Summary
A novel moderately thermophilic, facultatively anaerobic chemoorganotrophic bacterium strain
P3M
‐2
T
was isolated from a microbial mat developing on the wooden surface of a chute under the flow of hot water (46°
C
) coming out of a 2775‐m‐deep oil exploration well (
T
omsk region,
R
ussia). Strain
P3M
‐2
T
is a moderate thermophile and facultative anaerobe growing on mono‐, di‐ or polysaccharides by aerobic respiration, fermentation or by reducing diverse electron acceptors [nitrite,
F
e(
III
),
As
(
V
)]. Its closest cultivated relative (90.8%
rRNA
gene sequence identity) is
I
gnavibacterium album
, the only chemoorganotrophic member of the phylum
C
hlorobi
. New genus and species
M
elioribacter roseus
are proposed for isolate
P3M
‐2
T
. Together with
I
. album
, the new organism represents the class
I
gnavibacteria
assigned to the phylum
Chlorobi
. The revealed group includes a variety of uncultured environmental clones, the
16S rRNA
gene sequences of some of which have been previously attributed to the candidate division
ZB1
. Phylogenetic analysis of
M
. roseus
and
I
. album
based on their
23S rRNA
and
RecA
sequences confirmed that these two organisms could represent an even deeper, phylum‐level lineage. Hence, we propose a new phylum
Ignavibacteriae
within the
Bacteroidetes
–
C
hlorobi
group with a sole class
I
gnavibacteria
, two families
I
gnavibacteriaceae
and
M
elioribacteraceae
and two species
I
. album
and
M
. roseus
. This proposal correlates with chemotaxonomic data and phenotypic differences of both organisms from other cultured representatives of
C
hlorobi
. The most essential differences, supported by the analyses of complete genomes of both organisms, are motility, facultatively anaerobic and obligately organotrophic mode of life, the absence of chlorosomes and the apparent inability to grow phototrophically.