Natural volatile halocarbons are important as carriers of reactive hal
ogens to the troposphere and, in the case of the more stable compounds
, to the stratosphere. Bromoform (CHBr3) has been of particular intere
st as a potential source of bromine which might account for sudden ozo
ne depletion events in the Arctic boundary layer. The oceans have been
shown to be major contributors of volatile halocarbons to the atmosph
ere, but the sources of halocarbons within them have been unknown exce
pt for macrophytic algae which are normally confined to the coastal zo
ne. Here we report experiments that demonstrate that certain unialgal
cultures of marme phytoplankton produce a suite of halocarbons (CHBr3,
CHBr2Cl, CH2Br2). The production rate of each of these halocarbons is
dependent on both species and growth stage. Chloroiodomethane which a
lso appeared in the cultures could be attributable to photochemical pr
oduction from a precursor (CH2I2). Great caution should be shown in ex
trapolating the rates estimated from these controlled experiments to t
he marine environment.