Growth Kinetics of Hyphomicrobium and Thiobacillus spp. in Mixed Cultures Degrading Dimethyl Sulfide and Methanol

Abstract

The growth kinetics of Hyphomicrobium spp. and Thiobacillus spp. on dimethyl sulfide (DMS) and methanol (in the case of Hyphomicrobium spp.) in an enrichment culture created from a biofilter cotreating DMS and methanol were studied. Specific growth rates of 0.099 h ؊1 and 0.11 h ؊1 were determined for Hyphomicrobium spp. and Thiobacillus spp., respectively, growing on DMS at pH 7. These specific growth rates are double the highest maximum specific growth rate for bacterial growth on DMS reported to date in the literature. When the pH of the medium was decreased from pH 7 to pH 5, the specific growth rate of Hyphomicrobium spp. decreased by 85%, with a near 100-fold decline in the yield of Hyphomicrobium 16S rRNA gene copies in the mixed culture. Through the same pH shift, the specific growth rate and 16S rRNA gene yield of Thiobacillus spp. remained similar. When methanol was used as a substrate, the specific growth rate of Hyphomicrobium spp. declined much less over the same pH range (up to 30%) while the yield of 16S rRNA gene copies declined by only 50%. Switching from an NH 4 ؉ -N-based source to a NO 3 ؊ -N-based source resulted in the same trends for the specific growth rate of these microorganisms with respect to pH. This suggests that pH has far more impact on the growth kinetics of these microorganisms than the nitrogen source. The results of these mixed-culture batch experiments indicate that the increased DMS removal rates observed in previous studies of biofilters cotreating DMS and methanol are due to the proliferation of DMS-degrading Hyphomicrobium spp. on methanol at pH levels not conducive to high growth rates on DMS alone.Dimethyl sulfide (DMS) is a reduced sulfur compound that is emitted from both natural and anthropogenic sources. Natural DMS emissions are largely the result of the cleavage of dimethylsulfoniopropionate (9), the breakdown of the sulfurcontaining amino acids methionine and cysteine (9, 11), and the degradation of methoxylated aromatic compounds (3, 9). Anthropogenic DMS emissions tend to be the result of hightemperature industrial processes and are problematic due to the foul smell of DMS and its low odor threshold (34). Industries that are sources of anthropogenic DMS emissions include wastewater treatment (14), aerobic composting (40), animal rendering (23), and kraft pulping (35).In the environment, microbial degradation can be a significant sink for DMS. In seawater, approximately 90% of the DMS produced is removed biologically before it reaches the atmosphere (21). Removal of DMS in the environment can be carried out by a variety of pathways. Aerobic bacteria, such as Hyphomicrobium spp. The prevalence of bacteria in the environment capable of growth on DMS has created interest in developing low-cost biotechnological methods to remove DMS from industrial waste gas streams. One possible technology is biofiltration which involves passing waste air through a packed bed of microorganisms. Removal of DMS in these systems, however, has proved to be difficult. This is believed t...

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