| The hadal zone represent the deepest part of the ocean.Heterotrophic bacteria such as those from Alphaproteobacteria,Gammaproteobacteria,Marinimicrobia and Bacteroidetes dominate the hadal zones of oceans;however,we know little about the genomic characteristics and potential metabolic adaptations of hadal trench-dwelling bacteria.Here,we report multiple single amplified genomes(SAGs)belonging to Roseobacter,Alteromonas,Marinimicrobia,Polaribacter and Dadabacteria,recovered from the hadal zone of the Mariana Trench.Phylogenetic analyses were performed by using the reference sequences from public database.Besides,we present a comparative genomic analysis of hadal SAGs to characterize the potential metabolic adaptations of these typical marine heterotrophic populations to the hadal environment.The hadal Roseobacter and Alteromonas SAGs cluster with their surface relatives based on phylogenetic analyses of the bacteria core genes,however,an analysis of genomic recruitment indicates that they have higher relative abundances in the hadal zone of the Mariana Trench.Comparative genomic analyses between the hadal SAGs and reference genomes of closely related shallow-water relatives indicate that genes involved in the mobilome(prophages and transposons)are overrepresented among the unique genes of the hadal Roseobacter and Alteromonas SAGs;the functional proteins encoded by this category of genes also shows higher amino acid sequence variation than those encoded by other gene sets within the Roseobacter SAGs.We also found that genes involved in cell wall/membrane/envelope biogenesis,transcriptional regulation,and metal transport may be important for the adaptation of hadal Roseobacterand Alteromonas lineages.Instead of these hadal Roseobacter and Alteromonas SAGs,phylogenetic analyses of hadal Marinimicrobia and Dadabacteria SAGs show that these cluster with their relatives recovered from hadal zone.Phylogenetic analyses of Polaribacter estimate longer branch length of the hadal Polaribacter SAG,suggest the independent cluster that isolated from hadal zone.Searches against the MEROPS and dbCAN database identified many different types of peptidases and the peptidoglycan lyase in Marinimicrobia,Polaribacter and Dadabacteria SAGs.Comparative genomic analyses between the hadal SAGs and reference genomes of closely related shallowwater relatives indicate that genes involved in amino acids biosynthesis and recombination,cell wall/membrane/envelope biogenesis,as well as transcription are overrepresented among the unique genes of the hadal Marinimicrobia,Polaribacter and Dadabacteria SAGs,respectively.These results imply that the modification of cell surface-related proteins and transporters is the major direction of genomic evolution in hadal bacteria adapting to the hadal environment,and that prophages and transposons may be the key factors driving this process.Based on the presence of peptidases and the CAZymes,it can be assumed the organic matter as the metabolic substrate of hadal heterotrophic bacteria. |
