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.

During phototrophic growth on acetate and CO2Rhodospirillum rubrum 2R contained malate synthase but lacked isocitrate lyase. Acetate assimilation by R. rubrum cells was stimulated by pyruvate, propionate glyoxylate, CO2 and H2. Acetate photoassimilation by R. rubrum cells in the presence of bicarbonate was accompanied by glyoxylate secretion, which increased after addition of fluoroacetate and decreased after addition of malonate. When acetyl‐CoA was incubated with pyruvate in cell‐free extracts, citramalate was formed. Citramalate was also formed from propionyl‐CoA and glyoxylate. The existence in R. rubrum of a CO2‐dependent cyclic pathway of acetate oxidation to glyoxylate with citramalate as an intermediate is proposed. Inhibitor analysis of acetate and bicarbonate assimilation indicated that pyruvate synthase is not involved in acetate assimilation in R. rubrum. The possible anaplerotic sequences employed by R. rubrum during phototrophic growth on acetate are discussed.

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