Publications

4495

AbstractNitrogen (N2) fixation in oligotrophic surface waters is the main source of new nitrogen to the ocean1 and has a key role in fuelling the biological carbon pump2. Oceanic N2 fixation has been attributed almost exclusively to cyanobacteria, even though genes encoding nitrogenase, the enzyme that fixes N2 into ammonia, are widespread among marine bacteria and archaea3–5. Little is known about these non-cyanobacterial N2 fixers, and direct proof that they can fix nitrogen in the ocean has so far been lacking. Here we report the discovery of a non-cyanobacterial N2-fixing symbiont, ‘Candidatus Tectiglobus diatomicola’, which provides its diatom host with fixed nitrogen in return for photosynthetic carbon. The N2-fixing symbiont belongs to the order Rhizobiales and its association with a unicellular diatom expands the known hosts for this order beyond the well-known N2-fixing rhizobia–legume symbioses on land6. Our results show that the rhizobia–diatom symbioses can contribute as much fixed nitrogen as can cyanobacterial N2 fixers in the tropical North Atlantic, and that they might be responsible for N2 fixation in the vast regions of the ocean in which cyanobacteria are too rare to account for the measured rates.

A Gram-stain-positive, rod-shaped, aerobic, motile bacterium, J379T, was isolated from radioactive water spring C1, located in a former silver–uranium mine in the Czech Republic. This slow-growing strain exhibited optimal growth at 24–28 °C on solid media with <1 % salt concentration and alkaline pH 8–10. The only respiratory quinone found in strain J379T was MK-7(H4). C18 : 1 ω9c (60.9 %), C18 : 0 (9.4 %), C16 : 0 and alcohol-C18 : 0 (both 6.2 %) were found to be the major fatty acids. The peptidoglycan contained directly cross-linked meso-diaminopimelic acid. Phylogenetic reconstruction based on the 16S rRNA gene sequences and the core-genome analysis revealed that strain J379T forms a separate phylogenetic lineage within the recently amended order Solirubrobacterales. A comparison of the 16S rRNA gene sequences between strain J379T and other members of the order Solirubrobacterales showed <96 % similarity. This analysis revealed that the closest type strains were Parviterribacter kavangonensis D16/0 /H6T (95.2 %), Capillimicrobium parvum 0166_1T (94.9 %) and Conexibacter arvalis KV-962T (94.5 %). Whole-genome analysis showed that the closest type strain was Baekduia soli BR7-21T with an average nucleotide identity of 78 %, average amino acid identity of 63.2 % and percentage of conserved proteins of 48.2 %. The G+C content of the J379T genomic DNA was 71.7 mol%. Based on the phylogenetic and phylogenomic data, as well as its physiological characteristics, strain J379T is proposed to represent a type strain (DSM 113746T=CCM 9300T) of Svornostia abyssi gen. nov. sp. nov. within the family Baekduiaceae.

Although species of the order Nostocales have morphological similarities to each other, some Nostoc species have been considered cryptic and are classified in the families Nostocaceae, Nodulariaceae and Aphanizomenaceae. Since then, the phylogenetic and morphological evaluation of Nostoc-like organisms has led to the taxonomic revision of some genera in different families. In the present work a cyanobacterium morphologically identified as Nostoc was isolated from an extreme cold lotic environment, in a high mountain of the Transmexican Volcanic Belt, and was taxonomically and phylogenetically characterized based on a polyphasic approach. Analysis of the 16S rRNA gene sequence references: Reofilinostoc matlalcueyense gen. et sp. nov. Thus, establishing a new genus of the family Nodulariaceae. The morphology of Reofilinostoc in the wild is reminiscent of a mouse ear, it is of 0.4-1.2 cm in length and has a firm cartilaginous texture; colonies can be pale green, brown or dark green in colour. Through a microscopic inspection, the filaments resemble to Komarekiella atlantica, Desikacharya nostocoides and Minunostoc cylindricum. While, the phylogenetic approach yielded similar results and consistently showed that Reofilinostoc matlalcueyense F02 (OR724089) is closely related to D. nostocoides and M. cylindricum in the core clade of Nodulariaceae. This research revealed that some comparisons between genera of Nodulariaceae exhibited similarity values higher than 97%, such as Atlanticothrix vs. Goleter and Cyanocohniella vs. Anabaenopsis. Nevertheless, in comparison to these phylogenetically closer genera, a substantial difference in the length and structure of D1-D1’, Box-B and V3 helix was demostrated; where Reofilinostoc, Desikacharya, and Minunostoc are part of the Nodulariaceae family; leading to the establishment of Reofilinostoc gen. nov., with Reofilinostoc matlalcueyense sp. nov. as the type species.

Numerous bacteria in the human gut microbiome remain unknown and/or have yet to be cultured. While collections of human gut bacteria have been published, few strains have been made publicly available. A major hurdle in making strains publicly available is their deposition to public culture collections. We propose a framework for the bulk-deposition of strains to culture collections, which removes many of the barriers previously identified (www.dsmz.de/bulk-deposit). Using this bulk-deposition system we have created a publicly available collection of human gut isolates. The Human intestinal Bacteria Collection (HiBC) (www.hibc.rwth-aachen.de) contains 340 strains representing 198 species within 29 families and 7 phyla, of which 29 previously unknown species are taxonomically described and named. These included two butyrate-producing species of Faecalibacterium and new dominant species associated with health and inflammatory bowel disease, Ruminococcoides intestinale and Blautia intestinihominis, respectively. Plasmids were prolific within the HiBC isolates, with almost half (46%) of strains containing plasmids, with a maximum of six within a strain. This included a broadly occurring plasmid (pBAC) that exists in three diverse forms across Bacteroidales species. Megaplasmids were identified within two strains, the pMMCAT megaplasmid is globally present within multiple Bacteroidales species. This collection of easily searchable and publicly available gut bacterial isolates will facilitate functional studies of the gut microbiome.