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Eighteen strains of obligately oligotrophic bacteria that grow in a medium containing 1 mg of organic carbon per liter and do not grow in a rich medium (5 g/liter of nutrient) were isolated as dominant organisms from the oligotrophic water of Lake Biwa. The growth properties of these, especially of five strains, were examined. The maximum cell yield ranged from 8.5×10(4)/ml to 2.3×10(6)/ml, and their doubling times ranged from 6.6/h to 11.8/h in LT10(-4) medium (0.5 mg trypticase and 0.05 mg yeast extract in 1 liter of filtered and aged lake water). They also showed good growth in lake water medium without adding nutrients. The optimum concentrations for their growth were 5 mg/1, 5-50 mg/1, 50 mg/1, or 500 mg/1, depending on the strains. They utilized glutamate, glycine, serine, and glycolate, but not acetate, proline, or leucine. Several properties were examined. Their growth properties were very different from those of oligotrophs or oligocarbophiles isolated by other researchers.

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.

In oligotrophic waters, not only community structure but also physiological properties of heterotrophic bacteria are influenced by the concentration of organic matter.The relationship between growth rate of two facultatively oligotrophic strains ofAeromonas sp. No. 6 andFlavobacterium sp. M1 was studied in comparison with that of two eutrophic strains ofEscherichia coli 7020 andFlavobacterium sp. M2. These strains had two or three different substrate constants (Ks values) depending on substrate concentrations: Ks values for the two former were remarkably lower than those for the two latter. For instance, Ks value forAeromonas sp. No. 6 was about 8.9μM when substrate concentration was greater than 53μM and about 1.1μM when substrate concentration was less man 53μM. InE. coli the Ks value was about 260μM at greater than 5600μM and about 47μM at less than 5600μM substrate concentration.Uptake kinetics ofAeromonas sp. grown in a medium containing 2.7 mM glutamate (H-cell) and 0.11μM glutamate (L-cell) have been determined for the intact cells. H-cell had two distinct values of Km for glutamate assimilation and respiration, and L-cell had three distinct values of Km for glutamate assimilation and respiration: In H-cell Km of assimilation was 2.8×10(-7) M and 1.5×10(-4) M, and Km of respiration was 2.3×10(-7) M and 1.7×10(-4) M; in L-cell Km of assimilation was 7.4×10(-8) M, 8.3×10(-6) M, and 1.3×10(-4) M, and Km of respiration was 2.5×10(-7), 8.9×10(-6)M, and 1.7×10(-4) M. More than 60% of glutamate taken up by the H- and L-cells was respired when the substrate concentration was less than 10(-6) M, although at greater than 10(-6) M, 50% and 30% of glutamate was respired by H-cells and L-cells, respectively. These results suggest that the facultatively oligotrophic bacteria grow with high efficiency in environments with extremely low nutrient concentration, such as oligotrophic waters of lakes and ocean, as compared with in their growth in conditions of high nutrient concentraton, such as nutrient broth.

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