Quantitative assessment of the effects of metals on microbial degradation of organic chemicals

Abstract

Biodegradation inhibition of a benchmark chemical, 2,4-dichloro-phenoxyacetic acid methyl ester (2,4-DME), was used to quantify the inhibitory effects of heavy metals on aerobic microbial degradation rates of organic chemicals. This procedure used lake sediments and aufwuchs (floating mats) collected in the field or from laboratory microcosms. Effects of CuCl2, HgCl2, ZnCl2, Cd(NO3)2, and Cr(NO3)3 at initial concentrations ranging from 0.3 μM to 73 mM (approximately 0.1 to 10,000 mg liter-1) were investigated. In general, such metallic compounds appeared to be considerably more inhibitory to the biodegradation of an organic chemical than high concentrations of microbially toxic organics studied previously. Effects of various metal concentrations were evaluated based on the following: (i) estimated MICs, (ii) concentrations that caused a significant effect on biodegradation parameters (both a > 10% decrease in V(max) and a > 10% increase in t( 1/2 ) for 2,4-DME degradation), and (iii) concentrations that caused biodegradation half-life doublings (HLDs). The MICs of metals in sediment were lowest for Zn2+ (0.10 μM) and highest for Cd2+ and Cu2+ (0.9 and 1.2 μM, respectively). The MICs of metals in aufwuchs were lowest for Hg2+ (0.01 μM), intermediate for Cu2+ and Zn2+ (0.42 and 0.62 μM, respectively), and highest for Cr3+ and Cd2+ (3.4 and 5.6 μM, respectively). Compared with Cu2+ on aufwuchs, 70 times more Zn2+, 250 times more Cr3+, and 1,000 times more Cd2+ was required to significantly affect aufwuchs biodegradation rate parameters and coefficients (V(max) and t( 1/2 )). Aufwuchs was significantly affected by the lowest Hg2+ concentration tested (5 μM). The HLDs of metals in sediment were 0.17, 0.24, and 0.79 mM for Cu2+, Cd2+, and Zn2+, respectively. The HLDs of metals in aufwuchs were 2.0 and 3.0 μM for Cu2+ and Hg2+, respectively, and 0.14, 0.44, and 1.2 mM for Cr3+, Zn2+, and Cd2+, respectively.