Thermodesulfovibrio aggregans is a species of bacterium.
Discovery Thermodesulfovibrio aggregans was discovered by Sekiguchi et al. in 2008 while researching thermophilic sulfate reducing bacteria that was present in anaerobic wastewater treatment systems in Japan. Researchers studying bacterial communities in these wastewater treatment environments isolated the organism from Thermophilic methanogenic sludge granules which are dense microbial aggregates that form in wastewater treatment reactors in anaerobic conditions. In these dense wastewater bacterial communities, different microorganisms in these communities perform specialized metabolic roles such as sulfate reduction and methane production, allowing the ecosystem to function as a community under anaerobic conditions.
Isolation To study the bacteria, researchers in Japan cultured the bacterium under anaerobic laboratory conditions and conducted multi-step experiments to figure out the taxonomic classification. Sequencing Thermodesulfovibrio aggregans 16S rRNA gene proved that it is in the Thermodesulfovibrio genus, which are a group of bacteria that are sulfate-reducing associated with high-temperature anaerobic environments. However, genetic sequencing showed that the isolates of Thermodesulfovibrio aggregans were genetically different from previously discovered species within the genus. Because of these differences, this new organism was given the name Thermodesulfovibrio aggregans, with the specific strain name given as TGE-P1. The species name aggregans originates from Thermodesulfovibrio aggregans capability to form cell clusters during growth and replication.
Taxonomy and phylogeny Thermodesulfovibrio aggregans was originally placed in the Nitrospirae phylum according to 16s rRNA completed in 2008. Starting in 2015, however, a lot of bacteria phylum groups began adopting the suffix "ota" as large scaled genome projects began reorganizing bacterial taxonomy using whole genome comparisons. As a result of this change, the phylum Nitrospirae was reclassified as Nitrosporita. The phylum Nitrosporita primarily contains chemolithotrophs and anaerobes that partake in the nitrogen, sulfur, and iron cycle. There are three primary classes within Nitrosporita including Nitrospiria, Leptospirillia, and Thermodesulfovibrionia. The species T. aggregans falls into the Thermodesulfovibrionia class, and most of the bacteria within this class are thermophilic, anaerobic, and sulfate-reducing. Within this class, there is only one order, family, and genus which is Thermodesulfovibrionales, Thermodesulfovibrionaceae, and Thermodesulfovibrio respectively. Within the genus Thermodesulfovibrio, there are 8 published species including, T. aggregans, T. yellowstoni, T. Islandicus, T. hydrogenphilus, T. yellowstonii subsp. denitrificans, T. thiophilus, and T. acidaminovorans. T. yellowstoni is a thermophilic anaerobe and is the most well known and studied species within the Thermodesulfovibrio genus. T. yellowstoni was first discovered in the Yellowstone National Park Hotsprings in 1994 and demonstrated that sulfate reduction is possible in bacteria within the Nitrospirota phylum. Prior to this discovery, sulfate reduction was only viewed in the more commonly studied sulfate reducers Proteobacteria. Another prominent member of the Thermodesulfovibrio genus is T. islandicus. T. islandicus was first discovered in Icelandic hot spring sediments and similar to T. yellowstoni, T. islandicus is also thermophilic, anaerobic, and sulfate reducing. The species T. thiophilus is the closest neighbor species of T. aggregans. T. thiophilus was first discovered from samples of Thermophilic methanogenic sludge in anaerobic digesters. It was determined that T. aggregans is most closely related to T. thiophilus based on their phylogeny. 16s rRNA gene sequencing showed that T. thiophilius shares the highest sequence similarity to T. aggregans out of all of the species in the Thermodesulfovibrio genus. Furthermore, DNA–DNA hybridization showed moderate to high similarity between T. thiophilus and T. aggregans demonstrating that their closely related but still distinct species. T. yellowstoni, T. islandicus, T. thiophilus and T. aggregans all share a similar metabolism and physiology, however, they are found in different ecological niches. T. yellowstoni and T. islandicus are found in natural geothermal environments while T. aggregans and T. thiophilus are found in engineered environments such as thermophilic anaerobic sludge reactors.
Cell structure and morphology
Morphology T. aggregans has a curved rod (vibrioid) shape and is approximately 0.3–0.5 µm wide and 1.5–3.0 µm long. Its cells are elongated with a slender and comma-like appearance. T. aggregans is often found in aggregates and can form small clusters. T. aggregans also uses a flagella for motility, is gram-negative, and is non-spore forming.
Genome The genome of T.aggregans consists of 2.0-2.3 megabases and 34%-35% gc content. Additionally, its genome contains 1900-2100 protein-coding genes. One important gene in T. aggregans for sulfate reduction is sulfate adenylyltransferase which activates sulfate to adenosine 5 phosphosulfate. Another gene in T. aggregans essential for sulfate reduction is adenylylsulfate which converts adenosine 5 phosphosulfate to sulfite. Finally, the gene dissimilatory sulfite reductase reduces sulfite to hydrogen sulfide (H₂S).
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