Comparative metagenomic analysis unveil the coexistence of multiple endosymbionts and its physiological relationship with the tubeworm Lamellibrachia satsuma

Abstract

Although attempts have been made to reveal the relationship between endosymbiont and tubeworm, little is known about the acquisition and coexistence of multiple symbiotic members in vestimentiferan. To reveal this, we studied on Lamellibrachia satsuma which is a small vestimentiferan tubeworm, one of the common members of faunal communities in the Kagosima Bay, nourished by chemoautotrophic bacterial endosymbionts growing in a specialized tissue called the trophosome. Two samples from different cluster of L. satsuma of the seep were studied. Two libraries were constructed from different samples by Illumina HiSeq 2000 platform. Metagenomic sequences were annotated using MG-RAST pipelines and comparative analysis carried out by STAMP software. Analysis of 16S rRNA gene fragments retrieved from metagenomic sequences suggests that the tubeworms mostly harbored γ-proteobacterial endosymbiont closely related to endosymbiont of Vestimentifera of Shinkai irrespective of different location but, in one sample, the presence of &#61541 -proteobacterial endosymbiont closely related to ectosymbiont of Rimicaris exoculata was confirmed. The symbionts contain all genes required for sulfur-oxidizing metabolism including those needed for the oxidation of reduced sulfur compounds mediated through the cytoplasmic enzymes adenylylsulfate reductase (AprA/AprB), periplasmic sulfite oxidase enzyme complex (Sox). The membrane bound respiratory nitrate reductase (NarI), a cytochrome cd1/ nitrite reductase (Nir), nitric oxide reductase (Nor) and nitric-oxide reductase (Nos) genes required for nitrate assimilation are also found in these symbionts. Surprisingly, the endosymbiont harbors genes for two different carbon fixation pathways, the Calvin-Benson-Bassham (CBB) cycle as well as the reductive tricarboxylic acid (rTCA) cycle, as has been reported in the endosymbiont of the various tubeworms like Riftia pachyptila, Tevnia jerichonana, Escarpia laminata and Lamellibrachia spp. This study suggests that, endosymbiont uptakes inorganic materials supplied by tubeworm and transforms into organic compounds which fulfil the energy requirement of the tubeworm despite of the presence of different class of endosymbionts. The specific role of coexisting endosymbionts in the tubeworm is still unknown and is yet to be studied. The existence of &#61541 -proteobacterial endosymbiont provides a new direction regarding to the acquisition of endosymbiont by tubeworm based on environmental microflora surrounding the vestimentiferan habitat depending on different geographical location.