The marine environment is known to be a source of CHBr3 and CH2Br2 and hence ozone-depleting inorganic bromine to the troposphere but, to date, the dominant processes controlling their concentrations in seawater remain poorly understood. Here results are reported from a series of laboratory experiments designed to investigate bromocarbon dynamics in cultures of marine diatoms and bacteria isolated recently from the Rothera Time-Series (RaTS) site located in coastal waters of the western Antarctic Peninsula. The main focus of this work was an isolate of the centric diatom Thalassiosira sp. Different processes were found to control the concentrations of CHBr3 and CH2Br2 in this culture. The production of CHBr3 was restricted to the exponential phase of growth suggesting a link with a primary metabolic process and was a factor of 5–6 higher in cultures treated with antibiotics to reduce bacterial activity. 13CHBr3 additions confirmed that CHBr3 was not subject to significant bacterial breakdown and hence bacteria are likely to be inhibiting the production of this compound. The rate of 13CH2Br2 appearance in the cultures observed following 13CHBr3 addition suggests that the major source of CH2Br2 in the diatom culture was transformation from CHBr3. CD2Br2 additions revealed that CH2Br2 was subject to significant breakdown in cultures of both Thalassiosira sp. and a bacterial isolate with apparent loss rate constants ranging from 0.21 to 0.78 day− 1. These findings are used to produce an empirical scheme describing bromocarbon cycling in natural waters which is validated against measured concentration data from the RaTS site. The detailed process information and schemes presented provide a major step forward towards the development of biogeochemical modules that could be coupled to ecosystem models. These could then be used to predict how sea-to-air biogenic bromine emissions will change under future scenarios.