Decomposition of brominated organic halogens by cultures of marine proteobacteria: Phaeobacter, Roseobacter, and Rhodobacter

Brominated organic halogens such as bromoform (CHBr3) and dibromomethane (CH2Br2) play important roles as carriers of bromine from the ocean to the atmosphere. The bromine carried by these brominated methanes is released into the atmosphere, where it catalyzes ozone depletion. Previously, however, no microbial organisms capable of CHBr3 decomposition have been found in marine environments. In this study, we investigated bacteria to assess their capacity to decompose CHBr3 in temperate oceans. Bacteria were incubated with 13CHBr3 at 25°C and the concentrations of trace gases in the gas phase above the cultured samples were determined using dynamic headspace gas chromatography-mass spectrometry. Bacterial growth was monitored by measuring the optical density at 600 nm. 13CHBr3 was decomposed for several days by cultures of three different bacteria, Phaeobacter gallaeciensis, Roseobacter denitrificans, and Rhodobacter vinaykumarii, suggesting temperate marine bacteria can decompose CHBr3. The maximum 13CHBr3 decomposition rate was observed in the culture of P. gallaeciensis. The half-lives for 13CHBr3 decomposition by P. gallaeciensis, R. denitrificans, and R. vinaykumarii were 0.4 (0.3-0.5), 2.7 (2.5-2.9), and 2.0 (1.9-2.0) days, respectively. 13CH2Br2 and 13CHBr2Cl production were observed in the cultures, suggesting that 13CHBr3 was transformed into these compounds, and the production ratio of 13CH2Br2 was approximately 30% of the amount of 13CHBr3 spiked into the bacterial cultures. In contrast to CHBr3, CH2Br2 was not decomposed by the P. gallaeciensis, R. denitrificans, and R. vinaykumarii cultures. These results suggest that aquatic bacteria could act as a new sink for CHBr3 through decomposition in marine environments.