Desulfatibacillum alkenivorans AK-01
Description and Significance
Groundwater serves as water for more than 50% of people living in North America therefore a significant public resource. To date, major contamination of groundwater in North America are due to the release and use of chlorinated ethenes by industry. Examples of such toxic compounds are perchloroethene (PCE), trichloroethene (TCE). Carbon tetrachloride (CT) is also a major groundwater pollutant . These compounds were widely used as solvents for dry cleaning and in textile manufacturing. They are sufficiently water soluble and can travel through soil where they reach the groundwater. The relative high concentration of them here can be harmful . Ground water is also contaminated by pollutants that are not highly toxic, but can be utilized or modified by microorganisms to become more toxic. For instance over-fertilization in agriculture leads to an increased nitrate concentration which i.e. can cause the Blue Baby syndrome. This is seen in infants younger than six month old who rely on bacteria to digest their food. Some of these bacteria also convert nitrate, a component of fertilizer, to nitrite. In the blood nitrite reacts with hemoglobin interfering with its ability to carry oxygen. The babies show sign of suffocation and gets a bluish skin .
Microbial metabolism of groundwater pollutants
Co-metabolism and degradation of TCE
Some dehalorespiring organisms are capable of degrading PCE, TCE and CT into non-toxic compounds. Degradation of PCE is only known to happen through reductive dechlorination and only under anaerobic condition. TCE is, unlike PCE, able to be degraded under aerobic conditions. This can happen through cometabolism. In co-metabolism a compound is transformed by an organism that doesn’t use the compound as an energy or carbon source and reducing power is not provided. The organism relies on another compound to serve as an energy and carbon source  . Methanotrophic organisms grow on methane as a primary substrate and oxygen but some are also able to degrade TCE as a secondary substrate. This is because of nonspecific enzymatic activity of enzymes (methane monooxygenase, MMO) involved in degradation of the primary substrate. The degradation of TCE serves no beneficial purpose for these organisms. It generates an epoxide(cf. figure 1) which is transported out of the cell and here other heterotrophic organisms bring about the transformation into non-toxic compounds resulting in the formation of CO2. Several factors inhibit the aerobic degradation of TCE here among the concentration of contamination, the pH and the temperature. Because both TCE and methane bind to the same site in MMO competition between growth substrate and non-growth substrate also seems to limit degradation of TCE .
Desulfatibacillum alkenivorans AK-01 is capable of using sulfate, sulfite and thiosulfate, but not sulfur, as terminal electron acceptors during heterotrophic metabolism . Alkane degradation is initiated through the subterminal addition of the alkane carbon to the double bond of fumarate . The alkylsuccinate intermediate is further degraded through processes of carbon-skeleton rearrangement, decarboxylation, and beta-oxidation . The resultant acetate molecule is further oxidized to CO2 through the reversal of the Wood-Ljungdahl pathway . Strain AK-01 is able to completely oxidize alkanes to carbon dioxide when coupled with sulfate reduction . This was shown through the formation of radiolabeled CO2 from radiolabeled [1-14C]hexadecane and the corresponding loss of sulfate that was congruent with the theoretical stoichiometric ratio . Under sulfate-limiting conditions, only 3% of the total alkane present was degraded, demonstrating the dependence of alkane degradation on sulfate reduction . When sulfate concentrations were sufficient, 91% of the alkane present was degraded . The mechanism of initiation via addition to fumarate is not unique to D. alkenivorans AK-01. It has also been observed in the sulfate-reducer Desulfoglaeba alkanexedens , Desulfatibacillum aliphaticivorans CV2803 , and the denitrifying strain HxN1, a member of the Betaproteobacteria . This reaction involving fumarate represents an alternative mechanism of alkane activation compared to the more commonly known hydroxylation reaction by monooxygenases in aerobic environments .
From an evolutionary perspective, D. alkenivorans AK-01 is well adapted to occupy a variety of niches. The discovery of genes related to motility and oxygen detoxification suggests AK-01 may be able to adapt to a greater variety of environmental conditions than previously thought . Furthermore, AK-01 is capable of exhibiting three distinct modes of metabolism, giving it a distinct advantage over more specialized organisms . The microbial interactions of AK-01 in situ have yet to be analyzed. There are no known diseases associated with D. alkenivorans AK-01 .
Organisms capable of coupling alkane degradation with sulfate reduction play a key role in sites contaminated with hydrocarbons, as these sites typically become anoxic over time due to aerobic respiration and decomposition . Additionally, sulfate is relatively abundant in marine waters compared to nitrate and iron, which typically exist at low concentrations . In one study, analysis of sediment polluted with petroleum after 502 days showed strains most closely related to AK-01 were the most abundant , further emphasizing possible bio-remedial applications of D. alkenivorans AK-01 in the future. It has been experimentally shown that AK-01 can interact with methanogens to degrade alkanes syntrophically. This mechanism could be applied within the oil industry, as the current techniques used to retrieve fossil fuels are only able to extract 40% of the available resource . The residual oil could be used to harvest methane gas through syntrophic degradation as an alternative energy source . Current experiments expect a yield of 3 mmol methane gas per gram of residual oil. Taking into account the number of U.S. oil reservoirs, syntrophic alkane degradation could provide 17% of the natural gas consumed in the United States .
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