Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


The highest most probable numbers (MPNs) of methanogens (1.18 x 10('6) cells per 100 g sediment, dry weight) and sulfate-reducers (0.31 x 10('6) cells per 100 g sediment, dry weight) in unstable, gas-rich, Mississippi River delta sediments occurred together in samples taken 0.2m below the sediment-water interface. The presence of sulfate-reducers was directly related to dissolved sulfate in the interstitial water. Dissolved methane in the sediments was inversely related to the presence of sulfate-reducing bacteria. Methanogens and sulfate-reducers were recovered from maximum depths of 29.3 and 75.0 m, respectively. Two horizons of dissolved sulfate, with peaks of sulfate-reducer MPNs, indicated the presence of two sulfate-reducing zones in Site 1 sediments at depths of 0-2, and 10-12 m below the sediment-water interface. The extensive depth distributions of methanogens and sulfate-reducers, plus the occurrence of two sulfate-reducing zones, were the result of sediment mixing during undersea landslides that frequently occurred in the delta. In vitro methanogenesis was stimulated to a high degree by the addition of methylamine, dimethylamine, trimethylamine, methanol, and CO(,2) plus H(,2) (48.5-64.7 mol% conversion), and to a lesser degree by methanethiol, methylhydrazine, choline, and pectin. Although ('14)CH(,4) was produced from ('14)C-2-acetate, the addition of unlabeled acetate had no effect on in vitro methanogenesis. Methanogenesis was sensitive to 2-bromoethanesulfonate, chloroform, nitrate, air, and formaldehyde, but not to added (beta)-fluoroacetate, sulfide, or sulfate. The optimum temperature for methanogenesis in enrichments and in unadulterated sediments was 35-40(DEGREES)C and 45(DEGREES)C, respectively. In vitro methanogenesis in 0.2 m-deep sediments at 20(DEGREES)C (the in situ temperature) was 0.24 nmol CH(,4)/g sediment, dry weight/day. Actively methanogenic broth enrichments containing methanol and methylamine were dominated by irregularly coccoid cells that closely resembled Methanococcus mazei in morphology. Seventy-five strains of desulfovibrios closely resembling Desulfovibrio salexigens, and five strains of Desulfotomaculum sp. were isolated from 0.2 m-deep delta sediments. The Desulfotomaculum isolates did not closely resemble any described species and may constitute a new taxon. The production of methane from choline, in addition to the isolation of desulfovibrios capable of producing trimethylamine from choline, implicated trimethylamine as a possible methane precursor in the delta sediments.