SELECTION BY TEMPERATURE OF NITRATE-REDUCING BACTERIA FROM ESTUARINE SEDIMENTS - SPECIES COMPOSITION AND COMPETITION FOR NITRATE

Nitrate-metabolising bacteria were isolated from estuarine sediment in carbon-limited anaerobic chemostats with nitrate as the only electron acceptor, at constant low temperature (5 degrees C), constant high te mperature (20 degrees C), or square wave temperature cycle (5-20 degre es C over 24 h). The steady-state communities isolated were sampled ra ndomly, and isolates identified. At constant 20 degrees C and under cy cling temperature the communities were dominated by bacteria with obli gately fermentative metabolism; Klebsiella spp. at 20 degrees C and En terobacter spp. in cycling temperature, which were nitrate-ammonifiers . The dominant isolates at 5 degrees C were bacteria with the capacity for oxidative, respiratory metabolism, which were able to denitrify. The physiologies of representative isolates of these two distinct high and low temperature communities were examined further. An Arrhenius p lot for growth of the 20 degrees C isolate Al (Klebsiella oxytoca) inc reased linearly from 10 degrees C up to the optimum at 28 degrees C, b ut decreased above the optimum and at temperatures < 10 degrees C. Al was unable to grow at 5 degrees C. The unidentifiable 5 degrees C isol ate (E3) was capable of both fermentative and oxidative metabolism, bu t when respiring nitrate the Arrhenius plot for growth was linear betw een 3-18 degrees C with an optimum at 19 degrees C. A cross-over in mu (max) of these two isolates occurred at about 23 degrees C. The abilit y of each bacterium to scavenge nitrate was examined in anaerobic nitr ate-limited chemostats, measuring the specific affinity (a(A)) of each bacterium for nitrate at different temperatures. Specific affinity fo r nitrate increased with temperature in both bacteria, indicating an i mproved ability to sequester nitrate at higher temperatures. Isolate A l had a higher a(A). for nitrate than E3 at 20 degrees C, but E3 had a higher a(A) at low temperature. A crossover in the specific affinitie s of these two bacteria occurred between 5 and 20 degrees C, so that A l was better able to sequester nitrate at high temperature while E3 wa s more effective in sequestering nitrate at lower temperature. We prop ose that at low temperature denitrifying bacteria predominate in estua rine sediments as they are selected by their improved ability to scave nge limited concentrations of nitrate at low temperature, whereas ferm entative nitrate-ammonifiers are better competitors for nitrate at the higher summer temperatures. Seasonal changes in environmental tempera ture therefore result in seasonal selection of different nitrate-utili sing communities.