Physiology, phylogenetic relationships, and ecology of filamentous sulfate-reducing bacteria (genus Desulfonema)

Microscopy of organic-rich sulfidic sediment samples of marine and freshwat er origin revealed filamentous, multicellular microorganisms with gliding m otility. Many of these neither contained sulfur droplets such as the Beggia toa species nor exhibited the autofluorescence of the chlorophyll-containin g cyanobacteria. A frequently observed morphological type of filamentous mi croorganism was enriched under anoxic conditions in the dark with isobutyra te plus sulfate. Two strains of filamentous, gliding sulfate-reducing bacte ria, Tokyo 01 and Jade 02, were isolated in pure cultures. Both isolates ox idized acetate and other aliphatic acids. Enzyme assays indicated that the terminal oxidation occurs via the anaerobic C-1 pathway (carbon monoxide de hydrogenase pathway). The 16S rRNA genes of the new isolates and of the two formerly described filamentous species of sulfate-reducing bacteria, Desul fonema limicola and Desulfonema magnum, were analyzed. All four strains wer e closely related to each other and affiliated with the delta-subclass of P roteobacteria. Another close relative was the unicellular Desulfococcus mul tivorans. Based on phylogenetic relationships and physiological properties, Strains Tokyo 01 and Jade 02 are assigned to a new species, Desulfonema is himotoi. A new, fluorescently labeled oligonucleotide probe targeted agains t 16S rRNA was designed so that that it hybridized specifically with whole cells of Desulfonema species. Filamentous bacteria that hybridized with the same probe were detected in sediment samples and in association with the f ilamentous sulfur-oxidizing bacterium Thioploca in its natural habitat. We conclude that Desulfonema species constitute an ecologically significant fr action of the sulfate-reducing bacteria in organic-rich sediments and micro bial mats.