An analysis of the changes in soluble hydrogenase and global gene expression in Cupriavidus necator (Ralstonia eutropha) H16 grown in heterotrophic diauxic batch culture

Biological carbon capture from biogas streams: Insights into Cupriavidus necator autotrophic growth and transcriptional profile

Recycling carbon-rich wastes into high-value platform chemicals through biological processes provides a sustainable alternative to petrochemicals. Cupriavidus necator, known for converting carbon dioxide (CO2) into polyhydroxyalkanoates (PHA) was studied for the first time using biogas streams as the sole carbon source. The bacterium efficiently consumed biogenic CO2 from raw biogas with methane at high concentrations (50%) proving non-toxic. Continuous addition of H2 and O2 enabled growth trends comparable to glucose-based heterotrophic growth. Transcriptomic analysis revealed CO2-adaptated cultures exhibited upregulation of hydrogenases and Calvin cycle enzymes, as well as genes related to electron transport, nutrient uptake, and glyoxylate cycle. Non-adapted samples displayed activation of stress response mechanisms, suggesting potential lags in large-scale processes. These findings showcase the setting of growth parameters for a pioneering biological biogas upgrading strategy, emphasizing the importance of inoculum adaptation for autotrophic growth and providing potential targets for genetic engineering to push PHA yields in future applications.

Valorization of whey-based side streams for microbial biomass, molecular hydrogen, and hydrogenase production

Side streams of the dairy industry are a suitable nutrient source for cultivating microorganisms, producing enzymes, and high-value chemical compounds. The heterotrophic Escherichia coli and chemolithoautotroph Ralstonia eutropha are of major biotechnological interest. R. eutropha is a model organism for producing O₂-tolerant [NiFe]-hydrogenases (Hyds) (biocatalysts), and E. coli has found widespread use as an expression platform for producing recombinant proteins, molecular hydrogen (H₂), and other valuable products. Aiming at developing suitable cultivation media from side streams of the dairy industry, the pre-treatment (filtration, dilution, and pH adjustment) of cheese (sweet) whey (SW) and curd (acid) whey (AW), with and without the use of ß-glucosidase, has been performed. Growth parameters (oxidation-reduction potential (ORP), pH changes, specific growth rate, biomass formation) of E. coli BW25113 and R. eutropha H16 type strains were monitored during cultivation on filtered and non-filtered SW and AW at 37 °C, pH 7.5 and 30 °C, pH 7.0, respectively. Along with microbial growth, measurements of pH and ORP indicated good fermentative growth. Compared to growth on fructose-nitrogen minimal salt medium (control), a maximum cell yield (OD₆₀₀ 4.0) and H₂-oxidizing Hyd activity were achieved in the stationary growth phase for R. eutropha. Hyd-3-dependent H₂ production by E. coli utilizing whey as a growth substrate was demonstrated. Moreover, good biomass production and prolonged H₂ yields of ~ 5 mmol/L and cumulative H₂ ~ 94 mL g/L dry whey (DW) (ß-glucosidase-treated) were observed during the cultivation of the engineered E. coli strain. These results open new avenues for effective whey treatment using thermostable β-glucosidase and confirm whey as an economically viable commodity for biomass and biocatalyst production. KEY POINTS: • Archaeal thermostable β-glucosidase isolated from the metagenome of a hydrothermal spring was used for lactose hydrolysis in whey. • Hydrogenase enzyme activity was induced during the growth of Ralstonia eutropha H16 on whey. • Enhanced biomass and H₂ production was shown in a genetically modified strain of Escherichia coli.

L-amino acids affect the hydrogenase activity and growth of Ralstonia eutropha H16

Ralstonia eutropha H16 is a chemolithoautotrophic bacterium with O₂-tolerant hydrogenase (Hyds) enzymes. Hyds are expressed in the presence of gas mixtures (H₂, O₂, CO₂) or under energy limitation and stress conditions. O₂-tolerant Hyds are promising candidates as anode biocatalysts in enzymatic fuel cells (EFCs). Supplementation of 0.5% (w/v) yeast extract to the fructose-nitrogen (FN) growth medium enhanced H₂-oxidizing Hyd activity ~ sixfold. Our study aimed to identify key metabolites (L-amino acids (L-AAs) and vitamins) in yeast extract that are necessary for the increased synthesis and activity of Hyds. A decrease in pH and a reduction in ORP (from + 240 ± 5 mV to - 180 mV ± 10 mV values) after 24 h of growth in the presence of AAs were observed. Compared to the FN-medium control, supplementation of 7.0 μmol/ml of the L-AA mixture stimulated the growth of bacteria ~ 1.9 to 2.9 fold, after 72 h. The whole cells' H₂-oxidizing Hyd activity was not observed in control samples, whereas the addition of L-AAs, mainly glycine resulted in a maximum of ~ 22 ± 0.5 and 15 ± 0.3 U, g CDW^-1 activity after 24 h and 72 h, respectively. Our results suggest a correlation between ORP, pH, and function of Hyds in R. eutropha H16 in the presence of key L-AAs. L-AAs used in small amounts can be proposed as signaling molecules or key components of Hyd maturation. These results are important for the optimization of O₂-tolerant Hyds production as anode biocatalysts.

Improving carbon monoxide tolerance of Cupriavidus necator H16 through adaptive laboratory evolution

Background: The toxic gas carbon monoxide (CO) is abundantly present in synthesis gas (syngas) and certain industrial waste gases that can serve as feedstocks for the biological production of industrially significant chemicals and fuels. For efficient bacterial growth to occur, and to increase productivity and titres, a high resistance to the gas is required. The aerobic bacterium Cupriavidus necator H16 can grow on CO₂ + H₂, although it cannot utilise CO as a source of carbon and energy. This study aimed to increase its CO resistance through adaptive laboratory evolution. Results: To increase the tolerance of C. necator to CO, the organism was continually subcultured in the presence of CO both heterotrophically and autotrophically. Ten individual cultures were evolved heterotrophically with fructose in this manner and eventually displayed a clear growth advantage over the wild type strain. Next-generation sequencing revealed several mutations, including a single point mutation upstream of a cytochrome bd ubiquinol oxidase operon (cydA2B2), which was present in all evolved isolates. When a subset of these mutations was engineered into the parental H16 strain, only the cydA2B2 upstream mutation enabled faster growth in the presence of CO. Expression analysis, mutation, overexpression and complementation suggested that cydA2B2 transcription is upregulated in the evolved isolates, resulting in increased CO tolerance under heterotrophic but not autotrophic conditions. However, through subculturing on a syngas-like mixture with increasing CO concentrations, C. necator could also be evolved to tolerate high CO concentrations under autotrophic conditions. A mutation in the gene for the soluble [NiFe]-hydrogenase subunit hoxH was identified in the evolved isolates. When the resulting amino acid change was engineered into the parental strain, autotrophic CO resistance was conferred. A strain constitutively expressing cydA2B2 and the mutated hoxH gene exhibited high CO tolerance under both heterotrophic and autotrophic conditions. Conclusion: C. necator was evolved to tolerate high concentrations of CO, a phenomenon which was dependent on the terminal respiratory cytochrome bd ubiquinol oxidase when grown heterotrophically and the soluble [NiFe]-hydrogenase when grown autotrophically. A strain exhibiting high tolerance under both conditions was created and presents a promising chassis for syngas-based bioproduction processes.

Hydrogen-oxidizing bacteria and their applications in resource recovery and pollutant removal

Hydrogen oxidizing bacteria (HOB), a type of chemoautotroph, are a group of bacteria from different genera that share the ability to oxidize H₂ and fix CO₂ to provide energy and synthesize cellular material. Recently, HOB have received growing attention due to their potential for CO₂ capture and waste recovery. This review provides a comprehensive overview of the biological characteristics of HOB and their application in resource recovery and pollutant removal. Firstly, the enzymes, genes and corresponding regulation systems responsible for the key metabolic processes of HOB are discussed in detail. Then, the enrichment and cultivation methods including the coupled water splitting-biosynthetic system cultivation, mixed cultivation and two-stage cultivation strategies for HOB are summarized, which is the critical prerequisite for their application. On the basis, recent advances of HOB application in the recovery of high-value products and the removal of pollutants are presented. Finally, the key points for future investigation are proposed that more attention should be paid to the main limitations in the large-scale industrial application of HOB, including the mass transfer rate of the gases, the safety of the production processes and products, and the commercial value of the products.

Synthetic biology toolkit for engineering Cupriviadus necator H16 as a platform for CO2 valorization

BACKGROUND

CO₂ valorization is one of the effective methods to solve current environmental and energy problems, in which microbial electrosynthesis (MES) system has proved feasible and efficient. Cupriviadus necator (Ralstonia eutropha) H16, a model chemolithoautotroph, is a microbe of choice for CO₂ conversion, especially with the ability to be employed in MES due to the presence of genes encoding [NiFe]-hydrogenases and all the Calvin-Benson-Basham cycle enzymes. The CO₂ valorization strategy will make sense because the required hydrogen can be produced from renewable electricity independently of fossil fuels.

MAIN BODY

In this review, synthetic biology toolkit for C. necator H16, including genetic engineering vectors, heterologous gene expression elements, platform strain and genome engineering, and transformation strategies, is firstly summarized. Then, the review discusses how to apply these tools to make C. necator H16 an efficient cell factory for converting CO₂ to value-added products, with the examples of alcohols, fatty acids, and terpenoids. The review is concluded with the limitation of current genetic tools and perspectives on the development of more efficient and convenient methods as well as the extensive applications of C. necator H16.

CONCLUSIONS

Great progress has been made on genetic engineering toolkit and synthetic biology applications of C. necator H16. Nevertheless, more efforts are expected in the near future to engineer C. necator H16 as efficient cell factories for the conversion of CO₂ to value-added products.

Nanoparticle-associated single step hydrogen fermentation for the conversion of starch potato waste biomass by thermophilic Parageobacillus thermoglucosidasius

In the present study, starch-based potato peel waste biomass (PWB) was utilized as a potential substrate for hydrogen production via dark fermentation by the thermophillic amylase producing strain Parageobacillus thermoglucosidasius KCTC 33548. Supplementation of Fe₃O₄ nanoparticles (300 mg/L) led to a 4.15-fold increase in hydrogen production as compared to the control. The addition of optimized concentrations of both Fe₃O₄ nanoparticles (300 mg/L) and L-cysteine (250 mg/L) during hydrogen fermentation using pure starch and PWB generated maximum cumulative hydrogen yields of 167 and 71.9 mL with maximum production rates of 2.81 and 1.26 mL/h, respectively. Further, the correlation between Fe₃O₄ and the expression of hydrogenase isoforms and the related hydrogenase activity was explored. The possible mechanisms of the action of Fe₃O₄ on enhanced hydrogenase activity and hydrogen production was elucidated. To our knowledge, there are no such studies reported on enhanced hydrogen production from PWB in a single step.

Melatonin-induced changes in the bovine vaginal microbiota during maternal nutrient restriction

Altering the composition of the bovine vaginal microbiota has proved challenging, with recent studies deeming the microbiota dynamic due to few overall changes being found. Therefore, the objectives of this study were to determine whether gestational age, endogenous progesterone, maternal nutrient restriction, or dietary melatonin altered the composition of the bovine vaginal microbiota. Brangus heifers (n = 29) from timed artificial insemination to day 240 of gestation were used; at day 160 of gestation, heifers were assigned to either an adequate (ADQ; n = 14; 100% NRC requirements) or restricted (RES; n = 15; 60% NRC requirements) nutritional plane and were either supplemented with dietary melatonin (MEL; n = 15) or not supplemented (CON; n = 14). Samples for vaginal microbiota analysis were taken on day 0 (prior to artificial insemination), day 150 (prior to dietary treatments), and day 220 of gestation (60 d post-treatment initiation) using a double guarded culture swab. The vaginal bacterial overall community structure was determined through sequencing the V4 region of the 16S rRNA gene using the Illumina Miseq platform. Alpha diversity was compared via 2-way ANOVA; β diversity was compared via PERMANOVA. The linear discriminant analysis for effect size (LEfSe) pipeline was utilized for analysis of taxonomic rank differences between bacterial communities. Gestational age, progesterone concentration, and maternal nutritional plane did not alter α or β diversity of the vaginal microbiota. However, gestational age resulted in compositional changes at the order, family, and genus level. Moreover, dietary melatonin supplementation did not alter α diversity of the vaginal microbiota but did alter β diversity (P = 0.02). Specifically, melatonin altered the composition at the genus level and increased the prevalence of aerobic bacteria in the vaginal tract. To date, melatonin is the first hormone associated with altering the composition of the bovine vaginal microbiota.

Metabolic Engineering of Cupriavidus necator H16 for Sustainable Biofuels from CO2

Decelerating global warming is one of the predominant challenges of our time and will require conversion of CO₂ to usable products and commodity chemicals. Of particular interest is the production of fuels, because the transportation sector is a major source of CO₂ emissions. Here, we review recent technological advances in metabolic engineering of the hydrogen-oxidizing bacterium Cupriavidus necator H16, a chemolithotroph that naturally consumes CO₂ to generate biomass. We discuss recent successes in biofuel production using this organism, and the implementation of electrolysis/artificial photosynthesis approaches that enable growth of C. necator using renewable electricity and CO₂. Last, we discuss prospects of improving the nonoptimal growth of C. necator in ambient concentrations of CO₂.

Transcriptional Organization of the Stability Module of Broad-Host-Range Plasmid RA3, from the IncU Group

The broad-host-range (BHR) conjugative plasmids have developed diverse adaptive mechanisms defining the range of their promiscuity. The BHR conjugative RA3 plasmid, the archetype of the IncU group, can transfer between, replicate in, and be maintained in representatives of Alpha-, Beta-, and Gammaproteobacteria Its stability module encompasses ten open reading frames (ORFs) apparently organized into five operons, all transcribed in the same direction from several strong promoters that are tightly regulated either by autorepressors or by global plasmid-encoded regulators. In this paper, we demonstrate that owing to an efficient RNA polymerase (RNAP) read-through, the transcription from the first promoter, orf02p, may continue through the whole module. Moreover, an analysis of mRNA produced from the wild-type (WT) stability module and its deletion variants deprived of particular internal transcription initiation sites reveals that in fact each operon may be transcribed from any upstream promoter, giving rise to multicistronic transcripts of variable length and creating an additional level of gene expression control by transcript dosage adjustment. The gene expression patterns differ among various hosts, indicating that promoter recognition, regulation, and the RNAP read-through mechanisms are modulated in a species-specific manner.IMPORTANCE The efficiently disseminating conjugative or mobilizable BHR plasmids play key roles in the horizontal spread of genetic information between closely related and phylogenetically distant species, which can be harmful from the medical, veterinary, or industrial point of view. Understanding the mechanisms determining the plasmid's ability to function in diverse hosts is essential to help limit the spread of undesirable plasmid-encoded traits, e.g., antibiotic resistance. The range of a plasmid's promiscuity depends on the adaptations of its transfer, replication, and stability functions to the various hosts. IncU plasmids, with the archetype plasmid RA3, are considered to constitute a reservoir of antibiotic resistance genes in aquatic environments; however, the molecular mechanisms determining their adaptability to a broad range of hosts are rather poorly characterized. Here, we present the transcriptional organization of the stability module and show that the gene transcript dosage effect is an important determinant of the stable maintenance of RA3 in different hosts.

Protein Activity Sensing in Bacteria in Regulating Metabolism and Motility

Bacteria have evolved complex sensing and signaling systems to react to their changing environments, most of which are present in all domains of life. Canonical bacterial sensing and signaling modules, such as membrane-bound ligand-binding receptors and kinases, are very well described. However, there are distinct sensing mechanisms in bacteria that are less studied. For instance, the sensing of internal or external cues can also be mediated by changes in protein conformation, which can either be implicated in enzymatic reactions, transport channel formation or other important cellular functions. These activities can then feed into pathways of characterized kinases, which translocate the information to the DNA or other response units. This type of bacterial sensory activity has previously been termed protein activity sensing. In this review, we highlight the recent findings about this non-canonical sensory mechanism, as well as its involvement in metabolic functions and bacterial motility. Additionally, we explore some of the specific proteins and protein-protein interactions that mediate protein activity sensing and their downstream effects. The complex sensory activities covered in this review are important for bacterial navigation and gene regulation in their dynamic environment, be it host-associated, in microbial communities or free-living.

Growth of the facultative chemolithoautotroph Ralstonia eutropha on organic waste materials: growth characteristics, redox regulation and hydrogenase activity

BACKGROUND

The chemolithoautotrophic β-proteobacterium Ralstonia eutropha H16 (Cupriavidus necator) is one of the most studied model organisms for growth on H₂ and CO₂. R. eutropha H16 is also a biologically significant bacterium capable of synthesizing O₂-tolerant [NiFe]-hydrogenases (Hyds), which can be used as anode biocatalysts in enzyme fuel cells. For heterotrophic growth of R. eutropha, various sources of organic carbon and energy can be used.

RESULTS

Growth, bioenergetic properties, and oxidation-reduction potential (ORP) kinetics were investigated during cultivation of R. eutropha H16 on fructose and glycerol or lignocellulose-containing brewery spent grain hydrolysate (BSGH). BSGH was used as carbon and energy source by R. eutropha H16, and the activities of the membrane-bound hydrogenase (MBH) and cytoplasmic, soluble hydrogenase (SH) were measured in different growth phases. Growth of R. eutropha H16 on optimized BSGH medium yielded ~ 0.7 g cell dry weight L^-1 with 3.50 ± 0.02 (SH) and 2.3 ± 0.03 (MBH) U (mg protein)^-1 activities. Upon growth on fructose and glycerol, a pH drop from 7.0 to 6.7 and a concomitant decrease of ORP was observed. During growth on BSGH, in contrast, the pH and ORP stayed constant. The growth rate was slightly stimulated through addition of 1 mM K₃[Fe(CN)₆], whereas temporarily reduced growth was observed upon addition of 3 mM dithiothreitol. The overall and N,N'-dicyclohexylcarbodiimide-sensitive ATPase activities of membrane vesicles were ~ 4- and ~ 2.5-fold lower, respectively, upon growth on fructose and glycerol (FGN) compared with only fructose utilization (FN). Compared to FN, ORP was lower upon bacterial growth on FGN, GFN, and BSGH.

CONCLUSIONS

Our results suggest that reductive conditions and low ATPase activity might be signals for energy depletion, which, in turn, leads to increased hydrogenase biosynthesis to overcome this unfavorable situation. Addition of fructose or microelements have no, or a negative, influence on hydrogenase activity. Organic wastes (glycerol, BSGH) are promising carbon and energy sources for the formation of biomass harboring significant amounts of the biotechnologically relevant hydrogenases MBH and SH. The results are valuable for using microbial cells as producers of hydrogenase enzymes as catalysts in enzymatic fuel cells.

Hydrogen-Driven Cofactor Regeneration for Stereoselective Whole-Cell C=C Bond Reduction in Cupriavidus necator

The coupling of recombinantly expressed oxidoreductases to endogenous hydrogenases for cofactor recycling permits the omission of organic cosubstrates as sacrificial electron donors in whole-cell biotransformations. This increases atom efficiency and simplifies the reaction. A recombinant ene-reductase was expressed in the hydrogen-oxidizing proteobacterium Cupriavidus necator H16. In hydrogen-driven biotransformations, whole cells catalyzed asymmetric C=C bond reduction of unsaturated cyclic ketones with stereoselectivities up to >99 % enantiomeric excess. The use of hydrogen as a substrate for growth and cofactor regeneration is particularly attractive because it represents a strategy for improving atom efficiency and reducing side product formation associated with the recycling of organic cofactors.

Global changes in the proteome of Cupriavidus necator H16 during poly-(3-hydroxybutyrate) synthesis from various biodiesel by-product substrates

Synthesis of poly-[3-hydroxybutyrate] (PHB) by Cupriavidus necator H16 in batch cultures was evaluated using three biodiesel-derived by-products as the sole carbon sources: waste glycerol (REG-80, refined to 80 % purity with negligible free fatty acids); glycerol bottom (REG-GB, with up to 65 % glycerol and 35 % free fatty acids), and free fatty acids (REG-FFA, with up to 75 % FFA and no glycerol). All the three substrates supported growth and PHB production by C. necator, with polymer accumulation ranging from 9 to 84 % cell dry weight (cdw), depending on the carbon source. To help understand these differences, proteomic analysis indicated that although C. necator H16 was able to accumulate PHB during growth on all three biodiesel by-products, no changes in the levels of PHB synthesis enzymes were observed. However, significant changes in the levels of expression were observed for two Phasin proteins involved with PHB accumulation, and for a number of gene products in the fatty acid β-oxidation pathway, the Glyoxylate Shunt, and the hydrogen (H2) synthesis pathways in C. necator cells cultured with different substrates. The glycerol transport protein (GlpF) was induced in REG-GB and REG-80 glycerol cultures only. Cupriavidus necator cells cultured with REG-GB and REG-FFA showed up-regulation of β-oxidation and Glyoxylate Shunt pathways proteins at 24 h pi, but H2 synthesis pathways enzymes were significantly down-regulated, compared with cells cultured with waste glycerol. Our data confirmed earlier observations of constitutive expression of PHB synthesis proteins, but further suggested that C. necator H16 cells growing on biodiesel-derived glycerol were under oxidative stress.

Genomic, transcriptomic and proteomic analyses of Dehalobacter UNSWDHB in response to chloroform

Organohalide respiring bacteria (ORB) are capable of utilising organohalides as electron acceptors for the generation of cellular energy and consequently play an important role in the turnover of natural and anthropogenically-derived organohalides. In this study, the response of a Dehalobacter sp. strain UNSWDHB to the addition of trichloromethane (TCM) after a 50 h period of its absence (suffocation) was evaluated from a transcriptomic and proteomic perspective. The up-regulation of TCM reductive dehalogenase genes (tmrABC) and their gene products (TmrABC) was confirmed at both transcriptional and proteomic levels. Other findings include the upregulation of various hydrogenases (membrane-associated Ni-Fe hydrogenase complexes and soluble Fe-Fe hydrogenases), formate dehydrogenases, complex I and a pyrophosphate-energized proton pump. The elevated expression of enzymes associated with carbon metabolism, including complete Wood Ljungdahl pathway, during TCM respiration raises interesting questions on possible fates of intracellular formate and its potential role in the physiology of this bacterium. Overall, the findings presented here provide a broader view on the bioenergetics and general physiology of Dehalobacter UNSWDHB cells actively respiring with TCM.

Construction and use of a Cupriavidus necator H16 soluble hydrogenase promoter (PSH) fusion to gfp (green fluorescent protein)

Hydrogenases are metalloenzymes that reversibly catalyse the oxidation or production of molecular hydrogen (H₂). Amongst a number of promising candidates for application in the oxidation of H₂ is a soluble [Ni–Fe] uptake hydrogenase (SH) produced by Cupriavidus necator H16. In the present study, molecular characterisation of the SH operon, responsible for functional SH synthesis, was investigated by developing a green fluorescent protein (GFP) reporter system to characterise P_SH promoter activity using several gene cloning approaches. A P_SH promoter-gfp fusion was successfully constructed and inducible GFP expression driven by the P_SH promoter under de-repressing conditions in heterotrophic growth media was demonstrated in the recombinant C. necator H16 cells. Here we report the first successful fluorescent reporter system to study P_SH promoter activity in C. necator H16. The fusion construct allowed for the design of a simple screening assay to evaluate P_SH activity. Furthermore, the constructed reporter system can serve as a model to develop a rapid fluorescent based reporter for subsequent small-scale process optimisation experiments for SH expression.

Production and purification of a soluble hydrogenase from Ralstonia eutropha H16 for potential hydrogen fuel cell applications

The soluble hydrogenase (SH) from Ralstonia eutropha H16 is a promising candidate enzyme for H₂-based biofuel application as it favours H₂ oxidation and is relatively oxygen-tolerant. In this report, bioprocess development studies undertaken to produce and purify an active SH are described, based on the methods previously reported [1], [2], [3], [4]. Our modifications are:

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Upstream method optimizations were undertaken on heterotrophic growth media and cell lysis involving ultrasonication.

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Two anion exchangers (Q Sepharose and RESOURCE Q) and size exclusion chromatographic (Superdex 200) matrices were successfully employed for purification of a hexameric SH from R. eutropha.

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The H₂ oxidizing activity of the SH was demonstrated spectrophotometrically in solution and also immobilized on an EPG electrode using cyclic voltammetry.

Organohalide Respiring Bacteria and Reductive Dehalogenases: Key Tools in Organohalide Bioremediation

Organohalides are recalcitrant pollutants that have been responsible for substantial contamination of soils and groundwater. Organohalide-respiring bacteria (ORB) provide a potential solution to remediate contaminated sites, through their ability to use organohalides as terminal electron acceptors to yield energy for growth (i.e., organohalide respiration). Ideally, this process results in non- or lesser-halogenated compounds that are mostly less toxic to the environment or more easily degraded. At the heart of these processes are reductive dehalogenases (RDases), which are membrane bound enzymes coupled with other components that facilitate dehalogenation of organohalides to generate cellular energy. This review focuses on RDases, concentrating on those which have been purified (partially or wholly) and functionally characterized. Further, the paper reviews the major bacteria involved in organohalide breakdown and the evidence for microbial evolution of RDases. Finally, the capacity for using ORB in a bioremediation and bioaugmentation capacity are discussed.