Notes: The capacity of some bacteria to metabolize hydrocarbons in the absence of molecular oxygen was first recognized only about ten years ago. Since then, the number of hydrocarbon compounds shown to be catabolized anaerobically by pure bacterial cultures has been steadily increasing. This review summarises the current knowledge of the bacterial isolates capable of anaerobic mineralization of hydrocarbons, and of the biochemistry and molecular biology of enzymes involved in the catabolic pathways of some of these substrates. Several alkylbenzenes, alkanes or alkenes are anaerobically utilized as substrates by several species of denitrifying, ferric iron-reducing and sulfate-reducing bacteria. Another group of anaerobic hydrocarbon degrading bacteria are ‘proton reducers’ that depend on syntrophic associations with methanogens. For two alkylbenzenes, toluene and ethylbenzene, details of the biochemical pathways involved in anaerobic mineralization are known. These hydrocarbons are initially attacked by novel, formerly unknown reactions and oxidized further to benzoyl-CoA, a common intermediate in anaerobic catabolism of many aromatic compounds. Toluene degradation is initiated by an unusual addition reaction of the toluene methyl group to the double bond of fumarate to form benzylsuccinate. The enzyme catalyzing this first step has been characterized at both the biochemical and molecular level. It is a unique type of glycyl-radical enzyme, an enzyme family previously represented only by pyruvate-formate lyases and anaerobic ribonucleotide reductases. Based on the nature of benzylsuccinate synthase as a radical enzyme, a hypothetical reaction mechanism for the addition of toluene to fumarate is proposed. The further catabolism of benzylsuccinate to benzoyl-CoA and succinyl-CoA appears to occur via reactions of a modified β-oxidation pathway. Ethylbenzene is first oxidized at the methylene carbon to 1-phenylethanol and subsequently to acetophenone, which is then carboxylated to 3-oxophenylpropionate and converted to benzoyl-CoA and acetyl-CoA. Anaerobic mineralization of alkanes involves an oxygen-independent oxidation to fatty acids, followed by β-oxidation. In one strain of an alkane-mineralizing sulfate-reducing bacterium, the activation appears to proceed via a chain-elongation, possibly by addition of a C1-group at the terminal methyl group of the alkane. Finally, aspects concerned with the regulation and ecological significance of anaerobic hydrocarbon catabolic pathways are discussed.

Notes: Abstract The survival after oxygen stress was studied with eight species of sulfate-reducing bacteria. In the absence of sulfide all species tolerated 6 min of aeration without loss of viability. Even after 3 h of aeration the viability of four species (Desulfovibrio vulgaris, D. desulfuricans, D. salexigens and Desulfobacter postgatei) was not impaired. Four other species were sensitive to 3 h of aeration: the surviving fractions of Desulfotomaculum ruminis, D. nigrificans and Desulfococcus multivorans were about 1%, that of Desulfotomaculum orientis about 0.01%. Formation of spores resulted in oxygen resistance of D. orientis. Reducing agents did not protect the vegetative cells of this strain against oxygen toxicity. In contrast, sulfhydryl group-containing agents increased the oxygen sensitivity considerably.Growth of sulfate- and sulfur-reducing bacteria in oxygen-sulfide gradients in agar tubes was studied. In the gradients these strictly anaerobic bacteria revealed oxygen-dependent growth in sulfate- and sulfur-free medium. Three sulfate-reducing bacteria that could not use thiosulfate or sulfur as electron acceptor failed to grow in oxygen-sulfide gradients. Obviously, not directly molecular oxygen, but oxidation products of sulfide, such as thiosulfate or sulfur, were used as electron acceptors and were continuously regenerated in a cycling process from sulfide by autoxidation. The conceivable ecological significance of a short sulfur cycle driven by autoxidation of sulfide is discussed.

Notes: Abstract Methanosarcina barkeri cells were observed in ammonia-free anaerobic acetate enrichments for sulfate-reducing bacteria. The capacity of Methanosarcina to grow diazotrophically was proved with a pure culture in mineral media with methanol. The cell yields with N2 or NH4+ ions as nitrogen source were 2.2 g and 6.1 g dry weight, respectively, per mol of methanol. Growth experiments with 15N2 revealed that 84% of the cell nitrogen was derived from N2. Acetylene was highly toxic to Methanosarcina and only reduced at concentrations lower than 100 μmol dissolved per 1 of medium. Assimilation of N2 and reduction of acetylene were inhibited by NH4+ ions. The experiments show that N2 fixation occurs not only in eubacteria but also in archaebacteria. The ecological significance of diazotrophic growth of Methanosarcina is discussed.