Distribution of obligately oligotrophic bacteria in Lake Biwa.

The distribution and seasonal changes of obligate and facultative oligotrophs in the water of Lake Biwa were investigated by the MPN-14C-glutamate method with LT10-4 medium (ca 0.6 mg organics in one liter of aged lake water) by means of 14C-uptake techinique as a criterion for bacterial growth. In unpolluted waters the obligate oligotroph was detected as a dominant population, but not in polluted waters. The proportion of the obligate and facultative oligotrophs to total heterotrophs was very variable in epilimnion, compared with that in hypolimnion. The large population of those oligotrophs in an unpolluted water was observed in late spring to late autumn. The obligate oligotroph has planktonic property and prefers amion acids to glucose. The definition of the oligotrophs was discussed further.