Publications

4608

Summary The widespread occurrence and diversity of ammonia oxidizing Archaea suggests their contribution to the nitrogen cycle is of global significance. Their distribution appeared limited to low‐ and moderate‐temperature environments until the recent finding of a diagnostic membrane lipid, crenarchaeol, in terrestrial hot springs. We report here the cultivation of a thermophilic nitrifier (‘ Candidatus Nitrosocaldus yellowstonii’), an autotrophic crenarchaeote growing up to 74°C by aerobic ammonia oxidation. The major core lipid of this archaeon growing at 72°C is crenarchaeol, providing the first direct evidence for its synthesis by a thermophile. These findings greatly extend the upper temperature limit of nitrification and document that the capacity for ammonia oxidation is broadly distributed among the Crenarchaeota .

ABSTRACT Microscopic examination of the hemolymph from diseased daphniids in 17 lakes in southwestern Michigan and five rock pools in southern Finland revealed the presence of tightly coiled bacteria that bore striking similarities to the drawings of a morphologically unique pathogen, “ Spirobacillus cienkowskii ,” first described by Elya Metchnikoff more than 100 years ago. The uncultivated microbe was identified as a deeply branching member of the Deltaproteobacteria through phylogenetic analyses of two conserved genes: the 16S rRNA-encoding gene ( rrs ) and the β-subunit of topoisomerase ( gyrB ). Fluorescence in situ hybridization confirmed that the rRNA gene sequence originated from bacteria with the tightly coiled morphology. Microscopy and PCR amplification with pathogen-specific primers confirmed infections by this bacterium in four species of Daphnia : Daphnia dentifera, D. magna, D. pulicaria , and D. retrocurva . Extensive field surveys reveal that this bacterium is widespread geographically and able to infect many different cladoceran species. In a survey of populations of D. dentifera in lakes in Michigan, we found the bacterium in 17 of 18 populations studied. In these populations, 0 to 12% of the individuals were infected, with an average of 3% during mid-summer and early autumn. Infections were less common in rock pool populations of D. magna in southern Finland, where the pathogen was found in 5 of 137 populations. The broad geographic distribution, wide host range, and high virulence of S. cienkowskii suggest it plays an important role in the ecology and evolution of daphniids.

Summary The facultative symbiont of Riftia pachyptila , named here Candidatus Endoriftia persephone , has evaded culture to date, but much has been learned regarding this symbiosis over the past three decades since its discovery. The symbiont population metagenome was sequenced in order to gain insight into its physiology. The population genome indicates that the symbionts use a partial Calvin–Benson Cycle for carbon fixation and the reverse TCA cycle (an alternative pathway for carbon fixation) that contains an unusual ATP citrate lyase. The presence of all genes necessary for heterotrophic metabolism, a phosphotransferase system, and dicarboxylate and ABC transporters indicate that the symbiont can live mixotrophically. The metagenome has a large suite of signal transduction, defence (both biological and environmental) and chemotaxis mechanisms. The physiology of Candidatus Endoriftia persephone is explored with respect to functionality while associated with a eukaryotic host, versus free‐living in the hydrothermal environment.