Chromatium 

What you should know:

Chromatium are photoautotrophic Gram-negative rods known as the Purple Sulfur Bacteria. They carry out anoxygenic photosynthesis using H₂S as electron source and bacteriochlorophyll as the photosynthetic pigment. Despite the similarities to the green sulfur bacteria, they are only distantly related.

Classification: 

Domain Bacteria

    Phylum Proteobacteria

        Class a -Proteobacteria

Bergey's Manual Volume 2: The Proteobacteria