Diversity of bacterioplankton.

We do not know how many bacterial species exist in the world, or in any particular environment, because bacteria cannot be differentiated under the microscope; we do not even know the right order of magnitude. Thus, up to a few years ago, discussions of bacterial diversity were unwarranted. However, the development of a phylogenetic system based on rRNA sequences has provided tools to approach bacterial diversity and ecology in situ for the first time.