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The 13th Symposium on Polar Science/Ordinary sessions [OB] Polar biology, Wed. 16 Nov.

The class Deltaproteobacteria comprises an ecologically and metabolically diverse group of bacteria best known for dissimilatory sulphate reduction and predatory behaviour. Although this lineage is the fourth described class of the phylum Proteobacteria , it rarely affiliates with other proteobacterial classes and is frequently not recovered as a monophyletic unit in phylogenetic analyses. Indeed, one branch of the class Deltaproteobacteria encompassing Bdellovibrio-like predators was recently reclassified into a separate proteobacterial class, the Oligoflexia . Here we systematically explore the phylogeny of taxa currently assigned to these classes using 120 conserved single-copy marker genes as well as rRNA genes. The overwhelming majority of markers reject the inclusion of the classes Deltaproteobacteria and Oligoflexia in the phylum Proteobacteria . Instead, the great majority of currently recognized members of the class Deltaproteobacteria are better classified into four novel phylum-level lineages. We propose the names Desulfobacterota phyl. nov. and Myxococcota phyl. nov. for two of these phyla, based on the oldest validly published names in each lineage, and retain the placeholder name SAR324 for the third phylum pending formal description of type material. Members of the class Oligoflexia represent a separate phylum for which we propose the name Bdellovibrionota phyl. nov. based on priority in the literature and general recognition of the genus Bdellovibrio. Desulfobacterota phyl. nov. includes the taxa previously classified in the phylum Thermodesulfobacteria , and these reclassifications imply that the ability of sulphate reduction was vertically inherited in the Thermodesulfobacteria rather than laterally acquired as previously inferred. Our analysis also indicates the independent acquisition of predatory behaviour in the phyla Myxococcota and Bdellovibrionota, which is consistent with their distinct modes of action. This work represents a stable reclassification of one of the most taxonomically challenging areas of the bacterial tree and provides a robust framework for future ecological and systematic studies.

We previously showed that novel filterable bacteria remain in “sterile” (&lt, 0.2 μm filtered) terrestrial environmental samples from Japan, China, and Arctic Norway. Here, we characterized the novel filterable strain IZ6T, a representative strain of a widely distributed lineage. Phylogenetic analysis showed that this strain was affiliated with the Rhizobiales (now proposed as Hyphomicrobiales) of Alphaproteobacteria, but distinct from any other type strains. Strain IZ6T shared the following chemotaxonomic features with the closest (but distantly) related type strain, Flaviflagellibacter deserti SYSU D60017T: ubiquinone-10 as the major quinone, phosphatidylethanolamine, phosphatidylcholine, and phosphatidylglycerol as major polar lipids, and slightly high G+C content of 62.2 mol%. However, the cellular fatty acid composition differed between them, and the unsaturated fatty acid (C18:1ω7c/C18:1ω6c) was predominantly found in our strain. Moreover, unlike methyrotrophs and nitrogen-fixers of the neighboring genera of Hyphomicrobiales (Rhizobiales), strain IZ6T cannot utilize a one-carbon compound (e.g., methanol) and fix atmospheric nitrogen gas. These findings were consistent with the genome-inferred physiological potential. Based on the phylogenetic, physiological, and chemotaxonomic traits, we propose that strain IZ6T represents a novel genus and species with the name Terrihabitans soli gen. nov., sp. nov. (=NBRC 106741T = NCIMB 15058T). The findings will provide deeper insight into the eco-physiology of filterable microorganisms.

We performed several experiments using three strains of Virgibacillus salexigens, namely, P2, NT N53, and C-20MoT (DSM 11483T), which were isolated from completely different sources, in relation to bacteriocin production ability. Results of whole-genome sequencing analysis revealed that all strains have very similar sequences encoding class IId bacteriocin. Although a partial amino acid sequence of the purified bacteriocin produced by strain P2 isolated from fermented food was previously reported, whole-genome sequencing and the N-terminal sequencing results in this study showed that its complete amino acid sequence consisted of 48 residues, which corresponded to that of the hypothetical bacteriocin encoded by the gene in Virgibacillus massiliensis strain Vm-5T (DSM 28587T) isolated from the human gut. From the results of 16S rRNA gene sequencing and whole-genome sequencing analyses, we taxonomically confirmed Vm-5T to be a strain of V. salexigens, and its broth culture showed antibacterial activity. Strain NT N53 isolated from the deep-sea floor produced two bacteriocins, namely, NTN-A and NTN-B. The results of N-terminal sequencing, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and whole-genome sequencing analyses showed that their amino acid sequences differed in only one residue, and NTN-A showed the same sequence as the bacteriocin produced by strain P2. Although strain C-20MoT isolated from a solar saltern had the coding sequence very similar to that of NTN-A, its broth culture showed no antibacterial activity. This finding suggests that class IId bacteriocin-producing or bacteriocin-gene-encoding V. salexigens strains are widely distributed in distinct environment sources with different geographical and material properties.

Groundwater sampling is a critical step in subsurface microbial ecology. Here, we compared two different sampling methods: commonly used disposable bailers (unimproved sampler) and an impro...

A novel mesophilic sulfate-reducing bacterium, strain HN2T, was isolated from groundwater sampled from the subsurface siliceous mudstone of the Wakkanai Formation located in Horonobe, Hokkaido, Japan. The bacterium was Gram-negative and vibrio-shaped, and its motility was conferred by a single polar flagellum. Cells had desulfoviridin. Catalase and oxidase activities were not detected. It grew in the temperature range of 25–40 °C (optimum, 35 °C) and pH range of 6.3–8.1 (optimum, pH 7.2–7.6). It used sulfate, thiosulfate, dimethyl sulfoxide, anthraquinone-2,6-disulfonate, Fe3+, and manganese oxide, but not elemental sulfur, nitrite, nitrate, or fumarate as electron acceptors. The strain showed weak growth with sulfite as the electron acceptor. Fermentative growth with pyruvate, lactate and cysteine was observed in the absence of sulfate, but not with malate or fumarate. NaCl was not required, but the strain tolerated up to 40 g l–1. Strain HN2T did not require vitamins. The major cellular fatty acids were iso-C15 : 0 (23.8 %), C18 : 1 ω9t (18.4 %), C18 : 0 (15.0 %), C16 : 0 (14.5 %), and anteiso-C17 :0 (10.1 %). The major respiratory quinone was menaquinone MK-6(H2). The G+C content of the genomic DNA was 56.7 mol%. Based on 16S rRNA gene sequence analysis, the closest phylogenetic relative of strain HN2T is Desulfovibrio psychrotolerans JS1T (97.0 %). Digital DNA–DNA hybridization (dDDH) and average nucleotide identity (ANI) values of the strains HN2T and D. psychrotolerans JS1T were 22.2 and 79.8 %, respectively. Based on the phenotypic and molecular genetic evidence, we propose a novel species, D. subterraneus sp. nov. with the type strain HN2T (=DSM 101010T=NBRC 112213T).

The 11th Symposium on Polar Science/Ordinary sessions: [OB] Polar Biology, Poster presentations, Mon. 16 Nov. - Fri. 18 Dec.

The class

著者人数: 13名, Accepted: 2017-10-12, 資料番号: SA1180309000

Two biofilm samples were collected from anaerobic groundwater in the depth range of 158.8–199.4 m in a borehole drilled in the Tono area, Japan, to understand their effects on the migration behavio...

Saxicolous rock ripe lichens that grow on rocks in the East Antarctic fellfields were sampled for phylotypic characterization of its constituent mycobionts (fungi) and photobionts (algae and cyanobacteria). The rock tripe lichen-forming fungal and algal phylotypes were classified under the common lichen-forming genera of ascomycetes, namely, Umbilicaria, and green algae, namely, Trebouxia and Coccomyxa. However, phylotypes of the green algal chloroplasts and the lichen-associated cyanobacteria showed unexpectedly high diversity. The detected chloroplast phylotypes were not fully affiliated with the green algal genera Trebouxia or Coccomyxa. The predominant chloroplast phylotype demonstrated maximum resemblance to Neglectella solitaria, which is neither a known Antarctic species nor a typical lichen photobiont. Another dominant chloroplast phylotype belonged to the atypical Antarctic green algae family. Cyanobacterial phylotypes were dominated by those affiliated with the Microcoleus species rather than the well-known lichen-associates, Nostoc species. The occurrences of these Microcoleus-affiliated cyanobacterial phylotypes were specifically abundant within the Yukidori Valley site, one of the Antarctic Specially Protected Areas (ASPA). The ASPA site, along with another 50 km-distant site, yielded most of the cryptic diversity in the phylotypes of chloroplasts and cyanobacteria, which may contribute to the phenotypic variability within the rock tripe lichen photobionts.

The Antarctic continent has ice-free areas with many freshwater lakes that support life. These lakes are generally ultra-oligotrophic and possess simplified food chains dominated by microorganisms with algal and cyanobacterial mats often occurring in the lake bottoms. In association with such mats, aquatic mosses sometimes form unique towerlike structures called “moss pillars.” Previous microflora analysis revealed the presence of several key groups (e.g., Leptolyngbya and Bradyrhizobium species) and uncultivated novel lineages in the pillars and the fact that the bacterial communities differ among the pillar sections. A wide range of eukaryotic phylotypes associated with algae, ciliates, fungi, nematodes, rotifers, and tardigrades, as well as unclassified phylotypes, were detected in the pillars. Moss pillars colonizing the nutrient-limited lakes are likely formed by a synergistic association of diverse organisms including both primary producers and decomposers. Indeed, a potential functional zonation, possibly reflected by different redox conditions within the pillar structure, was identified during the analyses of functional genes (e.g., CO2 fixation-coding genes). Interestingly, multiple sequences related to moss pillar-derived sequences were also observed in other Antarctic habitats. These findings provide clues toward solving a conundrum pertaining to Antarctic lake ecosystems: biomass-rich communities existing in the nutrient-poor conditions.

The Ninth Symposium on Polar Science/Ordinary sessions: [OB] Polar biology, Wed. 5 Dec. / Entrance Hall (1st floor), National Institute of Polar Research

A Gram-stain-positive, aerobic, non-motile, curved (selenoid), rod-shaped actinobacterium, designated KNCT, was isolated from the 0.2 μm-filtrate of river water in western Japan. Cells of strain KNCT were ultramicrosized (0.04–0.05 μm3). The strain grew at 15–37 °C, with no observable growth at 10 °C or 40 °C. The pH range for growth was 7–9, with weaker growth at pH 10. Growth was impeded by the presence of NaCl at concentrations greater than 1 %. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain KNCT showed relatively high sequence similarity (97.2 %) to Alpinimonas psychrophila Cr8-25T in the family Microbacteriaceae. However, strain KNCT formed an independent cluster with cultured, but as-yet-unidentified, species and environmental clones on the phylogenetic tree. The major cellular fatty acids were anteiso-C15 : 0 (41.0 %), iso-C16 : 0 (21.8 %), C16 : 0 (18.0 %) and anteiso-C17 : 0 (12.9 %), and the major menaquinones were MK-11 (71.3 %) and MK-12 (13.6 %). The major polar lipids were phosphatidylglycerol and two unknown glycolipids. The cell-wall muramic acid acyl type was acetyl. The peptidoglycan was B-type, and contained 3-hydroxyglutamic acid, glutamic acid, aspartic acid, glycine, alanine and lysine, with the latter being the diagnostic diamino acid. The G+C content of the genome was unusually low for actinobacteria (52.1 mol%), compared with other genera in the family Microbacteriaceae. Based on the phenotypic characteristics and phylogenetic evidence, strain KNCT represents a novel species of a new genus within the family Microbacteriaceae, for which the name Aurantimicrobium minutum gen. nov., sp. nov. is proposed. The type strain of the type species is KNCT ( = NBRC 105389T = NCIMB 14875T).

The unusual chemo-organoheterotrophic proteobacterial strain MWH-Nonnen-W8redT was isolated from a lake located in the Black Forest (Schwarzwald), Germany, by using the filtration-acclimatization method. Phylogenetic analyses based on the 16S rRNA gene sequence of the strain could not provide clear hints on classification of the strain in one of the current classes of the phylum Proteobacteria . Whole-genome sequencing resulted in a genome size of 3.5 Mbp and revealed a quite low DNA G+C content of 32.6 mol%. In-depth phylogenetic analyses based on alignments of 74 protein sequences of a phylogenetically broad range of taxa suggested assignment of the strain to a new order of the class Oligoflexia . These analyses also suggested that the order Bdellovibrionales should be transferred from the class Deltaproteobacteria to the class Oligoflexia , that this order should be split into two orders, and that the family Pseudobacteriovoracaceae should be transferred from the order Bdellovibrionales to the order Oligoflexales . We propose to establish for strain MWH-Nonnen-W8redT (=DSM 23856T=CCUG 58639T) the novel species and genus Silvanigrella aquatica gen. nov., sp. nov. to be placed in the new family Silvanigrellaceae fam. nov. of the new order Silvanigrellales ord. nov.

Bacterial utilization of humic acids (HAs) was examined under aerobic conditions using Pseudomonas spp. from diatomite from a depth of 250 m below ground level in the Horonobe Underground Research Laboratory. HA decolorization and bacterial aggregation were observed during cultivation when an auxiliary carbon source was added. High-performance size-exclusion chromatography showed that high-molecular-weight HAs were produced. Fourier transform infrared spectroscopy indicated that carboxyl groups and polysaccharide-related substances in HAs were eliminated, while aliphatic structural units and amide groups were added to HAs. These results suggested that Pseudomonas spp. utilize and alter the molecular structure of HAs under aerobic conditions caused by the construction of underground facilities.

Oligoflexus tunisiensis Shr3T is the first strain described in the newest (eighth) class Oligoflexia of the phylum Proteobacteria. This strain was isolated from the 0.2-μm filtrate of a suspension of sand gravels collected in the Sahara Desert in the Republic of Tunisia. The genome of O. tunisiensis Shr3T is 7,569,109 bp long and consists of one scaffold with a 54.3% G + C content. A total of 6,463 genes were predicted, comprising 6,406 protein-coding and 57 RNA genes. Genome sequence analysis suggested that strain Shr3T had multiple terminal oxidases for aerobic respiration and various transporters, including the resistance-nodulation-cell division-type efflux pumps. Additionally, gene sequences related to the incomplete denitrification pathway lacking the final step to reduce nitrous oxide (N2O) to nitrogen gas (N2) were found in the O. tunisiensis Shr3T genome. The results presented herein provide insight into the metabolic versatility and N2O-producing activity of Oligoflexus species.

A methanogenic organism from the domain Archaea , designated strain T10T, was isolated from groundwater sampled from a deep diatomaceous shale formation located in Horonobe, Hokkaido, Japan. The strain utilized H2/CO2 and formate as substrates for methanogenesis. Cells were strictly anaerobic, Gram-negative-staining, flagellated, irregular coccoids, 0.7–1.6 µm in diameter, and occurred singly. The strain grew at 25–45 °C (optimum 37–42 °C), at pH 5.8–8.2 (optimum pH 6.7–6.8) and in the presence of 0–1.3 M NaCl (optimum 0.1–0.2 M NaCl). The G+C content of the genomic DNA was 62.9 mol%. 16S rRNA gene sequencing revealed that, although the strain is a member of the genus Methanoculleus , it clearly differed from all described species of this genus (95.5–98.3 % sequence similarity). Values for DNA–DNA hybridization with type strains of closely related Methanoculleus species were less than 50 %. Phenotypic and phylogenetic features of strain T10T clearly indicate that it represents a novel species of the genus Methanoculleus , for which the name Methanoculleus horonobensis sp. nov. is proposed. The type strain is T10T ( = DSM 21626T = JCM 15517T).

This study deals with the biogeography of Antarctic microflora (Antarctica acts as best model to study microbial biogeography) such as cyanobacteria and selected halophiles with special emphasis on Halomonas variabilis and Bacillus licheniformis . Halophiles are known to be resistant not only to salt stress, but also to extreme temperature, pressure, and aridity and they are capable of surviving in harsh environments such as polar regions, deep-sea habitats, and deserts. Many microbes are known to be resistant to hostile environmental conditions, and are capable of surviving in harsh environments. Our group has isolated 444 strains belonging to 28 genera of halophiles from various environments around the world. The 16S rRNA gene sequences revealed that many of the isolated strains from geographically distant habitats having different environmental conditions, were closely related to each other, with some strains possessing 100% identical sequences. Organisms possessing survival mechanism such as spore formation are usually ubiquitous. The genus Halomonas is represented by potentially endemic strains and the ubiquitous H. variabilis , while spore-forming B. licheniformis showed cosmopolitan distribution. One potentially endemic (moderate endemicity that is regional and/or continental distribution) strain was reported from Syowa station, East Antarctica, and Mario Zucchelli station, West Antarctica, which are geographically separated by 3000 km. Moreover, 15 strains having 100% similarity with B. licheniformis were considered cosmopolitans. The results of this work provide support for the middle-ground model that some microbes have moderate endemicity and others have cosmopolitan distribution. These results will contribute to a greater understanding of microbial biogeography with special emphasis on Antarctica.

Thraustochytrids have the capability to recycle refractory organic matter, with a resulting im- pact on carbon cycling in coastal and open seawaters. The abundance of thraustochytrids has traditionally been estimated by acriflavine direct counting. How- ever, this technique may lead to over- or underestima- tion. To accurately quantify the abundance of thraus- tochytrids, we developed a quantitative PCR (qPCR) system using 7 genus-specific primer sets targeting 7 genera (Aurantiochytrium, Botryochytrium, Oblon - gichytrium, Parietichytrium, Schizochytrium, Sicyoi - dochytrium, and Ulkenia) from the family Thraus- tochytriaceae. The high specificity was verified in silico and with culture strains of each genus. In addi- tion, we applied this qPCR assay to test for the pres- ence of thraustochytrids in coastal and open seawaters around Japan. We successfully detected the presence of Aurantiochytrium (in the range of 1.12 × 10 4 to 1.31 × 10 4 cells l �1 ) and Oblongichytrium (in the range of 1.02 × 10 4 to 3.14 × 10 4 cells l �1 ) in 8 surface water sam- ples from around Satsuma-Iwojima (western Japan) and off the Karakuwa in Sanriku (eastern Japan). We obtained higher estimates using qPCR than the tra - ditional acriflavine method in all cases, with a weak positive correlation between the 2 methods (r 2 = 0.495). Interestingly, we quantified thraustochytrids in 104 additional samples by direct count, but not by qPCR, possibly because of inhibition of the qPCR re- action and/or the presence of novel thraustochytrid groups. Although these trials are preliminary, our approach can provide the genus-specific value of abundance in the environment. It will also promote further advances in our understanding of thraus- tochytrid diversity.

Aurantimicrobium minutum type strain KNC T is a planktonic ultramicrobacterium isolated from river water in western Japan. Strain KNC T has an extremely small, streamlined genome of 1,622,386 bp comprising 1,575 protein-coding sequences. The genome annotation suggests that strain KNC T has an actinorhodopsin-based photometabolism.

Decomposition of humic substances (HSs) is a slow and cryptic but non-negligible component of carbon cycling in sediments. Aerobic decomposition of HSs by microorganisms in the surface environment has been well documented; however, the mechanism of anaerobic microbial decomposition of HSs is not completely understood. Moreover, no microorganisms capable of anaerobic decomposition of HSs have been isolated. Here, we report the anaerobic decomposition of humic acids (HAs) by the anaerobic bacterium Clostridium sp. HSAI-1 isolated from the deep terrestrial subsurface. The use of 14C-labelled polycatechol as an HA analogue demonstrated that the bacterium decomposed this substance up to 7.4% over 14 days. The decomposition of commercial and natural HAs by the bacterium yielded lower molecular mass fractions, as determined using high-performance size-exclusion chromatography. Fourier transform infrared spectroscopy revealed the removal of carboxyl groups and polysaccharide-related substances, as well as the generation of aliphatic components, amide and aromatic groups. Therefore, our results suggest that Clostridium sp. HSAI-1 anaerobically decomposes and transforms HSs. This study improves our understanding of the anaerobic decomposition of HSs in the hidden carbon cycling in the Earth’s subsurface.

Aquatic mosses of Leptobryum species form unique tower-like pillars of vegetation termed “moss pillars” in Antarctic lakes. Moss pillars have distinct redox-affected sections: oxidative exteriors and reductive interiors. We have proposed that a “pillar” is a community and habitat of functionally interdependent organisms and may represent a mini-biosphere. Batteries of 16S rRNA genotypes, or phylotypes, of eubacteria and cyanobacteria, but no archaea, have been identified in moss pillars. However, detailed identification or phylogenetic analyses of the moss and their associated eukaryotic microbiota have not been performed. This study analyzed near-full-length 18S rRNA gene sequences obtained from two whole moss pillars. In total, 28 PCR clone libraries from two whole moss pillars were constructed, and 96 clones from each library (total 2,688 clones) were randomly selected and sequenced. Molecular phylogenetic analysis revealed that the phylotype belonging to Bryophyta, considered to be derived from moss, was closely related (99.9 %) to the 18S rRNA gene sequence from Leptobryum pyriforme. Unexpectedly, phylotypes belonging to a novel clade of fungi dominated (approximately 27–75 %) the moss pillar libraries. This suggests that fungi may contribute to carbon cycling in the moss pillar as parasites or decomposers. In addition, phylotypes related to ciliates and tardigrades were subdominant in the exterior, while the phylotype of the ameba-like, single-celled eukaryote, Cercomonas (Cercozoa), was detected only in the interior. These features were shared by both moss pillars. The 18S rRNA gene-based profiles demonstrated that redox-related factors may control distribution of some eukaryotic microbes in a whole moss pillar.

第6回極域科学シンポジウム分野横断セッション：[IB2] 地球環境変動の解析と地球生命システム学の構築11月19日（木） 統計数理研究所 セミナー室1（D305）

第6回極域科学シンポジウム分野横断セッション：[IB2] 地球環境変動の解析と地球生命システム学の構築11月19日（木） 統計数理研究所 セミナー室1（D305）

第6回極域科学シンポジウム[OB] 極域生物圏11月17日（火） 統計数理研究所 セミナー室1（D305）

Aquatic mosses in the genera Bryum and Leptobryum form unique tower-like “moss pillars” underwater in some Antarctic lakes, in association with algae and cyanobacteria. These are communities with a two-layer structure comprising an oxidative exterior and reductive interior. Although habitats and photosynthetic properties of moss pillars have been reported, microfloral composition of the two-layer structure has not been described. Here we report fatty acid analysis of one moss pillar and molecular phylogenetic analysis, based on the 16S rRNA gene, of this and one other moss pillar. Cluster analysis of the phospholipid fatty acid composition showed three groups corresponding to the exterior, upper interior, and lower interior of the pillar. This suggested that species composition differed by section, with the exterior dominated by photosynthetic organisms such as mosses, algae, and cyanobacteria, the upper interior primarily containing gram-positive bacteria and anaerobic sulfate-reducing bacteria, and the lower interior dominated by gram-negative bacteria. Molecular phylogenetic analysis revealed that Proteobacteria dominate the moss pillar as a whole; cyanobacteria were found on the exterior and the gram-positive obligate anaerobe Clostridium in the interior, while gram-positive sulfate-reducing bacteria were present in the lowest part of the interior. Nitrogen-fixing bacteria and denitrifying bacteria were found in all sections. Thus, fatty acid analysis and genetic analysis showed similar patterns. These findings suggest that microorganisms of different phylogenetic groups inhabit different sections of a single moss pillar and form a microbial community that performs biogeochemical cycling to establish and maintain a structure in an oxidation–reduction gradient between exterior and interior.

An aragonite travertine at Nagano-yu hot spring, SW Japan, exhibits clear sub-millimeter-order lamination that resembles ancient ministromatolites. Thirty-three hours of continuous observation showed that the lamination is formed daily with no changes in physicochemical properties except light intensity. Phylotype analysis and fluorescence in situ hybridization indicate that Hydrogenophaga sp. is dominant and concentrated in diurnal layers containing abundant extracellular polymeric substances. Growth of Hydrogenophaga sp. is activated in the daytime, likely due to extracellular polymeric substance production by cyanobacterial photosynthesis. Daytime development of Hydrogenophaga-dominant biofilms, and the concurrent inhibiting effect on aragonite precipitation, explains the daily lamination observed.

We pyrosequenced the bulk DNA extracted from microorganisms that passed through 0.2-μm-pore-size filters and trapped by 0.1-μm-pore-size filters in the hydrothermal fluid of the Mariana Trough. Using the 454-FLX sequencer, we generated 202,648 sequences with an average length of 173.8 bases. Functional profiles were assigned by the SEED Annotation Engine. In the metagenome of the 0.2-μm-passable microorganisms, genes related to membrane function, including potassium homeostasis classified as membrane transport, and multidrug-resistance efflux pumps classified as virulence, were dominant. There was a higher proportion of genes pertinent to the subsystem of membrane transport in our metagenomic library than in other oceanic and hydrothermal vent metagenomes. Genes associated with a RND-type efflux transporter for exogenous substances were specifically identified in the present study. After a comparative analysis with the genome of the known ultramicrobacterium Sphingopyxis alaskensis RB2256, we discovered 1,542 cases of significant hits (E 1 × 10(-2)) in our metagenome, and 1,172 of those were related to the DNA repair protein RadA. In this way, the microbial functional profile of 0.2-μm-passable fraction in the present study differs from oceanic metagenomes in the 0.2-μm-trapped fractions and hydrothermal vent metagenomes reported in previous research.