Summary
Termite gut flagellates are typically colonized by specific bacterial symbionts. Here we describe the phylogeny, ultrastructure and subcellular location of ‘
Candidatus
Adiutrix intracellularis’, an intracellular symbiont of
Trichonympha collaris
in the termite
Zootermopsis nevadensis
. It represents a novel, deep‐branching clade of uncultured
Deltaproteobacteria
widely distributed in intestinal tracts of termites and cockroaches. Fluorescence
in situ
hybridization and transmission electron microscopy localized the endosymbiont near hydrogenosomes in the posterior part and near the ectosymbiont ‘
Candidatus
Desulfovibrio trichonymphae’ in the anterior part of the host cell. The draft genome of ‘
Ca
. Adiutrix intracellularis’ obtained from a metagenomic library revealed the presence of a complete gene set encoding the Wood–Ljungdahl pathway, including two homologs of
fdhF
encoding hydrogenase‐linked formate dehydrogenases (FDH
H
) and all other components of the recently described hydrogen‐dependent carbon dioxide reductase (HDCR) complex, which substantiates previous claims that the symbiont is capable of reductive acetogenesis from CO
2
and H
2
. The close phylogenetic relationship between the HDCR components and their homologs in homoacetogenic
Firmicutes
and
Spirochaetes
suggests that the deltaproteobacterium acquired the capacity for homoacetogenesis via lateral gene transfer. The presence of genes for nitrogen fixation and the biosynthesis of amino acids and cofactors indicate the nutritional nature of the symbiosis.