Production of methyl chloride and methyl bromide in laboratory cultures of marine phytoplankton

Laboratory cultures of three phytoplankton species (Phaeodactylum tricornutum, Phaeocystis sp., Thalassiosira weissflogii) were tested for methyl halide (monohalomethane) production by sparging and cryotrapping coupled with GC-ECD detection. Both axenic and xenic cultures were tested under various nutrient regimens. Production of methyl bromide (CH₃Br) and methyl chloride (CH₃Cl) was observed in all cultures. Methyl iodide (CH₃I) production was also observed but could not be quantified due to Chromatographic interference. No consistent differences in production rates were observed between axenic and xenic cultures or between nutrient regimens. Methyl halide production was not directly dependent on photosynthesis. Within each species, total methyl halide production was most closely correlated with biomass, measured as utilized nitrate (ΔDIN) or Chl a. Among the three species, Phaeocystis sp. had the highest production rates and T. weissflogii the lowest. In all cases, the biomass-normalized production rates were only a fraction of the levels needed for the ocean to constitute a significant global source of either CH₃Cl or CH₃Br. However, it must be noted that these data comprise results from a limited number of species and a limited range of conditions.