Early evolutionary relationships among known life forms inferred from elongation factor EF-2/EF-G sequences: Phylogenetic coherence and structure of the archaeal domain

Summary Phylogenies were inferred from both the gene and the protein sequences of the translational elongation factor termed EF-2 (for Archaea and Eukarya) and EF-G (for Bacteria). All treeing methods used (distance-matrix, maximum likelihood, and parsimony), including evolutionary parsimony, support the archaeal tree and disprove the eocyte tree (i.e., the polyphyly and paraphyly of the Archaea). Distance-matrix trees derived from both the amino acid and the DNA sequence alignments (first and second codon positions) showed the Archaea to be a monophyletia-holophyletic grouping whose deepest bifurcation divides a Sulfolobus branch from a branch comprising Methanococcus, Halobacterium, and Thermoplasma. Bootstrapped distance-matrix treeing confirmed the monophyly-holophyly of Archaea in 100% of the samples and supported the bifurcation of Archaea into a Sulfolobus branch and a methanogen-halophile branch in 97% of the samples. Similar phylogenies were inferred by maximum likelihood and by maximum (protein and DNA) parsimony. DNA parsimony trees essentially identical to those inferred from first and second codon positions were derived from alternative DNA data sets comprising either the first or the second position of each codon. Bootstrapped DNA parsimony supported the monophyly-holophyly of Archaea in 100% of the bootstrap samples and confirmed the division of Archaea into a Sulfolobus branch and a methanogen-halophile branch in 93% of the bootstrap samples. Distance-matrix and maximum likelihood treeing under the constraint that branch lengths must be consistent with a molecular clock placed the root of the universal tree between the Bacteria and the bifurcation of Archaea and Eukarya. The results support the division of Archaea into the kingdoms Crenarchaeota (corresponding to the Sulfolobus branch and Euryarchaeota). This division was not confirmed by evolutionary parsimony, which identified Halobacterium rather than Sulfolobus as the deepest offspring within the Archaea.Offprint requests to: P. Cammarano     

Effects of sequence diversity and recombination on the accuracy of phylogenetic trees estimated by kSNP

kSNP v2 is a powerful tool for single nucleotide polymorphism (SNP) identification from complete microbial genomes and for estimating phylogenetic trees from the identified SNPs. kSNP can analyse finished genomes, genome assemblies, raw reads or any combination of those and does not require either genome alignment or reference genomes. This study uses sequence evolution simulations to evaluate the topological accuracy of kSNP trees and to assess the effects of diversity and recombination on that accuracy. The accuracies of kSNP trees are strongly affected by increasing diversity, with parsimony accuracy > maximum‐likelihood accuracy > neighbour‐joining accuracy. Accuracy is also strongly influenced by recombination; as recombination increases accuracy decreases. Reliable trees are arbitrarily defined as those that have ≥ 90% topological accuracy. It is determined that the best predictor of topological accuracy is the ratio of r/m, a measure of the effect of recombination, to FCK (the fraction of core kmers), a measure of diversity. Tools are available to allow investigators to determine both r/m and FCK, and the relationship between topological accuracy and the ratio of r/m to FCK is determined. The practical implication of this study is that kSNP is an effective tool for estimating phylogenetic trees from microbial genome sequences provided that both recombination and sequence diversity are within acceptable ranges.     

Rooted phylogeny of the three superkingdoms

The traditional bacterial rooting of the three superkingdoms in sequence-based gene trees is inconsistent with new phylogenetic reconstructions based on genome content of compact protein domains. We find that protein domains at the level of the SCOP superfamily (SF) from sequenced genomes implement with maximum parsimony fully resolved rooted trees. Such genome content trees identify archaea and bacteria (akaryotes) as sister clades that diverge from an akaryote common ancestor, LACA. Several eukaryote sister clades diverge from a eukaryote common ancestor, LECA. LACA and LECA descend in parallel from the most recent universal common ancestor (MRUCA), which is not a bacterium. Rather, MRUCA presents 75% of the unique SFs encoded by extant genomes of the three superkingdoms, each encoding a proteome that partially overlaps all others. This alone implies that the common ancestor to the superkingdoms was very complex. Such ancestral complexity is confirmed by phylogenetic reconstructions. In addition, the divergence of proteomes from the complex ancestor in each superkingdom is both reductive in numbers of unique SFs as well as cumulative in the abundance of surviving SFs. These data suggest that the common ancestor was not the first cell lineage and that modern global phylogeny is the crown of a “recently” re-rooted tree. We suggest that a bottlenecked survivor of an environmental collapse, which preceded the flourishing of the modern crown, seeded the current phylogenetic tree.     

Analysis of mitochondrial and chloroplast genomes in two volvocine algae: Eudorina elegans and Eudorina cylindrica (Volvocaceae,Chlorophyta)

The colonial volvocine algae span the full range of organizational complexity, from four-celled species to multicellular species, and this group of algae is often used for the study of evolution. In recent years, many organelle genomes have been sequenced using the application of next generation sequencing technology; however, only a few organelle genomes have been reported in colonial volvocine algae. In this study, we determined the organelle genomes of Eudorina elegans and Eudorina cylindrica and analysed the organelle genome size, structure and gene content between these volvocine species. This provided useful information to help us understand the composition of colonial volvocine organelle genomes. Based on the chloroplast genome protein-coding genes, we conducted a phylogenomics analysis of the volvocine algae. The result revealed an unexpected phylogenetic relationship, namely, E. elegans is more closely related to Pleodorina starrii than to E. cylindrica. The substitution rate of volvocine algae was then calculated based on organelle genome protein-coding genes; our analysis suggested the possibility that the two Eudorina species may be under similar evolutionary pressure. Lastly, the synteny analysis of the mitochondrial genome showed that gene arrangements and contents are highly conserved in the family Volvocaceae, and the synteny analysis of the chloroplast genome indicated that the genus Eudorina may have experienced genomic changes.     

Genome sequence of a mesophilic hydrogenotrophic methanogen Methanocella paludicola, the first cultivated representative of the order Methanocellales

We report complete genome sequence of a mesophilic hydrogenotrophic methanogen Methanocella paludicola, the first cultured representative of the order Methanocellales once recognized as an uncultured key archaeal group for methane emission in rice fields. The genome sequence of M. paludicola consists of a single circular chromosome of 2,957,635 bp containing 3004 protein-coding sequences (CDS). Genes for most of the functions known in the methanogenic archaea were identified, e.g. a full complement of hydrogenases and methanogenesis enzymes. The mixotrophic growth of M. paludicola was clarified by the genomic characterization and re-examined by the subsequent growth experiments. Comparative genome analysis with the previously reported genome sequence of RC-I(MRE50), which was metagenomically reconstructed, demonstrated that about 70% of M. paludicola CDSs were genetically related with RC-I(MRE50) CDSs. These CDSs included the genes involved in hydrogenotrophic methane production, incomplete TCA cycle, assimilatory sulfate reduction and so on. However, the genetic components for the carbon and nitrogen fixation and antioxidant system were different between the two Methanocellales genomes. The difference is likely associated with the physiological variability between M. paludicola and RC-I(MRE50), further suggesting the genomic and physiological diversity of the Methanocellales methanogens. Comparative genome analysis among the previously determined methanogen genomes points to the genome-wide relatedness of the Methanocellales methanogens to the orders Methanosarcinales and Methanomicrobiales methanogens in terms of the genetic repertoire. Meanwhile, the unique evolutionary history of the Methanocellales methanogens is also traced in an aspect by the comparative genome analysis among the methanogens.     

Wo lebten die ersten Zellen – und wovon?

How, and where, did the first cells on Earth grow? The last universal common ancestor of all cells (Luca) was long considered as the common ancestor of bacteria, archaea and eukaryotes. New trees of life have a host for the origin of mitochondria (of eukaryotes) branching within the archaea, making Luca the common ancestor of bacteria and archaea. New comparative genomic investigations have reconstructed Luca's microbial ecology. The 355 protein families that trace back to Luca by phylogenetic criteria describe Luca as anaerobic, CO₂ ‐ and N₂ ‐fixing, H₂ ‐dependent and thermophilic. Luca's biochemistry was replete with FeS clusters and radical reaction mechanisms, its cofactors reveal an essential role for transition metals in its metabolism. Luca lived in an anaerobic geochemical active environment rich in H₂ , CO₂ and iron. This lifestyle is similar to modern acetogens (bacteria) and methanogens (archaea), the physiologically most ancient microbes.     

A mitochondrial genome phylogeny of the Neuropterida (lace-wings, alderflies and snakeflies) and their relationship to the other holometabolous insect orders

We present a mitochondrial (mt) genome phylogeny inferring relationships within Neuropterida (lacewings, alderflies and camel flies) and between Neuropterida and other holometabolous insect orders. Whole mt genomes were sequenced for Sialis hamata (Megaloptera: Sialidae), Ditaxis latistyla (Neuroptera: Mantispidae), Mongoloraphidia harmandi (Raphidioptera: Raphidiidae), Macrogyrus oblongus (Coleoptera: Gyrinidae), Rhopaea magnicornis (Coleoptera: Scarabaeidae), and Mordella atrata (Coleoptera: Mordellidae) and compared against representatives of other holometabolous orders in phylogenetic analyses. Additionally, we test the sensitivity of phylogenetic inferences to four analytical approaches: inclusion vs. exclusion of RNA genes, manual vs. algorithmic alignments, arbitrary vs. algorithmic approaches to excluding variable gene regions and how each approach interacts with phylogenetic inference methods (parsimony vs. Bayesian inference). Of these factors, phylogenetic inference method had the most influence on interordinal relationships. Bayesian analyses inferred topologies largely congruent with morphologically‐based hypotheses of neuropterid relationships, a monophyletic Neuropterida whose sister group is Coleoptera. In contrast, parsimony analyses failed to support a monophyletic Neuropterida as Raphidioptera was the sister group of the entire Holometabola excluding Hymenoptera, and Neuroptera + Megaloptera is the sister group of Diptera, a relationship which has not previously been proposed based on either molecular or morphological data sets. These differences between analytical methods are due to the high among site rate heterogeneity found in insect mt genomes which is properly modelled by Bayesian methods but results in artifactual relationships under parsimony. Properly analysed, the mt genomic data set presented here is among the first molecular data to support traditional, morphology‐based interpretations of relationships between the three neuropterid orders and their grouping with Coleoptera.     

Mitogenomic phylogeny of Trochoidea (Gastropoda: Vetigastropoda): New insights from increased complete genomes

Increased mitochondrial (mt) genomes can provide more sets of genome‐level characteristics for resolving deeper phylogeny. Limited information with respect to the Trochoidea mitochondrial genome organization is available; besides, monophyly and internal relationships of the superfamily still remain a matter of discussion. To resolve the monophyly and internal phylogenetic controversies of Trochoidea and expand our understanding for mt genomic characteristic evolution among Trochoidea, the phylogenetic trees were reconstructed using 13 newly sequenced complete mt genomes and 35 genomes from GenBank, and both the maximum likelihood and Bayesian inference analyses were highly supported. Vetigastropoda phylogenetic analyses recovered the monophyly of Trochoidea. Trochoidea phylogenetic analyses and genetic distances supported the non‐monophyly of Tegulidae and Tegula, indicating that the taxonomic status of several genera (Rochia, Tectus and Cittarium) should be revised and Tegula, Omphalius and Chlorostoma should be placed as a same genus. The close affinity between Tectus virgatus and Rochia was also revealed. Three‐nucleotide insertion in nad1, nine‐nucleotide insertion and six‐nucleotide deletion in nad5 are detected in Tegulidae, Tectus and Rochia, respectively. Gene orders within Trochoidea are stable, with gene rearrangements exclusive to tRNA genes observed. Homoplasious convergences because of trnT rearrangement display translocation in Turbinidae and reversion in Trochidae and Calliostomatida. For trnE and trnG, we identify 11 arrangement types, suggesting that the gene rearrangement history needs to be further evaluated. Our study emphasizes the importance of mt genomes in resolving phylogenetic relationships within Trochoidea. In addition, the mt genomic characters would contribute new insights into the classification of Trochoidea.     

Methane Production and Methanogenic Archaea in the Digestive Tracts of Millipedes (Diplopoda)

Methane production by intestinal methanogenic Archaea and their community structure were compared among phylogenetic lineages of millipedes. Tropical and temperate millipedes of 35 species and 17 families were investigated. Species that emitted methane were mostly in the juliform orders Julida, Spirobolida, and Spirostreptida. The irregular phylogenetic distribution of methane production correlated with the presence of the methanogen-specific mcrA gene. The study brings the first detailed survey of methanogens’ diversity in the digestive tract of millipedes. Sequences related to Methanosarcinales, Methanobacteriales, Methanomicrobiales and some unclassified Archaea were detected using molecular profiling (DGGE). The differences in substrate preferences of the main lineages of methanogenic Archaea found in different millipede orders indicate that the composition of methanogen communities may reflect the differences in available substrates for methanogenesis or the presence of symbiotic protozoa in the digestive tract. We conclude that differences in methane production in the millipede gut reflect differences in the activity and proliferation of intestinal methanogens rather than an absolute inability of some millipede taxa to host methanogens. This inference was supported by the general presence of methanogenic activity in millipede faecal pellets and the presence of the 16S rRNA gene of methanogens in all tested taxa in the two main groups of millipedes, the Helminthophora and the Pentazonia.     

A DNA topoisomerase IB in Thaumarchaeota testifies for the presence of this enzyme in the last common ancestor of Archaea and Eucarya

   

DNA topoisomerase IB (TopoIB) was thought for a long time to be a eukaryotic specific enzyme. A shorter version was then found in viruses and later on in several bacteria, but not in archaea. Here, we show that a eukaryotic-like TopoIB is present in the recently sequenced genomes of two archaea of the newly proposed phylum Thaumarchaeota. Phylogenetic analyses suggest that a TopoIB was present in the last common ancestor of Archaea and Eucarya. This finding indicates that the last common ancestor of Archaea and Eucarya may have harboured a DNA genome.     

Ancestral state reconstructions for genomes

The recent expansion of phylogenetic analysis from the traditional field of molecular evolution, analyzing histories of genes, to the nascent field of "genomic evolution", analyzing histories of entire genomes, enables the construction of trees based on genome information, the quantification of the key processes that shape genome content and, ultimately, plausible parsimony reconstructions of ancestral genomes. Thus, when genomes are considered as phylogenetic characters, it is possible to reconstruct not only the history of species but also the ancestral states in terms of genome structure or function. In the future, we might be able to accurately reconstruct--or retrodict--a chain of events that led to the emergence of a specific genome sequence and, ultimately, to synthesize ancestral genomes at will, creating a "Jurassic database" of genomes.     

Plastid genome sequencing,comparative genomics,and phylogenomics: Current status and prospects

Abstract More than 190 plastid genomes have been completely sequenced during the past two decades due to advances in DNA sequencing technologies. Based on this unprecedented abundance of data, extensive genomic changes have been revealed in the plastid genomes. Inversion is the most common mechanism that leads to gene order changes. Several inversion events have been recognized as informative phylogenetic markers, such as a 30‐kb inversion found in all living vascular plants minus lycopsids and two short inversions putatively shared by all ferns. Gene loss is a common event throughout plastid genome evolution. Many genes were independently lost or transferred to the nuclear genome in multiple plant lineages. The trnR‐CCG gene was lost in some clades of lycophytes, ferns, and seed plants, and all the ndh genes were absent in parasitic plants, gnetophytes, Pinaceae, and the Taiwan moth orchid. Certain parasitic plants have, in particular, lost plastid genes related to photosynthesis because of the relaxation of functional constraint. The dramatic growth of plastid genome sequences has also promoted the use of whole plastid sequences and genomic features to solve phylogenetic problems. Chloroplast phylogenomics has provided additional evidence for deep‐level phylogenetic relationships as well as increased phylogenetic resolutions at low taxonomic levels. However, chloroplast phylogenomics is still in its infant stage and rigorous analysis methodology has yet to be developed.     

Conservation of long-range synteny and microsynteny between the genomes of two distantly related nematodes

Background  

Comparisons between the genomes of the closely related nematodes Caenorhabditis elegans and Caenorhabditis briggsae reveal high rates of rearrangement, with a bias towards within-chromosome events. To assess whether this pattern is true of nematodes in general, we have used genome sequence to compare two nematode species that last shared a common ancestor approximately 300 million years ago: the model C. elegans and the filarial parasite Brugia malayi.     

Analysis of Methanogen Diversity in the Rumen Using Temporal Temperature Gradient Gel Electrophoresis: Identification of Uncultured Methanogens

A temporal temperature gradient gel electrophoresis (TTGE) method was developed to determine the diversity of methanogen populations in the rumen. Tests with amplicons from genomic DNA from 12 cultured methanogens showed single bands for all strains, with only two showing apparently comigrating bands. Fingerprints of methanogen populations were analyzed from DNA extracted from rumen contents from two cattle and four sheep grazing pasture. For one sheep, dilution cultures selective for methanogens were grown and the culturable methanogens in each successive dilution examined by TTGE. A total of 66 methanogen sequences were retrieved from bands in fingerprints and analyzed to reveal the presence of methanogens belonging to the Methanobacteriales, the Methanosarcinales, and to an uncultured archaeal lineage. Twenty-four sequences were most similar to Methanobrevibacter ruminantium, five to Methanobrevibacter smithii, four to Methanosphaera stadtmanae, and for three, the nearest match was Methanimicrococcus blatticola. The remaining 30 sequences did not cluster with sequences from cultured archaea, but when combined with published novel sequences from clone libraries formed a monophyletic lineage within the Euryarchaeota, which contained two previously unrecognized clusters. The TTGE bands from this lineage showed that the uncultured methanogens had significant population densities in each of the six rumen samples examined. In cultures of dilutions from one rumen sample, TTGE examination revealed these methanogens at a level of at least 10⁵ g⁻¹. Band intensities from low-dilution cultures indicated that these methanogens were present at similar densities to Methanobrevibacter ruminantium-like methanogens, the sole culturable methanogens in high dilutions (10⁶–10⁻¹⁰ g⁻¹). It is suggested that the uncultured methanogens together with Methanobrevibacter spp. may be the predominant methanogens in the rumen. The TTGE method presented in this article provides a new opportunity for characterizing methanogen populations in the rumen microbial ecosystem.     

Informatics issues in large‐scale sequence analysis: Elucidating the protein kinases of C. elegans

With the availability of the nearly complete genomic sequence of C. elegans, the first multicellular organism to be sequenced, molecular biology has definitely entered the postgenomic era. Annotation of the genomic sequence, which refers to identifying the genes and other biologically relevant sections of the genome, is an important and nontrivial next step. A first‐pass annotation will be necessarily incomplete but will drive further biological experiments, which in turn will help to annotate the genome better. Given the scale of the genome sequence analysis, it is clear that the annotation should be automated as much as possible without sacrificing the quality of analysis. In this work, we outline our approach to identifying the protein kinases of C. elegans from the genomic sequence. We describe new tools we have developed for analysis, management and visualization of genomic data. By developing modular and scalable solutions, this study has provided a framework for future analysis of the Drosophila and human genomes. J. Cell. Biochem. 80:181–186, 2000. © 2000 Wiley‐Liss, Inc.     

Molecular signatures for the Crenarchaeota and the Thaumarchaeota

Crenarchaeotes found in mesophilic marine environments were recently placed into a new phylum of Archaea called the Thaumarchaeota. However, very few molecular characteristics of this new phylum are currently known which can be used to distinguish them from the Crenarchaeota. In addition, their relationships to deep-branching archaeal lineages are unclear. We report here detailed analyses of protein sequences from Crenarchaeota and Thaumarchaeota that have identified many conserved signature indels (CSIs) and signature proteins (SPs) (i.e., proteins for which all significant blast hits are from these groups) that are specific for these archaeal groups. Of the identified signatures 6 CSIs and 13 SPs are specific for the Crenarchaeota phylum; 6 CSIs and >250 SPs are uniquely found in various Thaumarchaeota (viz. Cenarchaeum symbiosum, Nitrosopumilus maritimus and a number of uncultured marine crenarchaeotes) and 3 CSIs and ~10 SPs are found in both Thaumarchaeota and Crenarchaeota species. Some of the molecular signatures are also present in Korarchaeum cryptofilum, which forms the independent phylum Korarchaeota. Although some of these molecular signatures suggest a distant shared ancestry between Thaumarchaeota and Crenarchaeota, our identification of large numbers of Thaumarchaeota-specific proteins and their deep branching between the Crenarchaeota and Euryarchaeota phyla in phylogenetic trees shows that they are distinct from both Crenarchaeota and Euryarchaeota in both genetic and phylogenetic terms. These observations support the placement of marine mesophilic archaea into the separate phylum Thaumarchaeota. Additionally, many CSIs and SPs have been found that are specific for different orders within Crenarchaeota (viz. Sulfolobales—3 CSIs and 169 SPs, Thermoproteales—5 CSIs and 25 SPs, Desulfurococcales—4 SPs, and Sulfolobales and Desulfurococcales—2 CSIs and 18 SPs). The signatures described here provide novel means for distinguishing the Crenarchaeota and the Thaumarchaeota and for the classification of related and novel species in different environments. Functional studies on these signature proteins could lead to discovery of novel biochemical properties that are unique to these groups of archaea.     

Gene transfers from nanoarchaeota to an ancestor of diplomonads and parabasalids

Rare evolutionary events, such as lateral gene transfers and gene fusions, may be useful to pinpoint, and correlate the timing of, key branches across the tree of life. For example, the shared possession of a transferred gene indicates a phylogenetic relationship among organismal lineages by virtue of their shared common ancestral recipient. Here, we present phylogenetic analyses of prolyl-tRNA and alanyl-tRNA synthetase genes that indicate lateral gene transfer events to an ancestor of the diplomonads and parabasalids from lineages more closely related to the newly discovered archaeal hyperthermophile Nanoarchaeum equitans (Nanoarchaeota) than to Crenarchaeota or Euryarchaeota. The support for this scenario is strong from all applied phylogenetic methods for the alanyl-tRNA sequences, whereas the phylogenetic analyses of the prolyl-tRNA sequences show some disagreements between methods, indicating that the donor lineage cannot be identified with a high degree of certainty. However, in both trees, the diplomonads and parabasalids branch together within the Archaea, strongly suggesting that these two groups of unicellular eukaryotes, often regarded as the two earliest independent offshoots of the eukaryotic lineage, share a common ancestor to the exclusion of the eukaryotic root. Unfortunately, the phylogenetic analyses of these two aminoacyl-tRNA synthetase genes are inconclusive regarding the position of the diplomonad/parabasalid group within the eukaryotes. Our results also show that the lineage leading to Nanoarchaeota branched off from Euryarchaeota and Crenarchaeota before the divergence of diplomonads and parabasalids, that this unexplored archaeal diversity, currently only represented by the hyperthermophilic organism Nanoarchaeum equitans, may include members living in close proximity to mesophilic eukaryotes, and that the presence of split genes in the Nanoarchaeum genome is a derived feature.     

Biochemical and structural characterization of Cren7, a novel chromatin protein conserved among Crenarchaea

Archaea contain a variety of chromatin proteins consistent with the evolution of different genome packaging mechanisms. Among the two main kingdoms in the Archaea, Euryarchaeota synthesize histone homologs, whereas Crenarchaeota have not been shown to possess a chromatin protein conserved at the kingdom level. We report the identification of Cren7, a novel family of chromatin proteins highly conserved in the Crenarchaeota. A small, basic, methylated and abundant protein, Cren7 displays a higher affinity for double-stranded DNA than for single-stranded DNA, constrains negative DNA supercoils and is associated with genomic DNA in vivo. The solution structure and DNA-binding surface of Cren7 from the hyperthermophilic crenarchaeon Sulfolobus solfataricus were determined by NMR. The protein adopts an SH3-like fold. It interacts with duplex DNA through a β-sheet and a long flexible loop, presumably resulting in DNA distortions through intercalation of conserved hydrophobic residues into the DNA structure. These data suggest that the crenarchaeal kingdom in the Archaea shares a common strategy in chromatin organization.     

Prokaryotic diversity,composition structure,and phylogenetic analysis of microbial communities in leachate sediment ecosystems

In order to obtain insight into the prokaryotic diversity and community in leachate sediment, a culture-independent DNA-based molecular phylogenetic approach was performed with archaeal and bacterial 16S rRNA gene clone libraries derived from leachate sediment of an aged landfill. A total of 59 archaeal and 283 bacterial rDNA phylotypes were identified in 425 archaeal and 375 bacterial analyzed clones. All archaeal clones distributed within two archaeal phyla of the Euryarchaeota and Crenarchaeota, and well-defined methanogen lineages, especially Methanosaeta spp., are the most numerically dominant species of the archaeal community. Phylogenetic analysis of the bacterial library revealed a variety of pollutant-degrading and biotransforming microorganisms, including 18 distinct phyla. A substantial fraction of bacterial clones showed low levels of similarity with any previously documented sequences and thus might be taxonomically new. Chemical characteristics and phylogenetic inferences indicated that (1) ammonium-utilizing bacteria might form consortia to alleviate or avoid the negative influence of high ammonium concentration on other microorganisms, and (2) members of the Crenarchaeota found in the sediment might be involved in ammonium oxidation. This study is the first to report the composition of the microbial assemblages and phylogenetic characteristics of prokaryotic populations extant in leachate sediment. Additional work on microbial activity and contaminant biodegradation remains to be explored.