Study On Microbial Community Structures In Sediments From Lau Basin Deep-sea Hydrothermal Vents

The Lau Basin deep-sea hydrothermal fields display unique geochemical and biologic characteristics with high concentrations of sulfides and metal elements such as Fe, Mn, Cu, Zn, Cd and Pb in hydrothermal fluids, which are distinct from hydrothermal vent systems of the East Pacific Rise (EPR), Mid-Atlantic Ridge (MAR) and other well-studied vent sites. A variety of potential electron donors (sulfides, organic C, CH4 and Fe) and electron acceptors (O2, SO42- and CO2) have been identified in these environments, potentially supporting the growth of diverse heterotrophic and autotrophic prokaryotes. There are six active venting fields with large gradients of physicochemical characteristics along the Eastern Lau Spreading Centre (ELSC), making them ideally suited for comparative studies on the relationships between geochemical variables and microbial community structures and function in deep-sea hydrothermal vent environments.In this paper, microbial community structures and several major functional community structures were surveyed in the five sediment samples collected from deep-sea hydrothermal vents along the Eastern Lau Spreading Centre during round-the-world ocean research of Dayang Yihao (ocean No.1) in May 2007. Furthermore, we tried to analyze the relationships between environment parameters and microbial community structures.First, bacteria and archaea diversity in sediments from five active hydrothermal fields along ELSC was surveyed by denatured gradient gel electrophoresis. DGGE profiles revealed significant differences in banding patterns of 16S rRNA genes, which suggested that bacteria and archaea community structures differed among sediment samples from the five deep-sea hydrothermal sites. The bacterial sequences identified were clustered into six lineages, as follows:a-Proteobacteria, y-Proteobacteria,ε-Proteobacteria,δ-Proteobacteria, Bacteroidetes and Nitrospirae. Theα-Proteobacteria and y-Proteobacteria were the most abundant phylogenetic groups. Archaeal sequences were majorly grouped in Crenarchaeota and Euryarchaeote, and most of which were only related to uncultured archaeal clones detected from deep-sea hydrothermal fields and other marine environments. According to previous studies, the archaeal phylotypes recovered were associated with the following groups:the Miscellaneous Crenarchaeotic Group (MCG), Marine Group I (MGI), deep-sea hydrothermal vent Euryarchaeota 5 and 6 (DHVE5 and DHVE6). Principal component analysis showed the differences of microbial community structures were associated with geochemical characteristics of sediments from deep-sea hydrothermal vents along the ELSC to a certain degree.Considering the disadvantage of DGGE technique and in order to obtain more sequence information and more complete community composition, sample RVDY-4 and RVDY-5 with significant difference were chosen as a representative for clone libraries analysis of 16S rRNA genes.Six bacterial lineages detected in the DGGE analysis, a-Proteobacteria, y-Proteobacteria,ε-Proteobacteria,δ-Proteobacteria, Bacteroidetes and Nitrospirae, were also found in the two bacterial libraries. Although the proportions of these groups showed difference, y-Proteobacteria were the most prominent groups in the two bacterial libraries. Additionally, in RVDY-4BA clone library Planctomycetes was identified, and in RVDY-5BA clone library Planctomycetes and Firmicutes were identified.16S rRNA sequences in archaeal clone libraries were mainly divided into the Miscellaneous Crenarchaeotic Group (MCG), Marine Group I and III, Marine Benthic Group E (MBGE) and pSL12-related group. These sequences were only distantly related to known cultured archaea, which suggested there were some novel and unique microbial species not discovered in the other environments. The numbers of bacteria and archaea in deep-sea hydrothermal sediments were estimated by fluorescence in situ hybridization with specific probes of 16S rRNA. The FISH results indicated the sample RVDY-1, RVDY-2 and RVDY-4 had a higher proportion of bacteria; sample RVDY-3 had a similar proportion of bacteria and archaea; sample RVDY-5 showed a higher proportion of archaea. These results demonstrated that there was a high diversity of bacteria and archaea in deep-sea hydrothermal sediments along the ELSC, and the proportions of bacteria and archaea were obviously distinct among the different samples.Additionally, diversity and abundance of genes encoding theβsubunit of the dissimilatory sulfite reductase (dsrB), a subunit of the particulate methane monoxygenase (pmoA) and the methyl coenzyme M reductase (mcrA) in sediments from Lau Basin deep-sea hydrothermal vents were investigated by PCR-DGGE and real-time quantitative PCR. Analysis of sequences showed that there were a considerable diversity of dsrB, pmoA, mcrA in sediments from Lau Basin deep-sea hydrothermal vents, and there were certain differences among the different sites. The dsrB sequences were divided into the five clusters, in which the majority of sequences were unclassified. The pmoA sequences retrieved from the hydrothermal vent sites were found to be related to both TypeⅠmethanotrophic members of the y-proteobacteria and TypeⅡof the a-proteobacteria, and Type I methanotrophs were dominated. The mcrA sequences were related to Methanococcales, Methanosarcinales and"unidentified group". The results of real-time quantitative PCR revealed that a high abundance of dsrB, pmoA and mcrA genes existed in Lau Basin hydrothermal vent sediments, and their quantity was between 105 and 106 copies/g (fresh weight) of sediments. CCA analysis showed that the water depth, pH, organic carbon and acid volatile sulfide had considerable influence on dsrB, pmoA, mcrA genes. Results demonstrated that, there existed potentially sulfate reduction, methane oxidation and production activities drived by microorganisms in deep-sea hydrothermal environments along the ELSC; a number of sequences obtained in the course of this study had only a low similarity to these known sequences of cultivated organisms in the database, indicating that novel lineages of these genes of dsrB, pmoA, mcrA were detected in sediments from Lau Basin deep-sea hydrothermal vents.