Publications

3807

SummaryPhototrophic members of the Chloroflexota phylum are enigmas in the evolution of phototrophy. Although all previously characterized phototrophic Chloroflexota members use a Type II reaction center (RCII) to perform light energy conversion1,2, many members host chlorosomes, a light-harvesting apparatus only otherwise reported among Type I reaction center (RCI)-containing phototrophs3. Here we report the discovery and cultivation of “Candidatus Chlorohelix allophototropha”, the first known member of the Chloroflexota to use RCI, rather than RCII, for phototrophy. Cultivated from an iron-rich Boreal Shield lake, “Ca. Chx. allophototropha” cells contain chlorosomes and encode a novel fifth clade of RCI protein. Phylogenomic data demonstrate that “Ca. Chx. allophototropha” shares closest ancestry to RCII-utilizing relatives for multiple photosynthesis accessory genes, including chlorosomes. Thus, we propose that the enigmatic distribution of chlorosomes in the Chloroflexota phylum resulted from genetic interaction of RCI- and RCII-utilizing Chloroflexota members. Although undetected previously, metatranscriptome data demonstrate that RCI-utilizing Chloroflexota members can be among the most active populations in illuminated anoxic waters of Boreal Shield lakes, which number in the millions globally4. Our results establish the Chloroflexota as the only bacterial phylum containing RCI- and RCII-utilizing anoxygenic phototrophs, providing new context for understanding the origins of phototrophic life on Earth.

Vegetative grafting is a common method of transmitting and propagating ‘Candidatus Liberibacter asiaticus’, the bacterial species accepted as the causal agent of the citrus disease huanglongbing (HLB). Generally, infected tissue that is grafted to a receptor tree remains in place indefinitely to ensure transmission. In this study, individual HLB-symptomatic leaves were grafted as ‘Ca. L. asiaticus’ inoculum sources to receptor trees of six citrus types and removed after an inoculation period (IP) of 21, 51, or 81 days. The goal was to assess the effect of varying IPs on transmission of bacteria to the receptor tree and on the successful establishment of a new infection. Survival analysis of data from three trials showed a significantly reduced proportion of infected trees with an IP of 21 days compared with IPs of 51 and 81 days but that there was no significant difference in the proportion of infected trees between IPs of 51 and 81 days. In addition, the time to first detection of pathogen DNA in an infected tree was delayed significantly for the 21-day IP when compared with the 51- and 81-day IPs. Survival analysis showed that the probability of infection of sweet orange trees was significantly higher than for trees of five other citrus types throughout the experiment. There was no significant difference between the infection probabilities of these latter five citrus types. The data from this study show that successful infection by grafting is dependent upon the time of exposure to the inoculum, that shorter IPs increase the time needed to establish a systemic infection, and that citrus types vary in their overall susceptibility to infection by ‘Ca. L. asiaticus’.

We here present annotated lists of names ofCandidatustaxa of prokaryotes with ranks between subspecies and class, proposed between the mid-1990s, when the provisional status ofCandidatustaxa was first established, and the end of 2018. Where necessary, corrected names are proposed that comply with the current provisions of the International Code of Nomenclature of Prokaryotes and its Orthography appendix. These lists, as well as updated lists of newly published names ofCandidatustaxa with additions and corrections to the current lists to be published periodically in theInternational Journal of Systematic and Evolutionary Microbiology, may serve as the basis for the valid publication of theCandidatusnames if and when the current proposals to expand the type material for naming of prokaryotes to also include gene sequences of yet-uncultivated taxa is accepted by the International Committee on Systematics of Prokaryotes.