Ann Noowong

• This thesis focuses on the biogeochemical interactions and behaviors of dissolved organic matter (DOM), metal bioaccumulations, and iron species in the Kairei and Pelagia hydrothermal vent fields in the Indian Ocean. Deep-sea vents have recently been identified as sources of DOM and dissolved metals in the ocean but the knowledge of their fates and their roles in global geochemical cycles is still scarce. DOM compositions show that both vents are sources of DOS and reduced DOM in the Indian Ocean deep water. DOM compositions of both vent hot fluids were different than those which had undergone thermal abiotic alteration/degradation in previous experiments. This implies that additional processes and/or chemical reactions are involved during hydrothermal circulations. In term of Fe stabilization and transportation, the results indicate that Fe(II) is the dominant Fe species in all hot fluid samples. Fe(III) becomes prominent in all diffuse fluids which indicates sub-seafloor mixing of endmember fluids with seawater. High [DFe] were observed in both vent plume samples. The voltammetric titrations reveal that Fe organic ligand complexes found in the plumes play a significant role in driving Fe mobilization and bioavailability in the deep water. In both vents, dense benthic communities were found. Four ubiquitous benthic species were selected i.e., mussels (Bathymodolus septemdierum), scaly-foot snails (Chrysomallon squamiferum), shrimps (Rimicaris kairei) and crabs (Austinograea rodriguezensis) for the metal bioaccumulation study in the different tissue types. Most chalcophile elements are mainly accumulated in the organic tissues of the animals. Different degrees of metal partitioning were also observed between the various tissues and organs. High accumulations found in both vent benthic organisms suggest that the animals possess a similar advanced ability to acclimatize to high-and steady metal exposures like vent organisms in the Atlantic and Pacific Oceans.