Microbiology

Abstract Most autotrophs use the Calvin–Benson–Bassham (CBB) cycle for carbon fixation. In contrast, all currently described autotrophs from the Campylobacterota (previously Epsilonproteobacteria) use the reductive tricarboxylic acid cycle (rTCA) instead. We discovered campylobacterotal epibionts (“Candidatus Thiobarba”) of deep-sea mussels that have acquired a complete CBB cycle and may have lost most key genes of the rTCA cycle. Intriguingly, the phylogenies of campylobacterotal CBB cycle genes suggest they were acquired in multiple transfers from Gammaproteobacteria closely related to sulfur-oxidizing endosymbionts associated with the mussels, as well as from Betaproteobacteria. We hypothesize that “Ca. Thiobarba” switched from the rTCA cycle to a fully functional CBB cycle during its evolution, by acquiring genes from multiple sources, including co-occurring symbionts. We also found key CBB cycle genes in free-living Campylobacterota, suggesting that the CBB cycle may be more widespread in this phylum than previously known. Metatranscriptomics and metaproteomics confirmed high expression of CBB cycle genes in mussel-associated “Ca. Thiobarba”. Direct stable isotope fingerprinting showed that “Ca. Thiobarba” has typical CBB signatures, suggesting that it uses this cycle for carbon fixation. Our discovery calls into question current assumptions about the distribution of carbon fixation pathways in microbial lineages, and the interpretation of stable isotope measurements in the environment.

Huanglongbing (HLB) disease is threatening citrus production worldwide. The causative agent is “ Candidatus Liberibacter asiaticus.” Prior work using mapping-based approaches identified prophage-like sequences in some “ Ca. Liberibacter asiaticus” genomes but not all. Here, we utilized a de novo approach that expands the number of prophage-like elements found in “ Ca. Liberibacter asiaticus” from 16 to 33 and identified at least one prophage-like sequence in all “ Ca. Liberibacter asiaticus” strains. Furthermore, we identified a prophage-like sequence type that is a remnant of an integrated prophage—expanding the number of prophage types in “ Ca. Liberibacter asiaticus” from 3 to 4. Overall, the findings will help researchers investigate the role of prophage in the ecology, evolution, and pathogenicity of “ Ca. Liberibacter asiaticus.”

“ Candidatus Liberibacter solanacearum” is a pathogen transmitted by the potato psyllid Bactericera cockerelli (Šulc) (Hemiptera: Triozidae) in a persistent manner. In this study, we investigated the molecular interaction between “ Ca. Liberibacter solanacearum” and the potato psyllid at the gut interface. Specifically, we focused on the apoptotic response of potato psyllids to the infection by two “ Ca. Liberibacter solanacearum” haplotypes, LsoA and LsoB.