Publications

4504

AbstractThe aquatic symbiont “Candidatus Aquarickettsia rohweri” infects a diversity of non-bilaterian metazoan phyla. In the threatened coralAcropora cervicornis,Aquarickettsiaproliferates in response to increased nutrient exposure, resulting in suppressed growth and increased disease susceptibility and mortality. This study evaluated the extent, as well as the ecology and evolution ofAquarickettsiainfecting the Caribbean corals:Ac. cervicornisandAc. palmataand their hybrid (‘Ac. prolifera’). The bacterial parasiteAquarickettsiawas found in all acroporids, with host and sampling location impacting infection magnitude. Phylogenomic and genome-wide single nucleotide variant analysis foundAquarickettsiaclustering by region, not by coral taxon. Fixation analysis suggested within coral colonies,Aquarickettsiaare genetically isolated to the extent that reinfection is unlikely. Relative to other Rickettsiales,Aquarickettsiais undergoing positive selection, with Florida populations experiencing greater positive selection relative to the other Caribbean locations. This may be due toAquarickettsiaresponse to increased nutrient stress in Florida, as indicated by greaterin situreplication rates in these corals.Aquarickettsiadid not significantly codiversify with either coral animal nor algal symbiont, and qPCR analysis of gametes and juveniles from susceptible coral genotypes indicated absence in early life stages. Thus, despite being an obligate parasite,Aquarickettsiamust be horizontally transmitted via coral mucocytes, an unidentified secondary host, or a yet unexplored environmentally mediated mechanism. Importantly, the prevalence ofAquarickettsiainAc. cervicornisand high abundance in Florida populations suggests that disease mitigation efforts in the US and Caribbean should focus on preventing early infection via horizontal transmission.

AbstractCandidate phyla radiation (CPR) bacteria and DPANN archaea are unisolated, small-celled symbionts that are often detected in groundwater. The effects of groundwater geochemistry on the abundance, distribution, taxonomic diversity and host association of CPR bacteria and DPANN archaea has not been studied. Here, we performed genome-resolved metagenomic analysis of one agricultural and seven pristine groundwater microbial communities and recovered 746 CPR and DPANN genomes in total. The pristine sites, which serve as local sources of drinking water, contained up to 31% CPR bacteria and 4% DPANN archaea. We observed little species-level overlap of metagenome-assembled genomes (MAGs) across the groundwater sites, indicating that CPR and DPANN communities may be differentiated according to physicochemical conditions and host populations. Cryogenic transmission electron microscopy imaging and genomic analyses enabled us to identify CPR and DPANN lineages that reproducibly attach to host cells and showed that the growth of CPR bacteria seems to be stimulated by attachment to host-cell surfaces. Our analysis reveals site-specific diversity of CPR bacteria and DPANN archaea that coexist with diverse hosts in groundwater aquifers. Given that CPR and DPANN organisms have been identified in human microbiomes and their presence is correlated with diseases such as periodontitis, our findings are relevant to considerations of drinking water quality and human health.

Plant diseases caused by vector-borne pathogens are responsible for tremendous losses and threaten some of the most important agricultural crops. A good example is the citrus greening disease, which is caused by bacteria of the genus Liberibacter and is transmitted by psyllids; it has devastated the citrus industry in the United States, China, and Brazil.

“Candidatus Nitrosocaldaceae” are globally distributed in neutral or slightly alkaline hot springs and geothermally heated soils. Despite their essential role in the nitrogen cycle in high-temperature ecosystems, they remain poorly understood because they have never been isolated in pure culture, and very few genomes are available. In the present study, a metagenomics approach was employed to obtain “Ca. Nitrosocaldaceae” metagenomic-assembled genomes (MAGs) from hot spring samples collected from India and China. Phylogenomic analysis placed these MAGs within “Ca. Nitrosocaldaceae.” Average nucleotide identity and average amino acid identity analysis suggested the new MAGs represent two novel species of “Candidatus Nitrosocaldus” and a novel genus, herein proposed as “Candidatus Nitrosothermus.” Key genes responsible for chemolithotrophic ammonia oxidation and a thaumarchaeal 3HP/4HB cycle were detected in all MAGs. Furthermore, genes coding for urea degradation were only present in “Ca. Nitrosocaldus,” while biosynthesis of the vitamins, biotin, cobalamin, and riboflavin were detected in almost all MAGs. Comparison of “Ca. Nitrosocaldales/Nitrosocaldaceae” with other AOA revealed 526 specific orthogroups. This included genes related to thermal adaptation (cyclic 2,3-diphosphoglycerate, and S-adenosylmethionine decarboxylase), indicating their importance for life at high temperature. In addition, these MAGs acquired genes from members from archaea (Crenarchaeota) and bacteria (Firmicutes), mainly involved in metabolism and stress responses, which might play a role to allow this group to adapt to thermal habitats.

Huanglongbing (HLB) is the most destructive, yet incurable disease of citrus. Finding sources of genetic resistance to HLB-associated ‘CandidatusLiberibacter asiaticus’ (Las) becomes strategic to warrant crop sustainability, but no resistantCitrusgenotypes exist. SomeCitrusrelatives of the family Rutaceae, subfamily Aurantioideae, were described as full-resistant to Las, but they are phylogenetically far, thus incompatible withCitrus. Partial resistance was indicated for certain cross-compatible types. Moreover, other genotypes from subtribe Citrinae, sexually incompatible but graft-compatible withCitrus, may provide new rootstocks able to restrict bacterial titer in the canopy. Use of seedlings from monoembryonic species and inconsistencies in previous reports likely due to Las recalcitrance encouraged us to evaluate more accurately theseCitrusrelatives. We tested for Las resistance a diverse collection of graft-compatible Citrinae species using an aggressive and consistent challenge-inoculation and evaluation procedure. Most Citrinae species examined were either susceptible or partially resistant to Las. However,Eremocitrus glaucaand Papua/New GuineaMicrocitrusspecies as well as their hybrids and those withCitrusarose here for the first time as full-resistant, opening the way for using these underutilized genotypes as Las resistance sources in breeding programs or attempting using them directly as possible new Las-resistantCitrusrootstocks or interstocks.

Abstract BACKGROUND: Rickettsia spp. are human pathogens that cause a number of diseases and are transmitted by arthropods, such as ixodid ticks. Estonia is one of few regions where the distribution area of two medically important tick species, Ixodes persulcatus and I. ricinus, overlaps. The presence of the nidicolous rodent-associated I. trianguliceps has also recently been shown in Estonia. Although there is no data available in Estonia on human disease caused by tick-borne Rickettsia spp., the presence of three Rickettsia species in non-nidicolous ticks was also previously reported. The aim of this study was to detect, identify and partially characterize Rickettsia species in nidicolous and non-nidicolous ticks attached to rodents.RESULTS: Larvae and nymphs of I. ricinus (n = 1004), I. persulcatus (n = 75) and I. trianguliceps (n = 117) removed from rodents and shrews caught in different parts of Estonia were studied for the presence of Rickettsia spp. by nested PCR. Ticks were collected from 314 small animals of 5 species (Myodes glareolus (bank voles), Apodemus flavicollis (yellow necked mice), A. agrarius (striped field mice), Microtus subterranius (pine voles) and Sorex araneus (common shrews)). Rickettsial DNA was detected in 8.7% (103/1186) of the studied ticks. In addition to R. helvetica previously found in questing ticks, this study reports the first identification of the recently described I. trianguliceps-associated Candidatus R. uralica west of the Ural Mountains.