Quantitative ecology of psychrophilic bacteria in an aquatic environment and characterization of heterotrophic bacteria from permanently cold sediments

Aerobic heterotrophic bacteria, anaerobic heterotrophic bacteria, ammonifying bacteria, sulfur-oxidizing bacteria, and sulfate-reducing bacteria were quantitated in Fairbank Lake, an oligotrophic to mesotrophic lake with a permanently cold hypolimnion, as a function of depth in three seasons. Representatives of each physiological group were recovered at an incubation temperature of 2 °C and for all the physiological groups the 2 °C counts were usually higher than the 37 °C counts, although sulfate-reducing bacteria were not recoverable at an incubation temperature of 37 °C. In addition, the numbers of each physiological type were generally higher in the sediments than in the water column, except in the case of sulfate-reducing bacteria for which the counts were low and often below the detection limit. Aerobic heterotrophic bacteria usually outnumbered the other physiological groups surveyed, and winter minima were characteristic of some of the physiological groups. A relatively stable density of anaerobic heterotrophic bacteria, as a function of sediment depth, was observed when the incubation temperature was 2 °C. At 37 °C, these anaerobes were not detected, and this was true for sulfate-reducing bacteria at both temperatures.Heterotrophic bacterial isolates from the permanently cold sediments were examined with regard to Gram reaction, the obligate or facultative nature of anaerobes, ability to use ecologically important substrates, psychrophilic type, and temperature range for growth. Isolates recovered at 2 °C were predominantly Gram-negative bacilli, whereas isolates recovered at 37 °C were predominantly Gram-positive bacilli. The anaerobic isolates were mainly Gram-positive bacilli regardless of the isolation temperature, and most of those examined were obligately anaerobic. Many of the isolates tested were positive for gelatinase, chitinase, amylase, and lipase, but none was positive for cellulase. Most of the sediment isolates were facultatively psychrophilic and a considerable fraction of the 37 °C isolates were facultative psychrophiles.