Growth of phenol-mineralizing microorganisms in fresh water

Abstract

A method was developed to enumerate the procaryotic and eucaryotic phenol-mineralizing microorganisms present in samples of fresh water. Sixty-five percent or greater mineralization of [U-14C]phenol was considered a positive tube (contained phenol-mineralizing microorganisms) in the most-probable-number technique. Replicate most-probable-number tubes contained no microbial inhibitors, streptomycin and tetracycline, or cyclohexamide and nystatin plus 200 pg to 100 μg of phenol per ml. Phenol mineralization rates were obtained by measuring the amount of exogenous phenol that disappeared from solution over time in the presence or absence of the microbial inhibitors. Initially, less than 100 phenol-mineralizing bacteria per ml and 1 phenol-mineralizing fungus per ml were present at both 200 pg and 100 μg of phenol per ml. Phenol mineralization rates were 6.3 times greater for the mineralizing bacteria than for the fungi at 200 pg of phenol per ml. Phenol concentrations above 10 μg/ml were inhibitory to the microorganisms capable of mineralizing phenol. The phenol mineralizers grew in the water samples in the absence of phenol, indicating that there were sufficient indigenous nutrients in the lake water to support growth. There was no difference in the growth rate of these microorganisms in the presence or absence of 1 ng of phenol per ml, whereas the growth rate was more rapid at 1 μg of phenol per ml than in its absence. There was a correlation between microbial growth and the amount of phenol mineralized at 1 μg but not at 1 ng of phenol per ml. This indicates that 1 ng of phenol per ml was not, but 1 μg of phenol per ml was, a sufficient concentration to increase the rate of growth of the mineralizing microorganisms above that obtained with the indigenous nutrients alone. The possible existence of microorganisms capable of metabolizing phenol at both high and low concentrations (mesotrophic microorganisms) is discussed.