Bacterial oxidation of dibromomethane and methyl bromide in natural waters and enrichment cultures

Bacterial oxidation of¹⁴CH₂Br₂ and¹⁴CH₃Br was measured in freshwater, estuarine, seawater, and hypersaline-alkaline samples. In general, bacteria from the various sites oxidized similar amounts of¹⁴CH₂Br₂ and comparatively less ¹⁴CH₃Br. Bacterial oxidation of¹⁴CH₃Br was rapid in freshwater samples compared to bacterial oxidation of ¹⁴CH₃Br in more saline waters. Freshwater was also the only site in which methyl fluoride-sensitive bacteria (e.g., methanotrophs or nitrifiers) governed brominated methane oxidation. Half-life calculations indicated that bacterial oxidation of CH₂Br₂ was potentially significant in all of the waters tested. In contrast, only in freshwater was bacterial oxidation of CH₃Br as fast as chemical removal. The values calculated for more saline sites suggested that bacterial oxidation of CH₃Br was relatively slow compared to chemical and physical loss mechanisms. However, enrichment cultures demonstrated that bacteria in seawater can rapidly oxidize brominated methanes. Two distinct cultures of nonmethanotrophic methylotrophs were recovered; one of these cultures was able to utilize CH₂Br₂ as a sole carbon source, and the other was able to utilize CH₃Br as a sole carbon source.