Zetaproteobacteria is a class of Proteobacteria

Proteobacteria

The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....

, which is composed of a sole member, Mariprofundus ferrooxydans described in 2010, falling basal to the gamma and betaproteobacteria and confirmed by other studies
Following the naming convention of the classes of Proteobacteria

Proteobacteria

The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....

 (formerly "purple bacteria") started with Carl Woese

Carl Woese

Carl Richard Woese is an American microbiologist and physicist. Woese is famous for defining the Archaea in 1977 by phylogenetic taxonomy of 16S ribosomal RNA, a technique pioneered by Woese and which is now standard practice. He was also the originator of the RNA world hypothesis in 1977,...

, this is the 5th class to be described so received the 5th letter of the Greek alphabet, zeta and is not based on phylogenetic position in regards to the other classes.