Medium composition affects the degree and pattern of cadmium inhibition of naphthalene biodegradation

Metals have been reported to inhibit organic pollutant biodegradation; however, widely varying degrees and patterns of inhibition have been reported. To investigate the roles of medium composition and metal bioavailability on these different degrees and patterns of inhibition, we assessed the impact of cadmium on naphthalene biodegradation by a newly isolated strain of Comamonas testosteroni in three chemically-defined minimal salts media (MSM): Tris-buffered MSM, PIPES-buffered MSM, and Bushnell-Haas medium. Cadmium (total concentrations of 100 and 500 μM) inhibited biodegradation in each medium. Degrees of inhibition were different in each medium. Cadmium was most inhibitory in PIPES-buffered MSM and least inhibitory in Bushnell-Haas. For example, in Bushnell-Haas medium, 100 μM cadmium reduced the cell yield more than 4-fold compared to controls not containing cadmium. The same concentration of cadmium completely inhibited growth in PIPES-buffered MSM. No difference in inhibition was observed in any medium when cadmium was added 24 h before inoculation rather than when added within one minute of inoculation. Two patterns of inhibition were observed. Inhibition occurred in a dose dependent pattern in Tris- and PIPES-buffered MSM and in a non-dose dependent pattern in Bushnell-Haas. Specifically, in Bushnell-Haas, 100 μM total cadmium extended the lag phase by 23 ± 8.66 h, whereas 500 μM did not extend the lag phase. Soluble, ionic cadmium (Cd²⁺) concentrations were measured and modeled in each medium to assess cadmium bioavailability. In media containing 500 μM total cadmium, bioavailability was highest in Tris- and PIPES-buffered MSM and lowest in Bushnell-Haas. In Bushnell-Haas, cadmium bioavailability was initially higher in the 500 μM treatments (196 ± 21.2 μM) than in the 100 μM treatments (78.2 ± 2.04 μM); however, after 12 h, bioavailability was higher in the 100 μM treatments (56.4 ± 24.8 μM) than the 500 μM treatments (13.3 ± 1.2 μM). These data suggest that the type of medium determines the degrees and patterns by which metals inhibit biodegradation and emphasize the importance of coupling metal toxicity and bioavailability data.