Geology topics

J.B. Risatti

Publications and source records attributed to J.B. Risatti.

5 recordsLinked to original sources

USGS1994

Community structure of a microbial mat: The phylogenetic dimension

Traditional studies of microbial communities are incomplete because of the inability to identify and quantify all contributing populations. In the present study, we directly determine the abundance and distribution of sulfate-reducing bacterial populations in a microbial mat community by using hybridization probes complementary to the 16S-like rRNAs of major phylogenetic groups. Most of the major groups were found in this single community, distributed for the most part in nonoverlapping depth intervals of the mat. The reflection of the phylogenetic structure in the community structure suggests that those species making up the major phylogenetic groups perform specific interrelated metabolic functions in the community. Comparison of population profiles to previously observed rates of sulfate reduction suggests there are additional populations of sulfate-reducing bacteria both within the photooxic zone and deeper in the mat.

Proceedings of the National Academy of Sciences of↗

USGS1989

Carbon isotope effects associated with autotrophic acetogenesis

The carbon kinetic isotope effects associated with synthesis of acetate from CO 2 and H 2 during autotrophic growth of Acetobacterium woodii at 30°C have been measured by isotopic analyses of CO 2 , methyl-carbon, and total acetate. Closed systems allowing construction of complete mass balances at varying stages of growth were utilized, and the effects of the partitioning of carbon between CO 2 and HCO 3 − were taken into account. For the overall reaction, total carbonate → total acetate, isotope effects measured in replicate experiments ranged from −59.0 ± 0.9% to − 57.2 ± 2.3 z %. Taking into account all measurements, the weighted mean and standard deviation are −58.6 ± 0.7%. There is no evidence for intramolecular ordering in the acetate. The carbon isotopic composition of sedimentary acetate, otherwise expected to be near that of sedimentary organic carbon, is likely to be depleted in environments in which autotrophic acetogenesis is occurring.

Organic Geochemistry↗

USGS1988

Microbial oxidation of pyrrhotites in coal chars

The ability of Thiobacillus ferrooxidans to oxidize pyrrhotite minerals occurring in coal chars was investigated, to evaluate the feasibility of microbial char desulphurization. Bio-oxidation of pyrrhotites in chars produced by two different processes was demonstrated conclusively. Microbial removal of sulphur from a char and its parent coal proceeded at the rate of 3.5% and 12% day −1 , respectively with a total of 48% and 81% removal after 27 days. The pH of shake flask cultures containing the coal dropped naturally to a final value of 2.2, while the pH of cultures containing the corresponding char rose and had to be lowered artificially with additional acid. Amending char cultures with elemental sulphur to increase acidity upon bio-oxidation and prevent precipitation of ferric iron was successful; however, the extent of pyrrhotite removal, as demonstated by X-ray diffraction analysis, was not improved. As yet, there is no explanation for the failure of microbial removal of pyrrhotitic sulphur to go to completion.

Fuel↗

USGS1984

Stereochemical studies of acyclic isoprenoids—XII. Lipids of methanogenic bacteria and possible contributions to sediments

Abundant volatile lipids of Methanobacterium thermoautotrophicum and Methanosarcina barkeri include isoprenoid hydrocarbons (≤ C 30 ), and C 15 , C 20 and C 25 isoprenoid alcohols. M. barkeri contains 2,6,10,15,19-pentamethyleicosane, whose relative stereochemistry is the same as found in marine sediments, indicating that it is a marker of methanogenic activity. The C 20 , C 30 and C 25 alkenes in M. thermoautotrophicum also have a preferred sterochemistry; the latter have the 2,6,10,14,18-pentamethyleicosanyl skeleton, suggesting that the alkane in marine sediments may derive from methanogens. The stereochemistry of squalane in a marine sediment is also compatible with an origin in methanogens; in contrast, the stereochemistry of pristane in M. thermoautotrophicum indicates a fossil fuel contaminant origin, suggesting that this and certain other alkanes reported in archaebacteria might also be of contaminant origin. There is, therefore, little evidence at present that the pristane in immature marine sediments originates in methanogens. The C 15 and C 20 saturated alcohols in M. thermoautotrophicum have mainly the all-R configuration. If this is generally true for methanogens, the C 20 alcohol in the Messel shale may originate mainly from methanogens, whereas that in the Green River shale may originate mainly from photosynthetic organisms.

Organic Geochemistry↗

USGS1981

Fractionation of carbon and hydrogen isotopes by methane-oxidizing bacteria

Carbon isotopic analysis of methane has become a popular technique in the exploration for oil and gas because it can be used to differentiate between thermogenic and microbial gas and can sometimes be used for gas-source rock correlations. Methane-oxidizing bacteria, however, can significantly change the carbon isotopic composition of methane; the origin of gas that has been partially oxidized by these bacteria could therefore be misinterpreted. We cultured methane-oxidizing bacteria at two different temperatures and monitored the carbon and hydrogen isotopic compositions of the residual methane. The residual methane was enriched in both 13 C and D. For both isotopic species, the enrichment at equivalent levels of conversion was greater at 26°C than at 11.5°C. The change in δD relative to the change in δ 13 C was independent of temperature within the range studied. One culture exhibited a change in the fractionation pattern for carbon (but not for hydrogen) midway through the experiment, suggesting that bacterial oxidation of methane may occur via more than one pathway. The change in the δD value for the residual methane was from 8 to 14 times greater than the change in the δ 13 C value, indicating that combined carbon and hydrogen isotopic analysis may be an effective way of identifying methane which has been subjected to partial oxidation by bacteria.

Geochimica et Cosmochimica Acta↗