Muroto deep ocean water has been pumped up from a point within a 3 km distance from the coast and at depths of 320m or 344 m in the sea, in Muroto, Kochi, Japan. In this study, in order to clarify the characteristics of selenium in the Muroto deep ocean water, total selenium and selenium speciation were investigated. Both inorganic and organic selenium were detected, and two distinct features of the selenium were discovered. Compared to ocean surface and river waters, (1) Muroto deep ocean water contained a high level of total selenium, and (2) less toxic selenium species, namely selenite and organic selenide were dominant. The total selenium content of Muroto deep ocean water was in the same range of the North Pacific Ocean. The majority of selenite and selenate occurred in the dissolved fraction; however, around 60 % of the organic selenide was found in the particulate fraction (particles diameter>0.22μm). Selenium contained in the organic and inorganic particulates accounted for 55 and 45% of the total particulates selenium, respectively. Moreover, free and combined seleno-amino acids accounted for more than 70% of the dissolved organic selenide.
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