Symbiotic relationship revealed by transcriptomics and metagenomics in Lamellibrachia satsuma

Abstract

Lamellibrachia satsuma is a vestimentiferan tube worm, which are nourished by chemoautotrophic bacterial endosymbionts growing in a specialized tissue called the trophosome. Although several studies tried to understand the symbiotic relationship between tubeworm and its endosymbionts, most of them are focused on endosymbionts rather than tubeworms. In this study, we have analysed from both, endosymbiont and host, by metagenomic and transcriptomics analysis, respectively. The tubeworm L. satsuma was preserved in RNAlater immediately after collection from the sampling site to know the gene expression in the natural environment. cDNA library was constructed from the mRNA of the host tissues. Both cDNA and trophosome DNA from the tubeworm were sequenced by Illumina HiSeq 2000. Metagenomic data assembled by SOAP de novo were submitted to the IMG/MER for annotation. We examined quantitative changes in gene expression via high-throughput transcriptomics from the trophosome and the plume (the gas exchange organ in contact with surrounding environment) annotated by NCBI (Nr) database, KEGG, Swisprot and COG. Metagenomic analysis suggests that tubeworm harbors mostly γ-proteobacterial endosymbionts very closely related to the endosymbionts of the vent tubeworm Riftia pachyptila. The genes involved in carbon and sulfur metabolism indicate a sulfide-oxidizing chemoautotrophic endosymbiont. The symbionts contain all genes required for sulfur-oxidizing metabolism like adenylylsulfate reductase (AprA/AprB), periplasmic sulfite oxidase enzyme complex (Sox). The endosymbiont harbors genes for two different carbon fixation pathways, the CBB cycle as well as the rTCA cycle, as has been reported in the endosymbiont of the various tubeworms. Transcriptomic analysis suggest that, tubeworm possess the sulphite oxidase gene required for utilizing the oxidised sulphur (SO32-) produced by the endosymbiont which is not well studied in other vestimentiferan. We also found genes involved with oxygen and sulphide binding, namely haemoglobin A and B subunits which facilitate the diffusion of oxygen and sulphide in to the bacteriocyte. We have found genes that encode for carbonic anhydrase (CA) enzymes responsible for acid-base regulation to cope with highly variable changes in environmental pH and inorganic carbon acquisition. From this study, we found the tubeworm and endosymbiotic bacteria, symbiotically utilize reduced sulfur from the environment for the energy metabolism. The reduced sulfur transported to the bacteriocyte by tubeworm through blood vascular system and oxidised by endosymbiont which further utilised by tubeworm to fulfil the energy requirement.