Abstract

The present study was carried out to re-clarify the taxonomic relationship of Caldicellulosiruptor acetigenus, Caldicellulosiruptor lactoaceticus and Caldicellulosiruptor kristjanssonii. The 16S rRNA sequence similarities between these species of the genus Caldicellulosiruptor were above the threshold values (98.65%) for bacterial species delineation. Similarly, the digital DNA-DNA hybridization and average nucleotide and amino acid identity values were greater than the thresholds for bacterial species delineation. In phylogenetic (based on 16S rRNA gene sequences) and phylogenomic trees Caldicellulosiruptor acetigenus, Caldicellulosiruptor lactoaceticus and Caldicellulosiruptor kristjanssonii clade together. The results of our analysis indicated that these three taxa are conspecific. Therefore, Caldicellulosiruptor lactoaceticus Mladenovska et al. 1997 and Caldicellulosiruptor kristjanssonii Bredholt et al. 1999 should be reclassified as later heterotypic synonyms of Caldicellulosiruptor acetigenus (Nielsen et al. 1994) Onyenwoke et al. 2006.