A new evolutionary branch of bacteria has been discovered in human guts and in groundwater.

While characterizing microbial communities in human faeces and environmental samples, a team led by Jillian Banfield at the University of California, Berkeley, and Ruth Ley at Cornell University in Ithaca, New York, found DNA sequences that were very distantly related to photosynthetic cyanobacteria. Further sequencing produced three complete and three partial genomes for the bacteria. The authors assigned the microbes their own phylum — Melainabacteria.

The melainabacteria lack the genes required for photosynthesis and for coping with oxygen. They probably generate energy by fermenting various carbon-containing molecules, and may provide their human hosts with the vitamins B and K.

eLife 2, e01102 (2013)