The　submarine　hydrothermal　systems　are　extreme　environments　where　active　cycling　of　dissolved　organic　matter　(DOM)　may　occur.　However,　little　is　known　about　the　optical　properties　and　bioavailability　of　hydrothermal　DOM,　which　could　provide　valuable　insights　into　its　transformation　processes　and　biogeochemical　reactivity.　The　quantity,　quality,　and　bioavailability　of　DOM　were　investigated　for　four　very　different　hydrothermal　vents　east　of　Taiwan,　using　dissolved　organic　carbon　(DOC),　absorption　spectroscopy,　and　fluorescence　excitation-emission　matrices-parallel　factor　analysis　(EEM-PARAFAC).　The　DOC　and　absorption　coefficient　C1280　were　both　lower　in　the　two　hydrothermal　vents　off　the　Orchid　Island　and　on　the　Green　Island　than　in　the　surrounding　seawater　and　the　two　vents　off　the　Kueishantao　Island,　indicating　effective　removals　of　DOM　in　the　former　two　hydrothermal　systems　owing　to　possible　adsorption/co-precipitation　and　thermal　degradation　respectively.　The　four　hydrothermal　DOM　showed　notable　differences　in　the　absorption　spectral　slope　S275-295,　humification　index　HIX,　biological　index　BIX,　EEM　spectra,　and　the　relative　distributions　of　seven　PARAFAC　components.　The　results　demonstrated　a　high　diversity　of　chemical　composition　and　transformation　history　of　DOM　under　contrasting　hydro　thermal　conditions.　The　little　change　in　the　hydrothermal　DOC　after　28-day　microbial　incubations　indicated　a　low　bioavailability　of　the　bulk　DOM,　and　different　PARAFAC　components　showed　contrasting　bioavailability.　The　results　have　profound　implications　for　understanding　the　biogeochemical　cycling　and　environmental　effects　of　hydrothermal　DOM　in　the　marine　environments.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.