Introduction

It has been proposed that the members of the class Deltaproteobacteria (previously thought to be unrelated to Thermodesulfobacteria) be split into four new phyla: Desulfobacterota, Myxococcota, Bdellovibrionota, and SAR324 (a placeholder name pending the description of type material), and that the Thermodesulfobacteria be placed inside of the new phylum Desulfobacterota along with several of the members of the former Deltaproteobacteria.

Thermodesulfobacterium is a Group II sulfate-reducing bacterium because it can can utilize acetate and other fatty acids, oxidizing them completely. It also produces nonisoprenoid branched glycerol diethers (discussed below). Sulfate-reducing bacteria fall into over 20 separate genera and these can be classified by the types of substrates that are utilized. A convenient division is by the ability to utilize acetate as a source of electrons. Other Group II sulfate-reducers include Desulfosarcina and Desulfobacter. Thermodesulfobacterium commune expresses a unique type of dissimilatory sulfite reductase which is taxonomically distinct from that of related genera or other hyperthermophilic sulfate reducers.

A thermophilic, non-spore-forming, marine, sulfate-reducing bacterium, strain SL6T, was isolated from deep-sea hydrothermal sulfides collected at Guaymas Basin. The gram-negative-staining cells occurred singly or in pairs as small, highly motile rods. The temperature range for growth was 50-80 degrees C with an optimum at 75 degrees C. The pH range for growth at 70 degrees C was 6.3-6.8, with an optimum at 6.5. The NaCl concentration range for growth was 5-55 g l(-1), with an optimum at 30 g l(-1). H2 and CO2 were the only substrates for growth and sulfate reduction. However, growth was stimulated by several organic compounds. Sulfur, thiosulfate, sulfite, cystine, nitrate and fumarate were not used as electron acceptors. Pyruvate, lactate and malate did not support fermentative growth. Desulfoviridin was not detected.

Bacterial strain

Furthermore, strain R1Ha3 reduced nitrate and strain JSP reduced sulfite. Neither of the strains was able to oxidize lactate completely to CO2 and neither of the strains contained desulfoviridin. 16S rDNA sequencing placed strain JSP in the genus Thermodesulfobacterium and strain R1Ha3 in the genus Thermodesulfovibrio. Based on the DNA-DNA hybridization studies and differences in morphology and physiology to their closest relatives the two new isolates were considered as new species. Strain JSP is named Thermodesulfobacterium hveragerdense and strain R1Ha3 Thermodesulfovibrio islandicus.

Using a combination of coding potential prediction and similarity searches, 2159 protein-coding genes were predicted. Of these, 1458 genes were functionally assigned with different degrees of generalization and confidence, while the function of the remaining 701 genes could not be predicted from the deduced amino acid sequences. The properties and the statistics of the genome are summarized. Consistent with its affiliation to the phylum Thermodesulfobacteria, T. dismutans shares more than half of the proteome with that of its closest relative with sequenced genome, Thermodesulfatator indicus.

A new type of dissimilatory bisulfite reductase, desulfofuscidin, was isolated from the nonsporeforming thermophilic sulfate-reducing microorganism Thermodesulfobacterium commune. The molecular weight of the enzyme was estimated at 167,000 by sedimentation equilibrium, and the protein was pure by both disc electrophoresis and ultracentrifugation.

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