RAPID CONSUMPTION OF LOW CONCENTRATIONS OF METHYL-BROMIDE BY SOIL BACTERIA

A dynamic dilution system for producing low mixing ratios of methyl br omide (MeBr) and a sensitive analytical technique were used to study t he uptake of MeBr by various soils. MeBr was removed within minutes fr om vials incubated with soils and similar to 10 parts per billion by v olume of MeBr. Killed controls did not consume MeBr, and a mixture of the broad-spectrum antibiotics chloramphenicol and tetracycline inhibi ted MeBr uptake by 98%, indicating that all of the uptake of MeBr was biological and by bacteria. Temperature optima for MeBr uptake suggest ed a biological sink, yet soil moisture and temperature optima varied for different soils, implying that MeBr consumption activity by soil b acteria is diverse, The eucaryotic antibiotic cycloheximide had no eff ect on MeBr uptake, indicating that soil fungi were not involved in Me Br removal, MeBr consumption did not occur anaerobically, A dynamic fl owthrough vial system was used to incubate soils at MeBr mixing ratios as low as those found in the remote atmosphere (5 to 15 parts per tri llion by volume [pptv]), Soils consumed MeBr at all mixing ratios test ed. Temperate forest and grassy lawn soils consumed MeBr most rapidly (rate constant [k] = 0.5 min(-1)), yet sandy temperate, boreal, and tr opical forest soils also readily consumed MeBr. Amendments of CH, up t o 5% had no effect on MeBr uptake even at CH4:MeBr ratios of 10(7), an d depth profiles of MeBr and CH4 consumption exhibited very different vertical rate optima, suggesting that methanotrophic bacteria, like th ose presently in culture, do not utilize MeBr when it is at atmospheri c mixing ratios, Data acquired with gas flux chambers in the field dem onstrated the very rapid in situ consumption of MeBr by soils, Uptake of MeBr at mixing ratios found in the remote atmosphere occurs via aer obic bacterial activity, displays first-order kinetics at mixing ratio s from 5 pptv to similar to 1 part per million per volume, and is rapi d enough to account for 25% of the global annual loss of atmospheric M eBr.