Nostoc  

What you should know:

Nostoc is a Gram-negative coccus-shaped cyanobacterium that is arranged in long chains. Larger heterocysts for nitrogen fixation can also be seen in these chains. Nostoc floats in water with the aid of a slime coat that surrounds a colony of cells. Cyanobacteria such as Nostoc are photoautotrophs and carry out oxygenic photosynthesis with the aid of chlorophyll. 

Classification: 

Domain Bacteria

   Phylum Cyanobacteria

        Class "Cyanobacteria"

Bergey's Manual Volume 1: The Archaea and the deeply branching and phototrophic Bacteria