BackgroundLamellibrachia　luymesi　dominates　cold　sulfide-hydrocarbon　seeps　and　is　known　for　its　ability　to　consume　bacteria　for　energy.　The　symbiotic　relationship　between　tubeworms　and　bacteria　with　particular　adaptations　to　chemosynthetic　environments　has　received　attention.　However,　metabolic　studies　have　primarily　focused　on　the　mechanisms　and　pathways　of　the　bacterial　symbionts,　while　studies　on　the　animal　hosts　are　limited.ResultsHere,　we　sequenced　the　transcriptome　of　L.　luymesi　and　generated　a　transcriptomic　database　containing　79,464　transcript　sequences.　Based　on　GO　and　KEGG　annotations,　we　identified　transcripts　related　to　sulfur　metabolism,　sterol　biosynthesis,　trehalose　synthesis,　and　hydrolysis.　Our　in-depth　analysis　identified　sulfation　pathways　in　L.　luymesi,　and　sulfate　activation　might　be　an　important　detoxification　pathway　for　promoting　sulfur　cycling,　reducing　byproducts　of　sulfide　metabolism,　and　converting　sulfur　compounds　to　sulfur-containing　organics,　which　are　essential　for　symbiotic　survival.　Moreover,　sulfide　can　serve　directly　as　a　sulfur　source　for　cysteine　synthesis　in　L.　luymesi.　The　existence　of　two　pathways　for　cysteine　synthesis　might　ensure　its　participation　in　the　formation　of　proteins,　heavy　metal　detoxification,　and　the　sulfide-binding　function　of　haemoglobin.　Furthermore,　our　data　suggested　that　cold-seep　tubeworm　is　capable　of　de　novo　sterol　biosynthesis,　as　well　as　incorporation　and　transformation　of　cycloartenol　and　lanosterol　into　unconventional　sterols,　and　the　critical　enzyme　involved　in　this　process　might　have　properties　similar　to　those　in　the　enzymes　from　plants　or　fungi.　Finally,　trehalose　synthesis　in　L.　luymesi　occurs　via　the　trehalose-6-phosphate　synthase　(TPS)　and　trehalose-6-phosphate　phosphatase　(TPP)　pathways.　The　TPP　gene　has　not　been　identified,　whereas　the　TPS　gene　encodes　a　protein　harbouring　conserved　TPS/OtsA　and　TPP/OtsB　domains.　The　presence　of　multiple　trehalases　that　catalyse　trehalose　hydrolysis　could　indicate　the　different　roles　of　trehalase　in　cold-seep　tubeworms.ConclusionsWe　elucidated　several　molecular　pathways　of　sulfate　activation,　cysteine　and　cholesterol　synthesis,　and　trehalose　metabolism.　Contrary　to　the　previous　analysis,　two　pathways　for　cysteine　synthesis　and　the　cycloartenol-C-24-methyltransferase　gene　were　identified　in　animals　for　the　first　time.　The　present　study　provides　new　insights　into　particular　adaptations　to　chemosynthetic　environments　in　L.　luymesi　and　can　serve　as　the　basis　for　future　molecular　studies　on　host-symbiont　interactions　and　biological　evolution.