Separation of Marine Bacteria according to Buoyant Density by Use of the Density-Dependent Cell Sorting Method

The purpose of this study was to test whether some phylogenetic groups of natural marine bacteria have unique buoyant densities that allow them to be separated by the density-dependent cell sorting (DDCS) method. We first concentrated a natural bacterial assemblage to collect sufficient numbers of cells. They were separated into three fractions by DDCS, and the community structure in each was clarified by fluorescence in situ hybridization. The cells of Archaea tended to appear in the high-density fraction, whereas those of Cytophaga-Flavobacterium-Bacteroides were in the low-density fraction. We also calculated the sedimentation velocities of three typical marine bacteria (low density, middle density, and high density) using their buoyant density. The sedimentation velocities were approximately 10, 20, and 30 μm h⁻¹; these velocities have ecological implications when the heterogeneity of bacteria is considered at a microscale. To our knowledge, this is the first report on the buoyant density of natural marine bacteria.