Publications

4366

Summary Until now anaerobic oxidation of VFA at high salt‐pH has been demonstrated only at sulfate‐reducing conditions. Here, we present results of a microbiological investigation of anaerobic conversion of organic acids and alcohols at methanogenic conditions by syntrophic associations enriched from hypersaline soda lakes in Central Asia. Sediment incubation experiments showed active, albeit very slow, methane formation from acetate, propionate, butyrate and C 2 C 4 alcohols at pH 10 and various levels of salinity. Enrichments of syntrophic associations using hydrogenotrophic members of the genus Methanocalculus from soda lakes as partners resulted in several highly enriched cultures converting acetate, propionate, butyrate, benzoate and EtOH to methane. Most syntrophs belonged to Firmicutes , while the propionate‐oxidizer formed a novel lineage within the family Syntrophobacteraceae in the Deltaproteobacteria . The acetate‐oxidizing syntroph was identified as ‘ Ca . Syntrophonatronum acetioxidans’ previously found to oxidize acetate at sulfate‐reducing conditions up to salt‐saturating concentrations. Butyrate and a benzoate‐degrading syntrophs represent novel genus‐level lineages in Syntrophomonadales which are proposed as Candidatus taxons ‘Syntrophobaca’, ‘Syntrophocurvum’ and ‘Syntropholuna’. Overall, despite very slow growth, the results indicated the presence of a functionally competent syntrophic community in hypersaline soda lakes, capable of efficient oxidation of fermentation products to methane at extremely haloalkaline conditions.

Summary Termite gut flagellates are typically colonized by specific bacterial symbionts. Here we describe the phylogeny, ultrastructure and subcellular location of ‘ Candidatus Adiutrix intracellularis’, an intracellular symbiont of Trichonympha collaris in the termite Zootermopsis nevadensis . It represents a novel, deep‐branching clade of uncultured Deltaproteobacteria widely distributed in intestinal tracts of termites and cockroaches. Fluorescence in situ hybridization and transmission electron microscopy localized the endosymbiont near hydrogenosomes in the posterior part and near the ectosymbiont ‘ Candidatus Desulfovibrio trichonymphae’ in the anterior part of the host cell. The draft genome of ‘ Ca . Adiutrix intracellularis’ obtained from a metagenomic library revealed the presence of a complete gene set encoding the Wood–Ljungdahl pathway, including two homologs of fdhF encoding hydrogenase‐linked formate dehydrogenases (FDH H ) and all other components of the recently described hydrogen‐dependent carbon dioxide reductase (HDCR) complex, which substantiates previous claims that the symbiont is capable of reductive acetogenesis from CO 2 and H 2 . The close phylogenetic relationship between the HDCR components and their homologs in homoacetogenic Firmicutes and Spirochaetes suggests that the deltaproteobacterium acquired the capacity for homoacetogenesis via lateral gene transfer. The presence of genes for nitrogen fixation and the biosynthesis of amino acids and cofactors indicate the nutritional nature of the symbiosis.

“ Ca . Pelagibacter ubique” is a key driver of marine biogeochemistry cycles and a model for understanding how minimal genomes evolved in free-living anucleate organisms. This study explores the unusual sulfur acquisition strategy that has evolved in these cells, which lack assimilatory sulfate reduction and instead rely on reduced sulfur compounds found in oxic marine environments to meet their cellular quotas. Our findings demonstrate that the sulfur acquisition systems are constitutively expressed but the enzymatic steps leading to the essential sulfur-containing amino acid methionine are regulated by a unique array of riboswitches and genes, many of which are encoded in a rapidly evolving genome region. These findings support mounting evidence that streamlined cells have evolved regulatory mechanisms that minimize transcriptional switching and, unexpectedly, localize essential sulfur acquisition genes in a genome region normally associated with adaption to environmental variation.