Methyl iodide production from marine phytoplankton cultures

Unialgal cultures of 15 species of marine phytoplankton were examined for the production of methyliodide (CH 3 I). CH 3 I production was quantified during log and stationary phases of phytoplankton growth in f/2 medium and compared to CH 3 I levels in control vessels. Significant increases in CH 3 I production were seen in five cultures. Phytoplankton CH 3 I production varied from a low of 10 −4 amol CH 3 I cell −1 d −1 to a high of 8 × 10 3 amol CH 3 I cell −1 d −1 . In three diatom cultures, the increase in total CH 3 I and cell‐based CH 3 I production rates were much higher during the log phase than during the stationary phase, even though the bacterial numbers continued to increase, supporting the premise of phytoplankton CH 3 I production. In two cultures, it was more difficult to distinguish between phytoplankton and bacterial production. Additional biotic and abiotic sources and sinks of CH 3 I were considered in the analysis, including bacterial consumption, chemical removal, and photochemical production. The only significant CH 3 I sink during the course of the experiment was its nucleophilic substitution with Cl − .