Publications

4372

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.

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 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 .

Atalantia buxifolia (Poir.) Oliv., synonym Severinia buxifolia (Poir.) Ten. as commonly found in literature, is a common landscape plant and a popular Chinese medicinal herb known as Jiubingle or Dongfengjie. It remains unclear if this rutaceous plant could host ‘Candidatus Liberibacter asiaticus’, the pathogen of citrus Huanglongbing (HLB) in Guangdong, P. R. China. This information is important for HLB control in citrus because infected A. buxifolia could serve as a source of inoculum. In August of 1994, three A. buxifolia plants adjacent to a citrus experimental orchard of the South China Agricultural University at Guangzhou were found showing leaf mottle/yellowing symptoms. Two buds from each plant were grafted onto three mandarin trees (Citrus reticulata cv. Pongan) in a psyllid-proof screenhouse for indexing. By October of 1995, typical leaf mottle symptoms were observed in all three grafted trees compared with a healthy control. In March of 1996, one of the A. buxifolia plants was transferred to a screenhouse and has been maintained there. The leaf mottle/yellowing symptoms persisted but did not significantly affect plant growth. DNA was extracted from leaf samples in October 2006 by using the CTAB (cetyltrimethylammoniumbromide) method and assayed by nested-PCR using the general bacterial 16S rDNA primer set fDl/rD1 as the first round of amplification and primer set OI1/OI2c as second round amplification (1,3). After agarose gel electrophoresis and staining with ethidium bromide, an approximate 1.1-kb DNA band was detected in symptomatic samples but not healthy leaf samples of A. buxifolia and C. reticulata. XbaI digestion of the amplicons yielded approximate 500- and 600-bp fragments, characteristic of ‘Ca. L. asiaticus’. Similarly, a standard PCR with primer set A5/J2 (3) yielded an approximate 700-bp DNA band characteristic of ‘Ca. L. asiaticus’ from symptomatic samples only. To our knowledge, this is the first report of graft transmission and PCR detection of ‘Ca. L. asiaticus’ from A. buxifolia in Guangdong, P. R. China. This work also confirms the findings from Taiwan (2) that A. buxifolia could serve as a source of ‘Ca. L. asiaticus’. References: (1) X. Deng et al. Online publication. doi:10.1094/PHP-2007-0419-01-BR. Plant Health Progress, 2007. (2) T.-H. Hung et al. Eur. J. Plant Pathol. 107:183, 2001. (3) S. Jagoueix et al. Mol. Cell. Probes 10:43, 1996.