Publications

4537

Abstract Nitrobacter strain NHB1 is a nitrite-oxidizing bacterium previously demonstrated to form a consortium capable of nitrification under acidic conditions when cocultivated with a neutrophilic ammonia-oxidizing bacterium. Here, we characterize the growth of isolated NHB1 under different pH and nitrite (NO2−) concentrations, as well as its influence on the activity of obligately acidophilic soil ammonia-oxidizing archaea (AOA) isolated from acidic soils when grown in coculture. NHB1 is acidotolerant with optimal growth at pH 6.0 (range: 5.0–7.5) at an initial NO2− concentration of 500 μM. However, at lower NO2− concentrations, closer to those found in soil, its pH optimum decreases to 5.0 and with detectable growth extended to pH 3.5. In coculture, NHB1 enhances the growth of the acidophilic AOA Nitrosotalea devaniterrae Nd1 and Nitrosotalea sinensis Nd2, which are highly sensitive to NO2-derived compounds and typically oxidize only ~200 to 300 μM ammonia (NH3) when grown in batch cultures as isolates. However, in coculture with NHB1, both strains oxidized up to ~3 mM NH3, limited only by the buffering capacity of the medium, and their pH range was also extended downward by ~0.5 units. NHB1 also possesses a cyanase, enabling reciprocal cross-feeding through cyanate-derived NH3 production while utilizing AOA-derived NO2−. These findings suggest that NO2− removal is essential for ammonia oxidizer growth in acidic soils and emphasize the importance of considering substrate and metabolic product concentrations when characterizing ecophysiology. Genome analysis reveals that NHB1 is distinct from validated species, and we propose the name “Nitrobacter laanbroekii.”

Abstract Members of the phylum Deferribacterota inhabit diverse environments, but their symbiosis with protists has never been reported. We discovered an ectosymbiotic clade of Deferribacterota specifically associated with spirotrichonymphid protists in the guts of the termites Reticulitermes speratus and Hodotermopsis sjostedti and trichonymphid protists in the gut of the wood-feeding cockroach Cryptocercus punctulatus. The ectosymbiotic Deferribacterota were spiral shaped and attached to 16%–91% of the host protist cells. These formed a monophyletic cluster within an uncultured insect gut-associated family-level clade, which is sister to the vertebrate gut-associated family Mucispirillaceae. The complete genome of an ectosymbiotic Deferribacterota was obtained from a Trichonympha acuta cell in a C. punctulatus gut and analyzed together with a single-cell amplified genome of another ectosymbiotic Deferribacterota associated with Holomastigotes sp. in the gut of R. speratus. Genome analyses suggest that these Deferribacterota ferment monosaccharides and conduct fumarate and oxidative respiration with H2 as an electron donor. They thus possibly contribute to the removal of hydrogen and oxygen to protect the fermentative activity of the protist hosts. The ectosymbionts possess reduced signal transduction gene repertoires, implying that the association has provided a relatively stable environment for these bacteria. The ectosymbionts likely possess flagella with an unusually expanded number of flagellin variants up to 40, which may reflect an adaptation to their ectosymbiotic lifestyle. We propose a novel genus, Termitispirillum, for these ectosymbionts and a novel family, Termitispirillaceae, for the insect-gut clade, under SeqCode. Our findings provide new insights into the ecology and evolution of Deferribacterota.

An anaerobic, salt-tolerant, rod-shaped bacterium, strain JL216-2 T , was isolated from the shell of the hydrothermal gastropod Alviniconcha marisindica in the Tianxiu hydrothermal field on the Carlsberg Ridge, Indian Ocean. Cells of strain JL216-2 T were motile and 0.5×3–6 µm in size. Strain JL216-2 T grew at 10–32 °C (optimal 28 °C), pH 6.0–9.0 (optimal pH 7.0) and 0.5–12.0% (w/v) sodium chloride(NaCl, optimal 2.5%). Strain JL216-2 T exhibited metabolic versatility, being capable of heterotrophic growth on complex organic compounds. It also performed autotrophic growth by utilizing carbon dioxide (CO₂) fixation or sodium bicarbonate (NaHCO 3 ) as the sole carbon source, driven by hydrogen oxidation coupled with Fe(III) reduction. Phylogenetic analysis based on 16S rRNA gene sequences indicated that the strain was most closely related to Helicovermis profundi S502 T (90.43% sequence similarity). The genome size of strain JL216-2 T was 4.91 Mb, with a G+C content of 39.54 mol%. Phylogenomic analysis and overall genome relatedness indices values indicated a close relatedness of strain JL216-2 T to H. profundi S502 T and Fusibacter spp. The combined genotypic and phenotypic data showed that strain JL216-2 T represents a novel genus within the order Eubacteriales . The type strain is JL216-2 T (=MCCC M30262 T =KACC 23553 T ).