Narihiro, Takashi

ABSTRACT To verify whether members of the phylum Candidatus Patescibacteria parasitize archaea, we applied cultivation, microscopy, metatranscriptomic, and protein structure prediction analyses on the Patescibacteria-enriched cultures derived from a methanogenic bioreactor. Amendment of cultures with exogenous methanogenic archaea, acetate, amino acids, and nucleoside monophosphates increased the relative abundance of Ca . Patescibacteria. The predominant Ca . Patescibacteria were families Ca . Yanofskyibacteriaceae and Ca . Minisyncoccaceae, and the former showed positive linear relationships ( r 2 ≥ 0.70) Methanothrix in their relative abundances, suggesting related growth patterns. Methanothrix and Methanospirillum cells with attached Ca . Yanofskyibacteriaceae and Ca . Minisyncoccaceae, respectively, had significantly lower cellular activity than those of the methanogens without Ca . Patescibacteria, as extrapolated from fluorescence in situ hybridization-based fluorescence. We also observed that parasitized methanogens often had cell surface deformations. Some Methanothrix -like filamentous cells were dented where the submicron cells were attached. Ca . Yanofskyibacteriaceae and Ca . Minisyncoccaceae highly expressed extracellular enzymes, and based on structural predictions, some contained peptidoglycan-binding domains with potential involvement in host cell attachment. Collectively, we propose that the interactions of Ca . Yanofskyibacteriaceae and Ca . Minisyncoccaceae with methanogenic archaea are parasitisms. IMPORTANCE Culture-independent DNA sequencing approaches have explored diverse yet-to-be-cultured microorganisms and have significantly expanded the tree of life in recent years. One major lineage of the domain Bacteria, Ca . Patescibacteria (also known as candidate phyla radiation), is widely distributed in natural and engineered ecosystems and has been thought to be dependent on host bacteria due to the lack of several biosynthetic pathways and small cell/genome size. Although bacteria-parasitizing or bacteria-preying Ca . Patescibacteria have been described, our recent studies revealed that some lineages can specifically interact with archaea. In this study, we provide strong evidence that the relationship is parasitic, shedding light on overlooked roles of Ca . Patescibacteria in anaerobic habitats.

AbstractTo verify the parasitic lifestyle ofCandidatusPatescibacteria in the enrichment cultures derived from a methanogenic bioreactor, we applied multifaceted approaches combining cultivation, microscopy, metatranscriptomic, and protein structure prediction analyses. Cultivation experiments with the addition of exogenous methanogenic archaea with acetate, amino acids, and nucleoside monophosphates and 16S rRNA gene sequencing confirmed the increase in the relative abundance ofCa. Patescibacteria and methanogens. The predominantCa. Patescibacteria wereCa. Yanofskybacteria and 32-520 lineages (to which belongs to classCa. Paceibacteria) and positive linear relationships (r2≥ 0.70) between the relative abundance ofCa. Yanofskybacteria andMethanothrix, suggesting that the tendency of the growth rate is similar to that of the host. By fluorescencein situhybridization (FISH) observations, the FISH signals ofMethanothrixandMethanospirillumcells withCa. Yanofskybacteria and with 32-520 lineages, respectively, were significantly lower than those of the methanogens withoutCa. Patescibacteria, suggesting their parasitic interaction. The TEM and SEM observations also support parasitism in that the cell walls and plugs of these methanogens associated with submicron cells were often deformed. In particular, someMethanothrix-like filamentous cells were dented where the submicron cells were attached. Metatranscriptomic and protein structure prediction analyses identified highly expressed secreted genes from the genomes ofCa. Yanofskybacteria and 32-520, and these genes contain adhesion-related domains to the host cells. Considering the results through the combination of microscopic observations, gene expression, and computational protein modeling, we propose that the interactions betweenCa. Yanofskybacteria and 32-520 belonging to classCa. Paceibacteria and methanogenic archaea are parasitism.

Tripartite symbioses between anaerobic ciliated protists and their intracellular archaeal and bacterial symbionts are not uncommon, but most reports have been based mainly on microscopic observations. Deeper insights into the function, ecology, and evolution of these fascinating symbioses involving partners from all three domains of life have been hampered by the difficulties of culturing anaerobic ciliates in the laboratory and the frequent loss of their prokaryotic partners during long-term cultivation. In the present study, we report the isolation of an anaerobic scuticociliate, strain GW7, which has been stably maintained in our laboratory for more than 3 years without losing either of its endosymbionts. Unexpectedly, molecular characterization of the endosymbionts revealed that the bacterial partner of GW7 is phylogenetically related to intranuclear endosymbionts of aerobic ciliates. This strain will enable future genomic, transcriptomic, and proteomic analyses of the interactions in this tripartite symbiosis and a comparison with endosymbioses in aerobic ciliates.