Liu, Jun

Abstract Cable bacteria are filamentous microorganisms capable of centimeter‐scale electron transport, which have great impacts on sediment biogeochemistry, especially oxygen consumption and sulfide depletion. While 16S rRNA sequences related to known cable bacteria have been identified in saline lakes, their genomic diversity, metabolic potentials, and evolution remain unknown. Eight cable bacteria genomes were retrieved from 23 sediment metagenomes across four saline lakes, representing five novel species adapted to different salinity niches. A deep‐branching Electronema species, named Electronema qinghaiense , was found preferentially in brackish to saline environments, implying an ecological and evolutionary link between marine and freshwater lineages. Based on genome analysis, the three newly named cable bacteria species are likely mixotrophic diazotrophs capable of degrading diverse complex carbohydrates, while also participating in hydrogen metabolism via various groups 3 and 4 [NiFe]‐hydrogenases. Genome streamlining and horizontal gene transfer likely drove ecophysiological differentiation among these Electrothrix and Electronema species, including an interphylum horizontal transfer of glycine/sarcosine N‐methyltransferase ( gsmt ) and sarcosine/dimethylglycine N‐methyltransferase ( sdmt ) genes into their common ancestor. Subsequent loss of these genes in some descendants led to adaptation to different salinity niches. Given the inferred ancestral physiological properties, phylogenomic analysis and the evidence that “freshwater” Electronema species experienced stronger purification selection than “saline” Electronema and “hypersaline” Electrothrix species, the evolutionary progression of cable bacteria occurred most likely in the saline‐to‐freshwater direction. Additionally, cable bacteria ecotypes adapted to specific salinity niches likely formed from selective sweeps with low homologous recombination. Collectively, these findings deepen our understanding of the ecophysiology and evolution of cable bacteria.

Two novel bacterial strains, designated as SYSU D00823T and SYSU D00873T, were isolated from sandy soil of the Gurbantunggut Desert in Xinjiang, north-west China. SYSU D00823T and SYSU D00873T shared 99.0 % 16S rRNA gene sequence identity, and were both most closely related to Pedobacter xinjiangensis 12157T with 96.1 % and 96.0 % similarities, respectively. Phylogenetic and phylogenomic analyses revealed that the two isolates and P. xinjiangensis 12157T formed a separate distinct cluster in a stable subclade with the nearby species Pedobacter mongoliensis 1-32T, as well as the genera Pararcticibacter and Arcticibacter. Furthermore, P. mongoliensis 1-32T formed a separate deep-branching lineage and did not form a cluster with members of the genus Pedobacter. The average nucleotide identity and digital DNA–DNA hybridization values between SYSU D00823T and SYSU D00873T and related species were well below the thresholds for species delineation (<81.0 % and <24.0 %, respectively). The genomes of SYSU D00823T and SYSU D00873T were 6.19 and 6.43 Mbp in size with 40.4 % and 40.5 % DNA G+C contents, respectively. The predominant fatty acids (>10 %) of SYSU D00823T and SYSU D00873T were iso-C15 : 0, iso-C17 : 0 3-OH and summed feature 3 (C16 : 1 ω7c and/or C16 : 1 ω6c). Menaquinone-7 was the only respiratory quinone. The major polar lipids were phosphatidylethanolamine, glycosphingolipid, aminoglycolipid/glycolipid, aminophospholipid and three or four unidentified polar lipids. These data indicated that strains SYSU D00823T and SYSU D00873T should be assigned to two novel species of a new genus within the family Sphingobacteriaceae, for which the names Desertivirga arenae gen. nov., sp. nov. and Desertivirga brevis sp. nov. are proposed. The type strains are SYSU D00823T (=CGMCC 1.18630T=MCCC 1K04973T=KCTC 82278T) and SYSU D00873T (=CGMCC 1.18629T=MCCC 1K04974T=KCTC 82281T), respectively. Accordingly, the reclassification of P. xinjiangensis as Desertivirga xinjiangensis comb. nov., and P. mongoliensis as Paradesertivirga mongoliensis gen. nov., comb. nov. are also proposed.

Abstract Several abundant but yet uncultivated bacterial groups exist in extreme iron- and sulfur-rich environments, and the physiology, biodiversity, and ecological roles of these bacteria remain a mystery. Here we retrieved four metagenome-assembled genomes (MAGs) from an artificial acid mine drainage (AMD) system, and propose they belong to a new deltaproteobacterial order, Candidatus Acidulodesulfobacterales. The distribution pattern of Ca. Acidulodesulfobacterales in AMDs across Southeast China correlated strongly with ferrous iron. Reconstructed metabolic pathways and gene expression profiles showed that they were likely facultatively anaerobic autotrophs capable of nitrogen fixation. In addition to dissimilatory sulfate reduction, encoded by dsrAB, dsrD, dsrL, and dsrEFH genes, these microorganisms might also oxidize sulfide, depending on oxygen concentration and/or oxidation reduction potential. Several genes with homology to those involved in iron metabolism were also identified, suggesting their potential role in iron cycling. In addition, the expression of abundant resistance genes revealed the mechanisms of adaptation and response to the extreme environmental stresses endured by these organisms in the AMD environment. These findings shed light on the distribution, diversity, and potential ecological role of the new order Ca. Acidulodesulfobacterales in nature.