cyanobacterium \cyanobacterium\ pl. cyanobacteria
\cyanobacteria\ [from the pigment phycocyanin.]
   any of a group of photosynthetic autotrophic prokaryotic
   microorganisms possessing characteristics of both bacteria
   and plants. When classed as bacteria, they are assigned to
   the Cyanobacteria; when classed as plants, they are assigned
   to the Cyanophyta. They were earlier named blue-green
   algae, a term less used now in technical discussions. Since
   the chlorophyll within the cyanobacteria is diffused
   throughout the cell, rather than being contained in
   chloroplasts, they are no longer thought of as true plants.
   [PJC]