Dehalococcoides

Dehalococcoides
Dehalococcoides sp. strain UCH00
Scientific classification Edit this classification
Domain: Bacteria
Kingdom: Bacillati
Phylum: Chloroflexota
Class: Dehalococcoidia
Order: Dehalococcoidales
Family: Dehalococcoidaceae
Genus: Dehalococcoides
Löffler et al. 2013
Species:
D. mccartyi
Binomial name
Dehalococcoides mccartyi
Löffler et al. 2013
Synonyms
  • "Dehalococcoides" Maymo-Gatell et al. 1997
  • "Dehalococcoides ethenogenes" Maymo-Gatell et al. 1997

Dehalococcoides is a genus of obligately anaerobic bacteria within the class Dehalococcoidia. These microbes have an irregular coccus morphology and lack a peptidoglycan cell wall. They can commonly be found in environments lacking oxygen such as, contaminated soil, groundwater, etc. They obtain their energy via the oxidation of hydrogen and subsequent reductive dehalogenation of halogenated organic compounds. This metabolic characteristic makes them significant in the degradation of pollutants and especially valuable to research because of their potential with bioremediation efforts.

Morphology and structure

Dehalococcoides mccartyi is a species of small bacteria which have a flat, disc-like shape. They are usually nonmotile and are around typically around 0.3-1.0μm wide and 0.1-0.2μm in height. A study of a specific Dehalococcoides strain called MB concluded that these cells are commonly disc-shaped and can occur either alone or in pairs. More recent and detailed imaging of D. mccartyi has shown that these cells can also occur in deformed curved or straight shapes with sharp edges. Earlier studies of D. mccartyi showed evidence of irregular indentations in the surface of cells; however, recent Cryo-electron tomography has not been successful in identifying any similar features to those described by these earlier studies, suggesting that these findings were likely caused by sample handling and preparation procedures. Recent microscopic studies have revealed that D. mccartyi possesses a cell envelope that contains a cytoplasmic membrane, as well as a prominent S-layer. Cryo-electron tomography has also revealed that these cells possess bundled cytoskeletal filaments and short surface appendages that appear to be consistent and connected with their prominent S-layers.

Activities

Dehalococcoides are obligately organohalide-respiring bacteria, meaning that they can only grow by using halogenated compounds as electron acceptors. Currently, hydrogen (H2) is often regarded as the only known electron donor to support growth of dehalococcoides bacteria. However, studies have shown that using various electron donors such as formate and methyl viologen have also been effective in promoting growth for various species of dehalococcoides. To perform reductive dehalogenation processes, electrons are transferred from electron donors through dehydrogenases, and ultimately used to reduce halogenated compounds, many of which are human-synthesized chemicals acting as pollutants. Furthermore, it has been shown that most reductive dehalogenase activities lie within the extracellular and membranous components of D. ethenogenes, indicating that dechlorination processes may function semi-independently from intracellular systems. Currently, all known dehalococcoides strains require acetate for producing cellular material, however, the underlying mechanisms are not well understood as they appear to lack fundamental enzymes that complete biosynthesis cycles found in other organisms.

Dehalococcoides can transform many persistent compounds. This includes tetrachloroethylene (PCE) and trichloroethylene (TCE) which are transformed to ethylene, and chlorinated dioxins, vinyl chloride, benzenes, polychlorinated biphenyls (PCBs), phenols and many other aromatic contaminants.

Applications

Environmental remediation efforts

Dehalococcoides have the unique ability to transform many highly toxic and/or persistent compounds, in addition to halogenated compounds that other common organohalide respirators use. They are currently the only known dechlorinating bacteria capable of completely reducing recalcitrant contaminants such as tetrachloroethene (PCE) and Trichloroethylene (TCE) into more suitable for environmental conditions. While common compounds such as chlorinated dioxins, benzenes, PCBs, phenols and many other aromatic substrates can be reduced into less harmful chemical forms by other bacterium, mixed cultures can enhance the dehalogenation process by producing metabolic products that can be used by Dehalococcoides and others involved in the degradation process. For these reasons, several companies worldwide now use Dehalococcoides-containing mixed cultures in commercial remediation efforts. In the US, BAV1 was patented for the in situ reductive dechlorination of vinyl chlorides and dichloroethylenes in 2007. D. mccartyi in high-density dechlorinating bioflocs have also been used in ex situ bioremediation. Their capacity to grow by using contaminants allows them to proliferate in contaminated soil or groundwater, offering promise for in situ decontamination efforts.

Microbial reductive dechlorination

The process of transforming halogenated pollutants to non-halogenated compounds involves different reductive enzymes. The Dehalococcoides sp. strain WL works alongside Dehalobacter in a step-wise manner to degrade vinyl chloride. D. mccartyi strain BAV1 is able to reduce vinyl chloride, a contaminant that usually originates from landfills, to ethene by using a special vinyl chloride reductase thought to be coded for by the bvcA gene. A chlorobenzene reductive dehalogenase has also been identified in the strain CBDB1. Also, the addition of electron acceptors is needed – they are converted to hydrogen in situ by other bacteria present, which can then be used as an electron source by Dehalococcoides. MEAL (a methanol, ethanol, acetate, and lactate mixture) is documented to have been used as substrate.

Implications of bioremediation

Although dehalococcoides have been shown to reduce contaminants such as PCE and TCE, it appears that individual species have various dechlorinating capabilities which contributes to the degree that these compounds are reduced. This could have implications on the effects of bioremediation tactics. For example, particular strains of Dehalococcoides have shown preference to produce more soluble, intermediates such as 1,2–dichloroethene isomers and vinyl chloride that are harmful in nature. This is contrasting to bioremediation goals of Dehalococcoides. Therefore, an important aspect of current bioremediation tactics involves the use of multiple dechlorinating organisms to promote symbiotic relationships within a mixed culture to ensure complete reduction to ethene. As a result, studies have focused upon metabolic pathways and environmental factors that regulate reductive dehalogenative processes in order to better implement dehalococcoides for bioremediation tactics.

However, not all members of Dehalococcoides can reduce all halogenated contaminants. Certain strains cannot use PCE or TCE as electron acceptors (e.g. CBDB1) and some cannot use vinyl chloride as an electron acceptor (e.g. FL2). D. mccartyi strains 195 and SFB93 are inhibited by high concentrations of acetylene (which builds up in contaminated groundwater sites as a result of TCE degradation) via changes in gene expression that likely disrupt normal electron transport chain function. Another chemical inhibitor that can occur in contaminated soils is Chloroform, which can interfere with reductive dechlorination in certain D. mccartyi strains. However, strain GEO12 exhibits a greater tolerance than other tested strains, highlighting the significance of variation between strains. Even when D. mccartyi strains work well to turn toxic chemicals into harmless ones, treatment times range from months to decades. When selecting Dehalococcoides strains for bioremediation use, it is important to consider their metabolic capabilities and their sensitivities to different chemicals.

In 2022, the United States National Aeronautics and Space Administration (NASA) co-funded a US$1.9 million multi-year project with Arizona State University, the University of Arizona, and the Florida Institute of Technology to reduce perchlorates (such as those found in the regolith of Mars) to a useful form of soil for growing plants.

Genomes

Several strains of Dehalococcoides sp. has been sequenced. They contain between 14 and 36 reductive dehalogenase homologous (rdh) operons each consisting of a gene for the active dehalogenases (rdhA) and a gene for a putative membrane anchor (rdhB). Most rdh-operons in Dehalococcoides genomes are preceded by a regulator gene, either of the marR-type (rdhR) or a two-component system (rdhST). Dehalococcoides have very small genomes of about 1.4–1.5 Mio base pairs. This is one of the smallest values for free-living organisms.

Biochemistry

Unlike model organohalide–respiring bacteria—Desulfitobacterium and Sulfurospirillum multivorans—D. mccartyi lacks lipophilic quinones in the cytoplasmic membrane. This absence is attributable to the genomic lack of biosynthetic pathways for menaquinone and ubiquinone: menA–H, mqnA–D, and ubiA–X. Similarly, D. mccartyi lacks the genome clusters for cytochrome biosynthesis (hemA–H). This unique process operates within the atypical monoderm cell envelope. D. mccartyi has no outer membrane and it does not synthesize peptidoglycan, instead relying on an S–layer directly overlapping the plasma membrane. Rather than occurring in the periplasm, extracellular redox reactions occur in enzyme complexes between the monoderm’s outer leaflet and the S–layer, the pseudo–periplasm.

See also

References

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