Publications

4467

ABSTRACT Huanglongbing, or citrus greening, is a devastating disease of citrus plants recently spreading worldwide, which is caused by an uncultivable bacterial pathogen, “ Candidatus Liberibacter asiaticus,” and vectored by a phloem-sucking insect, Diaphorina citri . We investigated the infection density dynamics of “ Ca . Liberibacter asiaticus” in field populations of D. citri with experiments using field-collected insects to address how “ Ca . Liberibacter asiaticus” infection density in the vector insect is relevant to pathogen transmission to citrus plants. Of 500 insects continuously collected from “ Ca . Liberibacter asiaticus”-infected citrus trees with pathological symptoms in the spring and autumn of 2009, 497 (99.4%) were “ Ca . Liberibacter asiaticus” positive. The infections were systemic across head-thorax and abdomen, ranging from 10 3 to 10 7 bacteria per insect. In spring, the infection densities were low in March, at ∼10 3 bacteria per insect, increasing up to 10 6 to 10 7 bacteria per insect in April and May, and decreasing to 10 5 to 10 6 bacteria per insect in late May, whereas the infection densities were constantly ∼10 6 to 10 7 bacteria per insect in autumn. Statistical analysis suggested that several factors, such as insect sex, host trees, and collection dates, may be correlated with “ Ca . Liberibacter asiaticus” infection densities in field D. citri populations. Inoculation experiments with citrus seedlings using field-collected “ Ca . Liberibacter asiaticus”-infected insects suggested that (i) “ Ca . Liberibacter asiaticus”-transmitting insects tend to exhibit higher infection densities than do nontransmitting insects, (ii) a threshold level (∼10 6 bacteria per insect) of “ Ca . Liberibacter asiaticus” density in D. citri is required for successful transmission to citrus plants, and (iii) D. citri attaining the threshold infection level transmits “ Ca . Liberibacter asiaticus” to citrus plants in a stochastic manner. These findings provide valuable insights into understanding, predicting, and controlling this notorious citrus pathogen.

Summary To date, six candidate genera of anaerobic ammonium‐oxidizing (anammox) bacteria have been identified, and numerous studies have been conducted to understand their ecophysiology. In this study, we examined the physiological characteristics of an anammox bacterium in the genus ‘ C andidatus   J ettenia’. Planctomycete KSU ‐1 was found to be a mesophilic (20–42.5°C) and neutrophilic ( pH 6.5–8.5) bacterium with a maximum growth rate of 0.0020 h −1 . Planctomycete KSU ‐1 cells showed typical physiological and structural features of anammox bacteria; i.e. 29 N 2 gas production by coupling of 15 NH 4 + and 14 NO 2 − , accumulation of hydrazine with the consumption of hydroxylamine and the presence of anammoxosome. In addition, the cells were capable of respiratory ammonification with oxidation of acetate. Notably, the cells contained menaquinone‐7 as a dominant respiratory quinone. Proteomic analysis was performed to examine underlying core metabolisms, and high expressions of hydrazine synthase, hydrazine dehydrogenase, hydroxylamine dehydrogenase, nitrite/nitrate oxidoreductase and carbon monoxide dehydrogenase/acetyl‐ CoA synthase were detected. These proteins require iron or copper as a metal cofactor, and both were dominant in planctomycete KSU ‐1 cells. On the basis of these experimental results, we proposed the name ‘ C a . J ettenia caeni’ sp. nov. for the bacterial clade of the planctomycete KSU ‐1.

ABSTRACT Large sulfur-oxidizing bacteria in the family Beggiatoaceae are important players in the global sulfur cycle. This group contains members of the well-known genera Beggiatoa , Thioploca , and Thiomargarita but also recently identified and relatively unknown candidate taxa, including “ Candidatus Thiopilula” spp. and “ Ca . Thiophysa” spp. We discovered a population of “ Ca . Thiopilula” spp. colonizing cold seeps near Barbados at a ∼4.7-km water depth. The Barbados population consists of spherical cells that are morphologically similar to Thiomargarita spp., with elemental sulfur inclusions and a central vacuole, but have much smaller cell diameters (5 to 40 μm). Metatranscriptomic analysis revealed that when exposed to anoxic sulfidic conditions, Barbados “ Ca . Thiopilula” organisms expressed genes for the oxidation of elemental sulfur and the reduction of nitrogenous compounds, consistent with their vacuolated morphology and intracellular sulfur storage capability. Metatranscriptomic analysis further revealed that anaerobic methane-oxidizing and sulfate-reducing organisms were active in the sediment, which likely provided reduced sulfur substrates for “ Ca . Thiopilula” and other sulfur-oxidizing microorganisms in the community. The novel observations of “ Ca . Thiopilula” and associated organisms reported here expand our knowledge of the globally distributed and ecologically successful Beggiatoaceae group and thus offer insight into the composition and ecology of deep cold seep microbial communities.