FLOTATION AND SEDIMENTATION OF A SINGLE MICROCYSTIS FLOC COLLECTED FROM SURFACE BLOOM

Flotation and sedimentation behaviour of single Microcystis colonies taken from a dense surface bloom in Lake Kasumigaura were analyzed on the basis of a geometrical model. In the analysis, a Microcystis floc is assumed to have a self-similar structure composed of identical spheres, each of which represents a domain occupied by a single cell which is surrounded by a slime layer. Invoking Stoke's law, an applicability of this model to the vertical movement was examined by three different experiments: (1) measurement of the flotation velocity of single algal flocs as a function of floc diameter; (2) measurement of the settling velocity of single algal flocs whose gas vacuole was collapsed due to the application of high pressure; (3) enumeration of cells which constitute a single floc. The results of these experiments consistently indicated that the Microcystis floc has the fractal dimension of 2.5, implying the validity of the applied model and Stokes' law. On the basis of this result, further discussion to treat the size of the domain occupied by a cell surrounded by a slime layer and the density of a cell was conducted by using transmission electron micrographs which visualize the amount of gas vacuole in a cell.