Genome-based reclassification of Caldicellulosiruptor lactoaceticus and Caldicellulosiruptor kristjanssonii as later heterotypic synonyms of Caldicellulosiruptor acetigenus.

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.

Nitrogen-fixing Ability and Nitrogen Fixation-related Genes of Thermophilic Fermentative Bacteria in the Genus Caldicellulosiruptor.

Fermentative nitrogen-fixing bacteria have not yet been examined in detail in thermal environments. In the present study, we isolated the thermophilic fermentative bacterium, strain YA01 from a hot spring. This strain grew at temperatures up to 78°C. A phylogenetic analysis based on its 16S rRNA gene sequence indicated that strain YA01 belonged to the genus Caldicellulosiruptor, which are fermentative bacteria in the phylum Firmicutes, with 97.7-98.0% sequence identity to its closest relatives. Strain YA01 clearly exhibited N2-dependent growth at 70°C. We also confirmed N2-dependent growth in the relatives of strain YA01, Caldicellulosiruptor hydrothermalis 108 and Caldicellulosiruptor kronotskyensis 2002. The nitrogenase activities of these three strains were examined using the acetylene reduction assay. Similar activities were detected for all tested strains, and were slightly suppressed by the addition of ammonium. A genome analysis revealed that strain YA01, as well as other Caldicellulosiruptor, possessed a gene set for nitrogen fixation, but lacked the nifN gene, which encodes a nitrogenase iron-molybdenum cofactor biosynthesis protein that is commonly detected in nitrogen-fixing bacteria. The amino acid sequences of nitrogenase encoded by nifH, nifD, and nifK shared 92-98% similarity in Caldicellulosiruptor. A phylogenetic tree of concatenated NifHDK sequences showed that NifHDK of Caldicellulosiruptor was in the deepest clade. To the best of our knowledge, this is the first study to demonstrate the nitrogen-fixing ability of fermentative bacteria at 70°C. Caldicellulosiruptor may have retained an ancient nitrogen-fixing enzyme system.