| Summary: | Competition between two psychrotolerant bacteria was examined in glycerol-limited chemostat experiments subjected to non-steady-state conditions of temperature. One bacterium, a Brevibacterium sp. strain designated CR3/1/15, responded rapidly to temperature change, while a second, Hydrogenophaga pseudoflava, designated CR3/2/10, exhibited a lag in growth after a shift-down during a square-wave temperature cycle but not after a shift-up. The effects on competition and survival by these bacteria of both sine-wave and square-wave temperature changes between 2 and 16 degrees C over a 24-h cycle time were examined, as well as square-wave cycles over 12 and 96 h. The changing proportion of each bacterium in the chemostat was determined by plate counting at regular intervals. Under a sine-wave temperature cycle H. psedoflava outcompeted the Brevibacterium sp., but under square-wave temperature cycles the two bacteria coexisted because the lag by H. pseudoflava after the temperature shift-down favored the faster-responding Brevibacterium sp. The two bacteria thus exhibited different survival strategies, with H. pseudoflava adapted to effective competition under steady-state conditions and the Brevibacterium sp. adapted to rapid adaptation and survival in a changing environment. The degree of perturbation of the bacteria, expressed as a temperature challenge index (delta temp/delta time), was greater under a square-wave temperature cycle than under a sine-wave cycle of equivalent amplitude and frequency, and higher-temperature challenge favored the Brevibacterium sp. A computer model was developed to examine competition between the bacteria in transient environments. The frequency of the temperature cycle influenced competition, as with a longer cycle (96 h) the significance of the lag by H. pseudoflava decreased compared with that of a 24-h cycle, and H. pseudoflava predominated in a mixed culture with a 96-h cycle. The shift-down lag by H. pseudoflava, during which it adapted to low temperature, disadvantaged it in a ...
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