Sulfate-reducing bacteria and their activities in cyanobacterial mats of solar lake (Sinai, Egypt).

The sulfate-reducing bacteria within the surface layer of the hypersaline cyanobacterial mat of Solar Lake (Sinai, Egypt) were investigated with combined microbiological, molecular, and biogeochemical approaches. The diurnally oxic surface layer contained between 10(6) and 10(7) cultivable sulfate-reducing bacteria ml-1 and showed sulfate reduction rates between 1,000 and 2, 200 nmol ml-1 day-1, both in the same range as and sometimes higher than those in anaerobic deeper mat layers. In the oxic surface layer and in the mat layers below, filamentous sulfate-reducing Desulfonema bacteria were found in variable densities of 10(4) to 10(6) cells ml-1. A Desulfonema-related, diurnally migrating bacterium was detected with PCR and denaturing gradient gel electrophoresis within and below the oxic surface layer. Facultative aerobic respiration, filamentous morphology, motility, diurnal migration, and aggregate formation were the most conspicuous adaptations of Solar Lake sulfate-reducing bacteria to the mat matrix and to diurnal oxygen stress. A comparison of sulfate reduction rates within the mat and previously published photosynthesis rates showed that CO2 from sulfate reduction in the upper 5 mm accounted for 7 to 8% of the total photosynthetic CO2 demand of the mat.

Metabolism of cyclohexane carboxylic acid by the photosynthetic bacterium Rhodopseudomonas palustris.

Cyclohexane carboxylate supported relatively rapid growth (doubling times 7-8 h) of Rhodopseudomonas palustris under oxic or photosynthetic conditions, but did not serve as a substrate for either of the known aromatic CoA ligases. A CoA ligase that thioesterifies cyclohexane carboxylate was partially purified and did not cross react immunologically with the two CoA ligases purified previously from this bacterium. Crude extracts of R. palustris cells grown with a range of aromatic or alicyclic acids contained a dehydrogenase that reacted with cyclohexane carboxyl-CoA or cyclohex-1-ene carboxyl-CoA, using 2,6-dichlorophenolindophenol or ferricenium ion as electron carrier. This activity was not detected in extracts of adipate-, glutamate-, or succinate-grown cells. No oxidation or reduction of nonesterified cyclohexane carboxylate or cyclohexene carbocylate was detected in extracts of cells grown with aromatic or aliphatic substrates, neither aerobically nor anaerobically. A constitutively expressed thioesterase that hydrolyzed cyclohexane carboxyl-CoA and also some alicyclic and aliphatic CoA derivatives was purified and characterized. The enzyme had little or no activity on benzoyl-CoA or 4-hydroxybenzoyl-CoA. The presence of a thioesterase that effectively hydrolyzes cyclohexane carboxyl-CoA suggests that transient production of cyclohexane carboxylate is a physiological response to temporary excess of reductant during metabolism of aromatic compounds.

Microbial transformation of nitroaromatic compounds under anaerobic conditions.

The transformation of several mono- and dinitroaromatic compounds (tested at 50-200 microM) by methanogenic bacteria, sulphate-reducing bacteria and clostridia was studied. Some of the nitroaromatics tested were transformed chemically by 1.5 mM quantities of culture media reducing agents, like cysteine or sulphide. This abiotic reduction occurred at the o-nitro-groups preferentially. Nitrophenols, p-nitroaniline and p-nitrobenzoic acid were completely transformed biologically into the corresponding amino derivatives. The nitroaromatics were transformed by all of the bacterial strains tested. While growing cells of sulphate-reducing bacteria and Clostridium spp. carried out nitroreduction, methanogen cells lysed in the presence of nitroaromatics. Nevertheless these culture suspensions converted nitroaromatics to the corresponding amino derivatives. This was also confirmed by crude cell extracts of methanogenic bacteria. The rate of nitroreduction by sulphate-reducing bacteria depended on the electron donors supplied and the cell density, with molecular hydrogen being the most effective donor of reducing equivalents. The toxicity of p-nitrophenol to some of the organisms tested depended on the concentration of the nitroaromatic compound and the type of organism.

Complete oxidation of benzoate and 4-hydroxybenzoate by a new sulfate-reducing bacterium resembling Desulfoarculus.

A new sulfate-reducer "strain SAX" was isolated from an anaerobic marine sediment [Saxild, Denmark]. The isolate was a gram-negative, motile and non-spore-forming rod which sometimes appeared as a curved rod. Strain SAX differed from all described Desulfovibrio-, Desulfobotulus- and Desulfoarculus-species by the ability to degrade aromatic compounds such as benzoate, 4-hydroxybenzoate and phenol completely to CO2. Electron donors used included lactate, pyruvate, malate, fumarate, crotonate and butyrate, while pyruvate was fermented in the absence of an external electron acceptor. Sulfate, thiosulfate or sulfite served as electron acceptors with benzoate as the donor, while nitrate and nitrite did not. The sulfate-reducing bacterium required vitamins and NaCl-concentrations of about 20 g/l. The optimum temperature for growth of strain SAX was 30 degrees C and the optimum pH value was 7.3. The DNA base composition was 62.4 mol% G+C. The strain possessed cytochrome c3, but no desulfoviridin. On the basis of these characteristics and because strain SAX could not be ascribed to any of the existing species therefore assignment as a new species to the genus Desulfoarculus was suggested.