Chloroflexota

Background

The name Chloroflexota comes from the Latin plural of Chloroflexus, the phylum’s type genus, and was first used in the 2001 edition of Bergey’s Manual of Systematic Bacteriology. In 1987, Carl Woese grouped the genera Chloroflexus, Herpetosiphon, and Thermomicrobium into a division he called “green non-sulfur bacteria and relatives,” based on 16S ribosomal RNA sequences. This group was later renamed “Chloroflexi” in the first volume of Bergey’s Manual. At that time, Chloroflexota was considered a deep-branching phylum with only a single class of the same name.

Since 2001, newly discovered species have led to the creation of several additional classes within the phylum. The name “Dehalococcoidetes” was proposed as a placeholder by Hugenholtz and Stackebrandt in 2004, after the species Dehalococcoides ethenogenes, which was partially described in 1997. The first fully described species in this group was Dehalogenimonas lykanthroporepellens, reported by Moe and colleagues in 2009, but the class was not formally established at that time because the two species share only 90% 16S ribosomal RNA identity, meaning they might belong to different families or even orders.

More recent phylogenetic analyses have found only weak support for grouping all current classes of Chloroflexota together. In trees built from large sets of concatenated protein sequences, the six classes did not consistently form a well-supported clade, and no conserved signature indels unique to the entire phylum were identified. However, the classes Chloroflexi and Thermomicrobia consistently grouped together in both phylogenetic analyses and through shared conserved signature indels in the 50S ribosomal protein L19 and the enzyme UDP-glucose 4-epimerase. It has been suggested that Chloroflexi sensu stricto should include only these two classes, while the other four classes—Dehalococcoidetes, Anaerolineae, Caldilineae, and Ktedonobacteria—may represent one or more independent phyla that branch near Chloroflexi.