Adaptation of phytoplankton-degrading microbial communities to thermal reactor effluent in a new cooling reservoir.

In water column and sediment inocula from a nuclear reactor cooling reservoir, natural phytoplankton substrate labeled with C was used to determine aerobic and anaerobic mineralization rates for a range of temperatures (25, 40, 55, and 70 degrees C) expected during reactor operation. For experiments that were begun during reactor shutdown, aerobic decomposition occurred at temperatures of <55 degrees C. After 2 months of reactor operation, aerobic rates increased substantially at 55 and 70 degrees C, although maximum rates were observed at temperatures of </=40 degrees C. The temperature range for which maximum anaerobic mineralization (i.e., the sum of CH(4) and CO(2)) was observed was 25 to 40 degrees C when the reactor was off, expanding to 25 to 55 degrees C during reactor operation. Increased rates at 55 degrees C, but not 70 degrees C, correlated with an increase in the ratio of cumulative methane to carbon dioxide produced over 21 days. When reduced reactor power lowered the maximum temperature of the reservoir to 42 degrees C, aerobic decomposition at 70 degrees C was negligible, but remained substantial at 55 degrees C. Selection for thermophilic decomposers occurred rapidly in this system in both aerobic and anaerobic communities and did not require prolonged exposure to elevated temperatures.