This study describes reconstruction of two highly unusual archaeal genomes by de novo metagenomic assembly of multiple, deeply sequenced libraries from surface waters of Lake Tyrrell (LT), a hypersaline lake in NW Victoria, Australia. Lineage-specific probes were designed using the assembled genomes to visualize these novel archaea, which were highly abundant in the 0.1-0.8 lm size fraction of lake water samples. Gene content and inferred metabolic capabilities were highly dissimilar to all previously identified hypersaline microbial species. Distinctive characteristics included unique amino acid composition, absence of Gvp gas vesicle proteins, atypical archaeal metabolic pathways and unusually small cell size (approximately 0.6 lm diameter). Multi-locus phylogenetic analyses demonstrated that these organisms belong to a new major euryarchaeal lineage, distantly related to halophilic archaea of class Halobacteria. Consistent with these findings, we propose creation of a new archaeal class, provisionally named 'Nanohaloarchaea'. In addition to their high abundance in LT surface waters, we report the prevalence of Nanohaloarchaea in other hypersaline environments worldwide. The simultaneous discovery and genome sequencing of a novel yet ubiquitous lineage of uncultivated microorganisms demonstrates that even historically well-characterized environments can reveal unexpected diversity when analyzed by metagenomics, and advances our understanding of the ecology of hypersaline environments and the evolutionary history of the archaea.

Microbial populations inhabiting a natural hypersaline lake ecosystem in Lake Tyrrell, Victoria, Australia, have been characterized using deep metagenomic sampling, iterative de novo assembly, and multidimensional phylogenetic binning. Composite genomes representing habitat-specific microbial populations were reconstructed for eleven different archaea and one bacterium, comprising between 0.6 and 14.1% of the planktonic community. Eight of the eleven archaeal genomes were from microbial species without previously cultured representatives. These new genomes provide habitat-specific reference sequences enabling detailed, lineage-specific compartmentalization of predicted functional capabilities and cellular properties associated with both dominant and less abundant community members, including organisms previously known only by their 16S rRNA sequences. Together, these data provide a comprehensive, culture-independent genomic blueprint for ecosystem-wide analysis of protein functions, population structure, and lifestyles of co-existing, co-evolving microbial groups within the same natural habitat. The “assembly-driven” community genomic approach demonstrated in this study advances our ability to push beyond single gene investigations, and promotes genome-scale reconstructions as a tangible goal in the quest to define the metabolic, ecological, and evolutionary dynamics that underpin environmental microbial diversity.

The diversity population of microorganisms with the capability to use selenate as a terminal electron acceptor, reducing it to selenite and elemental selenium by the process known as dissimilatory selenate reduction, is largely unknown. The overall objective of this study was to gain an in-depth understanding of anaerobic biotransformation of selenium in the environment, particularly anaerobic respiration, and to characterize the microorganisms catalyzing this process. Here, we demonstrate the isolation and characterization of four novel anaerobic dissimilatory selenate-respiring bacteria enriched from a variety of sources, including sediments from three different water bodies in Chennai, India, and a tidal estuary in New Jersey. Strains S5 and S7 from India, strain KM from the Meadowlands, NJ, and strain pn1, categorized as a laboratory contaminant, were all phylogenetically distinct, belonging to various phyla in the bacterial domain. The 16S rRNA gene sequence shows that strain S5 constitutes a new genus belonging to Chrysiogenetes, while strain S7 belongs to the Deferribacteres, with greater than 98% 16S rRNA gene similarity to Geovibrio ferrireducens. Strain KM is related to Malonomonas rubra, Pelobacter acidigallici, and Desulfuromusa spp., with 96 to 97% 16S rRNA gene similarity. Strain pn1 is 99% similar to Pseudomonas stutzeri. Strains S5, S7, and KM are obligately anaerobic selenate-respiring microorganisms, while strain pn1 is facultatively anaerobic. Besides respiring selenate, all these strains also respire nitrate.

Based on unique, coherent properties of phylogenetic analysis, key amino acid substitutions and structural modeling, we have identified a new class of unusual microbial rhodopsins related to the Anabaena sensory rhodopsin (ASR) protein, including multiple homologs not previously recognized. We propose the name xenorhodopsin for this class, reflecting a taxonomically diverse membership spanning five different Bacterial phyla as well as the Euryarchaeotal class Nanohaloarchaea. The patchy phylogenetic distribution of xenorhodopsin homologs is consistent with historical dissemination through horizontal gene transfer. Shared characteristics of xenorhodopsin-containing microbes include the absence of flagellar motility and isolation from high light habitats.Reviewers: This article was reviewed by Dr. Michael Galperin and Dr. Rob Knight.

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Strain S5T , a novel bacterium that was isolated for its capability to respire selenate to elemental selenium, is described. In addition to selenate respiration, it was also capable of dissimilatory selenite, arsenate and nitrate reduction with short-chain organic acids such as pyruvate, lactate and acetate as the carbon sources and electron donors. The isolate was unable to grow fermentatively. Strain S5T was isolated from sediment of an estuarine canal in Chennai, India.Phylogenetic analysis of the 16S rRNA gene of this novel isolate revealed that it belonged to the family Chrysiogenaceae with sequence similarities of 92 and 98 %, respectively, with the type strains of Chrysiogenes arsenatis and Desulfurispirillum alkaliphilum, its closest known relatives. Strain S5 T and D. alkaliphilum were closely related in terms of their 16S rRNA gene phylogeny; however, they varied greatly in their genomic DNA G+C content (56 mol% versus 45 mol%) and cellular fatty acid compositions, as well as in many metabolic capabilities. Strain S5 T represents a novel species for which the name Desulfurispirillum indicum sp. nov. is proposed; the type strain is S5

Sediment profiles of total mercury (Hg) and monomethylmercury (MMHg) were determined from a 30-m drill hole located north of Venice, Italy. While the sediment profile of total Hg concentration was fairly constant between 1 and 10 m, that of the MMHg concentration showed an unexpected peak at a depth of 6 m. Due to the limited sulfate content (<1 mM) at the depth of 6 m, we hypothesized that the methylation of inorganic Hg(II) at this depth is associated with the syntrophic processes occurring between methanogens and sulfidogens. To test this hypothesis, anoxic sediment slurries were prepared using buried Venice Lagoon sediments amended with HgCl(2), and we monitored MMHg concentration in sediment slurries over time under two geochemical conditions: high sulfate (1-16 mM) and limited sulfate concentrations (<100 microM). After day 52 and onward from the addition of inorganic Hg(II), the MMHg concentrations were higher in sulfate-limited slurries compared to high sulfate slurries, along with methane production in both slurries. On the basis of these results, we argue that active methylation of inorganic Hg(II) occurs under sulfate-limited conditions possibly by syntrophic processes occurring between methanogens and sulfidogens. The environmental significance of syntrophic Hg(II) methylation should be further studied.

Strain KMT is a novel bacterium with the unique metabolic abilities of being able to respire selenate as the electron acceptor using acetate as the carbon substrate and possessing the ability to grow fermentatively on short-chain organic acids such as lactate, citrate and pyruvate. Strain KMT was isolated from a sediment enrichment culture of a highly impacted wetland system in New Jersey, USA. Strain KMT is able to reduce selenate as well as selenite to elemental selenium. The unique metabolic capabilities of strain KMT include the respiration of nitrate, poorly crystalline Fe(III) and anthraquinone disulfonate. Phylogenetic analysis of the 16S rRNA gene of the novel isolate indicates that strain KMT groups within the family Geobacteraceae in the class Deltaproteobacteria with approximately 96–97 % 16S rRNA gene sequence similarity to the closest known organisms Malonomonas rubra Gra Mal 1T, Pelobacter acidigallici Ma Gal 2T and species of the genus Desulfuromusa. Recognized species of the genera Malonomonas and Pelobacter cannot use any inorganic electron acceptors, while strains of the genus Desulfuromusa do not ferment organic substrates. This contrasts with the ability of strain KMT to ferment organic compounds as well as to couple selenate reduction to acetate utilization. Based on 16S rRNA gene phylogeny and metabolic properties, strain KMT represents a novel species for which the name Pelobacter seleniigenes sp. nov. (type strain KMT=DSM 18267T=ATCC BAA-1388T) is proposed. Based on the phylogenetic grouping of species of the genus Pelobacter within the Desulfuromusa cluster, it is suggested that Malonomonas rubra Gra Mal 1T should also be included in this group.