Dehalococcoides mccartyi
(
Dhc
) and
Dehalogenimonas
spp. (
Dhgm
) are members of the class
Dehalococcoidia
, phylum Chloroflexi, characterized by streamlined genomes and a strict requirement for organohalogens as electron acceptors. Here, we used cryo-electron tomography to reveal morphological and ultrastructural features of
Dhc
strain BAV1 and ‘
Candidatus
Dehalogenimonas etheniformans’ strain GP cells at unprecedented resolution.
Dhc
cells were irregularly shaped discs (890 ± 110 nm long, 630 ± 110 nm wide and 130 ± 15 nm thick) with curved and straight sides that intersected at acute angles, whereas
Dhgm
cells appeared as slightly flattened cocci (760 ± 85 nm). The cell envelopes were composed of a cytoplasmic membrane (CM), a paracrystalline surface layer (S-layer) with hexagonal symmetry and ∼22 nm spacing between repeating units, and a layer of unknown composition separating the CM and the S-layer. Cell surface appendages were only detected in
Dhc
cells, whereas both cell types had bundled cytoskeletal filaments. Repetitive globular structures, ∼5 nm in diameter and ∼9 nm apart, were observed associated with the outer leaflet of the CM. We hypothesized that those represent organohalide respiration (OHR) complexes and estimated ∼30,000 copies per cell. In
Dhgm
cultures, extracellular lipid vesicles (20 - 110 nm in diameter) decorated with putative OHR complexes but lacking an S-layer were observed. The new findings expand our understanding of the unique cellular ultrastructure and biology of organohalide-respiring
Dehalococcoidia
.
Importance:
Dehalococcoidia
respire organohalogen compounds and play relevant roles in bioremediation of groundwater, sediments and soils impacted with toxic chlorinated pollutants. Using advanced imaging tools, we have obtained 3-dimensional images at macromolecular resolution of whole
Dehalococcoidia
cells revealing their unique structural components. Our data detail the overall cellular shape, cell envelope architecture, cytoskeletal filaments, the likely localization of enzymatic complexes involved in reductive dehalogenation, and the structure of extracellular vesicles. The new findings expand our understanding of the cell structure-function relationship in
Dehalococcoidia
with implications for
Dehalococcoidia
biology and bioremediation.