Growth rate-dependent synthesis of halomethanes in marine heterotrophic bacteria and its implications for the ozone layer recovery.

Halomethanes (e.g., CH Cl, CH Br, CH I and CHBr ) are ozone-depleting compounds that, in contrast to the human-made chlorofluorocarbons, marine organisms synthesize naturally. Therefore, their production cannot be totally controlled by human action. However, identifying all their natural sources and understanding their synthesis regulation can help to predict their production rates and their impact on the future recovery of the Earth's ozone layer. Here we show that the synthesis of mono-halogenated halocarbons CH Cl, CH Br, and CH I is a generalized process in representatives of the major marine heterotrophic bacteria groups. Furthermore, halomethane production was growth rate dependent in all the strains we studied, implying uniform synthesis regulation patterns among bacterioplankton. Using these experimental observations and in situ halomethane concentrations, we further evaluated the potential production rates associated with higher bacterial growth rates in response to global warming in a coastal environment within the Southern California Bight. Our estimates show that a 3°C temperature rise would translate into a 35%-84% increase in halomethane production rate by 2100. Overall, these data suggest that marine heterotrophic bacteria are significant producers of these climate-relevant gases and that their contribution to the atmospheric halogen budget could increase in the future, impacting the ozone layer recovery.