria, nitrobacteria and nitrosobacteria, were determined with MPN method in the Taihu Lake. The most probable number [ n (MPN)] of denitrifying bacteria in the water covered by Alternanthera philoxeroides and Eichhornia crassipes are about 2~3 and 2~5 order of magnitudes higher than those in the water vegetated with other macrophytes and in the unvegetated water, respectively. There is a s...