An isolate of an acidophilic archaeon, strain Y T , was obtained from a bioleaching pilot plant. The organism oxidizes ferrous iron as the sole energy source and fixes inorganic carbon as the sole carbon source. The optimal pH for growth is 17, although growth is observed in the range pH 13 to 22. The cells are pleomorphic and without a cell wall. 16S rRNA gene sequence analysis showed this strain to cluster phylogenetically within the order ' Thermoplasmales ' sensu Woese, although with only 899 and 872 % sequence identity, respectively, to its closest relatives, Picrophilus oshimae and Thermoplasma acidophilum. Other principal differences from described species of the ' Thermoplasmales ' are autotrophy (strain Y T is obligately autotrophic), the absence of lipid components typical of the ' Thermoplasmales ' (no detectable tetraethers) and a lower temperature range for growth (growth of strain Y T occurs between 15 and 45 SC). None of the sugars, amino acids, organic acids or other organic compounds tested was utilized as a carbon source. On the basis of the information described above, the name Ferroplasma acidiphilum gen. nov., sp. nov. is proposed for strain Y T within a new family, the Ferroplasmaceae fam. nov. Strain Y T is the type and only strain of F. acidiphilum. This is the first report of an autotrophic, ferrous-ironoxidizing, cell-wall-lacking archaeon.

Comparative analysis of 16S rRNA gene sequences, DNA–DNA hybridization data and phenotypic properties revealed that ‘Sulfobacillus thermosulfidooxidans subsp. thermotolerans’ strain K1 is not a member of the genus Sulfobacillus. Phylogenetically, strain K1 is closely related to unclassified strains of the genus Alicyclobacillus: the 16S rRNA gene sequence of strain K1 is similar to that of Alicyclobacillus sp. AGC-2 (99·6 %), Alicyclobacillus sp. 5C (98·9 %) and Alicyclobacillus sp. CLG (98·6 %) and bacterium GSM (99·1 %). The 16S rRNA gene sequence similarity values for strain K1 and species of the genus Alicyclobacillus with validly published names were in the range 92·1–94·6 %, and for S. thermosulfidooxidans VKM B-1269T the value was 87·7 %. Sulfobacillus disulfidooxidans SD-11T was also phylogenetically related to strain K1 (92·6 % sequence similarity) and thus belonged to the genus Alicyclobacillus. Chemotaxonomic data, such as the major cell-membrane lipid components of strains K1 and SD-11T (ω-alicyclic fatty acids) and the major isoprenoid quinone (menaquinone MK-7) of strain K1, supported the affiliation of strains K1 and SD-11T to the genus Alicyclobacillus. Physiological and molecular biological tests allowed genotypic and phenotypic differentiation of strains K1 and SD-11T from the nine Alicyclobacillus species with validly published names. The G+C content of the DNA of strain K1 was 48·7±0·6 mol%; that of strain SD-11T was 53±1 mol%. DNA–DNA reassociation studies showed low relatedness (22 %) between strains K1 and SD-11T, and even lower relatedness (3–5 %) between these strains and Alicyclobacillus acidocaldarius subsp. acidocaldarius ATCC 27009T, DSM 446T. DNA reassociation of strains K1 and SD-11T with Alicyclobacillus cycloheptanicus DSM 4006T gave values of 15 and 21, respectively. Based on the phenotypic and phylogenetic characteristics of strains K1 and SD-11T, Alicyclobacillus tolerans sp. nov. (type strain, K1T=VKM B-2304T=DSM 16297T) and Alicyclobacillus disulfidooxidans comb. nov. (type strain, SD-11T=ATCC 51911T=DSM 12064T) are proposed.

Polyphasic genotypic analysis of 25 Acidithiobacillus ferrooxidans strains isolated from ores and ore concentrates collected in different regions of the world showed considerable strain heterogeneity. Restriction patterns of the chromosomal DNA of these strains obtained by PFGE were specific for each strain. According to the degree of DNA relatedness, 17 of the 23 strains studied were divided into four genomovars. Six independent, considerably divergent strains could not be assigned to any of the genomovars. A comparison of nearly complete nucleotide sequences of the 16S rDNA of five representatives of the genomovars (including the type strain of A. ferrooxidans, ATCC 23270 T ) with those of species of the genus Acidithiobacillus available from GenBank showed that most of the A. ferrooxidans strains, together with the type strain and some other strains of the species Acidithiobacillus thiooxidans, comprised a monophyletic cluster. Within this major cluster, A. ferrooxidans strains fell into four phylogenetic groups that were equidistant from the phylogenetic group of A. thiooxidans strains. In general, the distribution of strains among the phylogenetic groups correlated with their distribution among the genomovars, except that the representatives of two different genomovars fell into one phylogenetic group. Thus, at least two levels of phylogenetic heterogeneity for A. ferrooxidans have been found. The phylogenetic heterogeneity of A. ferrooxidans strains, which are phenotypically indistinguishable, suggests the occurrence of microevolutionary processes in different econiches. This should be taken into account in the biohydrometallurgical applications of A. ferrooxidans strains.

A thermotolerant, Gram-positive, aerobic, endospore-forming, acidophilic bacterium (strain Kr1 T )was isolated from the pulp of a gold-containing sulfide concentrate processed at 40 6C in a gold-recovery plant (Siberia

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