Existence of obligately oligotrophic bacteria as a dominant population in the South China Sea and West Pacific Ocean

The distribution of obligate oligotrophs in the South China Sea and the West Pacific Ocean was investigated by the I4C-MPN method with a medium containing 0.2 mg organic carbon I-'. In surface waters at all stations except Singapore harbour, the total number of heterotrophic bacteria ranged from 1.3 X 10' to 2 . 5 ~ 103 cells ml-l, and 45 to 98% of the total heterotrophs were obligate oligotrophs and the rest facultative oligotrophs. Protein hydrolysate-utilizing oligotrophs and glucoseutilizing oligotrophs were dominant, while acetate-utilizing bacteria were minor components. Bacterial numbers determined by the I4C-MPN method were more than an order of magnitude higher than those obtained by the plate count method. The heterotrophic activities (V,,) of glutamate were remarkably hlgh in the open ocean, while uptake of acetate was undetectable. These results emphasize the importance of obligate oligotrophs in the open ocean.